**Figure 6** shows the transient transmission spectra at different probe delays. The spectra at 1 ps probe delay is positive in the entire spectral region, and by comparing the spectrum with the absorption and the photoluminescence data, it can be concluded that the signal is an overlap between the bleaching of the ground state and stimulated emission. Increasing the probe delay (green line in **Figure 6**) results in a new photoinduced absorption band that appears at ∼600 and at 950 nm, and is attributed to charged states as previously reported (Banerji et al., 2010; Etzold et al., 2011; Provencher et al., 2014). Surprisingly the positive band extends until the near infrared region where a contribution from stimulated emission is evident. This can be concluded because the absorption signal is much less significant in this region. The overlap of the SE with the PIA band due to the photogenerated charges in this spectral region makes the PL efficiency very low and consequently undetectable in the PL measurements. As an aside, it is important to note from the transient transmission data that there is no indication of a signal coming from residual PVP surfactant present in the solution, as occurred in the photoluminescence spectrum, which indicates that its presence is negligible.

To confirm the origin of the positive transient signal, the dynamics at 520, 600, 670, and 950 nm are compared in **Figure 7**. The decays are similar in first 10 ps indicating that the decay of the singlet exciton states is the origin of the photoinduced signal. After 10 ps, 1T/T at 600 and 950 nm becomes negative; the positive signal at these wavelengths overlaps with a negative photoinduced absorption band that is attributed to charged-state formation as previously reported (Provencher et al., 2014).

#### Discussion

The transient transmission spectra for the different particle suspensions, as for the steady state absorption and emission spectra, are completely different. While the small polymer particles present a photophysics similar to a disordered polymer film, the large particles behave in a completely different way. There are three important differences: Firstly, in the LMM mesoparticle suspension there is no indication of the typical PCDTBT exciton energy transfer from the higher energetic states to the lowest state while in the nanoparticle suspension this effect is evident (see blue line in **Figure 4**). Secondly, the signal from the charged states (blue line in **Figure 7**) is weak and is not instantaneous in the LMM suspension while the charges are instantaneously generated in the HMM suspension (see black solid line in **Figure 4**). Finally, stimulated emission occurs in the near infrared region only for the LMM suspension. All of these properties indicate a less efficient interchain interaction strengthening the idea that the LMM particle suspension has a more J-aggregate-like behavior that is not observed in the HMM-nanoparticle suspension.

It has been demonstrated in a range of different polymer systems that the strength of interchain vs. intrachain interactions, resulting in modulation between predominately H- vs. Jaggregate-like behavior, can be tuned systematically by parameters such as solvent casting and annealing (Khan et al., 2004; Eder et al., 2017), pressure (Niles et al., 2012), regioregularity (Clark et al., 2007), and molar mass (Paquin et al., 2013). From the perspective of classical polymer science, it is established that polymers of low molar mass form unconnected, extended-chain crystals. Due to the non-entangled nature of these relatively short-chain macromolecules, this leads to a polycrystalline, one-phase morphology. In contrast, with materials of molar mass larger than the entanglement molar

mass, typically two-phase morphologies are obtained, which comprise crystalline moieties embedded in largely amorphous regions, whereby individual macromolecules bridge multiple domains of order (Wunderlich, 1976; Paquin et al., 2013). For poly(3-hexylthiophene) it was found that the macromolecules in aggregated regions of high molar mass polymer adopt a more planar conformation compared to low molar mass materials. This results in the observed increase in intrachain exciton coherence. In contrast, shorter chains seem to lead to more disordered architectures (Paquin et al., 2013). In contrast to those previous results, here the low molar mass particle suspension presents a J-aggregate-like behavior (i.e., more intrachain interaction) but not the high molar mass suspension.

These results indicate that the means of particle preparation affects not only the particle size but also the packing of the polymer chains inside the particles, which dramatically changes their spectroscopic properties. Given that the optical properties of the thin films have been shown to be similar (**Figure 1B** and **Figure 5S**), it is unlikely that the molar mass plays a significant role. It is known that polymer molar mass does affect crystallization (Cheng et al., 1992), and were this to be a factor in the preparation process, then low molar mass polymers may be less well-stacked or less planar than their larger counterparts due, for example, to free chain ends, which would increase the likelihood of localized excitons and greater energetic order. It is nevertheless difficult to envisage how particle size alone can control whether or not excitons are localized, and so the possibility that the method of preparing the PCDTBT particles plays a role cannot be discounted.

#### CONCLUSION

The photophysics of poly[N-9′ -heptadecanyl-2,7-carbazolealt−5,5-(4,7-di-2-thienyl-2′ ,1′ ,3′ - benzothiadiazole] (PCDTBT) nanoparticle and mesoparticle suspensions has been investigated. Virgili et al. Optical Properties of PCDTBT Particles

The nanoparticles present a similar behavior of the pristine polymer, with a large Stokes shift and the presence of many localized states near the band edge. The efficient energy transfer from the higher energetic state to the lowest one happens in a time constant of around 7 ps. In these nanoparticles no stimulated emission is detected due to the presence of a photoinduced absorption band related to charged states which overlap the stimulated emission spectral region. The mesoparticles have a completely different behavior. The peak of the absorption spectrum is red-shifted and narrower than that of the nanoparticle suspension, and the Stokes shift is reduced as well as the efficiency of charges formation indicating a more evident J-aggregate-like behavior (fewer interchain interactions) compared to the nanoparticle suspension. Moreover, stimulated emission is detected in the near infrared region for the mesoparticle suspension. It is concluded that the method of particle preparation is crucial not only to determine the particle size but also to change the packing state of the polymer backbones that strongly affects the photophysical properties of the PCDTBT polymer.

### AUTHOR CONTRIBUTIONS

The project was devised by TV and MG and supervised by TV, MG, and EP. Data contained in the paper were obtained and analysed by TV, MM, and ChB in Milan. Micro-Raman

#### REFERENCES

spectroscopy and preparatory and comparative absorption and photoluminescence data were obtained and analysed in Bordeaux by LP, EP, CyB, and EC. All authors contributed to the scientific discussion of the results. The manuscript was written by TV and MG, although all authors contributed and approved the final version.

#### ACKNOWLEDGMENTS

MG is grateful to the Politecnico di Milano and CNR-ISMAC for hosting him in Milan. TV and MM acknowledge the project TIMES from Regione Lombardia. In Bordeaux, LP, CyB, EC, EP, and GH acknowledge the financial support from the ADEME Project ISOCEL number 1182C0212 and from Region Nouvelle Aquitaine. This work was performed within the framework of the LCPO/Arkema/ANR Industrial Chair HOMERIC ANR-13- CHIN-0002-01 with grant number AC-2013-365. Micro-Raman spectroscopy experiments were performed at the Institute of Molecular Science of the University of Bordeaux with the help of Dr T. Tassaing.

#### SUPPLEMENTARY MATERIAL

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fchem. 2019.00409/full#supplementary-material

**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2019 Virgili, Botta, Mróz, Parrenin, Brochon, Cloutet, Pavlopoulou, Hadziioannou and Geoghegan. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

# Formation of Monolithic Ion-Selective Transport Media Based on "Click" Cross-Linked Hyperbranched Polyglycerol

Tobias Abrahamsson1†, David J. Poxson1†, Erik O. Gabrielsson1†, Mats Sandberg2† , Daniel T. Simon1 \* and Magnus Berggren1†

Edited by: John George Hardy, Lancaster University, United Kingdom

#### Reviewed by:

Basem Moosa, KAUST Catalysis Center (KCC), Saudi Arabia Radek Cibulka, University of Chemistry and Technology in Prague, Czechia

#### \*Correspondence:

Daniel T. Simon<EMAIL_ADDRESS>orcid.org/0000-0002-2799-3490

†Tobias Abrahamsson orcid.org/0000-0002-3615-1850 David J. Poxson orcid.org/0000-0002-4246-8723 Erik O. Gabrielsson orcid.org/0000-0002-0302-226X Mats Sandberg orcid.org/0000-0002-0631-3804 Magnus Berggren orcid.org/0000-0001-5154-0291

#### Specialty section:

This article was submitted to Organic Chemistry, a section of the journal Frontiers in Chemistry

Received: 21 March 2019 Accepted: 24 June 2019 Published: 10 July 2019

#### Citation:

Abrahamsson T, Poxson DJ, Gabrielsson EO, Sandberg M, Simon DT and Berggren M (2019) Formation of Monolithic Ion-Selective Transport Media Based on "Click" Cross-Linked Hyperbranched Polyglycerol. Front. Chem. 7:484. doi: 10.3389/fchem.2019.00484 1 Laboratory of Organic Electronics, Department of Science and Technology, Linköping University, Norrköping, Sweden, 2 RISE Acreo AB, Norrköping, Sweden

In the emerging field of organic bioelectronics, conducting polymers and ion-selective membranes are combined to form resistors, diodes, transistors, and circuits that transport and process both electronic and ionic signals. Such bioelectronics concepts have been explored in delivery devices that translate electronic addressing signals into the transport and dispensing of small charged biomolecules at high specificity and spatiotemporal resolution. Manufacturing such "iontronic" devices generally involves classical thin film processing of polyelectrolyte layers and insulators followed by application of electrolytes. This approach makes miniaturization and integration difficult, simply because the ion selective polyelectrolytes swell after completing the manufacturing. To advance such bioelectronics/iontronics and to enable applications where relatively larger molecules can be delivered, it is important to develop a versatile material system in which the charge/size selectivity can be easily tailormade at the same time enabling easy manufacturing of complex and miniaturized structures. Here, we report a one-pot synthesis approach with minimal amount of organic solvent to achieve cationic hyperbranched polyglycerol films for iontronics applications. The hyperbranched structure allows for tunable pre multi-functionalization, which combines available unsaturated groups used in crosslinking along with ionic groups for electrolytic properties, to achieve a one-step process when applied in devices for monolithic membrane gel formation with selective electrophoretic transport of molecules.

Keywords: hyperbranched polyglycerol, polyelectrolyte, multi-functionalization, thiol-ene, cross-linking, ion-selective, electrophoretic transport

#### INTRODUCTION

Organic bioelectronics (Wei, 1998; Berggren and Richter-Dahlfors, 2007; Someya et al., 2016) comprises the research and engineering field in which the coupling of biomolecules and electronic charges, in organic electro-active materials, is utilized to achieve sensing or actuation of processes and reactions in various biological systems. During the last decade, major interest in this field has included the development and engineering of devices that convert electronic addressing signals into the delivery of small-sized charged biomolecules. In one embodiment, planar drug delivery electrodes, based on conjugated polymers pre-loaded with a biomolecule, have been explored

**119**

(Kontturi et al., 1998; Abidian et al., 2006; Wadhwa et al., 2006). Upon electronically addressing these electrodes, their oxidation state changes, triggering release of the pre-loaded biomolecule. This effect is driven by the changes in electrostatic and morphological properties of the electrode bulk upon switching. In another embodiment, the mixed ion-electron conduction of two separated conjugated polymer electrodes are combined with the selective ion transport of a membrane to form a class of electrophoretic drug delivery devices, called organic electronic ion pumps (OEIPs) (Isaksson et al., 2007; Simon et al., 2009; Arbring Sjöström et al., 2018). By addressing the two polarizable electrodes of the OEIP an electric field is established along the ion selective channel, enabling the selective delivery of small (.200 g/mol) charged biomolecules, transported from a reservoir to a target system. Such OEIP devices have successfully been explored to deliver neurotransmitters and ions in various applications, especially targeting actuation and suppression of neuronal processes and signaling in vitro (Tybrandt et al., 2009; Williamson et al., 2015; Jonsson et al., 2016) and in vivo (Simon et al., 2009; Jonsson et al., 2015; Proctor et al., 2018).

The monolithic ion-selective channel of an OEIP should exhibit a high and selective conductivity for the desired charged (bio)molecule, along with manufacturability of miniaturized structures to enable application in vitro or in vivo. Furthermore, to match the scale of target biological systems and provide the highest degree of proximity and spatiotemporal precision, OEIPs are generally fabricated with ion-selective channels on the micrometer scale (width, approaching the scale of a single cell) and thickness of hundreds of nanometers (based on their thin-film nature). To date, most OEIPs and related iontronic devices (Arbring Sjöström et al., 2018) have utilized linear polyelectrolyte systems, such as polystyrenesulfonate or quaternized poly(vinylbenzyl chloride), processed into features using classical thin film processing techniques. As produced, these features are dry and when immersed into electrolytes and media, the ion-selective conductor swells due to considerable uptake of water and electrolyte solutes, leading to frequent mechanical failure of the devices and low production yield. Recently, our group has demonstrated capillary-based OEIPs, where the monolithic ion transport channel is formed inside a free-standing glass capillary (Poxson et al., 2019; Seitanidou et al., 2019). These devices take advantage of the glass capillary (generally ≤60µm outer diameter) as both the encapsulation and substrate for the ion channel. As such, the encapsulation is in place during fabrication, enabling the channel to remain hydrated (if stored properly) continuously from fabrication to end-use.

Formation and operation of ion-selective monolithic ion transport media in microfluidic or capillary-based channels places a range of demands on the design of the molecules and reactions forming the given transport medium. Most importantly, the medium should comprise a continuous porous structure through the monolithic body where the pore size should permit mobility of the targeted charged molecules. The pores should, however, not be so large as to permit macroscopic flow. Further, the number density of fixed charges should be sufficient for ion-selective transport, and the distribution of charges should be as homogeneous as possible. Moreover, the monolithic ion transport medium should be obtained by polymerization of molecules via reactions that can be triggered by external stimuli such as heat or actinic irradiation under atmospheric conditions to form bonds that are stable at the extreme pH-values that may occur in ion transport devices (Seitanidou et al., 2017). For straightforward filling of the microfluidic channel, the composition forming the ion transport medium (monomers and polymerization initiators) should exist as a single-phase fluid to enable filling using a minimum of solvent. Mechanically, the formed monolithic ion transport medium should exhibit a cohesive strength sufficient to tolerate solvent exchange and exhibit a good adhesion to the channel walls. We have selected thiol-ene click coupling to form the cross-links between monomers upon irradiation, since the thiolene coupling reaction tolerates the presence of air oxygen and proceeds to produce cross-links that are stable at the pH-values reached during OEIP operation (Hoyle and Bowman, 2010). As a structural building block, we have selected polyfunctional hyperbranched and dendrimeric molecules. These polyfunctional dendrimers can be substituted with groups for cross-linking as well as ionic groups with controlled degrees of substitution. It is of course important that the structural unit, the dendrimer, is formed by bonds that are stable to conditions of an OEIP under operation. Therefore, we have opted for polyether dendrimers having hydroxyl terminal groups since the ether groups are relatively stable to extreme pH values.

The use of thiol-ene chemistry calls for the presence of carbon-carbon double bonds as well as thiol groups. We have selected to functionalize the dendrimers with allyl groups for coupling with a second molecule, a thiol-terminated polyether. In addition to allyl-groups, the dendrimers are functionalized with groups carrying either positive or negative charges. To form the covalently cross-linked ion-transport medium with thiol-ene coupling, radical initiators must also be present, in addition to the allyl-functional dendrimers and thiol-functionalized crosslinkers, and these components must exist as a single-phase mixture with blending at the molecular level in the composition. For this reason, there is a need to add a minimal amount of solvent to form the single-phase composition. The amount of solvent should be minimal and constitute a minority component in the composition to avoid formation of large voids in the formed ion-transport channel, which would lead to breakdown of Donnan exclusion (charge selectivity) (Kontturi et al., 2008).

Here we report a new type of material synthesized and characterized based on a hyperbranched polyglycerol foundational structure, which has been multi-functionalized with different degrees of allyl and triphenylphosphonium groups. The click chemistry polymerization between the cationic polyelectrolyte mixed with tri-functional polythiol cross-linker has been achieved via thermal and photo-initiation. The sol-gel process of the macromolecules has formed insoluble free-standing membranes, capable of selective electrophoretic transportation of large aromatic molecules in OEIP structures. Due to solubility properties of the hyperbranched polyelectrolyte, these types of gel membranes can be prepared with a minimal amount of organic solvent with pre-hydrated states for OEIPs, iontronics, and other such organic (bio)electronic devices.

### RESULTS

#### Synthesis and Structural Characterization

The multi-functionalization synthesis of the hyperbranched polyglycerol (5,000 Da) was carried out step wise, utilizing the multiple terminal hydroxyl groups of the polymer (**Scheme 1**). In the initial step, allylation was performed on the polymer resulting in a degree of substitution of ∼ 10% (**Figure S1a**) and distinct alkene carbon signals were detected in C-NMR (**Figure S1b**). This characterization was calculated via 1H-NMR spectroscopy where the integral ratio of the specific functionalities' protons was compared to the integral ratio of the core polymer's structure protons (Equation 1), assuming the correlation with the number of terminal groups equal to the number of repeating units of the polymer (Gode et al., 2006; Burakowska and Haag, 2009).

$$DS\left(\%\right) = \left(\frac{I\_{H-\text{func.}}}{H\_{\text{func.}}} / \frac{I\_{H-\text{core}}}{H\_{\text{core}}}\right) \times 100\tag{1}$$

The remaining hydroxyl groups were then converted into esters by introducing chloroacetyl functionality, though signal overlapping occurred between the newly introduced protons located at the α-chloroacetate and the previously incorporated methylene protons neighboring to the alkene groups, thus somewhat obstructing the values of integration. Regardless, a well-defined characteristic peak was observed (**Figure S2a**) with an integral magnitude in accordance with a high degree of functionalization of ∼90% chloroacetate (**Figure S2a** equation). Likewise, the functionalization shifts of the outer layer protons located at secondary and tertiary carbons which previously had hydroxyl (primary, secondary) functionality, became ester (primary, secondary) functionalities resulting in downfield shifts. This made the further estimation of the different functionalities much more difficult. The hyperbranched polyglycerol's solubility changed drastically with the incorporation of the hydrophobic α-chloroacetylate groups to the polymer, rendering it insoluble in hydrophilic solvents, emphasizing the crucial role of the surface functionalities of the hyperbranched polymers role in interacting with its surrounding environment (Türk et al., 2007; Thota et al., 2016).

The relationship between surface functionalization and solubility was also observed in the final synthetic step of the polymer where ionic functionality was introduced to the macromolecular structure—rendering it again soluble in polar solvents—by nucleophilic substitution of the αchloride with triphenylphosphine, quaternizing the phosphor into positively charged phosphonium (Matveeva et al., 2003). This final synthetic step resulted in cationically-functionalized hyperbranched polyglycerol with molecular weights of 5 kDa (C-HPG5, **Scheme 1**, Compound 4). The values obtained from the NMR peak integration of the introduced triphenyl functionality (**Figure 1**) lead to yields of the quaternization reactions between 60 and 70% (**Figure S3**), though solvent peak signals in the polymer region along with functionality shift overlap complicate this estimation.

Peak splitting was observed in the NMR spectra for the peak associated with the six-membered phenyl rings of the phosphonium, which can be related to the presence of different structural conformation over phosphor (Aganova et al., 2014) seen in 31P- and 13C-NMR. 31P-NMR studies of C-HPG5 also showed solvent-dependent peak splitting (**Figure S3c**). There was a partial split of the phosphonium signal seen in methanol-d6, but not exhibited in D2O.

The amphiphilic nature displayed of the triphenylphosphonium hyperbranched polyelectrolyte (C-HPG5) made it easily solubilized in very small amounts of methanol solution with up to 3 g/mL (80 wt %) of C-HPG5 polymer. This demonstrates high solubility capabilities of the hyperbranched polyelectrolyte with significant amounts of charged surface functionalities (Kłos and Sommer, 2010). In combination with the hydrophobic triphenyl groups, this renders the C-HPG5 polymer soluble/processable from both protic and aprotic solvent mixtures. This important feature was utilized in the subsequent cross-linking, where components such as initiators and cross-linking agents exhibit much more lipophilic solubilities. The versatile solubility also makes the C-HPG5 polyelectrolyte able to interface the system of hydrophilic cationic charges and hydrophobic interfaces in low amounts of solution.

#### Thermal Cross-Linking

To obtain the cross-linked hyperbranched polyglycerol electrolyte, the triphenylphosphonium and allyl functionalized HPG (C-HPG5, Compound 4) was first mixed with Thiocure ETTMP 1300 (ethoxylated-trimethylpropan tri(3-mercaptopropionate)) with AIBN (2,2′ -azobis(2 methylpropionitrile)), and TATATO (1,3,5-triallyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione) in water/methanol mixtures or DMSO, applying high concentration kinetics before thermal-initiated crosslinking (**Figure 2**).

The thiol-ene click chemistry reaction (Hoyle and Bowman, 2010; Trey et al., 2010) allowed the formation of large amounts of the cross-linked insoluble free-standing gel from versatile, inexpensive starting materials. Our initial experiments showed that synthetic mixtures under high concentration kinetic control successfully cross-linked with both anionic or cationic functional groups (e.g., sulfonate or phosphonium) by applying photoinitiated surface chemistry for the formation of anionic or cationic exchange membrane (AEM or CEM) gel materials. Unfortunately, we found that the thermally-initiated crosslinking reaction did not proceed in solution for the cationic C-HPG5 without addition of TATATO (Kasprzak et al., 2009), independent of the inclusion or exclusion of the thiol cross-linker Thiocure 1300 (**Table 1**, TM1-3). With several different reaction conditions applied, the concentration was observed to play a crucial role for the success of the thermally-initiated sol-gel: lower concentration of C-HPG5 polyelectrolyte combined with the same equivalents of reactants (as TM1) did not lead to successful gel formation (**Table 1**, TM4-5). To ensure that the final gels consisted primarily of the C-HPG5 polyelectrolyte material, the concentrations of the other components were reduced to levels corresponding to theoretically calculated 1:1:1 ratios of allyl-thiol-allyl for the three components, in accordance with the ∼10–20% allyl functionalized C-HPG5. This formulation did successfully cross-link, emphasizing the high concentration kinetics requirement (**Table 1**, TM6, **Figure 2b**). The exact concentration limit of the polyelectrolyte for the reaction wasn't defined in methanol, though attempts with similar reaction conditions in DMSO proved able to form a continuous gel at 1.13 g mL−1 , nearly half the concentration as T1-3 or T6-7 (**Table 1**, TD1-2, **Figure 2c**). The lower degree of porosity was attributed to the higher boiling point of the solvent, which was also observed in TM1 with the crosslinking conducted in either methanol or 1-propanol (**Figure S4**).

Experiments with catalytic amounts of AIBN (0.2 wt%) were also conducted (**Table 1**, TM7). This formulation resulted in an insoluble gel, though without high amounts of visible porosity compared to the TM1 (6.5 wt%, **Figure 3**) formulation. Gelling with high amounts of AIBN initiator exhibited pronounced pores, formed from the boiling of solvent during the solidification. However, reducing AIBN concentration

```
gels were able to withstand solvent swelling in methanol (left) and solvent exchange to H2O (right).
```

TABLE 1 | Thermal cross-linking formulations.

to catalytic amounts enabled incorporating methanol more continuously into the gel. N2-gas evolution from AIBN initiator breakdown may also play a role in pore formation.

#### NMR Monitoring of Thermal Cross-Linking

Components were combined in DMSO-d6 according to formulation TD2 (**Table 1**) and transferred into an NMR-tube. Following 5 min of sonification and deaerating with N2-gas, the tube was sealed and placed inside a Varian Oxford 300 MHz NMR spectrometer. When the tube had reached a temperature of 70◦C, the first spectrum was collected, and we proceeded to monitor the crosslinking reaction. The process was carried out by locking all the parameters according to the initial spectrum (to prevent re-shimming) and spectra were then collected every minute (**Figure 4**). After the heating had proceeded for 11 min with no significant change in intensity between spectra, the gelification process began. The sol-gel reaction proceeded with a rapid velocity and with the spectrum collected at minute 12, a change in the integral intensity was already observed (intensity ∼27,000 → 20,000). Within the time frame of 2 min, the entire solution had solidified, characterized by a substantial loss of signal intensity (**Figure 4**, 12–14 min) and broadening of the spectrum's overall signatures. Due to a rapid build-up of high pressure inside the tube during the gelification process, the monitoring was terminated after 14 min. A somewhat increased integral intensity (∼8,000 → 9,000) between the last two time points (13–14 min) could be seen, which is related to the overall broadening of signal intensity across the entire spectrum.

#### Photo-Initiated Cross-Linking

A series of free-standing transparent membranes were synthesized by drop-casting solutions (**Table 2**) on glass, Teflon, or gold substrates followed by UV exposure for 10 min at 365 nm (10 mJ/cm2 ). These membranes were then peeled off from the substrate surfaces, displaying soft, flexible properties while remaining intact after soaking in solvents such as deionized water and methanol along with solvent exchange (**Figure S5**). It was also observed that when the membranes had been equilibrated in

FIGURE 3 | Top row: Catalytic amount of AIBN (0.2 wt%) resulted in a more continuous gel (left) while high amounts (6.5 wt%) led to more porosity (right). Bottom row: Respective gels swelled in methanol. The catalytic gel (left) was cut into pieces to illustrate that an insoluble gel formation had occurred.

deionized water, the mechanical strength was somewhat reduced, resulting in more brittle—but still flexible—gels.

In the series of drop-casting formulations (**Table 2**), the Thiocure 1300 component was reduced so as to reach the theoretically calculated 1:1:1 ratio with the unsaturated allyl functionality (∼10 mol %) of the C-HPG5 in relation to the allyl of TATATO and the thiol. We thus aimed to achieve a membrane consisting primarily of C-HPG5 polymer (UVC4). We also reduced the composition concentration (0.5 g/mL C-HPG5) to investigate if the photo-initiated surface crosslinking could proceed at lower concentrations in comparison with thermally-initiated cross-linking (UVC2-4). These formulations did proceed to gelification, and in the two latter cases we were able to use a 50/50 blend of water/methanol as solvent (**Figures S5**, **S6**). The addition of water was intended to both reduce the factor of organic solvent and to ensure less solvent evaporation (higher boiling point) during the UV-curing process. Not only was membrane formation still possible with the 50/50 water/methanol solvent, but the resulting membranes were in a pre-hydrated state due to incorporation of water. With minimum amounts of other reagent components such as catalytic amounts of photo-initiator (UVC3) and a predominant C-HPG5 composition mixture (UVC4), we demonstrate the ability to formulate and crosslink into membranes at both low and high concentrations. This is due to the solubility of the cationic hyperbranched polyelectrolyte, whereas the limiting factor otherwise lies with the miscibility of the initiator and cross-linker.

#### Utilization in Organic Electronic Ion Pumps

To characterize the C-HPG5 membrane's anion transport capabilities, we fabricated OEIPs using a micro-molding in capillaries (MIMIC) technique (Kim et al., 1996) (**Figure 5A**). OEIPs were manufactured by pouring and curing PDMS into ∼2 mm thick films on a glass master substrate with a (photolithographically) pre-patterned SU-8 pattern on top of it, defining the ion channel geometry (**Figure 5A**-1). The width and height of the SU-8 pattern were measured to be 85 and 43µm, respectively, using an optical profilometer (**Figure S7**). The PDMS was cut into small blocks, consisting of two separated electrolyte reservoirs connected by a capillary channel with an average length of 7.1 mm. After surface activation by treatment with O2 plasma, the PDMS was bonded to glass substrates (**Figure 5A**-3). The channels were filled with C-HPG5 cross-linking solution (formulation UVC4) by capillary force according to the MIMIC technique (**Figure S8a**). Following UV cross-linking as described above, the C-HPG5-based AEM was formed (**Figure 5A**-4) where the homogeneity and continuity was studied using scanning electron microscopy (**Figure S8b**). The bonded PDMS mold was left on the device to act as an encapsulation layer.

Devices were tested and characterized with source and target electrolyte solutions of 100 mM NaCl(aq) with PEDOT:PSS on polyethylene terephthalate (PET) strips used as replaceable electrodes. Cl− ion transport through the C-HPG5 membrane was achieved by applying a constant current of 100 nA between the PEDOT:PSS electrodes (**Figure S9**) (voltage was recorded continuously). Assuming perfect permselectivity and using the OEIP's channel geometry and voltage measurements (from 4 OEIP devices tested), the conductivity (ρ) for Cl− ions was calculated using the formula:

$$\rho = \frac{l \times I}{w \times h \times V} \tag{2}$$

where l, w, and h are the length, width, and height of the channel, and I and V the measured current and voltage. An average conductivity value of 0.24 ± 0.02 S m−1 for in the C-HPG5 polyelectrolyte channel was calculated (**Table 3**).

As a demonstration of the C-HPG5 polyelectrolyte's capability to electrophoretically transport larger and more rigid molecular structures, the dye molecule methyl blue2− (10mM(aq), **Figure 5B**), was used as a source electrolyte. The transport of methyl blue2− was readily observed visually as the blue dye gradually filled the channel (**Figure 5C**).

FIGURE 4 | Thermal initiated crosslinking with C-HPG5 at 70◦C in DMSO, monitored by collecting 1H-NMR (300 MHz) spectra every minute (right). Integral of the triphenyl functionalities intensity (gray region in spectra) plotted against time (left). When the reaction is initiated it occurs at an extremely fast rate, distinguished by the rapid loss of the signal intensity.

TABLE 2 | Photo-initiated cross-linking formulations.

FIGURE 5 | C-HPG5-based OEIP device. (A) Schematic of OEIP MIMIC fabrication: (1) SU-8 is patterned on glass substrate; (2) PDMS cured over SU-8 pattern; (3) PDMS with molded void is transferred to a UV-ozone-treated glass substrate; (4) C-HPG5 is filled into PDMS void via capillary force and UV cross-linked. (B) Schematic profile of fabricated C-HPG5 based OEIP and chemical structure of methyl blue. (C) Photograph of C-HPG5-based OEIP during operation. Methyl blue in the source reservoir is electrophoretically transported through the C-HPG5 AEM channel toward the target electrolyte, NaCl(aq), due to applied voltage between the PEDOT:PSS electrodes. At the time point of the photograph, the front of methyl blue had nearly reached the target electrolyte.

#### DISCUSSION

The polymer core of hyperbranched polyglycerol (5,000 Da) used in this study was chosen as it has been well-studied since its original synthesis (Sunder et al., 1999), with low polydispersity (Sunder et al., 2000), high degree of branching ("dendrimer-like") structure (Abbina et al., 2017), biocompatibility (Frey and Haag, 2002; Calderón et al., 2010; Wilms et al., 2010), and thoroughly elucidated structural properties (Baille et al., 2004; Kainthan et al., 2006; Schubert et al., 2016). This versatile material, coupled with the stability of the unreactive ether bonds, allowed the final polyelectrolyte membrane to operate at elevated pH and

TABLE 3 | C-HPG5-based OEIP characterization (100 mM NaCl(aq) as source and target electrolytes).

voltages without structural breakdown. Previous studies have reported hyperbranched polyglycerols with cationic side-group functionalization (Schwab and Mecking, 2005) as well as alkene functionalities for thiol-ene click chemistry reactions (Fu et al., 2008; Killops et al., 2008; Nilsson et al., 2008; Trey et al., 2010). In this study, we demonstrate the possibility of precisely tailoring the properties of the hyperbranched polyglycerols with multifunctionalization by controlled introduction of both charged cationic and allyl groups. This multi-functionalization enabled thiol-ene crosslinking with tri-thiolated polyethylene glycol (Thiocure ETTMP 1300), forming a uniform charged membrane network primarily consisting of branched polyethylene glycol structures. The resulting membranes were then incorporated as the ion channel in OEIPs and demonstrated to be able to sustain selective electrophoretic "ion pumping" in this device configuration.

With the initial step of allylation (**Scheme 1**), the degree of substitution (DS) was calculated from the 1H-NMR spectra (**Figure S1a**) to ∼10 % following addition of 0.2 eq. (to eq. – OH of HPG5) of NaH base followed by addition of 0.2 eq. allylbromide to the HPG5 in DMF solvent. Though the resulting allyl functionalization corresponded to half the amount of added reagents, this was ultimately attributed to the freshness of the NaH reagent used in the synthesis. Initial trial reactions (not included) resulted in a DS ∼15–20%. Likewise, a higher degree of allyl functionalization (40–60%) could be achieved in DMF by utilizing more equivalents of NaH and allylbromide reagents. Regardless, the resulting degree of 10% allyl groups was deemed sufficient, and repeated synthesis resulted in overall DS 10–20% with the calculated 1H-NMR estimation.

In the proceeding reaction, excess chloroacetyl chloride was used to ensure full conversion of the remaining hydroxyl groups of the (allyl-)HPG5, which could be clearly observed in the 1H- and 13C-NMR (**Figures S2a,b**). By 1H-NMR integral calculation via α-protons of the chloroacetyl signal compared to HPG5 core protons (**Figure S2a**), a DS could be estimated to >100% (clearly an overestimation). The overestimation arises due to signal overlapping from the allyl functionality (HPG-O-CH2-CH=CH2) with new chemical shifts arising from protons corresponding to HPG5 located close to the introduced ester functionalities (HPG-CH-COO-CH2Cl and HPG-CH2-COO-CH2Cl). By calculating the integral of the chloroacetyl α-proton signal integral toward the HPG5 including the allyl functionality signals (estimating 10% DS for 5H), a more realistic DS value of ∼90% was obtained.

In the final synthetic step of substituting triphenylphosphine and quaternizating the phosphor into charged phosphonium, the DS was calculated as above, with the triphenyl integral signals toward combining the other shifts, resulting in an estimation of ∼60% (**Figure S3a**). These large functionalization groups increased the molecular weight of the HPG5 considerably, from 5 to ∼20 kDa (approximated theoretical molecular weight calculation), and thus dominate the overall signal. An interesting observation in the 13C-NMR was the strong signals from the phenyl carbons (**Figure S3b**), which dominated the intensity and seemed to suppress the rest of the structure's spectra, making it hard to achieve and render a clear resolution for the total structure. With 31P-NMR, a distinct shift from the triphenylphosphine reagent signal of −5.6 ppm into a broad quaternized phosphonium signal ∼19–20 ppm was observed. Interestingly, an extra peak at 32 ppm was detecting in methanol-d4 solution, which was not seen in deuterium oxide solution of the C-HPG5 (**Figure S3c**). This was attributed to the possibility of solvent-dependent structural conformation of the triphenylphosphonium functionality (Aganova et al., 2014).

In studying the thermal cross-linking of the C-HPG5, various requirements for the success of the reaction were derived. First, it was found that adding a small amount of TATATO was crucial for cross-linking with Thiocure ETTMP 1300. Second, we observed a dependency on high concentration formulation (**Table 1**), which led to a very swift observable gel formation. A possible explanation could be connected with the survival of radical species for initiation of the reaction with AIBN. The substantial macromolecular sizes of both C-HPG5 polymer and Thiocure 1300 cross-linker alone could have rendered the arrangement of the system too slow for cross-linking radical reactions to take place. The smaller TATATO may diffuse more readily and associate with the macromolecules' functionalities to perform chain-transfer in the polymerization mechanism quickly enough for the radical species' survival. It was also observed that the equivalents of radical initiator AIBN were not associated with the success of the thermally initiated gel membrane formation, with 6.5 wt% and 0.2 wt% (**Figure 3**; **Table 1**, TM1 and TM7). But the AIBN equivalents did affect the pore size formation in the membrane by the evaporation of the low boiling point solvent. This may have been due to the crosslinking density formed and thiol-ene conversion efficiency of AIBN (Uygun et al., 2010) coupled with the solvent incorporation, and possibly related to N2-gas evolution from the radical species breakdown.

In situ monitoring of thermal cross-linking in DMSO-d6 (**Figure 4**) indicated that the reaction initiated after ∼12 min, and proceeded in an ultra-fast (within ∼1 min) polymerization with substantial signal intensity loss, ppm shift, and broadening of the entire spectrum. Since the sample was heated at 70◦C for thermal initiation of AIBN, DMSO evaporation was not an issue and was thus ruled out as a cause of signal intensity loss. The initiation delay may be related to the presence of oxygenforming less-reactive radicals (Cook et al., 2008) and/or the requirement for the macromolecular mixture components to arrange in a kinetically favorable orientation, with functionalities associated with each other. The resulting smooth homogenous gels (**Figure 2c**; **Table 1**, TD2DMSO−d6) were attributed to the high boiling point DMSO solvent, high concentration polymer mixture, and low amounts of AIBN initiator.

Free-standing membranes of C-HPG5 (**Figure S5**) were achieved in a straightforward manner using photo-initiation (Irgacure 2959, curing at 365 nm). The absence of elevated temperature enabled the use of a water/methanol solvent mixture, resulting in pre-hydrated membranes without large solvent evaporation pores. Adjusting various formulation and fabrication parameters with C-HPG5, such as solvent, polymer concentration, and thermal/photo-initiation, allows for a range of customizable hyperbranched polyelectrolyte membranes with specific nanocomposite properties and electrophoretic transport capabilities (Demina et al., 2010). Both the photo- and thermalinitiated gels comprising 70–75% C-HPG5 (excluding solvent content) were able to withstand drying, swelling, and solvent exchange such as methanol and water (**Figure S6**). These materials displayed soft, flexible characteristics, especially in the dry state, while being more brittle in the fully hydrated state.

One of the general interests in hyperbranched/dendritic polymers is their ability to contain solvents and molecules (Oudshoorn et al., 2006; Wang et al., 2010; Zhang et al., 2013; Pedron et al., 2017). The C-HPG5 polymer, with its large amount of triphenylphosphonium charged functionality, displayed amphiphilic solvating capabilities in a very small amount of solution (80% w/w in methanol), providing the possibility of processing the polymer with low amounts of organic solvents. A possible explanation could be related to structural arrangement, where the compact, spherical shape of hyperbranched macromolecules (Kainthan et al., 2006) makes them amenable to close-packing. By utilizing these covalentlyarranged three-dimensional polymer structures coupled with charged surface functionalities, well-defined/tuned cavities in the close-packing between the different macromolecules could provide selective ion transport channels. This leads to our more specific interest in hyperbranched/dendritic polymers: their use as ion transport materials for OEIPs and other iontronic devices (Arbring Sjöström et al., 2018). The C-HPG5 polymer was thus demonstrated as the ion transport channel in an OEIP system, with a specific formulation chosen to achieve molding via the MIMIC technique (**Table 2**, UVC4, **Figure S8**). Stable electrophoretic transport of Cl− was observed in the C-HPG5 channels, resulting in an average ionic conductivity of 0.24 ± 0.02 S/m. By employing methyl blue at the source, ionic exchange and transportation of the large anionic (799.81 g/mol) dye molecule were accomplished through the cationic hyperbranched polyelectrolyte membrane.

In conclusion, demands for new ionically charged organic materials capable of versatile manufacturing processing, selective ion transport including large/aromatic compounds, and biocompatibility sets increasing requirement on the polyelectrolytes used in organic (bio)electronics. We have synthesized a new type of multi-functionalized hyperbranched polyglycerol with cationic charges and functionalities for click chemistry cross-linking (thermal- and photo-initiated), to form monolithic membranes. The membrane network formation reaction was investigated, which by initiation proceeded in an ultra-rapid fashion to form insoluble gels capable of solvent exchange and large anionic electrophoretic transportation. The amphiphilic nature of the polyelectrolyte allows for simple implementation to create pre-formed hydrated/loaded membranes applied directly into devices.

#### MATERIALS AND METHODS

Hyperbranched polyglycerol used with molecular weight corresponding to 5,000 Da and degree of branching 53.6%, based on GPC and inverse gated 13C-NMR, respectively, was purchased from NanoPartica GmbH. Thiocure R ETTMP 1300 was obtained from Bruno Bock Chemische. All other chemicals were purchased from Merck-Sigma Aldrich with highest purity and used as received. Dialysis tubing Spectra/Por© Regenerated Cellulose with molecular weight cut-off (MWCO) were bought from Spectrum Laboratories. Synthesis reactions were carried out in nitrogen atmosphere, with glassware dried in a 110◦C oven. Reaction solvents DMF and chloroform were pre-dried (>16 h) with 4 Å molecular sieves and deaerated with nitrogen (30–60 min) prior to use. For the functionalization of hyperbranched polyglycerol 5,000 Da, reagent equivalents were calculated and used corresponding to relative hydroxyl functionality (67.5). 1H-, 13C- and 31P-NMR analysis were recorded at 300, 75, and 121 MHz, respectively, on a Varian Oxford 300 MHz spectrometer using internal solvent peaks as reference (Gottlieb et al., 1997). NMR FID files were processed with MestReNova v12.04-22023 (Mestrelab Research S.L.).

### Allyl-Hyperbranched Polyglycerol (Compound 2)

Hyperbranched polyglycerol (HPG, 5,000 Da, Compound 1) (2 g, 27.00 mmol –hydroxyl functionality) was dissolved in dry DMF (54 mL) followed by addition of sodium hydride (60% oil dispersion, 216 mg, 5.40 mmol, 0.2 equivalents) and was left to stir for 2 h under nitrogen atmosphere at room temperature. Allyl bromide (468 µL, 5.40 mmol, 0.2 equivalents) was added and the reaction was left overnight (16 h) until it was quenched with ice chips. The solvent was reduced via rotary evaporation and the residuals were re-dissolved in H2O (60 mL), filtered with a 0.45µm PVDF membrane, washed twice with ethyl acetate (2 × 60 mL). Dialysis in H2O was then performed with a regenerated cellulose (3,500 Da MWCO) membrane. Water was removed by

rotary evaporation and the product was placed in vacuo to yield a transparent white/yellow viscous gel (Compound 2).

1H NMR (300 MHz, Deuterium Oxide) δ: 6.17–5.94 (m, 1H, -OCH2CH=CH2), 5.53–5.29 (m, 2H, -OCH2CH=CH2), 4.32– 4.12 (m, 2H, -OCH2CH=CH2), 4.12–3.38 (m, 5H, dPG), 1.55– 1.36 (m, 2H, TMPcore -CH2CH3), 1.03–0.88 (m, 3H, TMPcore -CH2CH3).

1H NMR (300 MHz, DMSO-d6) δ: 6.03–5.75 (m, 1H, - OCH2CH=CH2), 5.35–5.02 (m, 2H, -OCH2CH=CH2), 4.94– 4.40 (HO-HPG), 4.14–3.86 (m, 2H, -OCH2CH=CH2), 3.86–2.87 (m, 5H, dPG), 1.37–1.16 (m, 2H, TMPcore -CH2CH3), 0.88–0.69 (m, 3H, TMPcore -CH2CH3).

13C NMR (75 MHz, DMSO-d6) δ: 135.88, 135.51, 116.61, 116.25, 80.00, 78.11, 72.99, 71.56, 71.03, 70.58, 69.65, 69.01, 68.73, 63.13, 61.09.

### Chloroacetate-Allyl-Hyperbranched Polyglycerol (Compound 3)

Allyl-HPG5 (Compound 2) (1,108 mg, 13.50 mmol) was dissolved in dry DMF (27 mL) and cooled with an ice bath. A solution of chloroacetyl chloride (2.68 mL, 33.75 mmol, 2.5 eq.) in dry DMF (18 mL) was prepared and added drop-wise to the polymer solution under stirring and nitrogen atmosphere, followed by slow drop-wise addition of triethylamine (1.88 mL, 13.50 mmol, 1 eq.). The reaction was left to stir for 16 h at room temperature, quenched with ice chips, and filtered through a 0.45µm PVDF membrane. Solvent volume was reduced via evaporation and the residuals were dissolved in chloroform (50 mL). The organic phase was washed with H2O (50 mL), brine (50 mL), and H2O (2 × 50 mL), and dried with MgSO4. The salts were filtered off, the chloroform was removed via rotary evaporation, and the product was placed in vacuo to yield a brown viscous oil (Compound 3).

1H NMR (300 MHz, Chloroform-d) δ: 5.96–5.76 (m, 1H, -OCH2CH=CH2), 5.31–5.02 (m, 2H, -OCH2CH=CH2), 4.52-4.22 (m, 3H, HPG-CHCOO-CH2Cl, HPG-CH2COO-CH2Cl), 4.14–4.03 (m, 2H, -COOCH2Cl), 4.21–3.91 (m, 2H, - OCH2CH=CH2), 3.87–3.19 (m, 5H, hPG), 1.50–1.26 (m, 2H, TMPcore -CH2CH3), 0.92–0.74 (m, 3H, TMPcore -CH2CH3);

13C NMR (75 MHz, Chloroform-d) δ 167.37, 167.09, 166.94, 166.85, 135.29, 134.66, 117.45, 116.94, 78.89, 77.58, 77.16, 76.74, 73.11, 72.27, 71.73, 71.33, 70.60, 69.91, 69.26, 69.13, 68.28, 66.79, 65.26, 64.04, 63.83, 41.05, 40.84, 40.68.

#### Triphenylphosphoniumacetate-Allyl-Hyperbranched Polyglycerol Chloride (Compound 4, C-HPG5)

Chloroacetate-allyl-HPG5 (Compound 3) (1.32 mg, 9.00 mmol) was dissolved in chloroform (18 mL) together with triphenylphosphine (4.73 g, 18.02 mmol, 2 eq.) in a microtube that was sealed with a cap and purged with N2-gas. The reaction mixture was left to stir for 16 h at 60◦C after which the solvent was removed by evaporation. Remaining crude product was dissolved in a minimum amount of methanol and trituration were performed by pouring cold diethyl ether to precipitate the product. The diethyl ether was decanted and the procedure was repeated four times after which remaining solvents were removed with rotary evaporation. The remaining product was dissolved in water and dialysis in the medium with a regenerated cellulose membrane (3,500 Da MWCO) was performed. Azeotropic evaporation with toluene (2 × 20 mL) were performed and the remains were placed in vacuo to yield a solid yellow crystalline product (Compound 4, C-HPG5).

1H NMR (300 MHz, Methanol-d4) δ: 8.22–7.45 (m, 15H, - P +Ph3), 6.00–5.71 (m, 1H, -OCH2CH=CH2), 5.33–4.90 (m, 2H, -OCH2CH=CH2; m, 1H, HPG-CHCOO-), 4.35–3.86 (m, 2H,- OCH2CH=CH2; m, 2H, HPG-CH2COO-), 3.84–2.91 (m, 2H, -COOCH2P +Ph3; m, 5H, HPG).

13C NMR (75 MHz, Methanol-d4) δ: 165.98, 136.57, 135.15, 135.01, 133.73, 133.11, 132.98, 131.58, 131.41, 130.04, 129.88, 120.00, 118.81, 117.44, 117.19, 79.87, 74.04, 73.27, 72.19, 71.17, 68.97, 42.17, 30.86.

31P NMR (121 MHz, Methanol-d4) δ 32.16, 20.64–19.56 (m). 31P NMR (121 MHz, Deuterium Oxide) δ: 19.94–19.06 (m).

#### Thermal Cross-Linking Gel

C-HPG5 (400 mg, 1.135 mmol, Compound 4) was dissolved in methanol in a vial (200 µL) and mixed together with Thiocure 1300 ETTMP (400 mg, 0.303 mmol), Triallyl-1,3,5 triazine-2,4,6(1H,3H,5H)-trione (21 mg, 0.084 mmol), and 2,2'- Azobis(2-methylpropionitrile) (2 mg, 0.012 mmol) by vortexing. The vial was deaerated, sealed and heated to 75 ◦C overnight into an insoluble continuous gel.

#### Thermal Cross-Linking Gel for NMR

C-HPG-5 (341 mg, Compound 4) was added into a vial and dissolved in DMSO-d6 (300 µL) by vortexing for 5 min. This was followed by addition of Thiocure 1300 ETTMP (89 mg), triallyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione (18 mg, 0.072 mmol), and 2,2′ -Azobis(2-methylpropionitrile) (17 mg, 0.104 mmol) which was vortexed an additional 5 min into a homogenous viscous solution. The mixture was transferred into an NMR-tube, deaerated with N2-gas, sealed with a cap, and heated for 15 min at 70◦C inside a Varian Oxford 300 MHz spectrometer, where the reaction was monitored by acquisition of a 1H-NMR spectra collected every minute.

#### Organic Electronic Ion Pump Fabrication PDMS Master Mold Fabrication

SU-8 3050 (MicroChem Corp.) was spin-coated (5 s spread at 500 rpm, 30 s spin at 3,000 rpm) on a 4 silicon wafer and soft baked on a leveled hot plate (1 min hold at 65◦C, increasing with 10◦C/min to 95◦C, and hold for 15 min). After cooling, the wafer was exposed through a plastic mask with 300 mJ/cm2 (measured at 365 nm) using a Karl Suss MA/BM 6 mask aligner equipped with an I-line filter. The exposed parts were crosslinked by ramped hard bake (as soft bake, but final hold at 95 ◦C for 5 min) and developed using two baths of mr-Dev 600 (Micro resist technology GmbH), each for 5 min, followed by a 10 s rinse in isopropanol and drying using N2. The final SU-8 pattern was additionally hard baked at 110◦C for 5 min.

#### PDMS Master Mold Characterization

The width and depth of the SU-8 pattern was measured with an optical profilometer (Sensofar PLu Neox) using a 50x confocal objective.

#### PDMS Mold

PDMS (DowSil Sylgard 184) was prepared at a 10:1 base/curing agent ratio, and mixed and degassed. The blend was poured on the PDMS master mold to a thickness of ∼2 mm and cured on a hot plate (1 h at 120◦C). The PDMS mold was carefully peeled off the master, and cut into pieces, with each piece containing a capillary channel connecting two electrolyte reservoirs.

#### Channel Filling

Glass wafers were cleaned using Hellmanex R III, rinsed, and baked prior to use. Glass wafers and PDMS pieces were O2 plasma treated (100 W at 800 mTorr for 5 min) and subsequently immediately bonded together. C-HPG5 (50 mg, Compound 4), Thiocure 1300 ETTMP (13 mg, 0.01 mmol), 1,3,5-triallyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione (2.6 mg, 0.010 mmol), and 2-hydroxy-4′ -(2-hydroxyethoxy)-2-methylpropiophenone (3.4 mg, 0.015 mmol) were dissolved in methanol (50 µL) and water (50 µL). The solution was vortexed for 2 min and the channels were filled by capillary action by placing 1 µL of membrane solution at one end. The polyelectrolyte was cross-linked for 10 min in a Karl Suss MA/BM 6 mask aligner (flood exposure, 10 mJ/cm2 measured at 365 nm) under nitrogen atmosphere. The channels were then soaked in 0.1 mM KCl solution by placing droplets at the inlet and outlet for 3 h.

#### Scanning Electron Microscopy Imaging

Channels were prepared as above but on a silicon wafer with 50 nm gold deposited by thermal evaporation. Further, to enable removal of the mold after filling and AEM crosslinking, neither the substrate or PDMS mold were activated by plasma. Finally, a 5 nm layer of gold was evaporated on top of the AEM. Images were acquired using a Zeiss Sigma 500 Gemini scanning electron microscope at 2 kV acceleration voltage and with the in-lens secondary electron detector.

#### Channel Characterization

The conductivity of C-HPG5-based OEIPs were determined by placing PEDOT:PSS (coated on polyethylene terephthalate)

#### REFERENCES

electrodes in 0.1 M NaCl(aq) electrolytes on both side of the channel, applying a current of 100 nA, and measuring the required driving voltage using a Keithley Source Meter 2612 (Tektronix, Inc.). To assess the transport of larger molecules, the source solution was exchanged to the dye molecule methyl blue2− (10 mM aqueous).

### DATA AVAILABILITY

The datasets generated for this study are available on request to the corresponding author.

### AUTHOR CONTRIBUTIONS

TA performed all synthesis and chemical characterization. DP and EG fabricated and characterized OEIP devices. MS advised and planned synthesis and chemical characterization. DS, MB, and MS conceived of the project. TA wrote the manuscript, with assistance from EG, MS, and DS.

#### FUNDING

Major funding for this project was provided by the Swedish Foundation for Strategic Research. Additional funding was provided by the Swedish Government Strategic Research Area in Materials Science on Advanced Functional Materials at Linköping University, the Önnesjö Foundation, and the Knut and Alice Wallenberg Foundation.

### ACKNOWLEDGMENTS

The authors wish to thank Dr. Roger Gabrielsson for his assistance in development and synthesis throughout this project, Prof. Peter Konradsson for guidance and fruitful discussions, and Marie Jakesova for assistance with scanning electron microscopy imaging.

#### SUPPLEMENTARY MATERIAL

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fchem. 2019.00484/full#supplementary-material

functionalized dendrimers. J. Polym. Sci. A Polym. Chem. 46, 1339–1348. doi: 10.1002/pola.22474

**Conflict of Interest Statement:** MS is employed by the independent state-owned research institute RISE Acreo AB. DP, DS, MB, and TA are shareholders in the small, researcher-controlled intellectual property company OBOE IPR AB (oboeipr.com), which owns the patents related to this research.

The remaining author (EG) declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2019 Abrahamsson, Poxson, Gabrielsson, Sandberg, Simon and Berggren. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

# A Study on the Stability of Water-Gated Organic Field-Effect-Transistors Based on a Commercial p-Type Polymer

Rosaria Anna Picca1,2 \*, Kyriaki Manoli 1,2, Eleonora Macchia1,3, Angelo Tricase1 , Cinzia Di Franco4 , Gaetano Scamarcio2,4,5, Nicola Cioffi1,2 and Luisa Torsi 1,2,3 \*

1 Dipartimento di Chimica, Università degli Studi di Bari "Aldo Moro", Bari, Italy, 2 Consorzio per lo Sviluppo dei Sistemi a Grande Interfase, Unità di Bari, Bari, Italy, 3 The Faculty of Science and Engineering, Center for Functional Materials, Åbo Akademi University, Turku, Finland, 4 CNR - Istituto di Fotonica e Nanotecnologie, Unità di Bari, Bari, Italy, 5 Dipartimento Interateneo di Fisica "M. Merlin", Università degli Studi di Bari "Aldo Moro", Bari, Italy

#### Edited by:

Wolfgang Knoll, Austrian Institute of Technology (AIT), Austria

#### Reviewed by:

Maria Daniela Angione, Trinity College Dublin, Ireland Sven Ingebrandt, RWTH Aachen University, Germany

#### \*Correspondence:

Rosaria Anna Picca<EMAIL_ADDRESS>Luisa Torsi<EMAIL_ADDRESS>
#### Specialty section:

This article was submitted to Organic Chemistry, a section of the journal Frontiers in Chemistry

Received: 21 January 2019 Accepted: 18 September 2019 Published: 10 October 2019

#### Citation:

Picca RA, Manoli K, Macchia E, Tricase A, Di Franco C, Scamarcio G, Cioffi N and Torsi L (2019) A Study on the Stability of Water-Gated Organic Field-Effect-Transistors Based on a Commercial p-Type Polymer. Front. Chem. 7:667. doi: 10.3389/fchem.2019.00667 Robust electrolyte-gated organic field-effect-transistors (OFETs) are particularly needed for the development of biosensing devices. However, when a FET biosensor operates in aqueous environments or even in real biological fluids, some critical issues may arise due to the possible lack of environmental long-term and/or operational stability. An important source of instability is associated with the degradation of the organic electronic channel materials such as for instance, poly-3-hexylthiophene (P3HT), a benchmark commercially available p-type organic semiconductor. In this work, the investigation of critical parameters, such as the control over spurious electrochemical phenomena as well as the operating conditions that can affect water-gated OFETs lifetime, is reported, together with a proposed modeling of the P3HT stability curve over 1 week in water. The investigation of possible morphological/chemical modifications occurring at the polymer surface after operating in water for 2 weeks was carried out. Moreover, it is proven how the addition of a gel layer can extend the P3HT based water-gated OFET shelf life up to 2 months.

Keywords: poly-3-hexylthiophene, electrolyte-gated OFET, degradation, pulsed mode, biosensors

### INTRODUCTION

The use of organic field-effect-transistors (OFETs) as sensors (Torsi et al., 2013; Li et al., 2018; Surya et al., 2019) has received a tremendous boost in the last decades thanks to the advent of low-cost fabrication strategies as well as of flexible and/or stretchable substrates (Kim et al., 2013; Manoli et al., 2015). However, their exploitation and commercialization as real-life sensors have been limited by the organic polymer degradation occurring when the device is operated in aqueous environments (Knopfmacher et al., 2014; Wang et al., 2016; Zhang et al., 2016). Extensive literature on the operational and environmental stability of organic semiconductors have been reported for devices working with solid-state dielectrics (Salleo et al., 2005; Sirringhaus, 2009; Bobbert et al., 2012; Nikolka et al., 2016; Lee et al., 2017; Jia et al., 2018) addressing the role of external agents (e.g., water, oxygen, light) in reducing the device performance. Evidences of device degradation encompass the increased hysteresis on transfer curves and leakage (IG) for successive measurements, threshold voltage (VT) shift, on current (Ion) decrease with time (Sirringhaus, 2009). Such behavior has been mainly ascribed to the uncompensated charged trapped in deep localized states in the semiconductor also caused by spurious electrochemical processes (Sirringhaus, 2005; Beatrup et al., 2014; Porrazzo et al., 2014). For example, the presence of polaronic states can cause an increased susceptibility to the electrochemical degradation (Wade et al., 2015). In fact, the presence of hole polarons under device operation is found to make polaron degradation pathways accessible (Beatrup et al., 2014). Moreover, a charge transfer complex may be formed with oxygen (favored under light) leading to instability (Bellani et al., 2014).

Some alternatives to overcome such drawbacks have been proposed, such as the inclusion of ZnO nanophases in the bioreceptor layer of OFET biosensors (Picca et al., 2018) or polymer encapsulation (Yu et al., 2016; Lee et al., 2017). More recently, electrolyte-gated devices (EGOFETs) have emerged as interesting devices that can operate in the sub-volt regime (Panzer and Frisbie, 2008; Kergoat et al., 2010; Cramer et al., 2013) thus paving the way to their application in label-free ultrasensitive biosensors (Mulla et al., 2015; Macchia et al., 2018a,b, 2019a,b).

In EGOFETs, two electrical-charge-double-layers (EDL) are built: one at the gate/electrolyte and the other at the electrolyte/organic semiconductor interfaces. When water serves as electrolyte, the EDL capacitance is remarkably high, being in the order of few µF ∗ cm−2 (Cramer et al., 2013). In such an architecture, the direct contact between the organic semiconductor (OS) layer and the aqueous environment rises concerns about the already investigated degradation induced in such polymers by electrochemical processes that occur in water (de Leeuw et al., 1997; Sharma et al., 2010; Bobbert et al., 2012), that are particularly detrimental leading to unstable EGOFETs on prolonged use (Cramer et al., 2013; Bellani et al., 2014; Porrazzo et al., 2014). Until now, only few studies about EGOFET devices have addressed this issue reporting on stability over time (Zhang et al., 2016) or under harsh environments (Porrazzo et al., 2014; Algarni et al., 2016). In this work, poly-3-hexylthiophene (P3HT) was chosen as benchmark ptype OS used to fabricate water-gated OFETs, employing a bulk gold foil as gate electrode. At first, the reduction of hysteretic behavior was demonstrated by setting up a suitable measuring protocol. Moreover, environmental (incubation in HPLC water) and operational stability (cycling mode) studies were carried out demonstrating the main role of the electrolyte over the measuring conditions in device degradation. A suitable cell was also designed to overcome the problem of water evaporation during measurements. Interestingly, P3HT degradation was modeled in terms of ID decrease with a two-phase exponential decay function indicating that a first process occurs within few hours, whereas a second one has a time constant in the order of 35 h. It was demonstrated that a similar trend is mainly associated to VT negative shift with time. A strategy to extend EGOFET use over the months was also investigated. Additional morphological and spectroscopic characterizations performed on as-prepared and used devices provided information about the response of P3HT film to water contact on a long time scale (2 weeks).

## MATERIALS AND METHODS

### Fabrication of EGOFET Devices

Highly n-doped silicon substrates, covered by 300-nm thermally grown SiO2, were purchased from Si-Mat R . Source (S) and drain (D) interdigitated electrodes were photo-lithographically patterned on the Si/SiO2 wafer. To this end, a Ti (99.995%) film (5 nm thick) was deposited as adhesion layer prior of the Au (99.99%) film (50 nm thick) deposition by electron-beam evaporation. The S&D interdigitated electrodes have a channel length (L) of 5µm and a channel width (W) of 10,560µm. Poly(3-hexylthiophene-2,5-diyl) (regioregular, electronic grade, 99.995% trace metal basis, average Mn ∼17,500, Sigma-Aldrich), P3HT, was used as p-type OS throughout the study. The polymer was first dissolved in chlorobenzene (2.6 mg/mL), and then the solution was sonicated for 20 min prior to filtration through a PTFE 0.2µm syringe filter. Before depositing the OS film, substrates bearing the interdigitated electrodes were cleaned under ultrasounds in an acetone bath (10′ ) followed by a similar step in 2-propanol and finally dried under nitrogen. P3HT film was spin coated at 2,000 rpm for 20 s and annealed at 90◦C for 10 min. A channel area of 6.4·10−3 cm2 was estimated (Macchia et al., 2018a). The gate electrode (G) consisted of an Au bulk sheet (total area ∼1.5 cm2 ) mounted in L-configuration, whose water immersed area was about 0.6 cm2 . When specified, a channel consisting of P3HT/ZnO nanoparticles hybrid layer was prepared by spin coating starting from a filtered P3HT solution mixed with ZnO nanoparticles (Picca et al., 2015) (1.5 mg/mL final concentration).

A suitable well for performing long-term studies under water was designed and fabricated to prevent possible water evaporation. It consists of a polystyrene (PS) cell glued with polydimethylsiloxane on the substrate. A PTFE tube connects the cell to a proper water reservoir in order to keep the water level constant in the well. The well is filled with 1.2 mL of water (HPLC-grade, Honeywell Riedel-de Haën) acting as gating medium. The scheme and the picture of the measuring set up are presented in **Figure 1**.

### EGOFET Electrical Characterization

All the devices were characterized by means of a Keithley 4200-SCS semiconductor characterization system in air at room temperature (20–22◦C) in a dark box. The drain current (ID) was acquired scanning the gate voltage (VG) between 0.1 and −0.5 V, keeping the drain voltage (VD) at −0.4 V. The voltage step was set at 0.01 V. The source is grounded as typical in common source configuration measurements. Forward and backward scans were performed to evidence the occurrence of hysteretic behavior. A suitable measuring protocol was also set up as described in the results section. The potential window was defined according to the need to prevent any electrochemical process to occur and affect the EGOFET. To this aim, gate current (IG) was also registered in order to monitor any spurious electrochemical process. Electrical figures-of-merit (µFET and VT) were extracted from the ID-VG transfer characteristics in the saturation regime according to a well-known procedure (Torsi and Dodabalapur, 2005) and are averaged on 10 different devices (n = 10) operated

in independent measurements. Error bars refer to one standard deviation calculated considering n = 10.

to a zoom at higher magnification (scale bar corresponds to 200 nm) of the exposed sample.

#### Morphological Analysis

Scanning electron microscopy (SEM) was applied to investigate the in-plane homogeneity of OS films deposited by spin coating. Analysis was performed by means of a Carl Zeiss 6igma field emission SEM on films prepared, both on interdigitated electrodes and on SiO2/Si substrates. The probing electron beam was set at an acceleration voltage of 3–5 kV acquiring images at different magnifications in top-view. SE2 type detector was used to acquire the overall image of the device (**Figures 2A,C**), whereas the detailed view of the samples was recorded with the in-lens detector (**Figures 2B,D**).

Atomic Force Microscopy (AFM) characterization was performed on P3HT-based EGOFETs, just after their preparation and after contact with water for 2 weeks. AFM images were collected with a NT-MDT mod. Ntegra microscope in semicontact mode using a tip apex size of 10 nm at a frequency f = 180 kHz. For each sample, areas of 3 × 2µm were investigated. The Image Analysis Software was used to evaluate the surface roughness as route mean squared (RMS) on at least three representative areas of the sample.

### X-ray Photoelectron Spectroscopy Surface Analysis

P3HT-based FETs were analyzed by means of X-ray photoelectron spectroscopy (XPS) using a PHI Versaprobe II Spectrometer. A monochromatized Al Kα radiation (1486.6 eV) was used. Survey spectra were acquired with a pass energy of 117.4 eV; whereas high-resolution (HR) spectra were acquired with a pass energy of 58.7 eV. C1s, O1s, Si2p, S2p, Au4f regions were investigated. Both as-prepared and used samples were characterized. MultiPakTM (v. <IP_ADDRESS>, PHI-ULVAC) software was used to process the data. Binding energy (BE) scale was corrected taking as reference C1s component at 284.8 eV. Three representative areas of the sample were collected to evaluate the chemical surface composition.

### RESULTS AND DISCUSSION

#### SEM Characterization of P3HT Films

P3HT organic semiconductor has been typically successfully used by our group as active channel in FET biosensors with different architectures (Angione et al., 2012; Magliulo et al., 2013; Macchia et al., 2016). As a general procedure, P3HT films were prepared by spin-coating starting from a chloroform solution. However, SEM investigation on similar samples showed that P3HT layers were not uniform due to the presence of some voids and agglomerates (Sportelli et al., 2017). Chlorobenzene was then selected because of its higher boiling point as alternative solvent to improve layer uniformity and crystallinity, in agreement with what previously reported in the literature (Kergoat et al., 2011). SEM images of typical P3HT films, freshly deposited on a plain SiO2/Si substrate and on gold electrodes, are presented in **Figures 2A,B**. It is evident that a rather uniform OS layer is formed confirming the advantages of processing P3HT from chlorobenzene. The very same samples were put in contact with water for 2 weeks and used as EGOFET devices (vide infra) and then characterized by SEM to evaluate a possible modification of layer uniformity. Additional features appear in the images (**Figures 2C,D**), though it seems that they are mainly related to the effect of water evaporation and presence of dust/salt particles (see the inset of the Figure). In fact, cracks and/or delamination of the polymer are not evident.

### AFM Characterization of the OS Layer

**Figure 3** shows two typical AFM images of an area of a P3HT FET, before (a) and after (b) 2-week exposure to water. RMS was calculated as an estimate of the roughness in both cases. A general increase in the roughness going from 0.5 ± 0.1 nm for pristine samples to 1.6 ± 0.2 nm after use was found. The low roughness observed on freshly prepared films is indicative of the goodness of the solvent using for processing the polymer in agreement with SEM results. After 2-week contact with water, the RMS increment can be attributed to a sort of polymer swelling. Nonetheless, dramatic changes in the surface morphology are not visible which account for the preservation of the FET modulation, at least in the investigated time scale.

#### XPS Characterization of P3HT Active Layers

The surface chemical composition of fresh and used P3HTbased devices was determined by XPS analysis (**Table 1**). The chemical composition does not seem particularly affected by contact with water, apart the higher oxygen content. Such finding is confirmed by spectral comparison of C1s, O1s, S2p regions (**Figure 4**) acquired before and after long exposure to water. Slight increment in carbon oxygenated components is observed (BE > 286 eV) suggesting a certain chemical degradation of the polymer. Nonetheless, the lineshape of S2p signal is preserved suggesting no oxidation on Sulfur. Interestingly, increment in the oxygen content is correlated to the presence of hydroxyl groups in addition to the O1s component ascribed to SiO2 (BE = 532.5 ± 0.2 eV) (National Institute of Standards and Technology, 2012). XPS results suggest then that a partial modification of the polymer occurs within the investigated period, accounting for

TABLE 1 | Typical surface chemical composition of fresh (t = 0) and aged (t = 15 days in water) P3HT samples.

Error on atomic percentages (At%) is ±0.2% for Au and ±0.5% for all the other elements.

the increased roughness and the reduced electrical performance upon long exposure to water.

### The Measuring Protocol

As anticipated, hysteresis, i.e., the difference between ID values measured in the forward and backward gate voltage sweep, is one of the main issues that can arise when OFET devices are operated. In these cases, this behavior has been often attributed to the presence of moisture in the OS layer or at the OS/dielectric interface, especially when SiO2 is used as gate dielectric (Bobbert et al., 2012). On the other hand, water-gated OFETs show minor hysteresis phenomena. This is mainly attributed to the absence of proton trapping in a dielectric and the occurrence of their neutralization by water autoprotolysis (Cramer et al., 2013). Hysteresis can be influenced by the sweeping potential window and by the scan rate of the applied potential (Panzer et al., 2005). For example, it was shown that scanning the VG from the off to the on state (and vice versa) by applying pulses of alternating polarities was a suitable method to get hysteresisfree transfer curves in P3HT based FETs (Manoli et al., 2014). In this work, a similar protocol was adopted as reported in the **Supplementary Material**. It was observed that, regarding VG sweep protocol, keeping the on time equal to 10 ms (+1 s cumulative measure time) as compared to the off time (1 s) and selecting −0.05 V as base voltage, gave the best results. This means that a slow scan rate of about 40 mV/s is applied to VG. Moreover, the VD set at a maximum value of −0.4 V, was also pulsed keeping the on time (1 s) longer than the off time (0.1 s) at 0 V. A comparison between the performances of water-gated P3HT FETs operated in the DC-mode (red curves) or in the pulse-mode (black curves) is presented in **Figure 5**. The drain and the leakage currents (IG) are reported as a function of VG for the two cases. Apparently, the pulse mode contributes to measure a slightly increase ID, and more relevantly to measure an almost hysteresis-free and reduced gate leakage current level. Taking the current values at VG = −0.5 V, the ratio ID/IG in the pulse-mode was about twice the one measured in the DC-mode.

#### Operational Stability

The definition of a proper measurements protocol was beneficial to improve EGOFET performance level; however, it was not sufficient in limiting the degradation phenomena and hence the device long-term stability and reliability. In this respect, it is worth to dwell into the operational stability as well. A general scheme reported in literature involves the use of the so-called bias-stress mode consisting in the application of a continuous gate bias over time (Sirringhaus, 2009), though other approaches based on the cycling measurements of transfer curves have also been applied (Hwang et al., 2011). In this work, the interest was focused on the aspects connected with the application of EGOFETs as biosensing platforms; therefore, the operational stability of P3HT based devices was studied in water. To this aim, the typical sensing protocol followed in our laboratory in singlemolecule detection (Macchia et al., 2018a) was simulated and the transfer curves of the fabricated devices were measured every half an hour using water and Au sheet as gating medium and gate electrode, respectively. A 30 min interval was chosen to mimic the time generally passing between two successive exposures to analyte solutions in a conventional sensing experiment. In fact, it is fundamental to get highly stable devices before using them in sensing platforms (Macchia et al., 2019b). P3HT based devices were then tested just after their fabrication. An example of the variation of the maximum current, ID(max) (measured at VG = −0.5 V), over 48 h is shown in **Figure 6A**. It can be seen that a dramatic decrease in current occurs in the first cycles, whereas the current level almost reaches a steady state after 24 h. This trend was not surprising considering that the devices are not operated in an inert atmosphere. An alternative approach was explored to discriminate both the role of the measuring environment and of the operational protocol. Some devices were kept in contact with water just after their preparation (pristine sample) without operating them for the first 18 h. In this case, ID(max) variation over time (blue hollow triangles) is presented in **Figure 6B**. Interestingly, the two experiments led to very similar results, thus suggesting that the pronounced initial ID decrease is essentially correlated to the adjustment of the polymer film in water environment. Quantitatively, a current decay of about 35% was generally observed for both the 18 h-cycled or the 18 h-waterincubated samples as reported in **Figure 7**. This fundamental aspect has never been clearly addressed in previous works about operational stability of OFET devices in water. It is worth noting that a similar decay was also reported for devices based on OS blended with polystyrene after 11 h of operation in water. In this case, a current decrease rate of about 3%/h was reported (Zhang et al., 2016). In this work, we observed that devices comprising a commercial P3HT seem more stable even in the first day after their preparation. After this period, the current drift is typically reduced and is around 1%/h allowing the P3HT based device to be very well suited to be used in ultra-sensitive biosensing experiments (Macchia et al., 2019b).

The timescale of this characterization was further extended taking into account the variation of the electrical figures-ofmerit, µFET, VT, and ID(max), over a time lease of 2 weeks. In this case, the devices were kept in water and measured in cycling mode for 5 h per day. The current values at t = 0 day and at t = 15 days are given in **Table 2**. The EGOFET devices comprising a P3HT channel material modified with an n-type nanostructured oxide were also tested under the same conditions. Such investigation was undertaken because in our previous studies ZnO nanoparticles were implemented in OFETs giving good performance in terms of electrical properties (Picca et al., 2015) and stability (Picca et al., 2018).

been normalized on the initial ID value (I0).

Picca et al. Stability of P3HT-Based EGOFETs

TABLE 2 | Device figures of merit estimated on fresh and aged samples (used for 2 weeks).

A comparison with P3HT devices bearing ZnO nanoparticles (P3HT/ZnO) is shown. Errors refer to one standard deviation for n = 10.

At a first inspection, it can be observed that the device degradation translates into a negative shift of the threshold voltage, VT. Interestingly, mobility seems almost unaffected by the prolonged device operation in water. Similar findings were reported in other systems, as well. Moreover, the electrical performance and the stability of the devices were not significantly improved by the addiction of the ZnO nanostructures. This is not so surprising since in the previous work ZnO was deposited as separate layer underneath the P3HT film in a functional biointerlayer OFET biosensor, thus behaving as capturing layer for water molecules at the interface between P3HT and the solid dielectric (Picca et al., 2018). On the other hand, recent data on water-gated devices based on P3HT films modified with WS2 nanotubes showed an increased operational stability, quite likely due to the nanoscale morphology of the active blend layer (Macchia et al., 2018c).

As expected, the degradation cannot be avoided but it is in the order of the one reported for EGOFETs based on other materials. In fact, the current level was reduced of about 30% of its original value after 7 days (**Figure 8**), in agreement with data available in literature (Zhang et al., 2016). Furthermore, the study was focused on the modeling of device degradation under the experimental conditions explored in this work. In the past, works related to devices operating with a solid-state dielectric under bias stress proved that the VT shift correlates with current

decay, whereas mobility is almost unchanged (Salleo et al., 2005). Bias stress instability translates into a voltage threshold shift, and hence into ID decay, which can be generally modeled with a single exponential decay function (Zhang et al., 2009). In the paper by Salleo et al. (2005) it was also observed that on a very short timescale (<100 s) almost all the charged traps could be recovered after bias removal indicating that these defect states have a short lifetime and are probably located in the more ordered regions of the organic semiconductor. In the same work, a longlived component of trapped charges was also invoked. These traps are characterized by slow kinetics as they can last for days, thus being responsible for the observed lower VT shift. It is believed that they are present in deep states, quite likely distributed in the disordered portions of P3HT.

In the field of EGOFETs, little is known about device degradation under operation and empirical approaches are typically employed to describe their behavior, typically carrying out the studies on a limited period (hours). Based on the experimental conditions employed in this work, the degradation of the prepared devices was followed in terms of ID(max) decay and VT variation over 7 days (**Figure 9**). A double exponential decay function, expressed as y = A1∗ exp(–x/t1) + A2∗ exp(–x/t2) + y0, was used to fit both parameters. It is worth mentioning that such expression was already used in other works to describe recovery in pulse duty cycle studies (Miyadera et al., 2008; Manoli et al., 2014). However, both previous papers referred to OFET systems. The two extracted time constants (t1, t2) are similar for both drain current and threshold voltage changes, in agreement with previous observations. In particular, t1 is 1.8 ± 0.5 h and t2 is 35 ± 3 h. This means that two distinct processes impact on P3HT degradation processes occurring at different times. The first decay is responsible for the "rapid" current loss in the initial time-frame right after the incubation in water. Such trend could be in principle recovered within few days after stress removal (Sirringhaus, 2009). The second process may result from the presence of uncompensated immobile charges in the OS

film. Obviously, due to continuous contact with the electrolyte, other effects such as the partial penetration of water in the P3HT film as well as formation of a passivating oxide layer on gold should be also taken into account. In particular, AFM and XPS results indicate that slight modifications in terms of morphology and composition of the polymer occur when devices are operated in water over a long period thus supporting the existence of degradation pathways responsible for the irreversible components of the decay model. It was generally reported that water molecules can penetrate the OS layer being then enclosed in the polymer nanovoids and causing trap formation (Nikolka et al., 2016; Zuo et al., 2019). This means that the latter component of degradation cannot be easily recovered and further studies will be necessary to clarify this behavior.

#### Shelf Life Extension

An important aspect for the development of EGOFET biosensors is related to their storage as well as to the possibility to use them several times. When continuously operated in water, it was observed that the very maximum time frame in which P3HT based OFETs can be used is about 20 days as upper limit. A protocol for improving their durability was then applied. As a proof-of-concept, agarose gel was employed as protecting layer for the P3HT channel when the device was not in use. In the proposed set up, 1%w agarose (300 µL) solution was poured on the P3HT film and let jellify directly in the measuring well. Then, the devices could be stocked in the dark at ambient conditions. The important point is that the device was constantly kept in contact with the water trapped in the gel-matrix. When needed, the agarose film could be easily removed and substituted with water for further device electrical characterization. After use, the gel layer could be deposited again. Shelf life was evaluated in terms of ID measured on different days by acquiring a transfer curve, as reported in **Figure 10**. It can be seen that currents fluctuate in the order of the microampere, thus suggesting that agarose can be suitable for this application. More extensive studies will be performed to consider alternative protecting agents and/or strategies to further extend EGOFET durability.

### CONCLUSIONS

The investigation of the degradation processes occurring on P3HT film serving as semiconducting channel material in water-gated field-effect-transistors was carried out adopting the measurement protocols successfully set in single-molecule biosensing experiments. A benchmark polymer without any additional treatment was used in the development of watergated FETs to study its reliability on prolonged use. It was observed that no hysteresis and limited leakage currents were achieved not only by operating the OS in a voltage range where typical parasitic electrochemical processes are absent (i.e., water electrolysis, polymer oxidation), but also by the suitable set up of the measuring protocol. Furthermore, it was shown that water incubation more than operation in cycling mode is responsible for the initial abrupt current loss occurring in the first 20 h. Such behavior accounts for the unavoidable presence of water in between the polymer chains. The role of degradation in water for long period was also highlighted by SEM, AFM, and XPS analyses demonstrating that roughness increases and additional hydroxyl moieties are present. A model of P3HT EGOFET degradation operating the device over 1 week continuously was proposed suggesting that the current loss is due to the VT shift by a bi-exponential decay. On the other hand, P3HT based devices could keep their electrical performance over a prolonged period if stored under ambient conditions with a protecting layer of agarose gel. Though we are aware that particular conditions and materials were applied in this study, this work can be a guide to investigate aspects of EGOFET device stability which could be critical for their use in transistor biosensors.

#### DATA AVAILABILITY STATEMENT

All datasets generated for this study are included in the manuscript/**Supplementary Files**.

### AUTHOR CONTRIBUTIONS

RP, KM, EM, and AT carried out the EGOFET fabrication, including measuring cell design, and the electrical characterization. RP wrote the first draft of the manuscript and performed XPS characterization. KM set up the measuring protocol. CD and GS were responsible for interdigitated electrode fabrication, SEM, and AFM analyses. NC contributed to the investigation of spurious electrochemical processes. LT supervised the overall study, coordinated the team, and revised the manuscript as approved by all the authors.

#### ACKNOWLEDGMENTS

RP acknowledges the financial support of Fondo di Sviluppo e Coesione 2007-2013–APQ Ricerca Regione Puglia Programma regionale a sostegno della specializzazione intelligente e della sostenibilità sociale ed ambientale– Future In Research project #B164PG8 (BEND: Biosensori elettronici intelligenti per la diagnosi precoce di malattie neurodegenerative). Additional grants are acknowledged for partial financial support: SiMBiT: Single molecule bioelectronic smart system array for clinical testing funded by the European Commission under H2020 program, grant #824946, MIUR PON - e-DESIGN: Combination of Design,

#### REFERENCES

Electronics and Multifunctional Materials for New Aesthetic Components (ARS01\_01158).

#### SUPPLEMENTARY MATERIAL

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fchem. 2019.00667/full#supplementary-material

electrosynthesized ZnO nanoparticles. Sens. Actuators B Chem. 274, 210–217. doi: 10.1016/j.snb.2018.07.128

**Conflict of Interest:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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