Introduction {#sec1}
============

In contrast to bulk silver, nanometric silver materials exhibit many extraordinary properties such as a high surface-to-volume ratio,^[@ref1],[@ref2]^ quantum tunneling effects,^[@ref3],[@ref4]^ an abundance of free electrons,^[@ref5]^ surface plasmon resonance,^[@ref2],[@ref6]−[@ref8]^ and antibacterial behaviors.^[@ref9],[@ref10]^ Because of these unique properties, noble silver nanomaterials are widely applied in diverse areas, including thermotherapy,^[@ref5],[@ref11]^ medicine,^[@ref2],[@ref5],[@ref12]^ sensors,^[@ref13]−[@ref16]^ surface-enhanced spectroscopy,^[@ref6],[@ref17]−[@ref20]^ biology,^[@ref2]^ catalysis,^[@ref21]−[@ref28]^ and electronics.^[@ref29]−[@ref31]^ Among these many applications, the catalysis of the reduction of nitroarenes to aromatic amines is increasingly attracting attention because of pharmaceutical needs and the importance of this industry.^[@ref25],[@ref26],[@ref32]−[@ref38]^ Various strategies have been proposed to reduce nitroarenes more efficiently and more rapidly using silver nanomaterials. These strategies include depositing silver nanoparticles (AgNPs) on supports used as heterogeneous catalysts,^[@ref24],[@ref26],[@ref39]−[@ref42]^ combining AgNPs with reduced graphene oxide or graphene oxide as catalysts,^[@ref43]−[@ref47]^ and using silver nanocolloids as a quasi-homogeneous nanocatalyst.^[@ref25],[@ref33],[@ref48]−[@ref54]^ All of the aforementioned catalytic approaches are efficient and selective. However, the reaction rate for heterogeneous catalysis is rather low and quasi-homogeneous catalysis suffers from possible aggregation of the nanocatalyst. Furthermore, the procedures for preparing such nanocatalysts are somewhat complex and time-consuming.

Applications of magnetic nanoparticles have also been extensively investigated in recent years because they feature many notable characteristics, such as a high surface-to-volume ratio, easy attraction and redispersion, and paramagnetism. Because of these crucial properties and advantages, the combination of AgNPs and magnetic nanoparticles has become one of the most favorable approaches for the catalytic reduction of nitroarenes.^[@ref55]−[@ref60]^ In this study, a simple but facile method was applied in a single step to prepare silver-doped magnetic nanoparticles (AgMNPs) for the catalytic reduction of nitroarenes through spontaneous oxidation--reduction and coprecipitation. When mixing Fe^2+^ with Ag^+^, a spontaneous reaction is caused by the difference in standard reduction potential between the ionic species. When Ag^+^ is reduced to Ag^0^, an equivalent number of moles of Fe^2+^ ions are simultaneously oxidized to Fe^3+^. After the addition of precipitation agents, AgNPs were coprecipitated with iron oxide magnetic nanoparticles, which led to the formation of AgMNPs. The proposed preparation can be achieved in a single step, and the prepared AgMNPs can subsequently be utilized as nanocatalysts for the reduction of *o*-nitroaniline (*o*-NA). The parameters (pH, temperature, and amount of nanocatalyst) that affect the morphology and composition of the prepared AgMNPs and efficiencies of the catalytic reduction were systematically studied to gain a greater understanding of the characteristics of the AgMNPs prepared using the method proposed in this study. Additionally, the catalytic activity of the AgMNPs prepared for the reduction of other nitroarenes and their recyclability were investigated to fully evaluate their potential for practical applications.

Results and Discussion {#sec2}
======================

Effect of Oxidation--Reduction Time on AgMNP Preparation {#sec2.1}
--------------------------------------------------------

[Figure S1](http://pubs.acs.org/doi/suppl/10.1021/acsomega.7b01987/suppl_file/ao7b01987_si_001.pdf) shows the typical measured hysteresis loops of the prepared AgMNPs, which confirmed that the prepared AgMNPs were paramagnetic and usable for further applications. [Figure [1](#fig1){ref-type="fig"}](#fig1){ref-type="fig"} depicts the transmission electron microscopy (TEM) images of AgMNPs obtained using various reaction times. From the images, dark-sphere-like Ag nanoparticles were mixed with light-colored Fe~3~O~4~ NPs because Ag has a higher electron density that allows fewer electrons to transmit.^[@ref61],[@ref62]^ The AgNPs formed after a 10 min reaction time were larger than those formed after a 2 min reaction time. Notably, the size of the Fe~3~O~4~ NPs was mostly unaffected by the reaction time.

![TEM images of prepared AgMNPs with oxidation--reduction times of (a) 2 min, (b) 8 min, and (c)--(f) 10 min, where the \[Fe^2+^\]~0~ to \[Ag^+^\]~0~ ratios are (a)--(c) 3:1, (d) 2:1, (e) 4:1, and (f) 6:1. \[Fe^2+^\]~0~ are all 12 mM, and the magnifications of the images are all 100 000×. The yellow arrows indicate the examples of AgNPs for each sample.](ao-2017-019876_0008){#fig1}

[Figure [2](#fig2){ref-type="fig"}](#fig2){ref-type="fig"}a shows the evolution of the UV--vis spectra for the reduction of *o*-NA catalyzed by AgMNPs over time. The absorbance peak at 412 nm, which corresponds to the characteristic *o*-NA peak,^[@ref63]^ decreased as the reaction proceeded. The variations of the spectra indicated that *o*-NA was reduced to 1,2-phenylenediamine (1,2-PPD).^[@ref41],[@ref64]^ The relative concentration (*C*~t~/*C*~0~) of *o*-NA was obtained by dividing the absorbance recorded at 412 nm at the specified time (*C*~t~) by the absorbance at 412 nm before the addition of AgMNPs (*C*~0~). The results in [Figure [2](#fig2){ref-type="fig"}](#fig2){ref-type="fig"}b were plotted using various AgMNPs prepared using various reduction durations. In [Figure [2](#fig2){ref-type="fig"}](#fig2){ref-type="fig"}b, the catalytic efficiency of the AgMNPs shows no significant difference between when the reaction time was 4--10 min and when more than 95% of *o*-NA was reduced within 240 s. This finding suggests remarkable catalytic activity. By contrast, achieving the same conversion percentage required more than 500 s when the reaction time to prepare the AgMNPs was 2 or 12 min. Because the catalytic efficiency of nanocatalysts depends on their size and the amount of catalyst loaded,^[@ref26]^ we concluded that the reaction time to achieve the optimal morphology and catalyst loading was 10 min. For comparison, the results of an experiment conducted in parallel, where AgMNPs were replaced with Fe~3~O~4~ NPs, are also plotted in [Figure [2](#fig2){ref-type="fig"}](#fig2){ref-type="fig"}b, showing that the reduction of *o*-NA can proceed only when AgNPs are doped. In the absence of AgNPs, the reduction of *o*-NA is suspended.

![(a) UV--vis spectra of 1 mM *o*-NA reduced by 30 mM NaBH~4~ at room temperature (RT) in the presence of 20 mg of AgMNPs with increasing time, where the pH was 9.8. The AgMNPs were prepared under the condition described in [Figure [1](#fig1){ref-type="fig"}](#fig1){ref-type="fig"}c. (b) *C*~t~/*C*~0~ of 1 mM *o*-NA (412 nm) versus the catalytic reduction time in the presence of 30 mM NaBH~4~ and AgMNPs prepared with (■) 2 min, (●) 4 min, (▲) 6 min, (▼) 8 min, (◆) 10 min, and (★) 12 min of oxidation--reduction time during the preparation, where the other conditions are the same as described in (a). Parallel experiment (□) uses 20 mg of Fe~3~O~4~ NPs as nanocatalysts, where the other conditions are the same as described as (a).](ao-2017-019876_0001){#fig2}

Effect of Fe^2+^/Ag^+^ on AgMNP Preparation {#sec2.2}
-------------------------------------------

As described in the previous section, catalytic efficiency is related to the size and amount of doped AgNPs. In addition, we expected the morphology and amount of doped AgNPs to be affected by the ratio of initial concentration of Fe^2+^ to Ag^+^ because AgNO~3~ acts as the oxidation agent in the formation of AgMNPs. [Figure [1](#fig1){ref-type="fig"}](#fig1){ref-type="fig"} shows the TEM images of the AgMNPs with different ratios of \[Fe^2+^\]~0~ to \[Ag^+^\]~0~. The images reveal that the morphologies of the AgNPs are similar. [Figure S2](http://pubs.acs.org/doi/suppl/10.1021/acsomega.7b01987/suppl_file/ao7b01987_si_001.pdf) shows the typical X-ray diffraction (XRD) spectra of the prepared AgMNPs and illustrates that the locations and intensity of the diffraction peaks were consistent with the standard patterns for JCPDS card no. (79-0417) magnetite and JPCPDS card no. (4-0783) standard Ag crystal. The size of AgNPs can by estimated using the Scherrer equation, and all AgNPs are approximately 20 nm.^[@ref10]^ To further explore the effect of the initial Fe^2+^ to Ag^+^ concentration ratio, energy-dispersive X-ray spectroscopy (EDS) analysis was performed and the typical spectra for the AgMNPs are shown in [Figure S3A](http://pubs.acs.org/doi/suppl/10.1021/acsomega.7b01987/suppl_file/ao7b01987_si_001.pdf). The atomic percentages of Ag, Fe, and O for various initial concentration ratios of Fe^2+^ to Ag^+^ obtained through EDS are plotted in [Figure S3B](http://pubs.acs.org/doi/suppl/10.1021/acsomega.7b01987/suppl_file/ao7b01987_si_001.pdf), which shows that the atomic percentage of Ag increased as the initial concentration ratio of Fe^2+^ to Ag^+^ decreased. When the initial concentration ratio of Fe^2+^ to Ag^+^ was 2.0, the atomic percentage of Ag in the AgMNPs was 8.23%. The atomic percentage decreased to 0.76% when the initial concentration ratio of Fe^2+^ to Ag^+^ was 6.0.

[Figure [3](#fig3){ref-type="fig"}](#fig3){ref-type="fig"} plots the *C*~t~/*C*~0~ of *o*-NA as a function of the reduction time in the presence of AgMNPs prepared with various ratios of \[Fe^2+^\]~0~ to \[Ag^+^\]~0~. The figure shows that the AgMNPs prepared with a smaller ratio of \[Fe^2+^\]~0~ to \[Ag^+^\]~0~ had a higher catalytic efficiency. Through correlation with the EDS results, we concluded that the highest catalytic efficiency was obtained for the highest AgNP loading. Notably, although the catalytic efficiency of the AgMNPs was even higher when the initial concentration ratio of Fe^2+^ to Ag^+^ was 1.5, the yield of AgMNPs was very low because of weak magnetization and most particles formed after the coprecipitation stage could not be collected by the magnet. Similar results were observed when the initial concentration ratio of Fe^2+^ to Ag^+^ was 6.0. Therefore, we can conclude that the initial concentration ratio of Fe^2+^ to Ag^+^ should be between 2 and 3 to ensure that a sufficient amount of Fe^2+^ is oxidized so that the ratio of Fe^2+^ to Fe^3+^ is close to 2 before the coprecipitation agent is added to form magnetic Fe~3~O~4~ NPs.^[@ref65]^

![*C*~t~/*C*~0~ of 1 mM *o*-NA versus catalytic reduction time in the presence of 30 mM NaBH~4~ and AgMNPs prepared with \[Fe^2+^\]~0~ to \[Ag^+^\]~0~ ratios of (■) 6:1, (●) 4:1, (▲) 3:1, (▼) 2:1, and (◆) 1.5:1, where the other conditions are the same as described in [Figure [2](#fig2){ref-type="fig"}](#fig2){ref-type="fig"}a.](ao-2017-019876_0002){#fig3}

Effects of pH and Temperature on the Catalytic Reaction {#sec2.3}
-------------------------------------------------------

The acceleration of the reduction reaction by AgMNPs originates in a relay between the nucleophile and electrophile.^[@ref39]^ Therefore, the catalyzed reduction rate is affected by the abundance of electrons in the reaction system. To observe the relationship between the electron abundance and reaction rate, the catalysis reaction was performed at various pH values. The relative *o*-NA concentration (*C*~t~/*C*~0~) was recorded as a function of reaction time at various pH values, and the results are plotted in [Figure [4](#fig4){ref-type="fig"}](#fig4){ref-type="fig"}a. The pH played a key role in the catalysis reaction; when the pH was 9.8, the reduction of *o*-NA was completed within 4 min. The reduction rate significantly decreased when the pH was set to 8.8 and less than 85% of *o*-NA was reduced after approximately 5 min of reaction. When the pH was lower than 8, almost no conversion of *o*-NA could be observed. Because the acceleration of the reduction reaction by AgMNPs originates in a relay between the nucleophile and electrophile,^[@ref39]^ alkaline conditions enriched the electron densities on the AgNP surfaces by adsorbing more OH^--^, which promoted the reduction of *o*-NA. When the pH was set to 10.8, the reduction of *o*-NA decelerated, possibly because of the formation of yellow-colored 2,3-diaminophenazine under extremely alkaline conditions.^[@ref66]^

![(a) *C*~t~/*C*~0~ of 1 mM *o*-NA versus catalytic reduction time in the presence of 30 mM NaBH~4~ under different pHs of (■) 6.8, (●) 7.8, (▲) 8.8, (▼) 9.8, and (◆) 10.8, where the other conditions are the same as described in [Figure [2](#fig2){ref-type="fig"}](#fig2){ref-type="fig"}a. (b) *C*~t~/*C*~0~ of 1 mM *o*-NA versus catalytic reduction time in the presence of 30 mM NaBH~4~ under different temperatures of (■) 0 °C, (●) 25 °C, and (▲) 40 °C, where the other conditions are the same as described in [Figure [2](#fig2){ref-type="fig"}](#fig2){ref-type="fig"}a.](ao-2017-019876_0003){#fig4}

The reaction rate was affected by the temperature of the reaction system because the reactants had more kinetic energy at higher temperatures and were able to cross the activation state more easily. [Figure [4](#fig4){ref-type="fig"}](#fig4){ref-type="fig"}b depicts the relationship between *C*~t~/*C*~0~ and reaction times at various reaction temperatures. As shown in [Figure [4](#fig4){ref-type="fig"}](#fig4){ref-type="fig"}b, the higher the temperature of the reaction system, the faster was the observed reduction of *o*-NA. Furthermore, when the temperature was 0 °C, the reduction rate of *o*-NA was similar to that at 25 °C in the first minute and *C*~t~/*C*~0~ was almost unchanged after the first minute, suggesting that the reduction was almost interrupted after 1 min at 0 °C.

Effect of the Catalyst Amount on the Catalytic Reaction {#sec2.4}
-------------------------------------------------------

As described in the previous sections, the conversion efficiency is related to the amount of AgNP loading. Accordingly, the conversion efficiency can also be related to the amount of AgMNPs used per experiment. [Figure [5](#fig5){ref-type="fig"}](#fig5){ref-type="fig"} plots the relationship between the *C*~t~/*C*~0~ of *o*-NA and reduction time when various amounts of AgMNPs were used. As shown in [Figure [5](#fig5){ref-type="fig"}](#fig5){ref-type="fig"}, when 1 mg of AgMNPs was used per experiment, approximately 20% of *o*-NA was reduced after 5 min of conversion and the conversion efficiency increased with the amount of AgMNPs used. When 20 mg of AgMNPs was used per experiment, the conversion of *o*-NA was almost 100% within 250 s at room temperature, which is promising for further applications.

![*C*~t~/*C*~0~ of 1 mM *o*-NA versus catalytic reduction time in the presence of 30 mM NaBH~4~ and different amounts of AgMNPs of (■) 1 mg, (●) 5 mg, (▲) 10 mg, (▼) 15 mg, and (◆) 20 mg, where the other conditions are the same as described in [Figure [2](#fig2){ref-type="fig"}](#fig2){ref-type="fig"}a.](ao-2017-019876_0004){#fig5}

Activity of AgMNPs for the Catalytic Reduction of Other Nitroarenes {#sec2.5}
-------------------------------------------------------------------

After studying the characteristics and catalytic properties of the prepared AgMNPs, we investigated the catalytic reduction of other nitroarenes, including *m*-NA, *p*-NA, and *p*-NP, through the same procedures to investigate the ability of the AgMNPs to accelerate the reduction of other nitroarenes. [Figure S4](http://pubs.acs.org/doi/suppl/10.1021/acsomega.7b01987/suppl_file/ao7b01987_si_001.pdf) plots the evolution of the UV--vis spectra for reducing *m*-NA, *p*-NA, and *p*-NP in the presence of AgMNPs over time. The absorption maxima, which were located at 360 nm for *m*-NA, 380 nm for *p*-NA, and 400 nm for *p*-NP, decreased as the catalytic reduction proceeded. The relationship between *C*~t~/*C*~0~, where *C*~0~ is the absorbance at the initial time and *C*~t~ is the absorbance after the specific reaction time, for the nitroarenes tested and the reaction time under optimal conditions is plotted in [Figure [6](#fig6){ref-type="fig"}](#fig6){ref-type="fig"}. As indicated in the figure, the reductions of the four nitroarenes examined in this study were all completed within 4 min, which suggested that the AgMNPs prepared in this study were capable of catalytically reducing various nitroarenes.

![*C*~t~/*C*~0~ of different 1 mM nitroarenes versus catalytic reduction time in the presence of 30 mM NaBH~4~ and 20 mg of AgMNPs (■) *o*-NA, (●) *m*-NA (358 nm), (▲) *p*-NA (380 nm), and (▼) *p*-NP (400 nm), where the other conditions are the same as described in [Figure [2](#fig2){ref-type="fig"}](#fig2){ref-type="fig"}a.](ao-2017-019876_0005){#fig6}

Furthermore, as reported in other works, the catalytic reduction of nitroarenes follows the pseudo-first-order reaction kinetics.^[@ref26],[@ref43],[@ref51]^ The linear relations between ln(*C*~0~/*C*~t~) of nitroarenes examined in this study against reaction time were obtained and the rate constant (*k*) can be estimated by calculating the slopes of the obtained lines. The calculated *k* values at room temperature are 0.0192, 0.0145, 0.0185, and 0.0196 s^--1^ for the catalytic reduction of *o*-NA, *m*-NA, *p*-NA, and *p*-NP, respectively. To compare the results obtained in this study to those reported recently, [Table [1](#tbl1){ref-type="other"}](#tbl1){ref-type="other"} tabulates the catalytic activities of various AgNP-based catalytic systems. As can be observed in the table, the rate constants of AgMNPs prepared in this study for the reduction of nitroarenes are as good as other reported nanocatalysts. Moreover, because the effects of temperatures have also been studied in the previous sections, the thermodynamic parameters for the catalytic reduction of *o*-NA by AgMNPs prepared in this study can be calculated by following the Arrhenius and Eyring equations.^[@ref67],[@ref68]^ The calculated activation energy (*E*~a~) is 39.88 kJ/mol, activation enthalpy (Δ*H*) is 37.34 kJ/mol, and activation entropy (Δ*S*) is −123.29 J/(mol K). These results suggest that the AgMNPs prepared in this study are excellent nanocatalysts for the reduction of nitroarenes. The catalytic reduction of nitroarenes by metal nanoparticles is generally explained by the Langmuir--Hinshelwood mechanism, where both reactants are adsorbed on nanocatalyst surfaces and reaction occurred after the adsorption.^[@ref67],[@ref68]^ As a result of conversion, products are formed and then desorb from nanocatalyst surfaces. According to the Langmuir--Hinshelwood mechanism, the rate of catalytic reduction depends on the surface coverage reducing agent and nitroarene molecules.^[@ref69]−[@ref71]^ This mechanism rationalizes the large rate constant and small activation energy obtained in this study because the high surface-to-volume ratio and the quasi-homogeneous reaction conditions increase the surface coverage of reactants significantly.

###### Comparison of Catalytic Activities of Several AgNP-Based Systems

  catalysts                                  nitroarene (final concentration; mM)   concentration of NaBH~4~ (mM)   temperature                          apparent rate constant (s^--1^)   ref
  ------------------------------------------ -------------------------------------- ------------------------------- ------------------------------------ --------------------------------- ------------
  AgNPs/polydopamine/anodic aluminum oxide   *o*-NA (1.13)                          400                             RT[a](#t1fn1){ref-type="table-fn"}   0.0013                            ([@ref40])
  biogenic AgNPs                             *p*-NP (0.20)                          10                              [b](#t1fn2){ref-type="table-fn"}     0.00406                           ([@ref41])
  AgNPs/partially reduced graphene oxide     *p*-NP (0.10)                          13                              RT                                   0.0374                            ([@ref43])
  AgNPs on porous glass filters              *o*-NA (1.00)                          30                              50 °C                                0.0094                            ([@ref26])
                                             *p*-NA (1.00)                          30                              50 °C                                0.0071                            ([@ref26])
  AgNPs in microgels                         *p*-NP (0.08)                          24                              22 °C                                0.0153                            ([@ref50])
  AgNPs in microgels                         *o*-NA (0.09)                          18                              22 °C                                0.0067                            ([@ref51])
                                             *p*-NA (0.09)                          18                              22 °C                                0.0101                            ([@ref51])
                                             *p*-NP (0.09)                          18                              22 °C                                0.0052                            ([@ref51])
  AgNPs on fibrous nanosilica                *o*-NA (0.17)                          22                              RT                                   0.0043                            ([@ref52])
                                             *p*-NP (0.099)                         83                              RT                                   0.01                              ([@ref52])
  Fe~3~O~4~\@SiO~2~/Ag nanocomposite         *p*-NP (0.06)                          6                               25 °C                                0.00767                           ([@ref59])
  AgNPs/HLaNb~2~O~7~                         *p*-NP (0.091)                         18                              [b](#t1fn2){ref-type="table-fn"}     0.00301                           ([@ref53])
  this study                                 *o*-NA (1)                             30                              RT                                   0.0192                             
                                             *p*-NA (1.00)                          30                              RT                                   0.0185                             
                                             *p*-NP (1.00)                          30                              RT                                   0.0196                             

Room temperature.

Not mentioned.

Recyclability of the Ag Nanocatalysts {#sec2.6}
-------------------------------------

The recyclability of the AgMNPs prepared in this study was evaluated by consecutively reusing the nanocatalysts for the catalytic reduction of *o*-NA. As shown in [Figure [7](#fig7){ref-type="fig"}](#fig7){ref-type="fig"}, only approximately 40% of *o*-NA was reduced to 1,2-PPD after 10 min of reaction in the second consecutive test. This is possibly related to the adsorption of *o*-NA or 1,2-PPD on the surface of the silver nanocatalysts, which consequently reduced the electron transferability.^[@ref26]^ To reactivate the silver nanocatalysts, we soaked the used AgMNPs in an aqueous solution at pH 3 for 20 min and rinsed them with neutral water before the next use. As shown in [Figure [7](#fig7){ref-type="fig"}](#fig7){ref-type="fig"}, more than 90% of the *o*-NA was reduced within 5 min and more than 95% was reduced within 8 min. Furthermore, after treating the reused AgMNPs with an aqueous acidic solution, the AgMNPs could be recycled and their performance was similar to that in the previous run. Therefore, the AgMNPs prepared in this study can be recycled after treatment with an acidic solution, which significantly extends the practical applicability of these silver nanocatalysts.

![*C*~t~/*C*~0~ time profile of 1 mM *o*-NA (■: first run; ●: second run without regeneration; ▲: third run after regeneration; and ▼: fourth run after regeneration) in the presence of 30 mM NaBH~4~ and 20 mg of AgMNPs, where the other conditions are the same as described in [Figure [2](#fig2){ref-type="fig"}](#fig2){ref-type="fig"}a.](ao-2017-019876_0006){#fig7}

Conclusions {#sec3}
===========

In this study, a simple but facile approach to prepare silver-doped magnetic hybrid nanoparticles was proposed based on a chemical reduction and coprecipitation method. The nanoparticles prepared through this method were used as nanocatalysts for the reduction of *o*-NA. Using the AgMNPs prepared in this study as nanocatalysts exploits the advantages of quasi-homogeneous reaction conditions and enables the easy removal of nanocatalysts from the solution with a magnet. The results indicated that the composition of the AgMNPs prepared can be tuned by adjusting the ratio of \[Fe^2+^\]~0~ to \[Ag^+^\]~0~ and the chemical reduction time during the production of AgMNPs. During the catalytic reduction of *o*-NA, the pH and temperature of the system affect the reduction rate, which is also affected by the amount of nanocatalyst used in the reaction. Furthermore, the prepared AgMNPs were applicable to the catalytic reduction of other nitroarenes. Finally, the silver-doped magnetic nanocatalysts proposed in this study have several advantages, namely, easy preparation, significant catalytic activity at room temperature, high conversion ability, and recyclability, all of which enhance their usefulness for real applications.

Experimental Section {#sec4}
====================

Materials {#sec4.1}
---------

Ferrous sulfate and ferric chloride were obtained from Showa Chemical (Tokyo, Japan). Silver nitrate, *o*-NA, *m*-nitroaniline (*m*-NA), *p*-nitroaniline (*p*-NA), and *p*-nitrophenol (*p*-NP) were purchased from Alfa Aesar (Ward Hill, MA). Sodium borohydride was obtained from Acros Organics (Geel, Belgium). Ammonium hydroxide (28--30%, v/v) and nitric acid were purchased from Fisher Scientific (Hampton, NH). All chemicals were of reagent grade and used as received without further purification. Deionized Milli-Q water (Simplicity, Millipore, Burlington, MA) was used throughout this study.

Preparation of AgMNPs {#sec4.2}
---------------------

The preparation of AgMNPs was based on a chemical reduction and coprecipitation method. Briefly, 100 mL of 12 mM ferrous aqueous solution was mixed with various volumes of 200 mM silver nitrate aqueous solution under vigorous stirring for a specified amount of time. During stirring, a spontaneous oxidation--reduction reaction occurred between Ag^+^ and Fe^2+^. Ag^+^ was reduced to Ag^0^ and an equivalent number of moles of Fe^2+^ ions were oxidized to Fe^3+^. After the specified reaction time, 50 mL of 1.44 M ammonia solution, which acted as the precipitating agent, was rapidly added to the solution under vigorous stirring for 10 min to complete the coprecipitation process. After 3 h in storage, the formed nanoparticles were collected with a magnet and washed three times with distilled water and ethanol. Finally, the washed AgMNPs were dried in an oven at 140 °C for 8 h before further use. As an alternative for comparison, magnetite nanoparticles (Fe~3~O~4~ NPs) without silver doping were prepared following previous reports.^[@ref72]^ We conducted transmission electron microscopy (TEM) with a Hitachi HT-7700 microscope operated at 100 kV, energy-dispersive X-ray spectroscopy (EDS) analysis with a Hitachi SU-8010 microscope at an accelerating voltage of 15.0 kV, and powder X-ray diffraction (PXRD) with a Siemens D5000 XRD system to characterize the morphologies and compositions of the prepared AgMNPs. Hysteresis loops of the prepared AgMNPs were recorded at room temperature with a Quantum Design MPMS 3 SQUID vibrating sample magnetometer system.

Reduction of Nitroaniline Catalyzed by AgMNPs {#sec4.3}
---------------------------------------------

The catalytic efficiency of the AgMNPs was evaluated using the nanoparticles as prepared for the catalytic reduction of *o*-NA. A specific amount of AgMNPs was mixed with 15 mL of an aqueous solution consisting of 1 mM *o*-NA and 30 mM NaBH~4~ at room temperature. Ultraviolet--visible (UV--vis) spectra of the solution were recorded at chosen intervals. All UV--vis spectra in this study were measured using a Thermo Fisher Scientific Genesys 10S Bio UV--Vis spectrometer with a 1 nm resolution. The spectra were recorded within a wavelength range of 250--550 nm. The optical path of the UV--vis cell was 3 mm.

The Supporting Information is available free of charge on the [ACS Publications website](http://pubs.acs.org) at DOI: [10.1021/acsomega.7b01987](http://pubs.acs.org/doi/abs/10.1021/acsomega.7b01987).Hysteresis loops, XRD spectrum, and EDS analysis results of the AgMNPs, and the UV--vis spectra of the reduction of *m*-NA, *p*-NA, and *p*-NP in the presence of AgMNPs with increasing times ([PDF](http://pubs.acs.org/doi/suppl/10.1021/acsomega.7b01987/suppl_file/ao7b01987_si_001.pdf))

Supplementary Material
======================

###### 

ao7b01987_si_001.pdf

The authors declare no competing financial interest.

The authors acknowledge the financial support from the Taiwan Ministry of Science and Technology under grant MOST106-2113-M-037-016. This work was also supported by the Kaohsiung Medical University Research Foundation under grant KMU-M106016.
