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Ultraviolet luminescence and creation of (WO4)3−-type centers under UV irradiation of PbWO4 crystals doped with trivalent rare-earth ionsUltraviolet luminescence and creation of (WO 4 ) 3--type centers under UV irradiation of PbWO 4 crystals doped with trivalent rare-earth ions
A Krasnikov
T Kärner
V Kiisk
V V Laguta
Institute for Problems of Material Science
Ukrainian AS
Krjijanovskogo 303142KievUkraine
Institute of Physics
Cukrovarnicka 10162 53PragueAS CRCzech Republic
M Nikl
Institute of Physics
Cukrovarnicka 10162 53PragueAS CRCzech Republic
I Sildos
Y Usuki
Material Research Laboratory
Furukawa Co. Ltd
305-0856TsukubaJapan
S Zazubovich
Institute of Physics
University of Tartu
Riia 14251014TartuEstonia
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Ultraviolet luminescence and creation of (WO4)3−-type centers under UV irradiation of PbWO4 crystals doped with trivalent rare-earth ionsUltraviolet luminescence and creation of (WO 4 ) 3--type centers under UV irradiation of PbWO 4 crystals doped with trivalent rare-earth ions
10.1088/1742-6596/249/1/012001
Luminescence characteristics and creation of (WO 4 ) 3and (WO 4 ) 3--A 3+ centers were studied in PbWO 4 crystals doped with trivalent rare-earth A 3+ ions (A 3+ : La 3+ , Lu 3+ , Y 3+ , Ce 3+ , Gd 3+ , Eu 3+ ) under selective irradiation in the 3.4-5.0 eV energy range at 5-80 K. Optically created centers were detected by the TSL and ESR methods. Creation spectra of the mentioned centers were measured and the activation energies needed for their creation were calculated. It was found that both the (WO 4 ) 3and (WO 4 ) 3--A 3+ centers can be created not only in the band-to-band transitions region but also in the 3.8 ± 0.4 eV range. The activation energy for their creation at T<40 K is close to zero. Under irradiation in the energy range around 3.8 eV at T>60 K, the activation energy for (WO 4 ) 3--A 3+ centers creation varies from 10 to 25 meV, depending on the irradiation energy. A new UV emission, peaking in the 3.05-3.20 eV range, was found whose thermal quenching takes place with the same activation energy. Radiative decay of defect-related states, as well as the processes of their nonradiative decay, resulting in creation of (WO 4 ) 3--A 3+ centers and thermal quenching of the UV emission, are considered to explain the effects observed.
Introduction
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Ultraviolet luminescence and creation of (WO4)3−-type centers under UV irradiation of PbWO4 crystals doped with trivalent rare-earth ionsSingle crystals of lead tungstate (PbWO 4 ) are well known as a fast and heavy scintillation material for high-energy physics experiments. Radiative and non-radiative decay of various exciton-and defectrelated states under selective UV excitation at T>80 K was systematically studied for many PbWO 4 crystals (see, e.g., [1][2][3][4] and references therein). It was found that the radiative decay of the self-trapped and various localized excitons of the (WO 4 ) 2type, arising from the regular crystal regions, is responsible for the complex blue B emission band. The complex low-temperature green G(I) emission band of undoped crystals was ascribed to excitons of the same type, but arising from the lead-deficient 5 To whom any correspondence should be addressed. crystal regions. Photo-thermally stimulated non-radiative decay of the exciton-and defect-related states results in the appearance in the crystal lattice of various electron and hole centers. Tunneling recombination of the electron centers connected with oxygen vacancies and the hole centers connected with lead vacancies is accompanied with the green G(II) emission. It was concluded that lead and oxygen vacancies play the most important role in the optically and thermally stimulated recombination processes. Co-doping with stable trivalent rare-earth A 3+ ions which substitute for Pb 2+ ions in the PbWO 4 crystal lattice [5] suppresses the lead-deficient structure and reduces the number of the isolated vacancies [2,6,7]. This leads to a strong suppression of both the G(I) emission and slow (μs-ms) tunnelling recombination G(II) emission and to an enhancement of the fast (2-4 ns at RT) B emission. As a result, considerable improvement of scintillation characteristics of PbWO 4 crystals is obtained. However, the peculiarities of the luminescence and defects creation processes in A 3+ -doped PbWO 4 crystals were not studied in detail. Therefore, the aim of the present work was to study the processes of luminescence and creation of electron (WO 4 ) 3and (WO 4 ) 3--A 3+ centers in PbWO 4 :A 3+ crystals (A 3+ : La 3+ , Lu 3+ , Y 3+ , Ce
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Ultraviolet luminescence and creation of (WO4)3−-type centers under UV irradiation of PbWO4 crystals doped with trivalent rare-earth ions3+ , Y 3+ , Ce 3+ , Gd 3+ , Eu 3+ ) under their selective irradiation in the 3.4-5.0 eV energy range at 5-80 K.
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Ultraviolet luminescence and creation of (WO4)3−-type centers under UV irradiation of PbWO4 crystals doped with trivalent rare-earth ionsExperimental procedure
Single crystals of PbWO 4 :A 3+ were grown in Japan by Furukawa Co. Ltd from 4N purity raw material, using the Czochralski method in air with a Pt crucible and following the third crystallization method [8]. The concentration of A 3+ ions was about 135 ppm in the melt. The crystals contain less than 1 ppm of other impurities [8] and also very few isolated (non-compensated) lead and oxygen vacancies [2][3][4][5][6].
Luminescence characteristics were studied with the use of the set-ups and procedures described in [1,3]. Decay kinetics were measured with a modified Spectrofluorometer 199S (Edinburgh Instruments) under excitation with a nanosecond coaxial hydrogen-filled flashlamp (IBH) or microsecond Xe lamp (IBH) and using two single grating monochromators. The detection was performed with a IBH-04 photomultiplier module using the method of time-correlated single photon counting. A deconvolution procedure (SpectraSolve software package) was applied to extract true decay times from multiexponential approximation. Time-resolved emission spectra were measured under excitation with a Nd:YAG laser-pumped optical parametric oscillator NT 342/1/UVE (EKSPLA, Lithuania). It allows to obtain pulsed monochromatic radiation in the 0.6-6.0 eV energy range with a pulse duration of 5 ns.
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Ultraviolet luminescence and creation of (WO4)3−-type centers under UV irradiation of PbWO4 crystals doped with trivalent rare-earth ionsOptically created centers were detected by the thermally stimulated luminescence (TSL) and electron spin resonance (ESR) methods. The TSL glow curves were measured with a heating rate of 0.2 K/s after selective UV-irradiation of a crystal for 15-30 min through a monochromator with spectral width of the slits ≈5 nm. The spectral integrated emission of a crystal was detected with a photomultiplier or a Hamamatsu (6240) photon counting system. The ESR spectra were measured at 18 K using a ERS 231spectrometer (Germany) after irradiation of a non-oriented PbWO 4 :Y 3+ crystal at different temperatures (10-80 K) with the Nd:YAG laser. After each TSL or ESR measurement the crystal was heated up to 150 K to destroy the optically created centers.
The experiments at low temperatures were carried out with the use of immersion helium cryostats and vacuum nitrogen cryostats.
Experimental results
Photoluminescence characteristics
In the PbWO 4 :A 3+ crystals studied, besides the intense B emission, the weak ultraviolet -UV, green -G(I) and red -1. different from the red emissions (1.57 eV and 1.48 eV) ascribed in [9] to complex lead-vacanciescontaining centers. These centers are absent in most of the crystals studied in the present work. Under 3.85 eV excitation, the intensities of the UV, G(I) and 1.9 eV emissions are comparable (figure 1a). The intensity ratio of the UV and G(I) bands depends on the crystal (table 1).
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Ultraviolet luminescence and creation of (WO4)3−-type centers under UV irradiation of PbWO4 crystals doped with trivalent rare-earth ionsIn the excitation spectrum of the UV emission, the exciton band located at about 4.1 eV and a complex band located in the 3.50-3.95 eV range are observed (figure 1b). The 4.1 eV band appears partly due to the overlap of the UV and B emission bands. Its maximum is shifted to lower energy with respect to the exciton band located at 4.13-4.15 eV in the excitation spectra of the other emissions. Table 1. Intensity ratios obtained from corrected emission spectra measured at 80 K under E exc =3.85 eV (for the UV and G(I) emissions) and E exc =4.5 eV (for the B emission). The activation energies E q for thermal quenching of the UV emission. The activation energies E a for the creation of the TSL peak at 97-107 K under irradiation at T irr >55 K in the exciton (4.1 eV) and in the defect-related (3.7 eV) energy range. A very fast weak ≈3.25 eV emission band was detected at RT in [10] and ascribed to the radiative decay of the non-relaxed self-trapped exciton state. If so, the B/3.25 eV emission intensity ratio should be sample-independent. However, this is not the case for the UV emission studied (table 1). Besides, according to [10], the intensity of the 3.25 eV emission decreases with decreasing temperature. Thus, the UV emission reported in the present paper is different from that observed before.
Thermally stimulated luminescence studies
Thermally stimulated luminescence of X-ray irradiated PbWO 4 :A 3+ crystals was studied in [5,[11][12][13][14][15].
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Ultraviolet luminescence and creation of (WO4)3−-type centers under UV irradiation of PbWO4 crystals doped with trivalent rare-earth ionsThe TSL peaks at about 50 K and 95-110 K were ascribed to the thermal destruction of (WO 4 ) 3and (WO 4 ) 3--A 3+ centers, respectively. The TSL intensity in the UV-irradiated crystals is weak. The characteristics of all the crystals studied are similar. In the present paper they are illustrated on the example of the PbWO 4 :Y 3+ crystal. After irradiation at T irr <40 K, peaks appear at 53 K and 107 K (figure 4a). They are most effectively created in the band-to-band transitions region and by about two orders of magnitude less effectively, in the 3.8 ± 0.4 eV range. The creation spectra of both TSL peaks coincide (figure 4b), and around 3.8 eV they are close to the excitation spectrum of the UV emission (compare figures 4c and 1b). From the dependences of the TSL peak intensity on the irradiation temperature T irr , the activation energies E a were determined for the TSL peaks creation under irradiation in the exciton (4.05 eV) and defect-related (3.7 eV) regions. For (WO 4 ) 3centers creation, E a =0 (figure 5a). At T irr <40 K, the E a value for the creation of (WO 4 ) 3--A 3+ centers is also close to zero (figure 5b) which means that these centers are created only at thermal destruction of (WO 4 ) 3centers. At T irr >60 K, (WO 4 ) 3--A 3+ centers are created with E a =10-25 meV (table 1) depending on the irradiation energy E irr (see the inset in figure 5b). In the inset, dependence of the activation energy E a for the 107 K peak creation on E irr obtained under irradiation at T irr >55 K.
Electron spin resonance studies
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Ultraviolet luminescence and creation of (WO4)3−-type centers under UV irradiation of PbWO4 crystals doped with trivalent rare-earth ionsElectron spin resonance studies
The ESR spectra of (WO 4 ) 3and (WO 4 ) 3--La 3+ centers in PbWO 4 crystals were studied in [16,17] and [5,13], respectively. The ESR characteristics of (WO 4 ) 3--Lu 3+ centers were described in [17]. In the present paper, the ESR signals from (WO 4 ) 3and (WO 4 ) 3--Y 3+ centers were detected. After irradiation of PbWO 4 :Y 3+ with E irr =4.05 eV at T irr <40 K, (WO 4 ) 3centers are mainly created, while (WO 4 ) 3--Y 3+ centers appear only after heating of the irradiated sample up to 60 K or after irradiation at 60-80 K ( figure 6). Similar results were obtained in [5,17] under irradiation of PbWO 4 :A 3+ crystals with the He-Cd laser (3.82 eV). From the dependence of the ESR signal intensity on the irradiation temperature, the activation energies E a were calculated for the creation of (WO 4 ) 3--Y 3+ centers. Under irradiation at T irr <40 K, the E a values are close to zero. Under irradiation at 4.05 eV in the 60-80 K temperature range, a value of E a =11 meV was obtained. This value is close to the E a value obtained for 3- Figure 6. The ESR spectra of (WO 4 ) 3and (WO 4 ) 3--Y 3+ centers measured at 18 K after irradiation of the PbWO 4 :Y 3+ crystal at T irr =20 K and T irr =60 K, respectively. E irr =4.05 eV. The orientation of the magnetic field was close to the [100] axis of the crystal.
Discussion
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Ultraviolet luminescence and creation of (WO4)3−-type centers under UV irradiation of PbWO4 crystals doped with trivalent rare-earth ionsDiscussion
The data obtained in [5,17,18] and in the present paper indicate that electron (WO 4 ) 3and (WO 4 ) 3--A 3+ centers in PbWO 4 crystals can be created by the photons of energies 3.8 ± 0.4 eV which are about 1 eV smaller than the band gap energy. The creation of (WO 4 ) 3centers was explained before by a tunnelling process of the electron from an excited Pb 2+ ion [17] or by the ionisation of (MoO 4 ) 2groups and the subsequent trapping of the released electrons at (WO 4 ) 2groups [18]. However, the PbWO 4 :A 3+ crystals studied do not contain the Mo impurity. In the undoped PbWO 4 crystals, the absorption in the energy range around 3.8 eV is usually ascribed to the excitons from Pb-deficient (e.g., Pb 7.5 W 8 O 32 [19]) crystal regions or to some defect-related centers. In [1][2][3][4] it was shown that at T irr >120 K, the photo-thermally stimulated disintegration of the mentioned exciton-and defect-related states takes place with the activation energies of about 0.2-0.4 eV which are much larger than the values of E q and E a obtained in the present paper. The UV emission band, whose excitation spectrum coincides with the creation spectrum of electron (WO 4 ) 3and (WO 4 ) 3--A 3+ centers in the energy range around 3.8 eV, is relatively wide. Its FWHM=0.60±0.04 eV, like for the B and G(I) emission bands which both are of exciton origin. However, the Stokes shift (≈0.7 eV) is smaller than that observed for the other exciton emission bands in PbWO 4 crystals. The presence in the decay kinetics of the UV emission of two (fast -35 ns and slow -350 ns at 80 K) components with practically coinciding emission spectra is characteristic for an emission, arising from the triplet relaxed excited state (see, e.g., [20] and references therein).
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Ultraviolet luminescence and creation of (WO4)3−-type centers under UV irradiation of PbWO4 crystals doped with trivalent rare-earth ionsWe assume that the effects observed are due to the radiative and non-radiative decay of an excited state of some defect-related center. Under excitation around 3.8 eV, not only the radiative decay of this state, accompanied with the weak UV emission, but also ionisation of the defect center is possible. At T irr <40 K, the electrons released into the conduction band are trapped at (WO 4 ) 2groups and electron (WO 4 ) 3centers are created with the activation energy E a =0. The energy level of (WO 4 ) 3is located at 50 meV below the bottom of the conduction band [16]. We assume that tunneling transitions of electrons from the (WO 4 ) 3centers into the excited state of the mentioned defect center can take place. This process results also in the appearance of the UV emission. At T irr >60 K, when the (WO) 4 centers are thermally unstable, the optically released and trapped at (WO 4 ) 2electrons are immediately thermally released from the created (WO 4 ) 3centers into the conduction band and retrapped at more deep (WO 4 ) 2--A 3+ traps. As a result of this dynamical process, electron (WO 4 ) 3--A 3+ centers are created with the activation energy which is smaller than 50 meV. This process leads to the decrease of the population of the excited state of the defect center and, consequently, to the thermal quenching of the UV emission with the same activation energy of 10-25 meV. Further studies are needed to make a justified conclusion on the origin of the defect center responsible for the effects observed and on the structure and dynamics of its excited states.
3-
International Conference on Defects in
Conclusions
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Ultraviolet luminescence and creation of (WO4)3−-type centers under UV irradiation of PbWO4 crystals doped with trivalent rare-earth ionsConclusions
The ESR and TSL studies of PbWO 4 :A 3+ crystals (A 3+ : La 3+ , Lu 3+ , Y 3+ , Ce 3+ , Gd 3+ , Eu 3+ ) irradiated in the 3.4-5.0 eV energy range at 5-80 K indicate that electron (WO 4 ) 3and (WO 4 ) 3--A 3+ centers can be created under irradiation of the crystals not only in the band-to-band transitions region but also in the energy range around 3.8 eV. The activation energies for their creation are E a =0 and E a =10-25 meV, respectively. Under excitation in this range, a new UV emission band peaking at 3.05-3.20 eV is found. At T>60 K, both thermal quenching of the UV emission and the photo-thermally stimulated creation of (WO 4 ) 3--A 3+ centers take place with the same activation energy. The radiative and nonradiative decay of a defect-related state are proposed as possible reasons of the effects observed.
9 eV emissions are observed (figure 1a). The position and FWHM of the UV band are slightly different in different crystals, varying from 3.05 eV to 3.20 eV and from 0.56 eV to 0.64 eV, respectively. This difference can be caused by a complex structure of this band and by its overlap with the B band. The B/UV emission intensity ratio depends on the crystal (table 1). The maximum position of the G(I) emission band varies in different crystals from 2.25 eV to 2.40 eV. The 1.9 eV emission is
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Ultraviolet luminescence and creation of (WO4)3−-type centers under UV irradiation of PbWO4 crystals doped with trivalent rare-earth ionsFigure 1 .
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Ultraviolet luminescence and creation of (WO4)3−-type centers under UV irradiation of PbWO4 crystals doped with trivalent rare-earth ions1(a) Corrected and normalized emission spectra of the PbWO 4 :135 ppm Y 3+ crystal measured at 80 K under excitation at 4.5 eV (solid line: the B band) and 3.85 eV (dashed line: the UV, G(I), and R bands). The B/UV emission intensity ratio is equal to 108 (seetable 1). (b) Corrected excitation spectrum of the UV emission at 80 K. Temperature dependence of the UV emission intensity (in the inset).Thermal quenching of the UV emission takes place at much lower temperatures (T q ≈130 K, see the inset in figure 1b) as compared with the B emission (T q ≈160-180 K [1]) (T q is the temperature where the emission intensity has decreased by a factor of 2). The activation energies E q for thermal quenching of this emission, calculated from the lnI(1/T) dependences, are about 10-16 meV and slightly depend on the crystal (seetable 1).At 80 K the decay kinetics of the UV emission of the PbWO 4 :Y 3+ crystal consists of three components with decay times of 35 ns, 350 ns (figure 2a) and 11-14 μs (figure 2b, curve 1). The time-spectra, shown in the inset of figure 2a, indicate that both the ns-components arise from the 3.15 eV band. In the emission spectrum of the μs-component, the UV band located at about 3.05 eV and the G(I) band are observed (see the inset infigure 2b). The decay time of the G(I) emission (figure 2b, curve 2) is close to the value (≈20 μs) observed in other undoped crystals[3]. The time-resolved emission spectra measured at 10 K at different time gates (figure 3) also indicate that the spectra of the fast (curves 1, 2) and the slow (curves 4, 5) decay components of the UV emission are located at about 3.15 eV and 3.05 eV, respectively. At 10 K, the decay time of the slow component is about 100 μs.
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Ultraviolet luminescence and creation of (WO4)3−-type centers under UV irradiation of PbWO4 crystals doped with trivalent rare-earth ionsFigure 2 .
2Luminescence decay kinetics in the PbWO 4 :Y 3+ crystal measured at 80 K for the (a) 3.15 eV; (b) 3.0 eV (curve 1) and 2.35 eV (curve 2) emission bands. In the insets: (a) the emission spectra of the ns-components; (b) the emission spectrum measured at t=40 μs after the excitation pulse. E exc =3.8 eV.
Figure 3 .
3Time-resolved emission spectra of the PbWO 4 :135 ppm Y 3+ crystal at 10 K. Time gates: 0-0.1 μs (curve 1), 0.1-1.1 μs (curve 2), 1-11 μs (curve 3), 10-110 μs (curve 4), and 100-1100 μs (curve 5). Excitation with the Nd:YAG laser at 3.87 K; for emission detection spectral width of slits 10 nm.
Figure 4 .
4(a) The TSL glow curve measured after irradiation of the PbWO 4 :Y 3+ crystal at 26 K with E irr =3.8 eV. (b, c) Creation spectra of the TSL peaks at 53 K (solid circles) and 107 K (open circles) (normalized at 4.7 eV) measured under irradiation (b) at 26 K and (c) at 75 K.
Figure 5 .
5Dependence of the TSL intensity on the irradiation temperature T irr measured for the TSL peaks located at (a) 53 K and (b) 107 K after irradiation of the PbWO 4 :Y 3+ crystal with E irr =4.05 eV (solid circles) or E irr =3.7-3.8 eV (open circles).
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Ultraviolet luminescence and creation of (WO4)3−-type centers under UV irradiation of PbWO4 crystals doped with trivalent rare-earth ionsInsulating Materials IOP Publishing Journal of Physics: Conference Series 249 (2010) 012001 doi:10.1088/1742-6596/249/1/012001
AcknowledgementsThis work was partly supported by the Estonian Science Foundation Grants No. 7507, 6999 and Czech Institutional Research Plan AV0Z10100521.
. A Krasnikov, M Nikl, S Zazubovich, Phys. Status Solidi B. 2431727Krasnikov A, Nikl M and Zazubovich S 2006 Phys. Status Solidi B 243 1727
. V V Laguta, Nikl , M Zazubovich, S , Rad. Measur. 42515Laguta V V, Nikl M and Zazubovich S 2007 Rad. Measur. 42 515
. V Babin, P Bohacek, A Krasnikov, M Nikl, A Stolovits, S Zazubovich, J. Lumin. 124113Babin V, Bohacek P, Krasnikov A, Nikl M, Stolovits A and Zazubovich S 2007 J. Lumin. 124 113
. V V Laguta, Nikl , M Zazubovich, S , IEEE Transactions on Nuclear Science. 551275Laguta V V, Nikl M and Zazubovich S 2008 IEEE Transactions on Nuclear Science 55 issue 3 part 1 1275
. V V Laguta, M Martini, F Meinardi, A Vedda, A Hofstaetter, B K Meyer, M Nikl, E Mihokova, Rosa J Usuki, Y , Phys. Rev. B. 6210109Laguta V V, Martini M, Meinardi F, Vedda A, Hofstaetter A, Meyer B K, Nikl M, Mihokova E, Rosa J and Usuki Y 2000 Phys. Rev. B 62 10109
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Ultraviolet luminescence and creation of (WO4)3−-type centers under UV irradiation of PbWO4 crystals doped with trivalent rare-earth ions. V V Laguta, M Martini, A Vedda, M Nikl, E Mihokova, P Bohacek, J Rosa, A Hofstaetter, B K Meyer, Y Usuki, Phys. Rev. B. 64165102Laguta V V, Martini M, Vedda A, Nikl M, Mihokova E, Bohacek P, Rosa J, Hofstaetter A, Meyer B K and Usuki Y 2001 Phys. Rev. B 64 165102
. Han Baoguo, Feng Xiqi, Hu Guanqin, Wang Pingchu, Yin Zhiwen, J. Appl. Phys. 842831Han Baoguo, Feng Xiqi, Hu Guanqin, Wang Pingchu and Yin Zhiwen 1998 J. Appl. Phys. 84 2831
. M Kobayashi, M Ishii, K Harada, Y Usuki, H Okuno, H Shimizu, T Yazawa, Nucl. Instr. Meth. Phys. Research A. 373333Kobayashi M, Ishii M, Harada K, Usuki Y, Okuno H, Shimizu H and Yazawa T 1996 Nucl. Instr. Meth. Phys. Research A 373 333
. P Bohacek, N Senguttuvan, V Kiisk, A Krasnikov, M Nikl, I Sildos, Y Usuki, S Zazubovich, Rad. Measur. 38623Bohacek P, Senguttuvan N, Kiisk V, Krasnikov A, Nikl M, Sildos I, Usuki Y and Zazubovich S 2004 Rad. Measur. 38 623
. G Tamulaitis, S Burachas, V P Martinov, V D Ryzhikov, H H Gutbrod, V Manko, M Terekhin, Phys. Status Solidi A. 161533Tamulaitis G, Burachas S, Martinov V P, Ryzhikov V D, Gutbrod H H, Manko V I and Terekhin M 1997 Phys. Status Solidi A 161 533
|
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Ultraviolet luminescence and creation of (WO4)3−-type centers under UV irradiation of PbWO4 crystals doped with trivalent rare-earth ions. M Martini, F Meinardi, G Spinolo, A Vedda, M Nikl, Y Usuki, Phys. Rev. B. 604653Martini M, Meinardi F, Spinolo G, Vedda A, Nikl M and Usuki Y 1999 Phys. Rev. B 60 4653
M Böhm, Proc. of the 5th Int. Conf. on Inorganic Scintillators and Their Applications. V V Mikhailinof the 5th Int. Conf. on Inorganic Scintillators and Their ApplicationsMoscow, Russia; MoscowMoscow State University Press619Böhm M et al. 2000 Proc. of the 5th Int. Conf. on Inorganic Scintillators and Their Applications (Moscow, Russia, August 16-20, 1999) ed V V Mikhailin (Moscow: Moscow State University Press) p 619
A Hofstaetter, Proc. of the 5th Int. Conf. on Inorganic Scintillators and Their Applications. V V Mikhailinof the 5th Int. Conf. on Inorganic Scintillators and Their ApplicationsMoscow, Russia; MoscowMoscow State University Press128Hofstaetter A et al. 2000 Proc. of the 5th Int. Conf. on Inorganic Scintillators and Their Applications (Moscow, Russia, August 16-20, 1999) ed V V Mikhailin (Moscow: Moscow State University Press) p 128
. M Nikl, P Bohacek, E Mihokova, M Martini, F Meinardi, A Vedda, P Fabeni, G P Pazzi, M Kobayashi, M Ishii, Y Usuki, J. Appl. Phys. 874243Nikl M, Bohacek P, Mihokova E, Martini M, Meinardi F, Vedda A, Fabeni P, Pazzi G P, Kobayashi M, Ishii M and Usuki Y 2000 J. Appl. Phys. 87 4243
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Ultraviolet luminescence and creation of (WO4)3−-type centers under UV irradiation of PbWO4 crystals doped with trivalent rare-earth ions. 10.1088/1742-6596/249/1/012001International Conference on Defects in Insulating Materials IOP Publishing Journal of Physics: Conference Series. 24912001International Conference on Defects in Insulating Materials IOP Publishing Journal of Physics: Conference Series 249 (2010) 012001 doi:10.1088/1742-6596/249/1/012001
. M Nikl, P Bohacek, E Mihokova, N Solovieva, M Martini, A Vedda, P Fabeni, G P Pazzi, M Kobayashi, M Ishii, Y Usuki, D Zimmerman, J. Cryst. Growth. 229312Nikl M, Bohacek P, Mihokova E, Solovieva N, Martini M, Vedda A, Fabeni P, Pazzi G P, Kobayashi M, Ishii M, Usuki Y and Zimmerman D 2001 J. Cryst. Growth 229 312
. V V Laguta, J Rosa, M I Zaritskii, Nikl , M Usuki, Y , J. Phys.: Condens. Matter. 107293Laguta V V, Rosa J, Zaritskii M I, Nikl M and Usuki Y 1998 J. Phys.: Condens. Matter 10 7293
. M Böhm, F Henecker, A Hofstaetter, M Luh, B K Meyer, A Scharmann, O Kondratiev, M V Korzhik, Eff. & Defects Solids. 15021Böhm M, Henecker F, Hofstaetter A, Luh M, Meyer B K, Scharmann A, Kondratiev O V and Korzhik M V 1999 Rad. Eff. & Defects Solids 150 21
. A Hofstaetter, M V Korzhik, V V Laguta, B K Meyer, Nagirnyi V Novotny, R , Rad. Measur. 33533Hofstaetter A, Korzhik M V, Laguta V V, Meyer B K, Nagirnyi V and Novotny R 2001 Rad. Measur. 33 533
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Ultraviolet luminescence and creation of (WO4)3−-type centers under UV irradiation of PbWO4 crystals doped with trivalent rare-earth ions. J M Moreau, Galez Ph, R E Gladyshevski, J Peigneux, M V Korzhik, J. Allows and Comp. 284104Moreau J M, Galez Ph, Gladyshevski R E, Peigneux J P and Korzhik M V 1999 J. Allows and Comp. 284 104
. A Fukuda, Phys. Rev. 14161Fukuda A 1970 Phys. Rev. B1 4161
. 10.1088/1742-6596/249/1/012001International Conference on Defects in Insulating Materials IOP Publishing Journal of Physics: Conference Series. 24912001International Conference on Defects in Insulating Materials IOP Publishing Journal of Physics: Conference Series 249 (2010) 012001 doi:10.1088/1742-6596/249/1/012001
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsNovel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal Ions
Published: 10 April 2017
Fang-Cheng Liang
Institute of Organic and Polymeric Materials
National Taipei University of Technology
10608TaipeiTaiwan
Institute of Glycosciences
Grenoble Alpes University
CNRS
5301, 38000GrenobleCERMAV UPRFrance
Yi-Ling Luo
Institute of Organic and Polymeric Materials
National Taipei University of Technology
10608TaipeiTaiwan
Chi-Ching Kuo
Institute of Organic and Polymeric Materials
National Taipei University of Technology
10608TaipeiTaiwan
Bo-Yu Chen
Chia-Jung Cho
Institute of Organic and Polymeric Materials
National Taipei University of Technology
10608TaipeiTaiwan
Fan-Jie Lin
Yang-Yen Yu [email protected]
Institute of Organic and Polymeric Materials
National Taipei University of Technology
10608TaipeiTaiwan
Department of Materials Engineering
Ming Chi University of Technology
24301New TaipeiTaiwan
Department of Chemical and Materials Engineering
Chang Gung University
33302TaoyuanTaiwan
Redouane Borsali [email protected]
Institute of Organic and Polymeric Materials
National Taipei University of Technology
10608TaipeiTaiwan
Institute of Glycosciences
Grenoble Alpes University
CNRS
5301, 38000GrenobleCERMAV UPRFrance
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsNovel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal Ions
Published: 10 April 201710.3390/polym9040136Received: 27 February 2017; Accepted: 6 April 2017;polymers Article Academic Editor: Po-Chih Yangelectrospun nanofibersheavy metal ionsmagneticfluorescent sensingchemosensory
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsNovel multifunctional switchable chemosensors based on fluorescent electrospun (ES) nanofibers with sensitivity toward magnetism, temperature, and mercury ions (Hg 2+ ) were prepared using blends of poly(N-isopropylacrylamide)-co-(N-methylolacrylamide)-co-(Acrylic acid), the fluorescent probe 1-benzoyl-3-[2-(2-allyl-1,3-dioxo-2,3-dihydro-1Hbenzo [de]isoquinolin-6-ylamino)ethyl]-thiourea (BNPTU), and magnetite nanoparticles (NPs), and a single-capillary spinneret. The moieties of N-isopropylacrylamide, N-methylolacrylamide, acrylic acid, BNPTU, and Iron oxide (Fe 3 O 4 ) NPs were designed to provide thermoresponsiveness, chemical cross-linking, Fe 3 O 4 NPs dispersion, Hg 2+ sensing, and magnetism, respectively. The prepared nanofibers exhibited ultrasensitivity to Hg 2+ (as low as 10 −3 M) because of an 80-nm blueshift of the emission maximum (from green to blue) and 1.6-fold enhancement of the emission intensity, as well as substantial volume (or hydrophilic to hydrophobic) changes between 30 and 60 • C, attributed to the low critical solution temperature of the thermoresponsive N-isopropylacrylamide moiety. Such temperature-dependent variations in the presence of Hg 2+ engendered distinct on-off switching of photoluminescence. The magnetic ES nanofibers can be collected using a magnet rather than being extracted through alternative methods. The results indicate that the prepared multifunctional fluorescent ES nanofibrous membranes can be used as naked eye sensors and have the potential for application in multifunctional environmental sensing devices for detecting metal ions, temperature, and magnetism as well as for water purification sensing filters.Polymers 2017, 9, 136 2 of 21 derivative probes based on 1,8-naphthalimide have been used extensively in fluorescence detection for HTMs such as Hg 2+ , Cd 2+ , and Cu 2+ because of their
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal Ions, and Cu 2+ because of their many excellent optical properties such as high quantum yields and high photo stability[5][6][7][8][9][10][11]. A new fluorescent chemodosimeter, namely a 1,8-naphthalimide-based colorimetric derivative, 1-benzoyl-3-[2-(2-allyl-1,3-dioxo-2,3dihydro-1Hbenzo[de]isoquinolin-6-ylamino)-ethyl]-thiourea (BNPTU), with high selectivity for Hg 2+ in aqueous solutions and based on the reactivity of thiourea derivatives toward Hg 2+ was reported by Liu et al.[5]. Mercury-triggered intramolecular cyclization of thiourea results in the formation of a highly blue fluorescent naphthalimide derivative, whereas the dosimeter itself fluoresces yellowish green.Poly(N-isopropyl acrylamide) (PNIPAM) is a well-known thermosensitive polymer that exhibits a low critical solution temperature (LCST) of 32 • C, close to the healthy human body temperature. PNIPAM exhibits a hydrophilic extended structure below its LCST, whereas above its LCST, it dehydrates and exhibits a compact structure[12]. Chen reported that introducing hydrophilic acrylic acid (AA) monomers with different compositions can change the thermal responses of PNIPAM microgels. Furthermore, the AA content of poly(NIPAM-co-AA) can be adjusted to change the LCST, with the hydrogen bonding interaction being the fundamental cause of the change[13]. However, all of the aforereferenced studies adopted solutions and polymer composites rather than nanofibers in sensory applications. The high surface-to-volume ratio of nanofibers could induce responses in metal ions-sensitive, temperature-sensitive, and multifunctional sensory materials.Electrospinning is an easy, versatile, and inexpensive technique that enables the assembly of various functional nanofibers[14][15][16][17]. The high surface-to-volume ratio of electrospun (ES) nanofibers has encouraged extensive research on sensory applications including sensors for pH levels[18], temperatures[19,20], nitric oxide[21], and metal ions[22][23][24][25][26]. Recently, various fluorescent sensor-based ES
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsRecently, various fluorescent sensor-based ES polymer nanofibers for sensing pH or different metal ions such as Hg 2+ , Fe 3+ , Zn 2+ , and Cu 2+ were successfully prepared by our research group[24][25][26]. These multifunctional fluorescent ES nanofibers exhibited distinct on-off switching capabilities such as quenching-enhancing photoluminescence (PL) intensity and changed-recovered PL colors for sensing various pH levels or metal ions.Poly(acrylic acid) (PAA) is a weak polyelectrolyte often used to change the surface properties of inorganic particles, because of the strong chelation of its numerous carboxyl groups with metal ions. Iron oxide (Fe 3 O 4 ) nanoparticles (NPs) obtained through a one-pot method with PAA can solve the agglomeration problem[27][28][29][30][31]. Recently, Liu reported the fabrication of Fe 3 O 4 NP, PAA, and polyvinyl alcohol composite ES nanofibers with a homogeneous dispersion of Fe 3 O 4 NPs and high water resistance[32]. In addition, Xu et al. developed a material for industrial wastewater treatment combining polydopamine-coated mesh with PAA. This material not only selectively detects Hg 2+ but also effectively separates oil and water, a process attributed to the hydrophilicity of PAA and the change in mesh wettability based on the chelation between Hg 2+ and PAA[33]. However, most studies on polymer-inorganic composites have focused on exploring their physical properties, which have seldom been studied in terms of their combination with metal ions, temperature, and magnetic mulfunctional materials in sensory applications. This paper reports the production of novel multifunctional switchable ES nanofiber sensors from blends of multifunctional copolymers and Fe 3 O 4 NPs. Comprising traditional polymer-inorganic composite ES nanofibers, our chemosensors can simultaneously detect metal ions, temperature, and magnetism.In this study, we developed magnetic fluorescent sensory ES nanofibers by using blends of poly(N-isopropylacrylamide)-co-(N-methylolacrylamide)-co-(acrylic acid) (poly(NIPAAm-co-NMAco-AA)), fluorescent BNPTU, and Fe
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal Ionsfluorescent BNPTU, and Fe 3 O 4 NPs and a single-capillary spinneret. The nanofibers, which can sense Hg 2+ , temperature, and magnetism simultaneously, were prepared by combining synthesis and electrospinning, and their optical applications and morphological characteristics were analyzed. The process for synthesizing fluorescent BNPTU probes is shown inFigure 1. The poly(NIPAAm-co-NMA-co-AA) was synthesized through free radical polymerization(Figure 2a). The moieties of N-isopropylacrylamide (NIPAAm), N-methylolacrylamide (NMA), AA, BNPTU, Polymers 2017, 9, 136
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsIntroduction
In recent years, identifying the presence and concentration of heavy metal ions has received gradually increasing levels of attention because of emerging environmental and human health issues [1,2]. Heavy transition metal (HTM) cations and their derivatives are used extensively in the industrial sector, and can cause adverse environment and health problems [3,4]; thus, developing sensitive and selective fluorescent chromogenic probes composed of chelating ligands that are used to detect HTM cations in biological and environmental sensory devices is crucial. Colorimetric and Fe 3 O 4 NPs were designed to achieve thermoresponsiveness, chemical cross-linking, hydrophilicity, Hg 2+ sensing, and magnetism, respectively. A single-capillary spinneret was used to fabricate ES nanofibers from poly(NIPAAm-co-NMA-co-AA) blended with BNPTU (10%) and Fe 3 O 4 NPs (5%). The nanofibers were then post-treated through chemical cross-linking to enhance their stability in water (Figure 2b). The fluorescence emission of BNPTU is highly selective and Hg 2+ dependent. When BNPTU is used to detect Hg 2+ , its fluorescence emission changes from green to blue, and the NIPAAm and Fe 3 O 4 NPs exhibit thermoresponsive magnetic properties, respectively (Figure 2c). The relationship between the morphology of the ES nanofibers and their photophysical properties together with their sensing behavior in aqueous solutions was systematically investigated. The favorable detection of Hg 2+ , temperature, and magnetism demonstrated by the experimental results suggests that ES nanofibrous membranes can be used as naked eye sensors, and have the potential for application in multifunctional environmental sensing devices.
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsPolymers 2017, 9,136 3 of 21 poly(NIPAAm-co-NMA-co-AA) blended with BNPTU (10%) and Fe3O4 NPs (5%). The nanofibers were then post-treated through chemical cross-linking to enhance their stability in water (Figure 2b). The fluorescence emission of BNPTU is highly selective and Hg 2+ dependent. When BNPTU is used to detect Hg 2+ , its fluorescence emission changes from green to blue, and the NIPAAm and Fe3O4 NPs exhibit thermoresponsive magnetic properties, respectively (Figure 2c). The relationship between the morphology of the ES nanofibers and their photophysical properties together with their sensing behavior in aqueous solutions was systematically investigated. The favorable detection of Hg 2+ , temperature, and magnetism demonstrated by the experimental results suggests that ES nanofibrous membranes can be used as naked eye sensors, and have the potential for application in multifunctional environmental sensing devices. poly(NIPAAm-co-NMA-co-AA) blended with BNPTU (10%) and Fe3O4 NPs (5%). The nanofibers were then post-treated through chemical cross-linking to enhance their stability in water (Figure 2b). The fluorescence emission of BNPTU is highly selective and Hg 2+ dependent. When BNPTU is used to detect Hg 2+ , its fluorescence emission changes from green to blue, and the NIPAAm and Fe3O4 NPs exhibit thermoresponsive magnetic properties, respectively (Figure 2c). The relationship between the morphology of the ES nanofibers and their photophysical properties together with their sensing behavior in aqueous solutions was systematically investigated. The favorable detection of Hg 2+ , temperature, and magnetism demonstrated by the experimental results suggests that ES nanofibrous membranes can be used as naked eye sensors, and have the potential for application in multifunctional environmental sensing devices.
Experimental Section
Materials
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsN-Isopropylacrylamide (NIPAAm) and N-methylolacrylamide (NMA) were provided by Tokyo Chemical Industry Co. Japan. NIPAAm was re-crystallized three times from hexane-toluene (10/1, v/v) prior to use. Acrylic acid (AA, Aldrich, Saint Louis, MO, USA, 99%) was dried over CaH 2 to distill in a round-bottomed flask overnight, then purified by passing it through a short aluminum-oxide column (50-200 µm), and stored at 4 • C prior to use. The radical initiator 2,2 -azobis(2-methylpropionitrile) (AIBN) was purchased from UniRegion Bio-Tech and was recrystallized twice in ethanol prior to use. Iron (II) (FeCl 2 ·4H 2 O) and Iron (III) (FeCl 3 ·6H 2 O) chloride hexahydrate (Aldrich, 99%), dichloromethane (Tedia, Fairfield, OH, USA, 99.9%); methanol (Tedia, HPLC/SPECTRO); NH 4 OH (Alfa, 28%); anhydrous tetrahydrofuran (99.9%); acetone (99.8%); calcium hydride (reagent grade 95%); diethyl ether (99%); 4-bromo-1,8-naphthalic anhydride (95%); benzoyl isothiocyanate (98%); allylamine (98%); 1,2-ethylenediamine (99%); chloroform (anhydrous, 99%); N,N-dimethylformamide (99%); 1,4-dioxane (99.5%); acetonitrile (99%); ethanol (99.8%); and hydrazine hydrate (reagent grade, N 2 H 4 50-60%) were used as received. Perchlorate salts of metal ions (Co 2+ , Cu 2+ , Fe 2+ , Mg 2+ , Hg 2+ , Pb 2+ , Zn 2+ , Cd
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal Ions, Zn 2+ , Cd 2+ , Ni 2+ ) were purchased from Sigma-Aldrich (Saint Louis, MO, USA).
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsSynthesis of 4-Bromo-N-allyl-1,8-naphthalimide
Fluorescent BNPTU was prepared according to a previously reported method [8]. The process of its synthesis is illustrated in Figure 1. A 50-mL round-bottomed flask was charged with 4-bromo-1,8naphthalic anhydride (1.385 g, 5 mmol), allylamine (0.317 g, 5.18 mmol), and 20 mL of 1,4-dioxane. The reaction mixture was stirred at reflux for 8 h. After cooling to room temperature, the suspension was poured into 600 mL of ice water and then filtrated. After drying in a vacuum oven overnight at room temperature, 4-bromo-N-allyl-1,8-naphthalimide (BN-Br) was obtained as a slightly gray solid (1.12 g, yield: 85%). 1
Experimental Section
Materials
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsN-Isopropylacrylamide (NIPAAm) and N-methylolacrylamide (NMA) were provided by Tokyo Chemical Industry Co. Japan. NIPAAm was re-crystallized three times from hexane-toluene (10/1, v/v) prior to use. Acrylic acid (AA, Aldrich, Saint Louis, MO, USA, 99%) was dried over CaH2 to distill in a round-bottomed flask overnight, then purified by passing it through a short aluminum-oxide column (50-200 μm), and stored at 4 °C prior to use. The radical initiator 2,2′-azobis(2methylpropionitrile) (AIBN) was purchased from UniRegion Bio-Tech and was recrystallized twice in ethanol prior to use. Iron (II) (FeCl2•4H2O) and Iron (III) (FeCl3•6H2O) chloride hexahydrate (Aldrich, 99%), dichloromethane (Tedia, Fairfield, OH, USA, 99.9%); methanol (Tedia, HPLC/SPECTRO); NH4OH (Alfa, 28%); anhydrous tetrahydrofuran (99.9%); acetone (99.8%); calcium hydride (reagent grade 95%); diethyl ether (99%); 4-bromo-1,8-naphthalic anhydride (95%); benzoyl isothiocyanate (98%); allylamine (98%); 1,2-ethylenediamine (99%); chloroform (anhydrous, 99%); N,N-dimethylformamide (99%); 1,4-dioxane (99.5%); acetonitrile (99%); ethanol (99.8%); and hydrazine hydrate (reagent grade, N2H4 50-60%) were used as received. Perchlorate salts of metal ions (Co 2+ , Cu 2+ , Fe 2+ , Mg 2+ , Hg 2+ , Pb 2+ , Zn 2+ , Cd 2+ , Ni 2+
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal Ions, Cd 2+ , Ni 2+ ) were purchased from Sigma-Aldrich (Saint Louis, MO, USA).
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsSynthesis of 4-Bromo-N-allyl-1,8-naphthalimide
Fluorescent BNPTU was prepared according to a previously reported method [8]. The process of its synthesis is illustrated in Figure 1. A 50-mL round-bottomed flask was charged with 4-bromo-1,8-naphthalic anhydride (1.385 g, 5 mmol), allylamine (0.317 g, 5.18 mmol), and 20 mL of 1,4-dioxane. The reaction mixture was stirred at reflux for 8 h. After cooling to room temperature, the suspension was poured into 600 mL of ice water and then filtrated. After drying in a vacuum oven overnight at room temperature, 4-bromo-N-allyl-1,8-naphthalimide (BN-Br) was obtained as a slightly gray solid (1.12 g, yield: 85%). 1
Synthesis of 4-(Aminoethylene)amino-N-allyl-1,8-naphthalimide
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsA 25-mL round-bottomed flask was charged with NP-Br (500 mg, 1.57 mmol) and an excess of 1,2-ethylenediamine (8 mL). The reaction mixture was stirred at reflux for 6 h. The residues were dissolved in CH2Cl2 (200 mL) and extracted with water. Finally, 4-(aminoethylene) amino-N-allyl-1,8naphthalimide (NP-NH2) was obtained as an orange crystal (400 mg, yield: 80%). 1 H-NMR (300 MHz, DMSO-d6, TMS, Figure 3b): δ = 5.33 (a, 2H, -CH2CHCH2-); δ = 5.91 (b, 1H, -CH2CHCH2-); δ = 4.58 (c, 2H, -CH2CHCH2-); δ = 8.56 (d, 1H, 7-CH); δ = 8.03 (e, 1H, 3-CH); δ = 8.64 (f, 1H, 5-CH); δ = 7.94 (g, 1H, 6-CH); δ = 8.45 (h, 1H, 2-CH); δ = 2.85 (i, 2H, -CH2NH2).
Synthesis of 1-Benzoyl-3-[2-(2-allyl-1,3-dioxo-2,3-dihydro-1Hbenzo[de]isoquinolin-6-ylamino)-ethyl]thiourea (BNPTU)
A 25-mL round-bottomed flask was charged with NP-NH2 (400 mg, 1.36 mmol), benzoyl isothiocyanate (0.22 g, 1.36 mmol), and 12 mL of acetone. The reaction mixture was stirred at reflux for 1 h. After cooling to room temperature, the solution was filtrated and washed with ethanol. The product was purified using chromatography with CH2Cl2 (320 mg, yield: 80%). 1
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsSynthesis of 4-(Aminoethylene)amino-N-allyl-1,8-naphthalimide
A 25-mL round-bottomed flask was charged with NP-Br (500 mg, 1.57 mmol) and an excess of 1,2-ethylenediamine (8 mL). The reaction mixture was stirred at reflux for 6 h. The residues were dissolved in CH 2 Cl 2 (200 mL) and extracted with water. Finally, 4-(aminoethylene) amino-N-allyl-1,8naphthalimide (NP-NH 2 ) was obtained as an orange crystal (400 mg, yield: 80%). 1 H-NMR (300 MHz, DMSO-d 6 , TMS, Figure 3b): δ = 5.33 (a, 2H, -CH 2 CHCH 2 -); δ = 5.91 (b, 1H, -CH 2 CHCH 2 -); δ = 4.58 (c, 2H, -CH 2 CHCH 2 -); δ = 8.56 (d, 1H, 7-CH); δ = 8.03 (e, 1H, 3-CH); δ = 8.64 (f, 1H, 5-CH); δ = 7.94 (g, 1H, 6-CH); δ = 8.45 (h, 1H, 2-CH); δ = 2.85 (i, 2H, -CH 2 NH 2 ).
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsSynthesis of 1-Benzoyl
-3-[2-(2-allyl-1,3-dioxo-2,3-dihydro-1Hbenzo[de]isoquinolin-6-ylamino)-ethyl]- thiourea (BNPTU)
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsA 25-mL round-bottomed flask was charged with NP-NH 2 (400 mg, 1.36 mmol), benzoyl isothiocyanate (0.22 g, 1.36 mmol), and 12 mL of acetone. The reaction mixture was stirred at reflux for 1 h. After cooling to room temperature, the solution was filtrated and washed with ethanol. The product was purified using chromatography with CH 2 Cl 2 (320 mg, yield: 80%). 1 H-NMR (300 MHz, DMSO-d 6 , TMS, Figure 3c): δ = 5.33 (a, 2H, -CH 2 CHCH 2 -); δ = 5.91 (b, 1H, -CH 2 CHCH 2 -); δ = 4.58 (c, 2H, -CH 2 CHCH 2 -); δ = 8.41 (d, 1H, 7-CH); δ = 7.62-7.88 (e + n, 3H, 6-CH; -COCCHCHCHCH-); δ = 8.72 (f, 1H, 5-CH); δ = 7.03 (g, 1H, 3-CH); δ = 8.26 (h, 1H, 2-CH); δ = 4.02 (i, 2H, -NCH 2 CH 2 N-); δ = 3.95 (j, 2H, -NCH 2 CH 2 N-); δ = 11.08 (k, 1H, -CSNH-); δ = 11.38 (l, 1H, -CONH-); δ = 7.48 (m, 1H, -COCCHCH-); δ = 7.93 (o, 2H, -COCCH-; -COC(CH) 4 CH-).
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsSynthesis of Magnetic Iron Oxide (Fe 3 O 4 ) Nanoparticles (NPs)
Magnetic nanoparticles, Fe 3 O 4 , were prepared by a coprecipitation method as reported [27]. A 100-mL round-bottomed flask was charged with FeCl 3 ·6H 2 O (380 mg, 1.4 mmol), FeCl 2 ·4H 2 O (140 mg, 0.7 mmol) and 80 mL of water. The reaction mixture was stirred at reflux under N 2 until the mixture had completely dissolved. The ammonium hydroxide (8 mL) was added dropwise under mechanical stirring at 60 • C for 30 min under N 2 . Then, the magnetic iron oxides were isolated from the solution by a magnet bar and dried in a vacuum oven at 40 • C for 24 h.
Synthesis of Poly(NIPAAm-co-NMA-co-AA)
Poly(NIPAAm-co-NMA-co-AA) was synthesized through free-radical copolymerization of the following three monomers: NIPAAm, NMA, and AA ( Figure 2) Poly(NIPAAm-co-NMA-co-AA) with different monomer ratios were denoted P1-P2, as listed in Table 1. The concentration of AIBN used as the initiator was 0.004 M. The reaction mixture, containing dimethylformamide (DMF) and monomers, was degassed by first bubbling nitrogen through for 30 min and then left to react at 70 • C for 24 h. The reaction mixture was quenched by exposure to air. The mixture was diluted with methanol to remove the unreacted monomers. The light white filtrate was concentrated, re-precipitated from diethyl ether, collected by filtration, and dried under vacuum to obtain the polymer product. The synthesis and characterization of P1-P2 are described in the following sections. The number-averaged molecular weight (M n ) and polydispersity index (PDI) estimated from gel permeation chromatography (GPC) (THF eluent) are listed in Table 1 and Figure S1.
Synthesis of Poly(NIPAAm-co-NMA) Random Copolymers (P1)
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsA reaction mixture of 1.131 g (9.99 mmol) of NIPAAm, 505 mg (4.99 mmol) of NMA, 5 mg (0.03 mmol) of AIBN, and 7.5 mL of EtOH was used to produce an orange solid (yield: 86%). Figure 4 and Table 1 present the molecular weight and chemical structure characterization of poly(NIPAAm-co-NMA) obtained using GPC with THF as the eluent and 1 H-NMR, respectively. The copolymer composition, estimated by performing peak integration, was consistent with the proposed structure. The estimated copolymer ratio of poly(NIPAAm-co-NMA) based on the NMR spectrum was 81:19, and the number-averaged molecular weight M n and PDI estimated using GPC were 26,856 and 2.01, respectively.
Synthesis of Poly(NIPAAm-co-NMA-co-AA) Random Copolymers (P2)
A reaction mixture of 1.131 g (9.99 mmol) of NIPAAm, 505 mg (4.99 mmol) of NMA, 0.204 mL (0.0029 mmol) of AA, 5 mg (0.03 mmol) of AIBN, and 9 mL of EtOH was used to produce an orange solid (yield: 82%). Figure 4 and Table 1 present the molecular weight and chemical structure characterization of poly(NIPAAm-co-NMA-co-AA) obtained using GPC with THF as the eluent and 1 H NMR, respectively. The copolymer composition, estimated by performing peak integration, was consistent with the proposed structure. The copolymer ratio of poly(NIPAAm-co-NMA-co-AA) based on the NMR spectrum was 72:15:13, and the number-averaged molecular weight Mn and PDI estimated using GPC were 18041 and 1.76, respectively.
Preparation of Electrospun (ES) Nanofibers
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsAs shown in Figure 2b, the ES nanofibers were prepared using a single-capillary spinneret in a procedure similar to that described in our previous papers [19][20][21][22][23][24][25][26]. The poly(NIPAAm-co-NMA) (P1), poly(NIPAAm-co-NMA-co-AA) (P2), BNPTU, and Fe3O4 NPs blend was dissolved in methanol (MeOH) as solvent at 250 mL h −1 and stirred overnight. The blend composition (wt %) of (P1 or P2), BNPTU, and Fe3O4 NPs for preparing the ES nanofibers was 85/10/5. The polymer solution was fed into a metallic needle using syringe pumps (Model 100, KD Scientific, Holliston, MA, USA) at a feed rate of 0.8-1.0 mL h −1 . The tip of the needle was connected to a Chargemaster CH30P high voltage power supply (Simco, Hatfield, PA, USA) set at 13.4 kV during electrospinning. A piece of aluminum foil or quartz was placed 15 cm below the tip of the needle for 30 min to collect the ES nanofibers. All experiments were performed at room temperature and a relative humidity of approximately 30%. P1 and P2 blended with the Fe3O4 NPs at a 5 wt % ratio are denoted by P1-5% and P2-5%. The ES nanofibers were annealed at 100 °C for 24 h in an oven for chemical cross-linking.
Characterization
1 H-NMR data were recorded at room temperature using an AM 300 (300 MHz) spectrometer (Bruker, Billerica, MA, USA) and the residual proton resonance of deuterated chloroform and deuterated dimethyl sulfoxide. High resolution electrospray ionization mass spectrometry spectra were recorded using an ion-trap time-of-flight liquid chromatograph mass spectrometer (Shimadzu, Kyoto, Japan). Gel permeation chromatography (GPC) analysis was performed using a Lab Alliance RI2000 instrument (two-column, MIXED-C and MIXED-D from Polymer Laboratories, Theale, UK)
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsSynthesis of Poly(NIPAAm-co-NMA-co-AA) Random Copolymers (P2)
A reaction mixture of 1.131 g (9.99 mmol) of NIPAAm, 505 mg (4.99 mmol) of NMA, 0.204 mL (0.0029 mmol) of AA, 5 mg (0.03 mmol) of AIBN, and 9 mL of EtOH was used to produce an orange solid (yield: 82%). Figure 4 and Table 1 present the molecular weight and chemical structure characterization of poly(NIPAAm-co-NMA-co-AA) obtained using GPC with THF as the eluent and 1 H NMR, respectively. The copolymer composition, estimated by performing peak integration, was consistent with the proposed structure. The copolymer ratio of poly(NIPAAm-co-NMA-co-AA) based on the NMR spectrum was 72:15:13, and the number-averaged molecular weight M n and PDI estimated using GPC were 18041 and 1.76, respectively. 1 H-NMR (300 MHz, DMSO-d 6 , TMS, Figure 4): δ = 0.82-1.12 (a, 6H, -CH(CH 3 ) 2 ); 1.23-1.46 (h + i + j, 6H, -CH 2 CH-, -CH 2 CH-, -CH 2 CH-); 1.84-2.10 (h + i + j, 3H, -CH 2 CH-,-CH 2 CH-, -CH 2 CH-); 5.34-5.53 (b, 1H, -NHCH 2 OH); 3.71-3.82 (c, 1H, -CH(CH 3 ) 2 ); 11.92-12.05 (d, 1H, -COCH 2 OH); 7.23-7.59 (e, 1H, -CONHCH-); 7.92-8.14 (f, 1H, -NHCH 2 OH); 4.36-4.65 (g, 2H, -NHCH 2 OH-).
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsPreparation of Electrospun (ES) Nanofibers
As shown in Figure 2b, the ES nanofibers were prepared using a single-capillary spinneret in a procedure similar to that described in our previous papers [19][20][21][22][23][24][25][26]. The poly(NIPAAm-co-NMA) (P1), poly(NIPAAm-co-NMA-co-AA) (P2), BNPTU, and Fe 3 O 4 NPs blend was dissolved in methanol (MeOH) as solvent at 250 mL h −1 and stirred overnight. The blend composition (wt %) of (P1 or P2), BNPTU, and Fe 3 O 4 NPs for preparing the ES nanofibers was 85/10/5. The polymer solution was fed into a metallic needle using syringe pumps (Model 100, KD Scientific, Holliston, MA, USA) at a feed rate of 0.8-1.0 mL h −1 . The tip of the needle was connected to a Chargemaster CH30P high voltage power supply (Simco, Hatfield, PA, USA) set at 13.4 kV during electrospinning. A piece of aluminum foil or quartz was placed 15 cm below the tip of the needle for 30 min to collect the ES nanofibers. All experiments were performed at room temperature and a relative humidity of approximately 30%. P1 and P2 blended with the Fe 3 O 4 NPs at a 5 wt % ratio are denoted by P1-5% and P2-5%. The ES nanofibers were annealed at 100 • C for 24 h in an oven for chemical cross-linking.
Characterization
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsCharacterization
1 H-NMR data were recorded at room temperature using an AM 300 (300 MHz) spectrometer (Bruker, Billerica, MA, USA) and the residual proton resonance of deuterated chloroform and deuterated dimethyl sulfoxide. High resolution electrospray ionization mass spectrometry spectra were recorded using an ion-trap time-of-flight liquid chromatograph mass spectrometer (Shimadzu, Kyoto, Japan). Gel permeation chromatography (GPC) analysis was performed using a Lab Alliance RI2000 instrument (two-column, MIXED-C and MIXED-D from Polymer Laboratories, Theale, UK) connected to a refractive index detector (Schambeck SFD GmbH, Bad Honnef, Germany). All GPC analyses were performed using a polymer and THF solution at a flow rate of 1 mL min −1 at 40 • C and calibrated with methyl methacrylate. The thermal decomposition temperature was determined using a Q50 thermal gravimetric analyzer (TGA) (TA Instruments, Lukens Drive, New Castle, DE, USA) over a heating range of 100-800 • C at a heating rate of 10 • C min −1 in a nitrogen atmosphere. The LCST of the prepared copolymer solution was recorded by monitoring the transmittance of a 520 nm light beam on Shimadzu UV-Vis spectrophotometer. The copolymer concentration in water was 1 wt %, and the temperature was raised from 10 to 70 • C in 2.5 • C increments every 10 min. A plot of the changes in optical transmittance as a function of temperature for polymers in water was made, the LCST corresponds to the first turning point of the transition curve [21]. Magnetic properties of Fe 3 O 4 nanoparticles or ES nanofibers were measured on a Lake Shore VSM-7407 instrument (MPMS (SQUID) VSM, Quantum Design, San Diego, CA, USA). The magnetization hysteresis loops were measured at 300 K [30].
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsThe morphologies of ES nanofibers were characterized using an S-520 scanning electron microscope (Hitachi, Tokyo, Japan) equipped with X-ray microanalysis capability. Samples were coated with platinum prior to scanning electron microscopy (SEM) characterization, and analysis was conducted at an increased voltage of 10 kV. Fluorescence optical microscopy images were captured using an LCS SP5 two-photon laser confocal microscope (Leica, Mannheim, Germany). The morphologies of the ES nanofibers were similar to those reported in our previous studies [15,20,24].
Ultraviolet-visible (UV-Vis) absorption and PL spectra were measured to examine photophysical properties and recorded using a UV-Vis spectrophotometer (Shimadzu) and a Fluorolog-3 spectrofluorometer (Horiba Jobin Yvon, Edison, NJ, USA), respectively. Variations in the optical absorption and PL of the prepared ES nanofibers with different metal ion concentrations are described as follows. To ensure that the beam excited the same point on the prepared samples during each measurement, the ES nanofibers were fixed in cuvettes with adhesive tape, and the cuvette was filled with a basic aqueous metal ion solution at 10 −4 -10 −2 M. Each measurement was maintained for 15 min to ensure that the chelating reaction reached equilibrium. All PL spectra of the ES nanofibers were recorded using the spectrofluorometer, and the samples were excited at a suitable wavelength, as described in our previous studies [19][20][21][22][23][24][25][26].
Results and Discussion
Characterization of BNPTU and Poly(NIPAAm-co-NMA-co-AA)
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsThe chemical structure of BNPTU was characterized using 1 H-NMR ( Figure 3). The synthesis routes of BNPTU ( Figure 1) are similar to those reported in a previous study [8]. Figure 2a shows the route by which poly(NIPAAm-co-NMA-co-AA) copolymers were synthesized through free radical polymerization. The two types of copolymers (P1 and P2) were synthesized and their composites are listed in Table 1. Figure 4 shows the 1 H-NMR spectrum of poly(NIPAAm-co-NMA) (P1) and poly(NIPAAm-co-NMA-co-AA) (P2). The proton peaks at 7.23-7.59 ppm (Peak e) and 3.71-3.82 ppm (Peak c) correspond to the methylene neighbors of the secondary amine moiety and the alkyl chains on NIPAAm, respectively. The proton peaks at 5.34-5.53 ppm (Peak b), 7.92-8.14 ppm (Peak f), and 4.36-4.65 ppm (Peak g) correspond to the methylene neighbors of the hydroxyl, secondary amine moiety, and alkyl chains on NMA, respectively. The proton peak at 11.92-12.05 ppm (Peak d) corresponds to the methylene neighbor of hydroxyl on the AA in P2. The peak at 0.82-1.12 ppm (Peak a) and those at 1.23-2.1 ppm (Peaks h, i, and j) correspond to the alkyl chains on the copolymers. The copolymer ratios of P1 and P2 were estimated from NMR spectra, and were 81:19 and 72:15:13, respectively. The favorable agreement between the feeding ratio and experimental composition suggests the successful preparation of the target copolymers. The molecular weights, thermal properties, and LCSTs of P1 and P2 are listed in Table 1. The number-averaged molecular weights and polydispersity indices (PDIs) of P1 and P2 were 26,856 and 2.01, and 18,041 and 1.76, respectively. Notably, Figure S1 shows that the accurate PDI
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsFigure S1 shows that the accurate PDI (M n /M w ) data of P1 were 26,856/53,980 equal to 2.01 and that of P2 was 18,041/31,752 equal to 1.76. The thermal decompositon curves of the prepared polymers are presented in Figure S2 (Supplementary Materials). The identical thermal decomposition temperature of 210 • C for P1 and P2 is attributable to their similar NIPAAm compositions. All decompostion temperatures for P1 and P2 were higher than 270 • C, and thus, they exhibited favorable and stable thermal properties. Figure S3 shows the typical optical transmittance (520 nm) versus temperature curves of P1 and P2, which exhibit thermoresponsive soluble-to-insoluble phase transitions in an aqueous medium. The copolymers are soluble in water below their LCSTs. The LCSTs of P1 and P2 are both approximately 55 • C, slightly higher than that of PNIPAM (32 • C) because of the hydrophilic characteristics of the NMA and AA compounds, as stated in our previous study [23] and one other study [34]. Figure 5a illustrates that BNPTU in CH 3 CN had a maximum in the UV-Vis absorption peak (λ abs max ) of 430 nm at pH 7 and emitted green fluorescence under UV light (inset) as it is a fluorescent dye. Figure 5b shows variations in the UV-Vis spectra of BNPTU in a CH 3 CN solution containing various metal ions at a concentration of 10 −5 M (pH 7). The λ abs max of BNPTU was blueshifted from 430 to 350 nm when the Hg 2+ ion was added. The Hg 2+ ion transformed the thiourea unit of BNPTU under aqueous conditions into an imidazoline moiety with considerably weakened electron-donating ability ( Figure 2c) [5,[35][36][37]. However, no change in the absorption peak was observed when other metal ions such as Co 2+ , Ni 2+ , Pb 2+ , Zn 2+ , Mg 2+ , Cu 2+ , Fe 2+ , and Cd 2+ were added, suggesting that BNPTU exhibited high
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal Ionswere added, suggesting that BNPTU exhibited high selectivity and sensitivity toward Hg 2+ . polymers are presented in Figure S2 (Supplementary Materials). The identical thermal decomposition temperature of 210 °C for P1 and P2 is attributable to their similar NIPAAm compositions. All decompostion temperatures for P1 and P2 were higher than 270 °C, and thus, they exhibited favorable and stable thermal properties. Figure S3 shows the typical optical transmittance (520 nm) versus temperature curves of P1 and P2, which exhibit thermoresponsive soluble-to-insoluble phase transitions in an aqueous medium. The copolymers are soluble in water below their LCSTs. The LCSTs of P1 and P2 are both approximately 55 °C, slightly higher than that of PNIPAM (32 °C) because of the hydrophilic characteristics of the NMA and AA compounds, as stated in our previous study [23] and one other study [34]. Figure 5a illustrates that BNPTU in CH3CN had a maximum in the UV-Vis absorption peak (λ abs max) of 430 nm at pH 7 and emitted green fluorescence under UV light (inset) as it is a fluorescent dye. Figure 5b shows variations in the UV-Vis spectra of BNPTU in a CH3CN solution containing various metal ions at a concentration of 10 −5 M (pH 7). The λ abs max of BNPTU was blueshifted from 430 to 350 nm when the Hg 2+ ion was added. The Hg 2+ ion transformed the thiourea unit of BNPTU under aqueous conditions into an imidazoline moiety with considerably weakened electron-donating ability (Figure 2c) [5,[35][36][37]. However, no change in the absorption peak was observed when other metal ions such as Co 2+ , Ni 2+ , Pb 2+ , Zn 2+ , Mg 2+ , Cu 2+ , Fe 2+ , and Cd 2+ were added, suggesting that BNPTU exhibited high selectivity and sensitivity toward Hg 2+ . Figure 6 presents field-emission SEM (FE-SEM) images of the ES nanofibers prepared using P1 and P2 at a solution concentration
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal Ionsusing P1 and P2 at a solution concentration of 250 mg mL −1 with MeOH. When dry, the P1 and P2 ES nanofibers had average diameters of 416 ± 31 and 437 ± 42 nm, respectively. The recorded average diameter was a statistical average of 50 fibers from each sample. Moreover, all of the ES nanofibers from pure MeOH solvent were smooth and nonporous. The diameters of the P1 and P2 ES nanofibers were similar when they were dry because the concentration of the ES solution was fixed and the molar ratios of the NMA and AA moieties in the copolymers were lower than that of NIPAAm. To observe the morphologies of the cross-linked P1 and P2 ES nanofibers in pure water at various temperatures, the fibers were collected on a small piece of aluminum foil and immersed in water at 30 • C or 60 • C. After 10 min, the samples were solidified by placing in a flask containing liquid nitrogen, and the residual water was removed using a vacuum for 30 min to retain the original morphology. Figure 6 shows the FE-SEM images of the P1 and P2 ES nanofibers in a wet state after they were immersed in water at 30 or 60 • C. As shown in Figure 6 (wet state), the P1 and P2 fibers exhibited diameters of 1250 ± 205 and 1420 ± 282 nm at 25 • C and 830 ± 124 and 920 ± 138 nm at 60 • C, respectively. The diameters were substantially enlarged after the nanofibers were immersed in water (416 ± 31, 437 ± 42 nm, respectively; Figure 6) because the hydrophilic NIPAAm chain swelled in the water. However, these swollen fibers maintained their cylindrical shape and did not dissolve in water, a phenomenon attributed to the efficient chemical cross-linking of the NMA moiety [23]. Furthermore, the fiber diameters at 30 • C were greater than those at 60 • C because of the hydrophilic NIPAAm chain, the temperature of which was below the LCST. By contrast, the nanofiber diameters decreased from approximately 1.5 µm to 1.0 µm as the temperature
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal Ionsto 1.0 µm as the temperature increased from 30 to 60 • C, due to the temperature of which was above the LCST resulting in the hydrophobic NIPAAm chain.
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsMorphology and Characterization of ES Nanofibers
Morphology and Characterization of ES Nanofibers
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsPolymers 2017, 9,136 10 of 21 a statistical average of 50 fibers from each sample. Moreover, all of the ES nanofibers from pure MeOH solvent were smooth and nonporous. The diameters of the P1 and P2 ES nanofibers were similar when they were dry because the concentration of the ES solution was fixed and the molar ratios of the NMA and AA moieties in the copolymers were lower than that of NIPAAm. To observe the morphologies of the cross-linked P1 and P2 ES nanofibers in pure water at various temperatures, the fibers were collected on a small piece of aluminum foil and immersed in water at 30 °C or 60 °C. After 10 min, the samples were solidified by placing in a flask containing liquid nitrogen, and the residual water was removed using a vacuum for 30 min to retain the original morphology. Figure 6 shows the FE-SEM images of the P1 and P2 ES nanofibers in a wet state after they were immersed in water at 30 or 60 °C. As shown in Figure 6 (wet state), the P1 and P2 fibers exhibited diameters of 1250 ± 205 and 1420 ± 282 nm at 25 °C and 830 ± 124 and 920 ± 138 nm at 60 °C, respectively. The diameters were substantially enlarged after the nanofibers were immersed in water (416 ± 31, 437 ± 42 nm, respectively; Figure 6) because the hydrophilic NIPAAm chain swelled in the water. However, these swollen fibers maintained their cylindrical shape and did not dissolve in water, a phenomenon attributed to the efficient chemical cross-linking of the NMA moiety [23]. Furthermore, the fiber diameters at 30 °C were greater than those at 60 °C because of the hydrophilic NIPAAm chain, the temperature of which was below the LCST. By contrast, the nanofiber diameters decreased from approximately 1.5 μm to 1.0 μm as the temperature increased from 30 to 60 °C, due to the temperature of which was above the LCST resulting in the hydrophobic NIPAAm chain. Figure 7c shows that the
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsFigure 7c shows that the Fe3O4 NPs precipitated out of the solution and dropped to the bottom because of higher density (left image) and then accumulated on the side of the bottle because of a magnetic bar placed next to the bottle (right image), indicating that magnetic Fe3O4 NPs can be absorbed using a magnet [38]. Figure 8a presents an SEM image of P2-5% ES nanofibers prepared from P2 copolymers blended with Fe3O4 NPs at a 5% weight ratio. The strong stretching force associated with electrospinning induces orientation of these Fe3O4 NPs along the axis of the fiber. The numerous carboxyl groups of PAA in P2 inhibited the aggregation of Fe3O4 NPs, a result similar to those of several previous studies [27][28][29][30][31]. The result can be attributed to P2-5% containing carboxyl groups of PAA that interact with the Fe3O4 NPs, thereby enabling the PAA to be easily adsorbed by Fe3O4 NPs. Figure 8b shows an SEM image of P2-5% ES nanofibers before elemental mapping, and Figure 8c,d shows the corresponding SEM images of P2-5% ES nanofibers after carbon (C) and iron (Fe) elemental mapping, respectively, by using energy dispersive X-ray spectroscopy (EDS). Notably, the C and Fe are contributed by the P2 and Fe3O4 NPs, respectively. The C and Fe mapping images in Figure 8b-d indicate that Fe3O4 NPs were present within the P2-5% ES nanofibers because no Fe species remained on the substrate, as shown by the dark region. The Fe content within the P2-5% ES nanofibers was 14.54 wt % as estimated according to the EDS spectrum ( Figure 8e). Furthermore, the amounts of Fe3O4 NPs within the P2-5% ES nanofibers were confirmed through TGA in an oxygen atmosphere ( Figure S4). Figure S4 shows the stages within the temperature range 100-750 °C, including the decomposition of poly(N-methylolacrylamide)(PNMA) and PAA, and the
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal Ionsand PAA, and the oxdiation of the Fe3O4 NPs. Weight loss initially occurred below 300 °C, and originated from either the decomposition of noncoordinated carboxyls or the evaporation of trace water. Weight loss that occurred between 300 and 450 °C was caused by the cleaving of PNIPAM chains [31]. Finally, the copolymers were degraded and Fe3O4 was completely transformed into Fe2O3 at 700 °C [30]. The calculated weight percentage of the remaining content, consisting of carbon residue and Fe2O3, was approximately 14 wt %. [27][28][29][30][31]. The result can be attributed to P2-5% containing carboxyl groups of PAA that interact with the Fe 3 O 4 NPs, thereby enabling the PAA to be easily adsorbed by Fe 3 O 4 NPs. Figure 8b shows an SEM image of P2-5% ES nanofibers before elemental mapping, and Figure 8c,d shows the corresponding SEM images of P2-5% ES nanofibers after carbon (C) and iron (Fe) elemental mapping, respectively, by using energy dispersive X-ray spectroscopy (EDS). Notably, the C and Fe are contributed by the P2 and Fe 3 O 4 NPs, respectively. The C and Fe mapping images in Figure 8b-d indicate that Fe 3 O 4 NPs were present within the P2-5% ES nanofibers because no Fe species remained on the substrate, as shown by the dark region. The Fe content within the P2-5% ES nanofibers was 14.54 wt % as estimated according to the EDS spectrum ( Figure 8e). Furthermore, the amounts of Fe 3 O 4 NPs within the P2-5% ES nanofibers were confirmed through TGA in an oxygen atmosphere ( Figure S4). Figure S4 shows the stages within the temperature range 100-750 • C, including the decomposition of poly(N-methylolacrylamide)(PNMA) and PAA, and the oxdiation of the Fe 3 O 4 NPs. Weight loss initially occurred below 300 • C, and originated from either the decomposition of noncoordinated carboxyls
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal Ionsthe decomposition of noncoordinated carboxyls or the evaporation of trace water. Weight loss that occurred between 300 and 450 • C was caused by the cleaving of PNIPAM chains [31]. Finally, the copolymers were degraded and Fe 3 O 4 was completely transformed into Fe 2 O 3 at 700 • C [30]. The calculated weight percentage of the remaining content, consisting of carbon residue and Fe 2 O 3 , was approximately 14 wt %.
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsHg 2+ Sensing, Thermoresponsiveness, and Magnetic Properties of ES Nanofibers
P1 or P2 blended with 10% BNPTU and 5% Fe 3 O 4 NP ES nanofibers, which can sense metal ions because of the ability of PAA and BNPTU to adsorb Hg 2+ , was systematically explored [8,33]. Figure 9a shows the PL spectra of ES nanofibers prepared from P1 and P2 blended with 10% BNPTU in an aqueous solution without Hg 2+ (blank) or with Hg 2+ at 10 −2 M and subjected to 430-nm excitation. The maximum emission peak (λ PL max ) of the P1 and P2 ES nanofibers blueshifted substantially from 530 nm in a non-Hg 2+ aqueous solution to 450 nm in an Hg 2+ aqueous solution. This change corresponded to the thiourea unit of BNPTU, which transformed the imidazoline moiety (with the Hg 2+ ion), thereby causing a significant reduction in electron delocalization within the fluorophore (Figure 2c) [5,[35][36][37]. Moreover, the PL intensity of the P2 ES nanofibers was 1.3 times that of P1 ES nanofibers, indicating that the AA moiety of P2 enhances the adsorption of mercury ions, similar to one report [33].
Polymers 2017, 9,136 13 of 21
Hg 2+ Sensing, Thermoresponsiveness, and Magnetic Properties of ES Nanofibers
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsP1 or P2 blended with 10% BNPTU and 5% Fe3O4 NP ES nanofibers, which can sense metal ions because of the ability of PAA and BNPTU to adsorb Hg 2+ , was systematically explored [8,33]. Figure 9a shows the PL spectra of ES nanofibers prepared from P1 and P2 blended with 10% BNPTU in an aqueous solution without Hg 2+ (blank) or with Hg 2+ at 10 −2 M and subjected to 430-nm excitation. The maximum emission peak (λ PL max) of the P1 and P2 ES nanofibers blueshifted substantially from 530 nm in a non-Hg 2+ aqueous solution to 450 nm in an Hg 2+ aqueous solution. This change corresponded to the thiourea unit of BNPTU, which transformed the imidazoline moiety (with the Hg 2+ ion), thereby causing a significant reduction in electron delocalization within the fluorophore (Figure 2c) [5,[35][36][37]. Moreover, the PL intensity of the P2 ES nanofibers was 1.3 times that of P1 ES nanofibers, indicating that the AA moiety of P2 enhances the adsorption of mercury ions, similar to one report [33]. Figure 9b shows the PL spectra of ES nanofibers prepared from P2 blended with 10% BNPTU and 5% Fe3O4 NPs. P2-5% was placed in aqueous solutions without Hg 2+ (blank), with Hg 2+ at 10 −4 , 10 −3 , and 10 −2 M, and under neutral conditions at 430-nm excitation. The presence of the highest Hg 2+ concentration (10 −2 M) led to a λ PL max blueshift from 530 nm to 450 nm in the emission spectra, similar to that of the P2 ES nanofibers without Fe3O4 NPs, indicating that the metal ion sensing ability was unaffected by Fe3O4 NPs. As shown in the figure, the concentration of Hg 2+ increasing from 0 (blank) to 10 −4 ,
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal Ions(blank) to 10 −4 , 10 −3 , and 10 −2 M resulted in clear blueshifts in the λ PL max from 530 to 500, 450, and 450 nm, respectively. Furthermore, when the mercury ion concentration was at the extremely dilute level of 10 −3 M, the emission maximum shift Δλmax was observed to be as high as 80 nm, resulting in a color change easily observable by the naked eye. Moreover, the lowest detectable Hg 2+ concentration for the ES nanofibers was 10 −4 M, suggesting that the P2-5% ES nanofibers were highly sensitive to Hg 2+ but not to general ions, even when the system contained Fe3O4 NPs. Figure 9c shows the changes of the fluorescence intensity ratio, I450/I530 (I450 is the fluorescence intensity of BNTPU after the detection of Hg 2+ emission at 537 nm, whereas I530 is the fluorescence intensity of BNTPU before the detection of Hg 2+ emission at 530 nm) of the P2-5% ES nanofibers when subjected to Hg 2+ . As the concentration of Hg 2+ was increased, BNTPU emission at 530 nm gradually decreased, and BNTPU-Hg 2+ emission at 450 nm gradually increased. These changes corresponded to the fluorescence of BNTPU chelated with Hg 2+ ions, causing the I450/I530 to increase from approximately 0.1 to 1.2 as the Hg 2+ ion concentration increased from 0 to 10 −2 M. The P2-5% ES nanofibers exhibited high sensitivity to Hg 2+ ions between 10 −4 and 10 −2 M. Moreover, the titration data of Figure 9c show that the Kd was calculated as 1.58 mM [24]. Figure 9d shows fluorescence variations in the P2-5% ES nanofibers in a 10 −2 M Hg 2+ environment as the temperature was varied from 30 to 60 °C. At 30 °C, the temperature at which the ES nanofibers chelate Hg 2+ , the λ PL max was blueshifted substantially from 530 nm
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal Ionsmax was blueshifted substantially from 530 nm (blank, green emission) to 450 nm (blue emission) in the emission spectra. When the temperature was increased from 30 to 45 °C, only a mild decrease in PL intensity at 450 nm (λ PL max) was observed, exhibiting almost no change, as shown in the enlarged inset image. However, when the temperature was increased from 45 to 50 °C, a dramatic quenching of the PL intensity of the ES nanofibers was observed. The PL intensity decreased as the temperature increased from 50 to 60 °C, reaching its lowest point at 60 °C. Notably, Figure S5 shows no changes in the PL intensity of the pristine BNPTU compound as the temperature was increased from 30 to 60 °C because the chemical stability of BNTPU is favorable at room temperature or higher temperatures (e.g., 60 °C), indicating that the quenching of the PL intensity of the ES nanofibers (Figure 9d) was not caused by the pristine BNPTU compound. Figure 9b shows the PL spectra of ES nanofibers prepared from P2 blended with 10% BNPTU and 5% Fe 3 O 4 NPs. P2-5% was placed in aqueous solutions without Hg 2+ (blank), with Hg 2+ at 10 −4 , 10 −3 , and 10 −2 M, and under neutral conditions at 430-nm excitation. The presence of the highest Hg 2+ concentration (10 −2 M) led to a λ PL max blueshift from 530 nm to 450 nm in the emission spectra, similar to that of the P2 ES nanofibers without Fe 3 O 4 NPs, indicating that the metal ion sensing ability was unaffected by Fe 3 O 4 NPs. As shown in the figure, the concentration of Hg 2+ increasing from 0 (blank) to 10 −4 , 10 −3 , and 10 −2 M resulted in clear blueshifts in the λ PL max from 530 to 500, 450, and 450 nm, respectively. Furthermore, when the mercury ion concentration was at the extremely dilute level of 10 −3 M, the emission maximum shift ∆λ max was
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal Ionsthe emission maximum shift ∆λ max was observed to be as high as 80 nm, resulting in a color change easily observable by the naked eye. Moreover, the lowest detectable Hg 2+ concentration for the ES nanofibers was 10 −4 M, suggesting that the P2-5% ES nanofibers were highly sensitive to Hg 2+ but not to general ions, even when the system contained Fe 3 O 4 NPs. Figure 9c shows the changes of the fluorescence intensity ratio, I 450 /I 530 (I 450 is the fluorescence intensity of BNTPU after the detection of Hg 2+ emission at 537 nm, whereas I 530 is the fluorescence intensity of BNTPU before the detection of Hg 2+ emission at 530 nm) of the P2-5% ES nanofibers when subjected to Hg 2+ . As the concentration of Hg 2+ was increased, BNTPU emission at 530 nm gradually decreased, and BNTPU-Hg 2+ emission at 450 nm gradually increased. These changes corresponded to the fluorescence of BNTPU chelated with Hg 2+ ions, causing the I 450 / I 530 to increase from approximately 0.1 to 1.2 as the Hg 2+ ion concentration increased from 0 to 10 −2 M. The P2-5% ES nanofibers exhibited high sensitivity to Hg 2+ ions between 10 −4 and 10 −2 M. Moreover, the titration data of Figure 9c show that the Kd was calculated as 1.58 mM [24]. Figure 9d shows fluorescence variations in the P2-5% ES nanofibers in a 10 −2 M Hg 2+ environment as the temperature was varied from 30 to 60 • C. At 30 • C, the temperature at which the ES nanofibers chelate Hg 2+ , the λ PL max was blueshifted substantially from 530 nm (blank, green emission) to 450 nm (blue emission) in the emission spectra. When the temperature was increased from 30 to 45 • C, only a mild decrease in PL intensity at 450 nm (λ PL max ) was observed, exhibiting almost no change, as
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal Ions) was observed, exhibiting almost no change, as shown in the enlarged inset image. However, when the temperature was increased from 45 to 50 • C, a dramatic quenching of the PL intensity of the ES nanofibers was observed. The PL intensity decreased as the temperature increased from 50 to 60 • C, reaching its lowest point at 60 • C. Notably, Figure S5 shows no changes in the PL intensity of the pristine BNPTU compound as the temperature was increased from 30 to 60 • C because the chemical stability of BNTPU is favorable at room temperature or higher temperatures (e.g., 60 • C), indicating that the quenching of the PL intensity of the ES nanofibers (Figure 9d) was not caused by the pristine BNPTU compound.
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsThe aforementioned thermoreversible luminescence characteristics are explained as follows. As shown in Figure 6, compared with those in the dry state, the ES fibers soaked in water (wet state) underwent morphological change at 30 • C (below the LCST). At 30 • C, the hydrophilic groups in the PNIPAM interacted easily with water molecules to form intermolecular hydrogen bonds. However, the swollen ES fibers were insoluble in water because of the chemically cross-linked NMA segment. When the temperature was raised to 60 • C (above the LCST), the intermolecular hydrogen bonds between the PNIPAM and water were broken, leading to the release of water molecules from the fibers. However, Hg 2+ was still present within the fibers. Furthermore, as the temperature was 60 • C (above the LCST), the PNIPAM was collapsed and densely packed, potentially suppressing absorption of incident light by the BNPTU moiety with Hg 2+ , resulting in a reduction of the PL intensity. Thus, the prepared ES fibers exhibited an on-off PL intensity profile with temperature variation (Figure 9d), indicating that the ES nanofibers have thermoresponsive properties.
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsThe selectivity of P2-5% ES nanofibers toward Hg 2+ over other common metal ions was also studied ( Figure 10). Figure 10a shows that among all tested metal ions, namely Hg 2+ , Pb 2+ , Co 2+ , Cd 2+ , Mg 2+ , Ni 2+ , Zn 2+ , Fe 2+ , and Cu 2+ (10 −2 M; all metal ion test solutions were controlled at pH 4), a substantial blueshift in PL was observed only in the presence of Hg 2+ . In Figure 8b, I 450 /I 530 indicates the ratio of a PL intensity of 530 nm (λ PL max ), corresponding to other metal ions (Hg 2+ , Pb 2+ , Co 2+ , Cd 2+ , Mg 2+ , Ni 2+ , Zn 2+ , Fe 2+ , and Cu 2+ ), to a PL intensity of 450 nm (λ PL max ), corresponding to Hg 2+ . The presence of Hg 2+ induced the most prominent I 450 /I 530 enhancement (approximately 2.5-fold), resulting in blue emission. For all of the other metal ions, the nanofibers exhibited reduced I 460 /I 510 values (approximately 0.1-fold), resulting in green emission (Figure 10b). In addition, the fluorescence spectra recorded in the presence of Hg 2+ ions and the other metal ions revealed that none of the other metal ions interfered with the Hg 2+ ion-induced fluorescence enhancement (inset Figure 10b), indicating that the sensing of Hg 2+ by the P2-5% ES nanofibers was virtually unaffected by commonly coexisting ions. The aforementioned thermoreversible luminescence characteristics are explained as follows. As shown in Figure 6, compared with those in the dry state, the ES fibers soaked in water (wet state) underwent morphological change at 30 °C (below the LCST). At 30 °C, the hydrophilic groups in the PNIPAM interacted easily with water molecules to form intermolecular hydrogen bonds. However, the swollen ES fibers were insoluble in water because of the chemically cross-linked NMA segment. When the
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal Ionschemically cross-linked NMA segment. When the temperature was raised to 60 °C (above the LCST), the intermolecular hydrogen bonds between the PNIPAM and water were broken, leading to the release of water molecules from the fibers. However, Hg 2+ was still present within the fibers. Furthermore, as the temperature was 60 °C (above the LCST), the PNIPAM was collapsed and densely packed, potentially suppressing absorption of incident light by the BNPTU moiety with Hg 2+ , resulting in a reduction of the PL intensity. Thus, the prepared ES fibers exhibited an on-off PL intensity profile with temperature variation (Figure 9d), indicating that the ES nanofibers have thermoresponsive properties.
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsThe selectivity of P2-5% ES nanofibers toward Hg 2+ over other common metal ions was also studied ( Figure 10). Figure 10a shows that among all tested metal ions, namely Hg 2+ , Pb 2+ , Co 2+ , Cd 2+ , Mg 2+ , Ni 2+ , Zn 2+ , Fe 2+ , and Cu 2+ (10 −2 M; all metal ion test solutions were controlled at pH 4), a substantial blueshift in PL was observed only in the presence of Hg 2+ . In Figure 8b, I450/I530 indicates the ratio of a PL intensity of 530 nm (λ PL max), corresponding to other metal ions (Hg 2+ , Pb 2+ , Co 2+ , Cd 2+ , Mg 2+ , Ni 2+ , Zn 2+ , Fe 2+ , and Cu 2+ ), to a PL intensity of 450 nm (λ PL max), corresponding to Hg 2+ . The presence of Hg 2+ induced the most prominent I450/I530 enhancement (approximately 2.5-fold), resulting in blue emission. For all of the other metal ions, the nanofibers exhibited reduced I460/I510 values (approximately 0.1-fold), resulting in green emission (Figure 10b). In addition, the fluorescence spectra recorded in the presence of Hg 2+ ions and the other metal ions revealed that none of the other metal ions interfered with the Hg 2+ ion-induced fluorescence enhancement (inset Figure 10b), indicating that the sensing of Hg 2+ by the P2-5% ES nanofibers was virtually unaffected by commonly coexisting ions. Figure 11a shows the CIE coordinates of the P2-5% ES nanofibers in 0-10 −2 M Hg 2+ aqueous solutions. All of the inset figures show corresponding photographs captured under UV light. The CIE coordinates show a strong blueshift from (0.24, 0.61) (blank) to (0.16, 0.08) (pH 4, Hg 2+ 10 −2 M) as the concentration of Hg 2+ was varied from 0
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsHg 2+ was varied from 0 to 10 −2 M because of the detection of BNPTU by Hg 2+ , resulting in a color change from green to blue in the P2-5% ES nanofibers. Furthermore, as shown in the confocal microscopy images in Figure 11b, the color emission of the P2-5% ES nanofibers varied from green to blue as the concentration of Hg 2+ was increased. A microfluidics system was constructed, in which a P2-5% ES nanofiber filter membrane with an area of 3 cm 2 placed in the middle of a tube was used to rapidly absorb and sense Hg 2+ in a solution flowing through the tube (Figure 11c). Figure 11d depicts the measured time-dependent variation in the solution conductivity. The prepared Hg 2+ solution contained 1 ppm Hg 2+ (6.3 × 10 −3 M Hg 2+ in 0.5 L of water) and the conductivity of the Hg 2+ solution in its initial state (0 min) was 131.2 µS cm −1 . The solution conductivity gradually decreased to 101.4 uS cm −1 . Table S1 contains raw data on the conductivity changes over 20 min, and Figure 11d depicts C t /C 0 (%) versus time (C 0 : original conductivity at 0 min; C t : conductivity at time t). The solution conductivity decreased as the flow time increased, indicating that an increasing number of Hg 2+ ions was absorbed by the P2-5% ES nanofiber membrane, yielding less Hg 2+ in the solution. Thus, the 100% conductivity of the solution in its initial state (0 min) had decreased to 77.2% after 20 min. This rapid change in conductivity was caused by the high surface-to-volume ratio of the P2-5% ES nanofibers. The sensory filter membrane based on the P2-5% ES nanofibers specifically absorbed Hg 2+ in an aqueous solution that contained a variety of metal ions and had a dual fluorescent chemosensory function for Hg 2+ . Figure 11a shows the CIE coordinates of the P2-5% ES nanofibers in
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsES nanofibers in 0-10 −2 M Hg 2+ aqueous solutions. All of the inset figures show corresponding photographs captured under UV light. The CIE coordinates show a strong blueshift from (0.24, 0.61) (blank) to (0.16, 0.08) (pH 4, Hg 2+ 10 −2 M) as the concentration of Hg 2+ was varied from 0 to 10 −2 M because of the detection of BNPTU by Hg 2+ , resulting in a color change from green to blue in the P2-5% ES nanofibers. Furthermore, as shown in the confocal microscopy images in Figure 11b, the color emission of the P2-5% ES nanofibers varied from green to blue as the concentration of Hg 2+ was increased. A microfluidics system was constructed, in which a P2-5% ES nanofiber filter membrane with an area of 3 cm 2 placed in the middle of a tube was used to rapidly absorb and sense Hg 2+ in a solution flowing through the tube (Figure 11c). Figure 11d depicts the measured time-dependent variation in the solution conductivity. The prepared Hg 2+ solution contained 1 ppm Hg 2+ (6.3 × 10 −3 M Hg 2+ in 0.5 L of water) and the conductivity of the Hg 2+ solution in its initial state (0 min) was 131.2 μS cm −1 . The solution conductivity gradually decreased to 101.4 uS cm −1 . Table S1 contains raw data on the conductivity changes over 20 min, and Figure 11d depicts Ct/C0 (%) versus time (C0: original conductivity at 0 min; Ct: conductivity at time t). The solution conductivity decreased as the flow time increased, indicating that an increasing number of Hg 2+ ions was absorbed by the P2-5% ES nanofiber membrane, yielding less Hg 2+ in the solution. Thus, the 100% conductivity of the solution in its initial state (0 min) had decreased to 77.2% after 20 min. This rapid change in conductivity was caused by the high surfaceto-volume ratio of the
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal Ionsby the high surfaceto-volume ratio of the P2-5% ES nanofibers. The sensory filter membrane based on the P2-5% ES nanofibers specifically absorbed Hg 2+ in an aqueous solution that contained a variety of metal ions and had a dual fluorescent chemosensory function for Hg 2+ . Figure 12a shows the saturation magnetization and coercivity of the P2-5% ES nanofibersnanocomposite, which were determined using a vibrating sample magnetometer in the field range of ±20,000 Oe at 300 K. The results indicate that the P2-5% ES nanofibers exhibited magnetic properties, and the saturation magnetization was 4.8 emu g −1 . Moreover, the magnetic P2-5% ES nanofibers specifically adsorbed Hg 2+ in an aqueous solution containing a variety of metal ions and had a fluorescent chemosensory response to Hg 2+ ( Figure 11). Thus, filter membranes based on the P2-5% ES nanofibers with porous architectures can specifically chelate with Hg 2+ in an aqueous solution containing many types of metal ion and serve as fluorescent chemosensors for Hg 2+ . These results could assist researchers in cleaning water while simultaneously chelating and sensing Hg 2+ . In addition, rather than removal through alternative methods, a magnet can directly attract the P2-5% ES nanofibers because of the magnetism of the Fe3O4 NPs. The results in Figure 12b and Figure 2c indicate that P2-5% ES nanofibers have the potential for application in multifunctional sensory filter membrane devices for HTM ion chelation, temperature sensing, and magnetism. In conclusion, these copolymer inorganic NP-based sensory fibers have considerable potential for application in water purification sensing filters for the filtration of industrial wastewater with HTM, and may assist researchers in purifying water while simultaneously chelating and sensing Hg 2+ ; moreover, used magnetic fluorescent switchable chemosensors can be collected using magnets (noncontact force). Figure 12a shows the saturation magnetization and coercivity of the P2-5% ES nanofibers-nanocomposite, which were determined using a vibrating sample magnetometer
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal Ionswhich were determined using a vibrating sample magnetometer in the field range of ±20,000 Oe at 300 K. The results indicate that the P2-5% ES nanofibers exhibited magnetic properties, and the saturation magnetization was 4.8 emu g −1 . Moreover, the magnetic P2-5% ES nanofibers specifically adsorbed Hg 2+ in an aqueous solution containing a variety of metal ions and had a fluorescent chemosensory response to Hg 2+ ( Figure 11). Thus, filter membranes based on the P2-5% ES nanofibers with porous architectures can specifically chelate with Hg 2+ in an aqueous solution containing many types of metal ion and serve as fluorescent chemosensors for Hg 2+ . These results could assist researchers in cleaning water while simultaneously chelating and sensing Hg 2+ . In addition, rather than removal through alternative methods, a magnet can directly attract the P2-5% ES nanofibers because of the magnetism of the Fe 3 O 4 NPs. The results in Figures 12b and 2c indicate that P2-5% ES nanofibers have the potential for application in multifunctional sensory filter membrane devices for HTM ion chelation, temperature sensing, and magnetism. In conclusion, these copolymer inorganic NP-based sensory fibers have considerable potential for application in water purification sensing filters for the filtration of industrial wastewater with HTM, and may assist researchers in purifying water while simultaneously chelating and sensing Hg 2+ ; moreover, used magnetic fluorescent switchable chemosensors can be collected using magnets (noncontact force). (b) Schematic of a filter sensory membrane prepared from P2-5% ES nanofibers composed of poly(NIPAAm-co-NMA-co-AA)), BNPTU, and Fe3O4 NPs blends to simultaneously chelate and sense Hg 2+ . A magnet can directly attract the P2-5% ES nanofibers because of the magnetism of Fe3O4 NPs.
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsConclusions
Novel magnetic fluorescent switchable chemosensors for the simultaneous detection of temperature, magnetism, and Hg 2+ based on fluorescent ES nanofibers were prepared using blends of poly(NIPAAm-co-NMA-co-AA), BNPTU, and Fe3O4 NPs by employing a single-capillary spinneret. The NIPAAm, NMA, AA, BNPTU, Fe3O4 NPs moieties were designed to provide thermoresponsiveness, chemical cross-linking, dispersion of Fe3O4 NPs, sensing of Hg 2+ , and magnetism, respectively. Crosslinked ES nanofibers maintained their structure in water and exhibited sensitivity toward temperature variations and Hg 2+ because of the sufficient NMA composition. The fluorescence emission of BNPTU within the ES nanofibers exhibited strong selectivity toward Hg 2+ with green emission in aqueous solutions without Hg 2+ (thiourea-derived), shifting to blue emission in aqueous solutions with Hg 2+ (imidazoline-derived)). The P2-5% ES nanofibers exhibited considerable blueshifts in photoluminescence spectra and enhanced emission intensity for detecting an extremely dilute concentration of Hg 2+ (10 −3 M). Furthermore, the LCST of the NIPAAm moiety in the P2-5% nanofibers showed a significant temperature-dependent variation in PL intensity due to fiber volume change (or hydrophilic to hydrophobic change), engendering distinct on-off switching of photoluminescence when the nanofibers were exposed to Hg 2+ . Furthermore, a magnet can directly attract the P2-5% ES nanofibers because of the magnetism of the Fe3O4 NPs, which serves as a substitute for removal through other methods. The present study demonstrated that the prepared
Conclusions
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsConclusions
Novel magnetic fluorescent switchable chemosensors for the simultaneous detection of temperature, magnetism, and Hg 2+ based on fluorescent ES nanofibers were prepared using blends of poly(NIPAAm-co-NMA-co-AA), BNPTU, and Fe 3 O 4 NPs by employing a single-capillary spinneret. The NIPAAm, NMA, AA, BNPTU, Fe 3 O 4 NPs moieties were designed to provide thermoresponsiveness, chemical cross-linking, dispersion of Fe 3 O 4 NPs, sensing of Hg 2+ , and magnetism, respectively. Cross-linked ES nanofibers maintained their structure in water and exhibited sensitivity toward temperature variations and Hg 2+ because of the sufficient NMA composition. The fluorescence emission of BNPTU within the ES nanofibers exhibited strong selectivity toward Hg 2+ with green emission in aqueous solutions without Hg 2+ (thiourea-derived), shifting to blue emission in aqueous solutions with Hg 2+ (imidazoline-derived). The P2-5% ES nanofibers exhibited considerable blueshifts in photoluminescence spectra and enhanced emission intensity for detecting an extremely dilute concentration of Hg 2+ (10 −3 M). Furthermore, the LCST of the NIPAAm moiety in the P2-5% nanofibers showed a significant temperature-dependent variation in PL intensity due to fiber volume change (or hydrophilic to hydrophobic change), engendering distinct on-off switching of photoluminescence when the nanofibers were exposed to Hg 2+ . Furthermore, a magnet can directly attract the P2-5% ES nanofibers because of the magnetism of the Fe 3 O 4 NPs, which serves as a substitute for removal through other methods. The present study demonstrated that the prepared magnetic fluorescent ES nanofibers can be used as naked eye sensors and have potential for application in multifunctional environmental sensing devices.
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsSupplementary Materials: The following are available online at www.mdpi.com/2073-4360/9/4/136/s1. Figure S1: GPC profiles of P1 and P2 copolymers. Figure S2: TGA curves of P1 and P2 copolymers with a heating rate of 10 • C min −1 in a nitrogen atmosphere. Figure S3: Variations in optical transmittance of P1 and P2 in pH 7 water solutions with temperatures between 30 and 60 • C. Figure S4: TGA curves of P2-5% blended with 5 wt % Fe 3 O 4 NPs nanofibers with a heating rate of 10 • C min −1 in a nitrogen atmosphere. Figure S5: PL intensity of pristine BNPTU compound as the temperature is increased from 30 to 60 • C. Table S1: Time-dependent solution conductivity of the prepared Hg 2+ solution.
Figure 1 .
1Synthesis of BNPTU fluorescent monomer.
Figure 2 .
2Design of multifunctional sensory electro spun (ES) nanofibers synthesized from poly(NIPAAm-co-NMA-co-AA), BNPTU, and Fe3O4 blends with magnetic fluorescence emission. (a) Polymerization and chemical structure of poly(NIPAAm-co-NMA-co-AA), BNPTU, and Fe3O4 particles. (b) Fabrication of ES nanofibers from the blends. (c) Change in the chemical structure of BNPTU in solutions containing Hg 2+ . The fluorescence emission from the ES nanofibers exhibited color changes. A magnet can directly attract the ES nanofibers because of the magnetism of Fe3O4 NPs.
Figure 1 .
1Synthesis of BNPTU fluorescent monomer.
Figure 1 .
1Synthesis of BNPTU fluorescent monomer.
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsFigure 2 .
2Design of multifunctional sensory electro spun (ES) nanofibers synthesized from poly(NIPAAm-co-NMA-co-AA), BNPTU, and Fe3O4 blends with magnetic fluorescence emission. (a) Polymerization and chemical structure of poly(NIPAAm-co-NMA-co-AA), BNPTU, and Fe3O4 particles. (b) Fabrication of ES nanofibers from the blends. (c) Change in the chemical structure of BNPTU in solutions containing Hg 2+ . The fluorescence emission from the ES nanofibers exhibited color changes. A magnet can directly attract the ES nanofibers because of the magnetism of Fe3O4 NPs.
Figure 2 .
2Design of multifunctional sensory electro spun (ES) nanofibers synthesized from poly(NIPAAm-co-NMA-co-AA), BNPTU, and Fe 3 O 4 blends with magnetic fluorescence emission. (a) Polymerization and chemical structure of poly(NIPAAm-co-NMA-co-AA), BNPTU, and Fe 3 O 4 particles. (b) Fabrication of ES nanofibers from the blends. (c) Change in the chemical structure of BNPTU in solutions containing Hg 2+ . The fluorescence emission from the ES nanofibers exhibited color changes. A magnet can directly attract the ES nanofibers because of the magnetism of Fe 3 O 4 NPs.
H-NMR (300 MHz, CDCl 3 , TMS, Figure 3a): δ = 5.33 (a, 2H, -CH 2 CHCH 2 -); δ = 5.91 (b, 1H, -CH 2 CHCH 2 -); δ = 4.58 (c, 2H, -CH 2 CHCH 2 -); δ = 8.56 (d, 1H, 7-CH); δ = 8.03 (e, 1H, 3-CH); δ = 8.64 (f, 1H, 5-CH); δ = 7.94 (g, 1H, 6-CH ); δ = 8.45 (h, 1H, 2-CH).
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsH-NMR (300 MHz, CDCl3, TMS, Figure 3a): δ = 5.33 (a, 2H, -CH2CHCH2-); δ = 5.91 (b, 1H, -CH2CHCH2-); δ = 4.58 (c, 2H, -CH2CHCH2-); δ = 8.56 (d, 1H, 7-CH); δ = 8.03 (e, 1H, 3-CH); δ = 8.64 (f, 1H, 5-CH); δ = 7.94 (g, 1H, 6-CH ); δ = 8.45 (h, 1H, 2-CH).
Figure 3 .Figure 3 .
33Cont1 H-NMR spectra recorded for (a) BN-Br in CDCl3, (b) BN-NH2 in DMSO-d6, and (c) BNPTU monomer in DMSO-d6.
H-NMR (300 MHz, DMSO-d6, TMS, Figure 3c): δ = 5.33 (a, 2H, -CH2CHCH2-); δ = 5.91 (b, 1H, -CH2CHCH2-); δ = 4.58 (c, 2H, -CH2CHCH2-); δ = 8.41 (d, 1H, 7-CH); δ = 7.62-7.88 (e + n, 3H, 6-CH; -COCCHCHCHCH-);
Figure 3 .
31 H-NMR spectra recorded for (a) BN-Br in CDCl 3 , (b) BN-NH 2 in DMSO-d 6 , and (c) BNPTU monomer in DMSO-d 6 .
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal Ions1 H-NMR (300 MHz, DMSO-d 6 , TMS, Figure 4): δ = 0.82-1.12 (a, 6H, -CH(CH 3 ) 2 ); 1.23-1.46 (h + i, 4H, -CH 2 CH-, -CH 2 CH-); 1.84-2.10 (h + i, 2H, -CH 2 CH-, -CH 2 CH-,); 5.34-5.53 (b, 1H, -NHCH 2 OH); 3.71-3.82 (c, 1H,-CH(CH 3 ) 2 ); 7.23-7.59 (e, 1H, -CONHCH-); 7.92-8.14 (f, 1H, -NHCH 2 OH); 4.36-4.65 (g, 2H, -NHCH 2 OH-).
Figure 4 .
41 H-NMR spectrum of poly(NIPAAm-co-NMA-co-AA) in DMSO.
1 H-NMR (300 MHz, DMSO-d6, TMS, Figure 4): δ = 0.82-1.12 (a, 6H, -CH(CH3)2); 1.23-1.46 (h + i + j, 6H, -CH2CH-, -CH2CH-, -CH2CH-); 1.84-2.10 (h + i + j, 3H, -CH2CH-,-CH2CH-, -CH2CH-); 5.34-5.53 (b, 1H, -NHCH2OH); 3.71-3.82 (c, 1H, -CH(CH3)2); 11.92-12.05 (d, 1H, -COCH2OH); 7.23-7.59 (e, 1H, -CONHCH-); 7.92-8.14 (f, 1H, -NHCH2OH); 4.36-4.65 (g, 2H, -NHCH2OH-).
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsFigure 4 .
41 H-NMR spectrum of poly(NIPAAm-co-NMA-co-AA) in DMSO.
Figure 5 .
5UV-Vis spectra of (a) BNPTU in CH3CN solution (10 −5 M) and (b) variation of UV-Vis spectra of BNPTU CH3CN solution (10 −5 M, pH 7) with different metal ions at 10 −4 M. The corresponding inset figures show the color changes under visible light and 254-nm UV light.
Figure 6
6presents field-emission SEM (FE-SEM) images of the ES nanofibers prepared using P1 and P2 at a solution concentration of 250 mg mL −1 with MeOH. When dry, the P1 and P2 ES nanofibers had average diameters of 416 ± 31 and 437 ± 42 nm, respectively. The recorded average diameter was
Figure 5 .
5UV-Vis spectra of (a) BNPTU in CH 3 CN solution (10 −5 M) and (b) variation of UV-Vis spectra of BNPTU CH 3 CN solution (10 −5 M, pH 7) with different metal ions at 10 −4 M. The corresponding inset figures show the color changes under visible light and 254-nm UV light.
Figure 6 .
6Field-emission scanning electron microscopy (FE-SEM) images of copolymers. (a) P1 and (b) P2 cross-linked nanofibers at 120 °C in a dry state and treated with water at 30 and 60 °C.
Figure 7a ,
7ab shows FE-SEM images and transmission electron microscopy images of the Fe3O4 NPs, respectively. The diameters of the nanoparticles were monodispersed within the range 15-25 nm.
Figure 6 .
6Field-emission scanning electron microscopy (FE-SEM) images of copolymers. (a) P1 and (b) P2 cross-linked nanofibers at 120 • C in a dry state and treated with water at 30 and 60 • C.
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsFigure 7a ,
7ab shows FE-SEM images and transmission electron microscopy images of the Fe 3 O 4 NPs, respectively. The diameters of the nanoparticles were monodispersed within the range 15-25 nm.
Figure 7cshows that the Fe 3 O 4 NPs precipitated out of the solution and dropped to the bottom because of higher density (left image) and then accumulated on the side of the bottle because of a magnetic bar placed next to the bottle (right image), indicating that magnetic Fe 3 O 4 NPs can be absorbed using a magnet[38].
Figure 7 .
7(a) FE-SEM and (b) Transmission electron microscopy (TEM) images of Fe3O4 NPs synthesized through coprecipitation. (c) Magnetic Fe3O4 NPs in the solution (left: Fe3O4 precipitated out of the solution and dropped to the bottom; right: Fe3O4 accumulated on the side of the bottle because of the magnet).
Figure 7 .
7(a) FE-SEM and (b) Transmission electron microscopy (TEM) images of Fe 3 O 4 NPs synthesized through coprecipitation. (c) Magnetic Fe 3 O 4 NPs in the solution (left: Fe 3 O 4 precipitated out of the solution and dropped to the bottom; right: Fe 3 O 4 accumulated on the side of the bottle because of the magnet).
Figure 8a
8apresents an SEM image of P2-5% ES nanofibers prepared from P2 copolymers blended with Fe 3 O 4 NPs at a 5% weight ratio. The strong stretching force associated with electrospinning induces orientation of these Fe 3 O 4 NPs along the axis of the fiber. The numerous carboxyl groups of PAA in P2 inhibited the aggregation of Fe 3 O 4 NPs, a result similar to those of several previous studies
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsFigure 8 .
8(a) SEM image of Fe3O4 NPs blended with nanofibers at a 5% weight ratio. (b) FE-SEM image of P2-5% cross-linked nanofibers in the presence of Fe 3+ . (c,d) Energy dispersive X-ray spectroscopy EDS maps of C and Fe within the confined area in (b). (e) EDS spectrum recorded within the region defined in (b).
Figure 8 .
8(a) SEM image of Fe 3 O 4 NPs blended with nanofibers at a 5% weight ratio. (b) FE-SEM image of P2-5% cross-linked nanofibers in the presence of Fe 3+ . (c,d) Energy dispersive X-ray spectroscopy EDS maps of C and Fe within the confined area in (b). (e) EDS spectrum recorded within the region defined in (b).
Figure 9 .
9Cont.
Figure 9 .
9(a) Photoluminescence (PL) spectra of P1 and P2 nanofibers blended with 10 wt % BNPTU. (b) Comparison of P2-5% nanofibers with different Hg 2+ concentrations between 10 −2 , 10 −3 and 10 −4 M in aqueous solution. (c) Relative fluorescence intensity changes (I450/I530) of P2-5% ES nanofibers in aqueous solutions with various Hg 2+ concentrations. (d) PL spectra of P2-5% ES nanofibers in 10 −2 M Hg 2+ solution with a temperature increase from 30 to 60 °C.
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsFigure 9 .
9(a) Photoluminescence (PL) spectra of P1 and P2 nanofibers blended with 10 wt % BNPTU. (b) Comparison of P2-5% nanofibers with different Hg 2+ concentrations between 10 −2 , 10 −3 and 10 −4 M in aqueous solution. (c) Relative fluorescence intensity changes (I 450 /I 530 ) of P2-5% ES nanofibers in aqueous solutions with various Hg 2+ concentrations. (d) PL spectra of P2-5% ES nanofibers in 10 −2 M Hg 2+ solution with a temperature increase from 30 to 60 • C.
Figure 10 .
10(a) Variation in the normalized PL spectra of P2-5% ES nanofibers in aqueous solutions with various metal ions (10 −2 M) and no cation (blank). (b) Fluorometric responses (I450/I530) of P2-5% ES nanofibers to various cations at 10 −2 M aqueous solutions. From left to right: Hg 2+ , Pb 2+ , Co 2+ , Cd 2+ , Mg 2+ , Ni 2+ , Zn 2+ , Fe 2+ , and Cu 2+ . All the inset figures show corresponding photographs recorded under UV light. Inset of (b): fluorimetric response of the P2-5% ES nanofibers to solutions containing various metal ions at 10 −2 M (as in (b)) when Hg 2+ at 10 −2 M is present.
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsFigure 10 .
10(a) Variation in the normalized PL spectra of P2-5% ES nanofibers in aqueous solutions with various metal ions (10 −2 M) and no cation (blank). (b) Fluorometric responses (I 450 /I 530 ) of P2-5% ES nanofibers to various cations at 10 −2 M aqueous solutions. From left to right: Hg 2+ , Pb 2+ , Co 2+ , Cd 2+ , Mg 2+ , Ni 2+ , Zn 2+ , Fe 2+ , and Cu 2+ . All the inset figures show corresponding photographs recorded under UV light. Inset of (b): fluorimetric response of the P2-5% ES nanofibers to solutions containing various metal ions at 10 −2 M (as in (b)) when Hg 2+ at 10 −2 M is present.
Figure 11 .
11Cont.
Figure 11 .
11(a) CIE coordinates of P2-5% ES nanofibers in pH 7 aqueous solutions and after the detection of Hg 2+ at 10 −2 M aqueous solutions. (b) Confocal microscopy images of the ES nanofibers. All the inset figures show corresponding photographs recorded under UV light. (c) Schematic of a sensory filter microfluidics system for real-time metal ion sensing using an ES nanofiber membrane. (d) Relative conductivity versus time of the prepared Hg 2+ solution in the microfluidics system.
Figure 11 .
11(a) CIE coordinates of P2-5% ES nanofibers in pH 7 aqueous solutions and after the detection of Hg 2+ at 10 −2 M aqueous solutions. (b) Confocal microscopy images of the ES nanofibers. All the inset figures show corresponding photographs recorded under UV light. (c) Schematic of a sensory filter microfluidics system for real-time metal ion sensing using an ES nanofiber membrane. (d) Relative conductivity versus time of the prepared Hg 2+ solution in the microfluidics system.
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsFigure 12 .
12(a) Hysteresis loops of P2-5% ES nanofibers (5 wt % Fe3O4 NPs) measured at 25 °C.
Figure 12 .
12(a) Hysteresis loops of P2-5% ES nanofibers (5 wt % Fe 3 O 4 NPs) measured at 25 • C. (b) Schematic of a filter sensory membrane prepared from P2-5% ES nanofibers composed of poly(NIPAAm-co-NMA-co-AA)), BNPTU, and Fe 3 O 4 NPs blends to simultaneously chelate and sense Hg 2+ . A magnet can directly attract the P2-5% ES nanofibers because of the magnetism of Fe 3 O 4 NPs.
Table 1 .
1Polymerization conditions and molecular weights of poly(NIPAAm-co-NMA-co-AA) random copolymers.Polymer No.
Composition a
NIPAAm:NMA:AA
M n
b
PDI
T d ( • C)
LCST ( • C)
(in pH = 7)
P1
10:5:0
26,856
2.01
375
54.5
P2
10:5:3
18,041
1.76
374
58.1
a Molar ratio (%) estimated from 1 H-NMR spectra. b Determined using THF eluent.
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsPolymers 2017,9, 136
© 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
Conflicts of Interest:The authors declare no conflict of interest.
Rigidization, Preorientation and Electronic Decoupling-The 'Magic Triangle' for the Design of Highly Efficient Fluorescent Sensors and Switches. K Rurack, U Resch-Genger, 10.1039/b100604pChem. Soc. Rev. 31PubMedRurack, K.; Resch-Genger, U. Rigidization, Preorientation and Electronic Decoupling-The 'Magic Triangle' for the Design of Highly Efficient Fluorescent Sensors and Switches. Chem. Soc. Rev. 2002, 31, 116-127. [CrossRef] [PubMed]
Light-emitting molecular devices based on transition metals. V Amendola, L Fabbrizzi, F Foti, M Licchelli, C Mangano, P Pallavicini, A Poggi, D Sacchi, A Taglietti, 10.1016/j.ccr.2005.04.022Coord. Chem. Rev. 250Amendola, V.; Fabbrizzi, L.; Foti, F.; Licchelli, M.; Mangano, C.; Pallavicini, P.; Poggi, A.; Sacchi, D.; Taglietti, A. Light-emitting molecular devices based on transition metals. Coord. Chem. Rev. 2006, 250, 273-299. [CrossRef]
Ecological Effects, Transport, and Fate of Mercury: A General Review. D W Boening, 10.1016/S0045-6535(99)00283-0Chemosphere. 40Boening, D.W. Ecological Effects, Transport, and Fate of Mercury: A General Review. Chemosphere 2000, 40, 1335-1351. [CrossRef]
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsThe Role of Radical Reactions in Organomercurials Impact on Lipid Peroxidation. E R Milaeva, 10.1016/j.jinorgbio.2006.02.014J. Inorg. Biochem. 100PubMedMilaeva, E.R. The Role of Radical Reactions in Organomercurials Impact on Lipid Peroxidation. J. Inorg. Biochem. 2006, 100, 905-915. [CrossRef] [PubMed]
A Selective Fluorescent Ratiometric Chemodosimeter for Mercury Ion. B Liu, H Tian, 10.1039/b501913cChem. Commun. PubMedLiu, B.; Tian, H. A Selective Fluorescent Ratiometric Chemodosimeter for Mercury Ion. Chem. Commun. 2005, 3156-3158. [CrossRef] [PubMed]
Colorimetric Detection of Mercuric Ion (Hg 2+ ) in Aqueous Media Using Chemodosimeter-Functionalized Gold Nanoparticles. B Leng, L Zou, J Jiang, H Tian, 10.1016/j.snb.2009.03.074Sens. Actuators B. 140Leng, B.; Zou, L.; Jiang, J.; Tian, H. Colorimetric Detection of Mercuric Ion (Hg 2+ ) in Aqueous Media Using Chemodosimeter-Functionalized Gold Nanoparticles. Sens. Actuators B 2009, 140, 162-169. [CrossRef]
Chemodosimeters for Mercury(II) and Methylmercury(I) Based on 2, 1, 3-Benzothiadiazole. Q Zou, H Tian, 10.1016/j.snb.2010.06.040Sens. Actuators B. 149Zou, Q.; Tian, H. Chemodosimeters for Mercury(II) and Methylmercury(I) Based on 2, 1, 3-Benzothiadiazole. Sens. Actuators B 2010, 149, 20-27. [CrossRef]
|
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsTracking of Mercury Ions in Living Cells with a Fluorescent Chemodosimeter under Single-or Two-Photon Excitation. Z J Lua, P N Wang, Y Zhang, J Y Chen, S Zhen, B Leng, H Tian, 10.1016/j.aca.2007.06.057Anal. Chim. Acta. 597PubMedLua, Z.J.; Wang, P.N.; Zhang, Y.; Chen, J.Y.; Zhen, S.; Leng, B.; Tian, H. Tracking of Mercury Ions in Living Cells with a Fluorescent Chemodosimeter under Single-or Two-Photon Excitation. Anal. Chim. Acta 2007, 597, 306-312. [CrossRef] [PubMed]
Simple Naphthalimide-Based Fluorescent Sensor for Highly Sensitive and Selective Detection of Cd 2+ and Cu 2+ in Aqueous Solution and Living Cells. W Wang, Y Wen, Y Zhang, X Fei, Y Li, Q Yang, X Xu, 10.1039/C2DT32279JDalton Trans. 42PubMedWang, W.; Wen, Y.; Zhang, Y.; Fei, X.; Li, Y.; Yang, Q.; Xu, X. Simple Naphthalimide-Based Fluorescent Sensor for Highly Sensitive and Selective Detection of Cd 2+ and Cu 2+ in Aqueous Solution and Living Cells. Dalton Trans. 2013, 42, 1827-1833. [CrossRef] [PubMed]
Nanomolar Cu 2+ and F − Naked-Eye Detection with a 1,8-Naphthalimide-Based Colorimetric Probe. C Liu, J Xu, F Yang, W Zhou, Z Li, M Wei, M Yu, 10.1016/j.snb.2015.02.010Sens. Actuators B. 212Liu, C.; Xu, J.; Yang, F.; Zhou, W.; Li, Z.; Wei, M.; Yu, M. Nanomolar Cu 2+ and F − Naked-Eye Detection with a 1,8-Naphthalimide-Based Colorimetric Probe. Sens. Actuators B 2015, 212, 364-370. [CrossRef]
|
84
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsSynthesis of Some Polymerisable Fluorescent Dyes. Dyes Pigment. L G F Patrick, A Whiting, 10.1016/S0143-7208(02)00067-055Patrick, L.G.F.; Whiting, A. Synthesis of Some Polymerisable Fluorescent Dyes. Dyes Pigment. 2002, 55, 123-132. [CrossRef]
Stimuli-Reponsive Polymers and Their Bioconjugates. E S Gil, S M Hudson, 10.1016/j.progpolymsci.2004.08.003Prog. Polym. Sci. 29Gil, E.S.; Hudson, S.M. Stimuli-Reponsive Polymers and Their Bioconjugates. Prog. Polym. Sci. 2004, 29, 1173-1222. [CrossRef]
Preparation and Thermal Response Behavior of Poly(N-isopropylacrylamide-coacrylic acid) Microgels via Soap-Free Emulsion Polymerization Based on AIBN Initiator. S Chen, L Jiang, Y Dan, 10.1002/app.33938J. Appl. Polym. Sci. 121Chen, S.; Jiang, L.; Dan, Y. Preparation and Thermal Response Behavior of Poly(N-isopropylacrylamide-co- acrylic acid) Microgels via Soap-Free Emulsion Polymerization Based on AIBN Initiator. J. Appl. Polym. Sci. 2011, 121, 3322-3331. [CrossRef]
Electrospun Nanofibers of Blends of Conjugated Polymers: Morphology, Optical Properties, and Field-Effect Transistors. A Babel, D Li, Y Xia, S A Jenekhe, 10.1021/ma047529rMacromolecules. 38Babel, A.; Li, D.; Xia, Y.; Jenekhe, S.A. Electrospun Nanofibers of Blends of Conjugated Polymers: Morphology, Optical Properties, and Field-Effect Transistors. Macromolecules 2005, 38, 4705-4711. [CrossRef]
|
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsMorphology and Photophysical Properties of Light-Emitting Electrospun Nanofibers Prepared from Poly(fluorene) Derivative/PMMA Blends. C C Kuo, C H Lin, W C Chen, 10.1021/ma071182lMacromolecules. 40Kuo, C.C.; Lin, C.H.; Chen, W.C. Morphology and Photophysical Properties of Light-Emitting Electrospun Nanofibers Prepared from Poly(fluorene) Derivative/PMMA Blends. Macromolecules 2007, 40, 6959-6966. [CrossRef]
Novel Luminescent Electrospun Fibers Prepared from Conjugated Rod-Coil Block Copolymer of Poly. C C Kuo, Y C Tung, C H Lin, W C Chen, block-Poly29methyl methacrylateKuo, C.C.; Tung, Y.C.; Lin, C.H.; Chen, W.C. Novel Luminescent Electrospun Fibers Prepared from Conjugated Rod-Coil Block Copolymer of Poly[2,7-(9,9-dihexylfluorene)]-block-Poly(methyl methacrylate).
. 10.1002/marc.200800491Macromol. Rapid Commun. 29Macromol. Rapid Commun. 2008, 29, 1711-1715. [CrossRef]
|
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsMultifunctional Polyacrylonitrile-ZnO/Ag Electrospun Nanofiber Membranes with Various ZnO Morphologies for Photocatalytic, UV-shielding, and Antibacterial Applications. Y Y Chen, C C Kuo, B U Chen, P C Chiu, P C Tsai, 10.1002/polb.23621J. Polym. Sci. Part B Polym. Phys. 53Chen, Y.Y.; Kuo, C.C.; Chen, B.U.; Chiu, P.C.; Tsai, P.C. Multifunctional Polyacrylonitrile-ZnO/Ag Electrospun Nanofiber Membranes with Various ZnO Morphologies for Photocatalytic, UV-shielding, and Antibacterial Applications. J. Polym. Sci. Part B Polym. Phys. 2015, 53, 262-269. [CrossRef]
Morphology and pH Sensing Characteristics of New Luminescent Electrospun Fibers Prepared from Poly(phenylquinoline)-block-Polystyrene/Polystyrene Blends. C C Kuo, Y C Tung, W C Chen, 10.1002/marc.200900566Macromol. Rapid Commun. 31PubMedKuo, C.C.; Tung, Y.C.; Chen, W.C. Morphology and pH Sensing Characteristics of New Luminescent Electrospun Fibers Prepared from Poly(phenylquinoline)-block-Polystyrene/Polystyrene Blends. Macromol. Rapid Commun. 2010, 31, 65-70. [CrossRef] [PubMed]
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsThermoresponsive Luminescent Electrospun Fibers Prepared From Poly(DMAEMA-co-SA-co-StFl) Multifunctional Random Copolymers. Y C Chiu, C C Kuo, J C Hsu, W C Chen, 10.1021/am100760aACS Appl. Mater. Interfaces. 11PubMedChiu, Y.C.; Kuo, C.C.; Hsu, J.C.; Chen, W.C. Thermoresponsive Luminescent Electrospun Fibers Prepared From Poly(DMAEMA-co-SA-co-StFl) Multifunctional Random Copolymers. ACS Appl. Mater. Interfaces 2010, 11, 3340-3347. [CrossRef] [PubMed]
Synthesis, Morphology, and Sensory Applications of Multifunctional Rod-Coil-Coil Triblock Copolymers and Their Electrospun Nanofibers. Y C Chiu, Y Chen, C C Kuo, S H Tung, T Kakuchi, W C Chen, 10.1021/am300315vACS Appl. Mater. Interfaces. 4PubMedChiu, Y.C.; Chen, Y.; Kuo, C.C.; Tung, S.H.; Kakuchi, T.; Chen, W.C. Synthesis, Morphology, and Sensory Applications of Multifunctional Rod-Coil-Coil Triblock Copolymers and Their Electrospun Nanofibers. ACS Appl. Mater. Interfaces 2012, 4, 3387-3395. [CrossRef] [PubMed]
|
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsN-isopropylacrylamide)-co-(N-hydroxymethylacrylamide)) and Their Blends with 1,2-Diaminoanthraquinone for NO Gas Detection. L N Chen, Y C Chiu, J J Hung, C C Kuo, W C Chen, 10.1002/macp.201300604Multifunctional Electrospun Nanofibers Prepared from Poly. 215Macromol. Chem. Phys.Chen, L.N.; Chiu, Y.C.; Hung, J.J.; Kuo, C.C.; Chen, W.C. Multifunctional Electrospun Nanofibers Prepared from Poly((N-isopropylacrylamide)-co-(N-hydroxymethylacrylamide)) and Their Blends with 1,2-Diaminoanthraquinone for NO Gas Detection. Macromol. Chem. Phys. 2014, 215, 286-294. [CrossRef]
Preparation of Fluorescent Nanofibrous Film as a Sensing Material and Adsorbent for Cu 2+ in Aqueous Solution via Copolymerization and Electrospinning. W Wang, Q Yang, L Sun, H Wang, C Zhang, X Fei, M Sun, Y Li, 10.1016/j.jhazmat.2011.07.083J. Hazard. Mater. 194PubMedWang, W.; Yang, Q.; Sun, L.; Wang, H.; Zhang, C.; Fei, X.; Sun, M.; Li, Y. Preparation of Fluorescent Nanofibrous Film as a Sensing Material and Adsorbent for Cu 2+ in Aqueous Solution via Copolymerization and Electrospinning. J. Hazard. Mater. 2011, 194, 185-192. [CrossRef] [PubMed]
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsUltra Metal Ions and pH Sensing Characteristics of Thermoresponsive Luminescent Electrospun Nanofibers Prepared from Poly(HPBO-co-NIPAAm-co-SA). L N Chen, C C Kuo, Y C Chiu, W C Chen, 10.1039/C4RA07422JRSC Adv. 4Chen, L.N.; Kuo, C.C.; Chiu, Y.C.; Chen, W.C. Ultra Metal Ions and pH Sensing Characteristics of Thermoresponsive Luminescent Electrospun Nanofibers Prepared from Poly(HPBO-co-NIPAAm-co-SA). RSC Adv. 2014, 4, 45345-45353. [CrossRef]
Novel Highly Selective and Reversible Chemosensors Based on Dual-Ratiometric Fluorescent Electrospun Nanofibers with pH-and Fe 3+ -Modulated Multicolor Fluorescence Emission. B Y Chen, C C Kuo, Y S Huang, S T Lu, F C Liang, D H Jiang, 10.1021/am508029xACS Appl. Mater. Interfaces. 7PubMedChen, B.Y.; Kuo, C.C.; Huang, Y.S.; Lu, S.T.; Liang, F.C.; Jiang, D.H. Novel Highly Selective and Reversible Chemosensors Based on Dual-Ratiometric Fluorescent Electrospun Nanofibers with pH-and Fe 3+ -Modulated Multicolor Fluorescence Emission. ACS Appl. Mater. Interfaces 2015, 7, 2797-2808. [CrossRef] [PubMed]
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsPyrene or Rhodamine Derivative-Modified Surfaces of Electrospun Nanofibrous Chemosensors for Colorimetric and Fluorescent Determination of Cu 2+ , Hg 2+ , and pH. C J Cho, S T Lu, C C Kuo, F C Liang, B Y Chen, C C Chu, 10.1016/j.reactfunctpolym.2016.05.019React. Funct. Polym. Cho, C.J.; Lu, S.T.; Kuo, C.C.; Liang, F.C.; Chen, B.Y.; Chu, C.C. Pyrene or Rhodamine Derivative-Modified Surfaces of Electrospun Nanofibrous Chemosensors for Colorimetric and Fluorescent Determination of Cu 2+ , Hg 2+ , and pH. React. Funct. Polym. 2016, 137-147. [CrossRef]
Novel Highly Sensitive and Reversible Electrospun Nanofibrous Chemosensor-Filters Composed of poly(HEMA-co-MNA) and Bpy-F-Bpy with Metal-Ionmodulated Multicolor Fluorescence Emission. C C Hung, C C Kuo, N K Weng, W C Wu, B Y Chen, C J Cho, I J Hsu, Y C Chiu, C C Chen, Polym. J. 48Hung, C.C.; Kuo, C.C.; Weng, N.K.; Wu, W.C.; Chen, B.Y.; Cho, C.J.; Hsu, I.J.; Chiu, Y.C.; Chen, C.C. Novel Highly Sensitive and Reversible Electrospun Nanofibrous Chemosensor-Filters Composed of poly(HEMA-co-MNA) and Bpy-F-Bpy with Metal-Ionmodulated Multicolor Fluorescence Emission. Polym. J. 2016, 48, 439-449.
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsSelf-Assembly Behaviors of Thermaland pH Sensitive Magnetic Nanocarriers for Stimulitriggered Release. C Y Kuo, T Y Liu, A Hardiansyah, C F Lee, M S Wang, W Y Chiu, 10.1186/1556-276X-9-520Nanoscale Res. Lett. 9PubMedKuo, C.Y.; Liu, T.Y.; Hardiansyah, A.; Lee, C.F.; Wang, M.S.; Chiu, W.Y. Self-Assembly Behaviors of Thermal- and pH Sensitive Magnetic Nanocarriers for Stimulitriggered Release. Nanoscale Res. Lett. 2014, 9, 520. [CrossRef] [PubMed]
One-Step Hydrothermal Synthesis of Highly Water-Soluble Secondary Structural Fe 3 O 4 Nanoparticles. X Yang, W Jiang, L Liu, B Chen, S Wu, D Sun, F Li, 10.1016/j.jmmm.2012.02.111J. Magn. Magn. Mater. 324Yang, X.; Jiang, W.; Liu, L.; Chen, B.; Wu, S.; Sun, D.; Li, F. One-Step Hydrothermal Synthesis of Highly Water-Soluble Secondary Structural Fe 3 O 4 Nanoparticles. J. Magn. Magn. Mater. 2012, 324, 2249-2257. [CrossRef]
Size-Controlled Synthesis of Magnetite Nanoparticles in the Presence of Polyelectrolytes. S Si, A Kotal, T K Mandal, S Giri, H Nakamura, T Kohara, 10.1021/cm049205nChem. Mater. 16Si, S.; Kotal, A.; Mandal, T.K.; Giri, S.; Nakamura, H.; Kohara, T. Size-Controlled Synthesis of Magnetite Nanoparticles in the Presence of Polyelectrolytes. Chem. Mater. 2004, 16, 3489-3496. [CrossRef]
|
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsHigh Pervaporation Dehydration Performance of the Composite Membrane with an Ultrathin Alginate/Poly(acrylic acid)-Fe 3 O 4 Active Layer. C Zhao, Z Jiang, J Zhao, K Cao, Q Zhang, F Pan, 10.1021/ie403437gInd. Eng. Chem. Res. 53Zhao, C.; Jiang, Z.; Zhao, J.; Cao, K.; Zhang, Q.; Pan, F. High Pervaporation Dehydration Performance of the Composite Membrane with an Ultrathin Alginate/Poly(acrylic acid)-Fe 3 O 4 Active Layer. Ind. Eng. Chem. Res. 2014, 53, 1606-1616. [CrossRef]
Synthesis of PAA-g-PNIPAM Well-Defined Graft Oolymer by Sequential RAFT and SET-LRP and its Application in Preparing Size-Controlled Super-Paramagnetic Fe 3 O 4 Nanoparticles as a Stabilizer. X Jiang, S Zhai, X Jiang, G Lu, X Huang, 10.1016/j.polymer.2014.05.050Polymer. 55Jiang, X.; Zhai, S.; Jiang, X.; Lu, G.; Huang, X. Synthesis of PAA-g-PNIPAM Well-Defined Graft Oolymer by Sequential RAFT and SET-LRP and its Application in Preparing Size-Controlled Super-Paramagnetic Fe 3 O 4 Nanoparticles as a Stabilizer. Polymer 2014, 55, 3703-3712. [CrossRef]
Preparation of Magnetic Polyvinyl Alcohol Composite Nanofibers with Homogenously Dispersed Nanoparticles and High Water Resistance. F Liu, Q Q Ni, Y Murakami, 10.1177/0040517512444334Text. Res. J. 83Liu, F.; Ni, Q.Q.; Murakami, Y. Preparation of Magnetic Polyvinyl Alcohol Composite Nanofibers with Homogenously Dispersed Nanoparticles and High Water Resistance. Text. Res. J. 2013, 83, 510-518. [CrossRef]
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsMercury Ion Responsive Wettability and Oil/Water Separation. L Xu, N Liu, Y Cao, F Lu, Y Chen, X Zhang, L Feng, L Wei, 10.1021/am5038214ACS Appl. Mater. Interfaces. 6PubMedXu, L.; Liu, N.; Cao, Y.; Lu, F.; Chen, Y.; Zhang, X.; Feng, L.; Wei, L. Mercury Ion Responsive Wettability and Oil/Water Separation. ACS Appl. Mater. Interfaces 2014, 6, 13324-13329. [CrossRef] [PubMed]
Synthesis and Self-Assembly Behaviors of Three-Armed Amphiphilic Block Copolymers via RAFT Polymerization. W Zhang, W Zhang, N Zhou, Z Cheng, J Zhu, X Zhu, 10.1016/j.polymer.2008.08.034Polymer. 49Zhang, W.; Zhang, W.; Zhou, N.; Cheng, Z.; Zhu, J.; Zhu, X. Synthesis and Self-Assembly Behaviors of Three-Armed Amphiphilic Block Copolymers via RAFT Polymerization. Polymer 2008, 49, 4569-4575. [CrossRef]
Solid-Phase Synthesis of Guanidinium Derivatives from Thiourea and Isothiourea Functionalities. J C Manimala, E V Anslyn, 10.1002/1099-0690(200212)2002:23<3909::AID-EJOC3909>3.0.CO;2-JEur. J. Org. Chem. Manimala, J.C.; Anslyn, E.V. Solid-Phase Synthesis of Guanidinium Derivatives from Thiourea and Isothiourea Functionalities. Eur. J. Org. Chem. 2002, 3909-3922. [CrossRef]
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Novel Magnet and Thermoresponsive Chemosensory Electrospinning Fluorescent Nanofibers and Their Sensing Capability for Metal IonsImproved Method for the Preparation of Guanidines. K S Kim, L Qian, 10.1016/S0040-4039(00)61537-XTetrahedron Lett. 34Kim, K.S.; Qian, L. Improved Method for the Preparation of Guanidines. Tetrahedron Lett. 1993, 34, 7677-7680. [CrossRef]
A General Synthetic Method for the Formation of Substituted 5-Aminotetrazoles from Thioureas: A Strategy for Diversity Amplification. R A Batey, D A Powell, 10.1021/ol006465bOrg. Lett. 2PubMedBatey, R.A.; Powell, D.A. A General Synthetic Method for the Formation of Substituted 5-Aminotetrazoles from Thioureas: A Strategy for Diversity Amplification. Org. Lett. 2000, 2, 3237-3240. [CrossRef] [PubMed]
Structural Elution and characteristics of the phase transformations between a-Fe 2 O 3 , Fe 3 O 4 and c-Fe 2 O 3 nanoparticles under reducing and oxidizing atmospheres. X Zhang, Y Niu, X Meng, Y Li, J Zhao, 10.1039/c3ce41269eCrystEngComm. 15Zhang, X.; Niu, Y.; Meng, X.; Li, Y.; Zhao, J. Structural Elution and characteristics of the phase transformations between a-Fe 2 O 3 , Fe 3 O 4 and c-Fe 2 O 3 nanoparticles under reducing and oxidizing atmospheres. CrystEngComm 2013, 15, 8166-8172. [CrossRef]
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Electrochemical and structural characterization of Azotobacter vinelandii flavodoxin IISupplementary Material for Electrochemical and Structural Characterization of Azotobacter vinelandii Flavodoxin II
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Electrochemical and structural characterization of Azotobacter vinelandii flavodoxin IIHelen M Segal
Thomas Spatzal
Michael G Hill
Andrew K Udit
Douglas C Rees
Supplementary Material for Electrochemical and Structural Characterization of Azotobacter vinelandii Flavodoxin II
1 This PDF includes: Supplementary Materials and Methods Supplementary Text Figures S1-S5 Tables S1-S2
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Electrochemical and structural characterization of Azotobacter vinelandii flavodoxin IIAzotobacter vinelandii Cell Growth and Flavodoxin II Purification. Cell growths and proteinpurifications were carried out as previously described for isolation of Azotobacter vinelandii nitrogenase 53,54 . The nitrogenase component proteins were eluted from a HiTrap Q anion exchange column (GE Healthcare) with a gradient from 150 mM NaCl to 1 M NaCl over thirty column volumes. The flavodoxin eluted from the column at conductivity values between 39 to 40 mS/cm. The protein was further purified on a size exclusion column (Superdex200, 26/60, GE Healthcare). The protein concentration was quantified by Bradford assay using a BSA standard curve.Strain Construction for Overexpression of Flavodoxin II in E. coli. The flavodoxin II overexpression plasmid was synthesized by Genescript USA, Inc. In this plasmid, the Azotobacter vinelandii nifF gene was cloned into the pET21b(+) vector between the NdeI-BamHI restriction enzyme cut sites. A stop codon was included at the C-terminus of the gene so that the protein was expressed without affinity tags. The nifF gene was optimized for E. coli codon usage. The plasmid was transformed into E. coli BL21 (DE3) cells for overexpression of flavodoxin II.E. coli Cell Growth and Purification of Recombinant Flavodoxin II. The protocol for purification ofAzotobacter vinelandii flavodoxin II was adapted from previously published protocols 19,55 . The following buffers were prepared, adjusted to the correct pH at room temperature, and filtered with a glass fiber filter: anion exchange buffer A (50 mM Tris-HCl, pH 7.5, 150 mM NaCl, 2 mM dithiotreitol (DTT)), anion exchange buffer B (50 mM Tris-HCl, pH 7.5, 1 M NaCl, 2 mM DTT), and size exclusion buffer (50 mM Tris-HCl, pH 7.5, 200 mM NaCl). DTT was added to all buffers
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Electrochemical and structural characterization of Azotobacter vinelandii flavodoxin IIthe day of purification. Dioxygen was removed from the size exclusion column buffer prior to use in the purification by iterative cycles of vacuum followed by argon filling.
An overnight starter culture of the flavodoxin II overexpression strain supplemented with 100 µg/mL ampicillin was incubated at room temperature with shaking overnight. The starter culture was diluted 1:100 into 1 L of LB medium containing 50 µg/mL ampicillin. The cultures were grown to late exponential phase (OD 600 ~ 0.6-0.8) at 37 °C with shaking. Overexpression of flavodoxin II was induced by addition of IPTG to a final concentration of 0.5 mM in each culture. The cultures were incubated for 16 hours at 37 °C with shaking. The cells were pelleted via centrifugation at 6238 x g for ten minutes. The cell pellets were stored at -80 °C until purification.
All lysis steps were carried out on ice at room temperature. The BL21 (DE3) cell pellets were thawed on ice. The cell pellets were re-suspended in anion exchange buffer A with a homogenizer. The lysis buffer also contained complete protease inhibitor tablets (Roche) at a concentration of one tablet per 50 mL of buffer. The cells were lysed with an Avestin Emulsiflex C5 homogenizer. The cell debris was pelleted by centrifugation in a floor centrifuge at 13,000 x g at 4 °C for 35 minutes.
The cleared cell lysate was loaded onto two 5 mL HiTrapQ HP anion exchange columns connected in tandem using an Akta FPLC (GE Healthcare). The protein was purified by anion exchange chromatography as described previously 19,55 . Following the anion exchange column, the protein was loaded onto a Superdex200 size exclusion column. The protein that eluted from the column was collected, and was concentrated in an Amicon filter centrifuge tube (10,000 molecular weight cutoff) to a final concentration of about 10 µM. This protein was flash frozen in liquid nitrogen and stored under liquid nitrogen.
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Electrochemical and structural characterization of Azotobacter vinelandii flavodoxin IIThe concentration of protein was estimated based on the absorption of the sample at 452 nm, using an extinction coefficient of 11,300 M -1 cm -1 55 . The total protein yield was about 0.2 mg of protein per gram of cell paste. The purity of the isolated protein was analyzed on a denaturing polyacrylamide gel. The concentration of protein with bound flavin mononucleotide cofactor was estimated from the ratio of the absorption of the sample at 274 nm to the absorption of the sample at 452 nm. The ratio of Abs 274 /Abs 452 was typically ~7, which was higher than the ratio of 4.6-4.8 reported for the oxidized form of this protein 55 , suggesting the presence of some apo or partial reduced forms of the flavodoxin.
Preparation of Flavodoxin II for Electrochemistry Experiments.
Prior to electrochemistry experiments, sodium dithionite was removed from all protein samples using a PD10 column (GE Healthcare). This procedure was performed in a McCoy anaerobic chamber with a 95 % / 5 % Ar / H 2 atmosphere. The column was equilibrated in five column volumes of electrochemistry buffer (50 mM potassium phosphate, pH 7.5, 150 mM NaCl) with 10 mM sodium dithionite. This step allowed for the reduction of reactive oxygen species prior to introduction of the protein to the column. Then, the column was equilibrated in five column volumes of electrochemistry buffer to remove all sodium dithionite. All buffers were filtered with a 0.2 micron filter, and oxygen was removed from the buffer by iterative cycles of vacuum followed by filling with argon.
The protein was exchanged into electrochemistry buffer using the PD10 column. UV-visible absorption spectroscopy was used to quantify protein concentration, and to determine the extent of protein oxidation during the course of sample preparation. Following this procedure, the protein was flash frozen in liquid nitrogen, and when necessary was transported under liquid nitrogen.
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