WaterâChloroform Interface Assisted Microstructure Tuning of

Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Water−Chloroform Interface Assisted Microstructure Tuning of Polypyrrole−Silver Sheets Subin Kaladi Chondath,† Rasha Rahman Poolakkandy,† Roshima Kottayintavida,§ Aswani Thekkangil,‡ Nishanth Karimbintherikkal Gopalan,§ Suchithra Tharamel Vasu,‡ Sujith Athiyanathil,† and Mini Mol Menamparambath*,† †

Department of Chemistry and ‡School of Biotechnology, National Institute of Technology Calicut, Calicut 673601, Kerala, India Materials Science and Technology Division, CSIR-National Institute for Interdisciplinary Science and Technology (NIIST), Thiruvananthapuram 695 019, Kerala, India

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S Supporting Information *

ABSTRACT: The liquid−liquid interface of two immiscible solvents remarkably controls the morphology of polymeric nanostructures as compared to the polymerization in single solvent systems. The polymerization of pyrrole in the water− chloroform medium using silver nitrate (AgNO3) as oxidant yields polypyrrole/silver (PPy/Ag) sheets. The water−chloroform interface acts as a template for the growth of PPy/Ag hybrids into sheets by preventing the secondary growth of silver associated pyrrole oligomers in a three-dimensional (3D) manner. On the contrary, the 3-D growth of pyrrole oligomers into spherical shapes at the water−chloroform interface is observed when ammonium persulfate (APS) is used as the oxidant. Transmission electron microscopic and scanning electron microscopic images reveal the sheetlike morphology of PPy/Ag with a relatively uniform distribution of Ag NPs (∼100 nm) on PPy sheets. The ratio of aqueous−organic bisolvent and the concentration/type of oxidant have a distinct effect on morphology, crystallinity, and electrical properties of PPy/Ag sheets. The dispersed PPy/Ag sheets are stable in moderately polar solvents up to 2 weeks. The electrochemical behavior of PPy/Ag sheets is confirmed by H2O2 sensing capability through cyclic voltammetry experiments. The antibacterial activity toward E. coli and S. aureus is quantitatively assessed using the minimum bactericidal concentration (MBC) determination. KEYWORDS: polymerization at interface, morphology tuning, bisolvent, polypyrrole, PPy/Ag sheets

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PPys with different nanostructures such as nanowires,14 nanotubes,15−17 and nanoparticles2,18 have been utilized as templates for nanoparticle synthesis. Among the PPy/MNPs composites, PPy/silver (PPy/Ag) nanostructures are widely used for various applications such as flexible electronics19 and multifunctional wearable textiles20 because of their unique optoelectronic21,22 and antibacterial properties.14,23 The reinforcement in electrical, optical, and antimicrobial properties of the composites is mainly governed by the percolative efficiency, which is related to the geometry (including size and aspect ratio),13 dispersion,24 and uniform distribution of fillers13 in the polymer matrix. From the comparison of aspect ratio and percolation threshold of differently shaped fillers, nonspherical fillers (flakes, tubes, and rods) are found to be more effective reinforcers than spherical fillers due to their efficient network formation.25 Therefore, controlling the

INTRODUCTION Polymers possessing conjugated π-electron systems along the polymer backbone and exhibiting intrinsic conductivity are classified as conductive polymers, for example, polypyrrole (PPy).1 PPy and its derivatives are of particular interest because of high conductivity,2 stability in the oxidized state,3 attractive redox properties,3 and ease of synthesis via chemical, photochemical, and electrochemical routes.4 These unique properties of PPy make it suitable for applications in various fields, such as rechargeable batteries,5,6 drug delivery,7,8 membrane separation,9 supercapacitors,10 and sensors.1,11,12 However, the marginal physical and mechanical properties and insolubility in common organic solvents restrict the use of PPy composites for several applications.13 Incorporation of metal nanoparticles (MNPs)2 or nanostructures of high aspect ratio13 with conductive polymers has successfully emerged to overcome these hurdles. In an attempt to disperse nanostructures onto the PPy matrix, the greatest challenge is to avoid the agglomeration of nanostructures to retain its unique properties. Interestingly, © XXXX American Chemical Society

Received: October 29, 2018 Accepted: December 18, 2018

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DOI: 10.1021/acsami.8b18943 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. (A) Schematic illustration of the formation of PPy/Ag sheets by the polymerization of PPy at the water−chloroform interface. Optical and TEM images of PPy/Ag are provided. (B) Schematic illustration of the preparation of the modified glassy carbon electrode (GCE) using PPy/ Ag sheets. (C) Schematic illustration of the antibacterial studies of PPy/Ag sheets.

morphology of PPy/Ag structures is very important to enhance the electrical, optical, and antimicrobial properties of the PPy/ Ag composites. In earlier reports, the structural control of PPy/ Ag structures was mainly achieved by varying the type/ concentration of surfactants which act as structure guiding agents in polymerization reactions.25 Polymerization of conductive polymers at the interface of two immiscible solvents has received much attention over the past few years.26,27 The bisolvent interface offers effective tuning of nanostructure over conventional polymerization techniques in the presence of stabilizers like poly(vinyl alcohol),13 polyvinylpyrrolidone,13 and methyl orange.16 The three-dimensional approaches of oligomers or small molecules result in the secondary growth of polymers or nanomaterials to a multidimensional structure. In a bisolvent system, the interfacial tension at the liquid−liquid interface provides a scaffold for the organization of polymeric nanomaterials, inorganic nanomaterials, and viruses.27 Moreover, the twodimensional (2-D) liquid−liquid interface boundary can be considered as an imaginary membrane where a threedimensional approach of oligomers is restricted. Therefore, the interface of two immiscible solvents promotes the growth of polymers into sheetlike morphology. The present work focuses on the polymerization of pyrrole in bisolvent (water−chloroform) medium to control the nanostructure of polypyrrole sheets decorated uniformly with Ag NPs (PPy/Ag sheets). The importance of mass transfer and

redox properties of oxidant and monomer molecules at the bisolvent interface in controlling the nanostructure of PPy/Ag sheets is focused in this work. The method presented here is a significant step in the field of nanostructure control of polymer/nanomaterial hybrids using chemical reactions at bisolvent interfaces. Interestingly, the PPy/Ag sheets presented here exhibit promising multifunctionalities such as electrochemical response toward hydrogen peroxide and antibacterial activity toward E. coli and S. aureus bacteria.

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METHODS/EXPERIMENTAL SECTION

Chemicals and Reagents. Pyrrole (Sigma-Aldrich, 131709, 98%), silver nitrate (Spectrochem, 0119128, 99.9%), chloroform (Fisher Scientific, 33515, 99.5%), ethanol (Analytical reagent, 99.9%), N-methylpyrrolidone (Spectrochem, 011328, 99%) and ammonium persulfate (Sigma-Aldrich, 98%) were used as received. All the reagents were of analytical grade purity, and deionized water was used throughout the synthesis. Synthesis of PPy/Ag Sheets. The bisolvent assisted in situ oxidative interfacial polymerization was employed for the synthesis of PPy/Ag sheets. In a typical experiment, 1 mL (14.4 mmol) of pyrrole monomer was dissolved in chloroform followed by the addition of aqueous solution of AgNO3 (0.01 − 0.10 M). The 30 mL water− chloroform reaction medium was kept for constant stirring (250 rpm) for 24 h at room temperature. The product obtained after vacuum filtration was washed several times with water and ethanol. It was then dried in a vacuum oven for about 24 h at room temperature. A schematic illustration of the synthesis of PPy/Ag sheets by the polymerization of pyrrole at the water−chloroform interface is shown B


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Scheme 1. Mechanism of Oxidative Polymerization of Pyrrole to Polypyrrole/Silver (PPy/Ag) Sheets Using Silver Nitrate

in Figure 1A. For the synthesis of PPy/Ag sheets with varying solvent ratio, the water−chloroform ratio was ranged from 40:60 to 80:20. Pure polypyrrole (PPy) was also prepared by the same method in varying water−chloroform solvent ratio (40:60−80:20) using APS as initiator (Figure S1). Detection of Hydrogen Peroxide Using Cyclic Voltammetry. PPy/Ag sheets were employed to create a modified glassy carbon electrode for the detection of hydrogen peroxide. The glassy carbon electrode (GCE, 5 mm diameter) was polished with slurries of alumina polishing powder prior to usage. To prepare the modified electrode, PPy/Ag sheets and pure PPy were ultrasonically suspended in distilled water, and 4 μL of it was transferred onto the pretreated glassy carbon electrode using a micropipet. Then 3 μL of 0.5 wt % Nafion solution was coated on the surface of GCE and allowed to dry at room temperature. The sample loading was 0.563 mg/cm2 (Figure 1B). Cyclic voltammetry (CV) tests were performed within a potential range of −0.6 to +0.6 V at a scan rate of 25 mV/s in a N2-saturated 0.1 M potassium phosphate buffer (K2HPO4 and KH2PO4) solution of pH 7 containing 0.5 M KCl. The CV responses of the modified electrodes were recorded. Antibacterial Studies of PPy and PPy/Ag. The antibacterial activity of pure PPy (PPy 2) and PPy/Ag sheets (SA 3 and SR 2) was investigated against Escherichia coli and Staphylococcus aureus by the disk diffusion method.28 The bacterial suspension was grown overnight in Mueller Hinton broth (Himedia, India), adjusted to 0.5 McFarland turbidity standards. Whatman No. 1 paper disk was loaded with 50 μL of the solution containing 15 mg of pure PPy (PPy 2) and PPy/Ag sheets (SA 3 and SR 2) in 1 mL of acetone. The tests were performed in triplicates on Mueller Hinton agar (Himedia, India) plates incubated overnight at 37 °C, and zones of inhibition were measured. The minimum bactericidal concentrations were determined by incorporating different concentrations of PPy/Ag and pure PPy to MHA plates (Figure 1C). In detail, 20 mL of the MHA was fortified with different concentrations of PPy/Ag (0.15, 0.3, 1, 2, 3, 4, 5, and 15 mg/mL) and pure PPy (0.5, 1, 1.5, 3, 4.5, and 9 mg/mL) to prepare known concentrations. The test bacteria were cultured using the spread plate technique. The bacteria were allowed to grow in MHA plates without PPy/Ag as a control. The bacterial suspension was grown overnight in Mueller Hinton broth (Himedia, India), adjusted to 0.5 McFarland turbidity standards. An equal quantity of inoculum (50 μL) was tested against all test and control plates. The culture plates were incubated at 37 °C overnight, and growth was compared. Instrumentation. The morphology and composition of PPy/Ag sheets were characterized using a field emission scanning electron microscope (FESEM, SU6600 HITACHI), a scanning electron microscope (JEOL Model JSM - 6390LV) equipped with an energy dispersive spectrometer (EDS, OXFORD XMX N), and a highresolution transmission electron microscope (HRTEM, JEOL/JEM 2100). Fourier transform infrared spectroscopy (FTIR) studies were performed in KBr pressed pellets using a spectrum JASCO 4700 FTIR spectrometer. The powder X-ray diffraction (XRD) studies were performed using a Rigaku Miniflex-600 diffractometer with Cu Kα radiation. The electrical conductivity of the products was measured by

a standard four-probe method (Keithley 2450). Electrochemical measurements were performed using a conventional three-electrode system using the PPy/Ag sheet coated glassy carbon electrode (GCE) as the working electrode, Ag/AgCl as the reference electrode, and a platinum wire as the counter electrode.

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RESULTS AND DISCUSSION In this work, bisolvent assisted interfacial polymerization was employed for the synthesis of PPy/Ag sheets (Figure 1A). During this process, the pyrrole monomer from the organic phase and oxidant from aqueous phase were allowed to react at the water−chloroform interface. The major steps involved in the interfacial chemical oxidation polymerization are as follows: (a) the diffusion of reactants to the interface followed by adsorption onto the surface, (b) oxidative polymerization at the interface, and (c) movement of products away from the interface and diffusion to either of the two phases.29 The mechanism of polymerization of pyrrole at the bisolvent interface coupled with the reduction of silver ions to silver atoms is shown in Scheme 1. An interface was developed between the two immiscible solvents (water and chloroform) during the course of the polymerization reaction. In addition, the electrode potential for the reduction of Ag+ to Ag0 is 0.8 V,27 which is higher than that of the pyrrole reduction potential (−0.9 V).30 The significant difference in reduction potential favors the spontaneous reduction of Ag+ ions on the PPy structures at the interface followed by the subsequent growth of Ag atoms to Ag nanoparticles. The attachment of crystalline Ag NPs on the PPy backbone prevents the diffusion of PPy oligomers to highly polar aqueous phase or highly nonpolar organic phase as observed in Figure 1A. The Ag nanoparticle attached oligomers tend to localize to the boundary between two immiscible solvents where the interfacial tension is high.31 Similar observations are also reported for silicon oxide nanoparticles, which adsorb to the liquid−liquid interface, thus stabilizing the water-in-oil and oilin-water emulsions.32 Therefore, the three-dimensional growth of polymer and nanomaterials is restricted, which results in a sheetlike morphology of the composites. Figure 1A demonstrates the optical and transmission electron microscopy (TEM) images, which show the powdery nature of microscale PPy/Ag sheets. The microstructure tuning of PPy/Ag sheets in the water−chloroform bisolvent medium was studied by varying (a) AgNO3 amounts and (b) the volume ratio of solvents with sample codes SA and SR, respectively. The concentration of pyrrole and AgNO3, volume of solvents, and the sample codes are tabulated (Table S1). On the contrary, the diffusion of oligomer to the aqueous phase was observed when APS is used as oxidant (Figure S1). The charged C


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Figure 2. Scanning electron microscopic images of PPy/Ag sheets: (A, B) SA 3, (E, F) SR 2, and (H) pure PPy. Transmission electron microscopy images of PPy/Ag sheets: (C) SA 3 and (G) SR 2. (D) Elemental composition of PPy/Ag sheets estimated from EDAX analysis.

Figure 3. FTIR spectra (A, B) and XRD spectra (C, D) of as-prepared PPy/Ag sheets. (E) Optical images of PPy/Ag (SR 2) sheets dispersed in solvents NMP, ethanol, and chloroform at different time intervals.

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using transmission electron microscopy (TEM) images of SA 3 and SR 2 as shown in Figures 2C and 2G. Furthermore, the encapsulation of relatively uniform sized Ag NPs in PPy matrix is more visible in the TEM images. The Fourier transform infrared spectroscopy analysis illustrates the chemical characteristics of the as-prepared PPy/Ag sheets (Figures 3A,B). The characteristic peaks at 1550 cm−1 (CC stretching mode),35 1481 cm−1 (C−N stretching mode),35 922 cm−1 (C−H wagging),18 787 cm−1 (ring deformation),18 and 3450 cm−1 (N−H stretching mode)35 represent the PPy backbone. Additionally, the peaks at 1292 and 1177 cm−1 are attributed to =C−H in-plane bending36 and C−N wagging18 vibration modes of PPy. A very strong absorption peak at 1381 cm−1 corresponding to NO3− ions2 confirms the oxidation of PPy and doped NO3− ions during polymerization. The presence of counteranions on the PPy backbone promotes the electrostatic interaction between Ag+ ions and PPy, further enhancing the encapsulation of Ag NPs in the PPy matrix. The FTIR spectra of the PPy/Ag sheets synthesized with ranging concentration of AgNO3 (varied from 0.01 to 0.1 M) and the intensity of the peak at 1381 cm−1 from SA 1 to SA 6 are shown in Figure 3A. Similarly, in Figure 3B, the intensity corresponding to the peak at 1381 cm−1 decreases from SR 1 to SR 4 with an increase in dilution, irrespective of the amount of AgNO3 in the reaction medium. The FTIR spectra of pure PPy are presented (Figure S4A). The crystalline behavior of PPy/Ag sheets with varying silver nitrate concentration and solvent ratio are analyzed using powder X-ray diffraction patterns. The intense diffraction peaks (Figures 3C,D) corresponding to 2θ at 38.1°, 44.2°, 64.2°, 77.2°, and 81.4° represent the (111), (200), (220), (311), and (222) planes of silver, respectively.18,37 Crystal planes derived from Bragg’s reflections of face-centered cubic silver imply the presence of crystalline silver in the PPy/Ag sheets. The broad diffraction peak at 2θ = 25° is the characteristic of amorphous polypyrrole18 (Figure S4B); however, the intensity of this peak is not prominent in PPy/ Ag sheets. The degree of crystallinity of PPy/Ag sheets is calculated using the formula38 crystallinity (%) = (total area of crystalline peaks/total area of all peaks) × 100 and tabulated (Table S3). The crystallinity of samples increases from 48.77% to 65.92% on increasing the amount of AgNO3 from 0.01 to 0.10 M, which is due to the increase in the formation of crystalline silver during the reaction. However, when the water−chloroform ratio is ranged from 40:60 to 80:20, the crystallinity decreased from 67.90% to 54.72%, which indicates a decrease in crystallinity of the Ag NPs with increase in dilution of AgNO3. The Ag NPs growth could be reduced due to the minimal supply of Ag+ ions at the water−chloroform interface. This trend is similar to the FTIR results, which reveal that the mass transfer of monomer and oxidant is vital for the formation of PPy/Ag at the bisolvent interface. The stability of dispersion of nanostructures in the solvent phase and solid phase is important to exploit the unique properties of filler in composites. The dispersion stability of PPy/Ag sheets in N-methylpyrrolidone (NMP), ethanol, and chloroform were performed as shown in Figure 3E. The asprepared PPy/Ag sheets were dispersed with mild sonication using a probe sonicator (40 MHz) for 10 min. The PPy/Ag started aggregating after 2 h in NMP, 0.6 h in ethanol, and 0.5 h in chloroform; however, mild shaking of solution restored the dispersion even after 2 weeks. The stability of dispersion is attributed to the solvent polarity.39 NMP and ethanol are

oligomers freely diffuse to the aqueous phase; however, the charges on the oligomers are controlled by the doping levels.27 The reaction parameters for the synthesis of pure PPy and the corresponding optical and scanning electron microscopic (SEM) images are presented (Table S2 and Figure S1). These results indicate that the type of oxidant is also crucial on controlling the morphology of polymeric nanostructures in polymerizations at liquid−liquid interfaces. The recent developments on the microstructure tuning of PPy nanostructures confirm the importance of oxidants on the self-assembly of mesoscale networks.33,34 The control on the morphology of conductive polymers and their hybrids is mainly achieved through various capping agents or structure guiding agents which are added as additives in the reaction medium.25 The primary focus of this study is to tune the nanostructure of PPy/Ag composites as the morphology can be varied from microglobules25 to nanotubes17 and nanowires14 using various methods. In this study, the bisolvent medium and oxidant are evidently controlling two-dimensional sheet morphology of PPy/Ag sheets, which is confirmed by field emission scanning electron microscope (FESEM) analysis (Figures 2A,B,E,F). At low silver nitrate concentrations (Figure S2A) the sheets are small in size and discontinuous. As the concentration of AgNO3 increases (Figure S2B, Figures 2A,B, and Figure S2C) the sheets become more thin, continuous, and fluffy. However, the sheetlike morphology of PPy/Ag sheets got distorted with increasing silver concentrations (Figures S2D,E), which results from the tearing of PPy sheets with a higher amount of attached crystalline silver. Subsequently, from the samples with varying solvent ratio (Figures 2E,2F and Figures S2F−H), it is clear that solvent medium and AgNO3 are controlling the sheet morphology of the samples and the PPy sheets itself act as capping agents for the uniform encapsulation of Ag nanoparticles. The formation of sheetlike PPy/Ag can be explained in terms of the surface energy of nanoparticles and surface tension at the interface. The high surface tension at the interface drags the nanoparticle associated pyrrole oligomers to the interface,31,32 where they try to agglomerate due to the increased surface energy.23 However, the PPy oligomers capsulate the Ag NPs, which decrease the surface energy of nanoparticles, thereby preventing the agglomeration.23 These collective forces and interactions at the interface favor the morphology tuning of PPy/Ag to sheetlike structures. There is a relatively uniform distribution of Ag NPs in the sheets, though the structure of Ag NPs is not consistent in all the samples. To investigate the effect of oxidant on controlling the morphology, pure PPy was synthesized in a similar methodology, and the FESEM images of pure polypyrrole are obtained as agglomerated irregular spheres as shown in Figure 2H and Figures S2I−L. The spherelike PPy structure was due to the three-dimensional diffusion and secondary growth of PPy oligomers in the aqueous phase (Figure S1).29 This result indicates that Ag NPs embedded in the PPy matrix prevents the diffusion of PPy to aqueous phase from the site of oligomer formation (bisolvent interface). The elemental analysis of samples SA 3 and SR 2 using a scanning electron microscope (JEOL Model JSM-6390LV) from another set of samples further confirms the presence of carbon, silver, nitrogen, and oxygen in the PPy/Ag sheets (Figure 2D and Figure S3). The amount of silver present in the sample SA 3 is 19.14% and SR 2 is 26.22%, as listed in Figure 2D. The sheetlike morphology and the uniform distribution of Ag NPs were further confirmed E


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Figure 4. Electrical conductivity plots (A) of PPy/Ag Sheets at varying AgNO3 amounts (A1) and at varying solvent ratio (A2). Cyclic voltammograms (B, C) of bare GCE and modified electrodes in the presence of H2O2 scanned at 25 mV/s. (D) Optical images of the inhibition zones of PPy/Ag sheets against S. aureus and E. coli bacteria and the corresponding bacterial inhibition zone diameter plots. (E) Images of bacterial colonies grown on MHA plates incorporated with varying concentrations of PPy/Ag sheets (SR 2), and the minimum bactericidal concentrations (MBCs) are 3 and 4 mg/mL for E. coli and S. aureus, respectively.

The PPy/Ag films of dimensions 1.5 cm × 0.5 cm were fabricated on a prepatterned PET surface, and the conductivity of as-prepared samples was measured using the four-probe method (Figure 4A). The resistivity was calculated using the formula ρ = 2.3541 × (V/I) × t, where V is the measured voltage, I is the source current, and t is the sample thickness.42 The conductivity values increased from 0.00014 S/cm to the highest conductivity value of 0.0039 S/cm (SA 3) with an increase in the amount of silver nitrate during polymerization. Similarly, the conductivity values increased from 0.0023 S/cm

sufficiently polar solvents having dipole moments (4.09 D,39 1.75 D40) capable of dispersing PPy/Ag sheets. The doped NO3− ions can induce polarity in PPy/Ag sheets, leading to a dipole−dipole interaction between PPy/Ag and polar solvents. A relatively less polar solvent like chloroform (1.04 D41) also disperses PPy/Ag during sonication; however, the PPy/Ag aggregates within 30 min. This result indicates that solvents with moderate dipole moment (1.75−4.09 D) might be suitable for forming effective dispersions of PPy/Ag sheets. F


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can be attributed to higher Ag wt % as observed in EDAX analysis (Figure 2D). Therefore, SR 2 samples were preferred for MBC determination, since the extent of bacterial killing depends on Ag content in the composite as reported. The MBC studies were conducted for the quantitative analysis of the antibacterial activity of PPy/Ag sheets (SR 2) and pure PPy (PPy 2) as shown in Figure 4E as well as Figures S7 and S8. The results indicate that the pure PPy (Figure S7) is not antibacterial as reported.14 Therefore, the antibacterial activity (Figure 4E and Figure S8) is purely due to the in situ reduced nanosized Ag particles in the PPy sheets. The lowest PPy/Ag sheet concentration showing 100% killing of Gram-positive/ Gram-negative bacteria is considered as MBC. In MBC studies with the same load of bacteria, the control plate without test compound showed mat growth, whereas test plates with PPy/ Ag sheets showed a proportional decrease in the bacterial growth with increasing concentration of the test compound. The MBC of PPy/Ag against E. coli and S. aureus was found to be 3 and 4 mg/mL, respectively. The detailed investigations on the mechanism of action of silver and the kinetics of bacterial inhibition are beyond the scope of this work. The promising antibacterial activity along with the electrical conductivity of PPy/Ag can be explored for applications aiming at biomedical treatments and diagnostics.

to the highest conductivity value of 0.014 S/cm (SR 2), with a change in the solvent ratio (Table S1). It is reported that the conductivity of the PPy/Ag depends on the synthesis methods27 and the oxidizing agents.13 The highest conductivity of the order of 1000 S/cm2 at a maximum loading of 77.1 wt % Ag was reported for PPy globular structures, which is due to the high wt % of Ag loading to PPy structures. The electrical conductivity of the PPy/Ag films can be attributed to the aspect ratio of PPy/Ag sheets and the total Ag loading in the sheets. The PPy nanotubes of relatively higher aspect ratio attached with a small value of Ag loading may exhibit a high value of conductivity. The literature comparison on the conductivities for the present work is provided (Figure S5). For example, PPy/Ag nanotubes with 21.1 wt % of Ag loading is reported with a conductivity of 35.7 S/cm,15 and 10 wt % of Ag loading is reported with a conductivity of 30 S/ cm.16 Gemini-like PPy/Ag nanostructures reported conductivity in the order of 10−4 S/cm13 at Ag loading of 16.5 wt %. The conductivity of PPy/Ag sheets are in the order of 10−3−10−2 S/cm, which is due to the low Ag loading (SR 2: 26.22 wt %, 0.014 S/cm and SA 3: 19.14 wt %, 0.0039 S/cm) as observed in EDAX analysis (Figure 2D). The pure PPy synthesized using APS as the oxidizing agent in the water−chloroform medium is found to be insulating. This further confirms the contribution of silver toward the conductivity of PPy/Ag sheets, even at a lower loading. The electrochemical activity of PPy/Ag sheets and pure PPy (PPy 2) was investigated using cyclic voltammetric (CV) techniques in 0.1 M phosphate buffer solution (K2HPO4 and KH2PO4) with a pH of 7 in the presence of 1 mM H2O2 (Figures 4B,C). The CV was scanned over a potential range of −0.6 to +0.6 V at a scan rate of 25 mV/s. The results showed good electrical response of the SR 2 and SA 3 electrodes toward H2O2 reduction as compared to PPy 2. The drastic increase of reduction peak current to −2250 μA (SR 2) and −1636 μA (SA 3) at an applied potential of −0.25 and −0.21 V as compared to PPy 2 (−3 μA at −0.09 V) indicates the strong sensitivity of the electrodes toward the direct reduction of H2O2. The redox behavior of PPy/Ag sheets is attributed to its sheetlike morphology (high surface area sheets), the presence of Ag atoms, and overoxidized PPy backbones. The phenomena of direct reduction of H2O2 by the modified electrodes (SR 2/GCE and SA 3/GCE) can be due to the synergistic effect of Ag and polypyrrole.43 The Ag NPs react with H2O2 in the solution according to the following equations:43

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CONCLUSIONS

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ASSOCIATED CONTENT

The sheetlike morphology of PPy/Ag sheets was achieved with the choice of two immiscible solvents water and chloroform. The effects of solvents and nature of oxidant on controlling the morphology of PPy/Ag hybrids were systematically investigated. The morphology of the hybrid sheets and the uniform distribution of Ag NPs to PPy matrix were analyzed using scanning electron and transmission electron microscopy. The PPy/Ag sheets showed conductivity in the range of 10−3−10−2 S/cm due to the uniformly attached Ag NPs in the PPy sheets. Furthermore, the PPy/Ag was explored to investigate the electrochemical activity toward hydrogen peroxide and exhibited excellent reduction peak current (−2250 μA at −0.25 V and −1636 μA at −0.21 V) on H2O2 reduction. The PPy/Ag sheets exhibited promising antibacterial activity against E. coli and S. aureus as the zone diameter increased after 24 h incubation. The MBC of PPy/Ag against E. coli and S. aureus was found to be 3 and 4 mg/mL, respectively. Therefore, the polymerization at the interface of two immiscible solvents presented in this study could be a promising tool to design multifunctional nanostructures for diverse applications.

2PPy/Ag(0) + H 2O2 → 2PPy/Ag(I) − OH

2PPy/Ag(I) − OH + 2H+ + 2e− → 2PPy/Ag(0) + 2H 2O

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Thus, the PPy/Ag sheets can be a potential candidate for electrochemical sensor electrode toward hydrogen peroxide. Furthermore, the nanosized Ag particles exhibit the same antibacterial activity as silver ions.14 The as-synthesized PPy (PPy 2) and PPy/Ag sheets (SA 3 and SR 2) are subjected to antibacterial studies against E. coli (Gram-negative bacteria) and S. aureus (Gram-positive bacteria). The results are presented in Figure 4D, Figure S6, and Table S4. PPy/Ag sheets (SA 3 and SR 2) showed better activity against both bacteria as compared to pure PPy, which is evident from the bacterial zone diameter values. The initial zone diameter of 4 mm was increased to 7.66 ± 0.58 mm after 24 h of incubation. The high antibacterial activity of SR 2 in comparison to SA 3

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b18943. Experimental parameters for the synthesis of PPy/Ag sheets and pure PPy; schematic illustration of pure PPy synthesis; SEM images of PPy/Ag and pure PPy of all reaction conditions; FTIR and XRD spectra of pure PPy; degree of crystallinity from XRD analysis; literature comparison on the conductivity of PPy/Ag nanostructures; antibacterial activity of PPy/Ag and its components (PDF) G


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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected] (M.M.M.). ORCID

Nishanth Karimbintherikkal Gopalan: 0000-0001-7293-5946 Mini Mol Menamparambath: 0000-0001-6845-0815 Funding

M.M.M. greatly acknowledge the funding from INSPIRE faculty award (DST/INSPIRE/04/2015/002050) by the Department of Science and Technology (DST), India. Notes

The authors declare no competing financial interest.

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REFERENCES

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