In Situ Electrochemical Polymerization at Air–Water Interface: Surface

Article pubs.acs.org/JPCC

In Situ Electrochemical Polymerization at Air−Water Interface: Surface-Pressure-Induced, Graphene-Oxide-Assisted Preferential Orientation of Polyaniline Ganganahalli K. Ramesha, A. Vijaya Kumara, and Srinivasan Sampath* Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560 012, India S Supporting Information *

ABSTRACT: In situ electrochemical polymerization of aniline in a Langmuir trough under applied surface pressure assists in the preferential orientation of polyaniline (PANI) in planar polaronic structure. Exfoliated graphene oxide (EGO) spread on water surface is used to bring anilinium cations present in the subphase to air−water interface through electrostatic interactions. Subsequent electrochemical polymerization of aniline under applied surface pressure in the Schaefer mode results in EGO/PANI composite with PANI in planar polaronic form. The orientation of PANI is confirmed by electrochemical and Raman spectroscopic studies. This technique opens up possibilities of 2-D polymerization at the air−water interface. Electrochemical sensing of hydrogen peroxide is used to differentiate the activity of planar and coiled forms of PANI toward electrocatalytic reactions.

■

INTRODUCTION Graphene oxide (GO), the oxidized form of graphene, is a unique material possessing interesting properties like tunable charge-density based on surface functional groups, amenability for the formation of single-layer colloidal suspensions and chemical functionalization, ionic conductivity, amphiphilic nature, and so on.1,2 The functional groups present on GO such as carboxyl, epoxy, and hydroxyl groups that are randomly formed on the hexagonal carbon network3 have given rise to various possibilities of formation of composites of GO and polymers.4,5 Among the conducting polymers known in the literature, polyaniline (PANI) is one of the most studied polymers due to various advantages, such as ease of synthesis, low cost of the monomer, environmental stability, tunable conductivity, and so on.6 Different oxidation states of PANI include fully reduced leucoemaraldine (yellow, LE), half-oxidized emaraldine (blue, ES), and fully oxidized pernigraniline (purple, PE) forms.7 Recently, charge-transfer complex based on GO and PANI, which lies in the intermediate state between ES and LE, has been reported.8 Among the three redox states, the emaraldine form is the most conducting one. The nature of the species (polaron lattice or bipolaron) responsible for conduction of PANI has been the subject of several investigations.9 It is observed that the polaronic form is the most conducting state depending on chain linearity.10 Hence, preferential orientation of PANI (polaronic or bipolaronic) has attracted considerable attention, particularly in recent years.9−14 Effects of various parameters on the orientation of PANI such as varying the anions in the electrolyte,10 influence of phenazine structure,11 and effect of composite formation15 have been studied. Graphene/PANI composite is reported to predominantly © 2012 American Chemical Society

orient the polymer in the bipolaronic form when polymerized chemically in a liquid electrolyte medium.14 Most of the reported procedures for the preparation of PANI-based composites involve chemical polymerization consisting of both components in the medium.16 The present study explores the in situ interfacial electrochemical polymerization of aniline under surface pressure in a LB trough wherein the monomer is stabilized at air−water interface. Polymerization of monomers at air−water interface through chemical17−21 or enzymatic22 polymerization has been previously reported. In these studies, the oxidizing agent/ enzyme responsible for oxidation of the monomer is present in the subphase. Other related studies in Langmuir trough include that of Bard and coworkers, wherein a change in lateral connectivity of silver particles at the air−water interface has been reported.23 Insulator−metal transition of polymer monolayer has been observed by Unwin and coworkers at the air−water interface.24 However, to the best of our knowledge, there is no report where electrochemical polymerization has been carried out at air−water interface in a Langmuir−Schaefer mode, which explores the effect of surface pressure on the properties of resulting polymer. This may lead to a 2-D polymerization process. Present study uses the amphiphilic nature of exfoliated graphene oxide (EGO) to bring anilinium ion to air−water interface; subsequently, it is electropolymerized under surface pressure in Langmuir−Schafer configuration. This process Received: March 23, 2012 Revised: June 8, 2012 Published: June 11, 2012 13997

dx.doi.org/10.1021/jp302782w | J. Phys. Chem. C 2012, 116, 13997−14004

The Journal of Physical Chemistry C

Article

Figure 1. Schematics showing experimental setup for (a) external polymerization and (b) in situ polymerization. The working electrode used for experiment (a) is modified with EGO Langmuir monolayer.

water mixture again and was centrifuged at a rate of 2500 rpm for 10 min. The supernatant containing monodisperse EGO sheets was used for all Langmuir−Blodgett (LB) experiments. Scanning Tunneling Microscopy. Scanning tunneling microscopy (STM) measurements were carried out using Agilent 5500 atomic force microscope using electrochemically etched tungsten wire tip. Langmuir−Blodgett Studies. Pressure−area (π−A) isotherms were determined using LB trough (model 611, Nima, U.K.). The EGO colloid (7 mL) obtained after centrifugation was spread at air−water interface using micropipet. The surface pressure at the interface was measured using the Wilhelmy plate technique. The spreading solvent was allowed to evaporate for ∼1 h before the 2-D compression of the monolayer was carried out. All π−A isotherms were carried out at 25 °C. The subphase contained anilinium hydrochloride in water. Once EGO/anilinium complex was stable at the interface, potentiodynamic electrochemical polymerization was carried out in situ under applied surface pressure (schematics shown in Figure 1) in the Schafer mode using GCE. This is referred to as in situ polymerized sample. External electrochemical polymerization was carried out on EGO Langmuir film-modified GCE in a regular three-electrode cell containing 5 mM anilinium hydrochloride in 0.1 M HCl (schematics shown in Figure 1). This is referred to as ex situ polymerized sample. The EGO/ PANI composite was characterized by electrochemistry, UV− visible, and Raman spectroscopy. Brewster Angle Microscopy. Brewster angle microscopic (BAM) images of organized monolayer of EGO at the air− water interface were obtained using Brewster angle microscope (EP3BAM, Nanofilm Technologie GMH, Germany) equipped with Nd:YAG laser source. The p-polarized laser beam was made to incident on the surface at a Brewster angle of 53°, and the reflected image from water surface was detected using a CCD camera. The 10× objective provided limited lateral resolution of ∼2 μm. This was used in conjunction with Nima LB trough to study monolayers in situ. Images at different surface pressures were obtained by compressing the barrier to different pressures at a rate 50 cm2/min. The BAM pictures were taken after allowing 10 min for stabilization of the monolayer at the desired pressure. Raman Spectroscopy. Raman spectra were obtained using Horiba Jobin Yvon LabRAM Raman spectrometer with 50× objective using different excitation wavelengths of 514.5, 632, and 785 nm. Laser power at the sample was measured to be

induces preferential polaronic orientation of PANI on the electrode surface.

■

EXPERIMENTAL SECTION Reagents and Materials. Natural graphite obtained from Arthur and Branwell, U.K. was used for the preparation of exfoliated graphite (EG). Aniline hydrochloride was purchased from Aldrich, USA. Analytical-grade potassium permanganate, hydrogen peroxide, sulfuric acid, nitric acid, hydrochloric acid, and methanol were used in all experiments. All solutions were prepared using water with resistivity of 18 MΩcm (output from Millipore water purification system). Glassy carbon electrode (GCE) of rectangular shape with dimensions, 1 × 1 cm2 obtained from Structure Probe, USA was used. Preparation of Exfoliated Graphene Oxide. EGO was prepared by oxidizing EG using modified Hummers method.25 In a typical preparation procedure, 10 g of graphite particles (300−400 μm size) were intercalated with bisulphate anions by immersing in 100 mL of 3:1 mixture of conc. H2SO4:HNO3 mixture for 24 h at room temperature. The intercalated material was washed very well with distilled water and dried. Exfoliation was then carried out by introducing the dried intercalated graphite into a preheated furnace at 800 °C for a few minutes. This results in fluffy, fiber-like, porous material. This material is referred to as EG. EG was subjected to ultrasonication for 1 h in acetone and dried well. Chemical oxidation was then carried out by treating 200 mg of EG in 46 mL of conc. H2SO4, followed by the addition of 6 g of KMnO4 slowly at temperatures below 5 °C. After the addition of KMnO4, the mixture was heated to 37 °C for ∼30 min. Subsequently, 92 mL of distilled water was slowly added (caution: there is chance of bumping of solution mixture) and the temperature was increased to 97 °C and maintained for a further 15 min. Subsequently, 280 mL of distilled water was added, followed by cooling the mixture. This was followed by the addition of 20 mL of 30% H2O2. Bright-yellow precipitate was formed, confirming the preparation of EGO. The solid was then separated and washed well with 5% HCl until it was free of sulfate ions. The EGO was then characterized using spectroscopic and microscopic techniques as previously reported.25−27 EGO colloid (0.5 mg/mL) was prepared by dispersing EGO in 5:1 volume ratio of methanol/water mixture, as previously reported.28 The as-prepared colloid was centrifuged at a rate of 8000 rpm for 20 min. The supernatant was removed, and the precipitate was redispersed in methanol/ 13998

dx.doi.org/10.1021/jp302782w | J. Phys. Chem. C 2012, 116, 13997−14004

The Journal of Physical Chemistry C

Article

Figure 2. STM images of (a) HOPG and (b) EGO.

Figure 3. (i) Compression−expansion isocycles for EGO monolayer. (ii) Pressure−area isotherms for (a) 5 mM anilinium chloride in 0.1 M HCl in the subphase (no material present at the interface), (b) EGO at the interface (water as the subphase), and (c) EGO at the interface and 5 mM anilinium chloride in 0.1 M HCl as the subphase.

of the Supporting Information). Figure 3i shows the isocycles corresponding to EGO monolayer at air−water interface. As observed, after a few cycles, the isotherms are stable over a long period of time similar to the previously reported data.28 The EGO flakes at the air−water interface are far from each other at low surface pressures, whereas they form a uniform, wellpacked monolayer at high surface pressures, resulting in solid phase of the pressure−area isotherm (Figure S1 of Supporting Information). Uniform distribution and stability of EGO at air− water interface is further evidenced by depositing EGO layer on solid surfaces and following its characteristics using absorption spectroscopy. Figure S2 of the Supporting Information shows the UV−visible spectra of EGO layers deposited on quartz substrate using Langmuir trough (numbers indicate number of EGO layer deposited) along with plot of absorbance versus number of EGO monolayers. Absorption spectrum of EGO shows a maximum at 232 nm with a shoulder at 300 nm corresponding to n to π* and π to π* transitions, respectively.29 As can be seen from Figure S2 of the Supporting Information, absorbance increases with the increase in number of layers deposited linearly, indicating the same amount of EGO deposited on substrate during each deposition. When aqueous subphase is replaced with aqueous 0.1 M HCl containing 5 mM anilinium chloride, EGO binds electrostatically with anilinium ions and stabilizes at the air−water interface, resulting in high surface pressure as compared with that observed in pure aqueous subphase (Figure 3ii). The corresponding BAM images are shown in Figure S1 of the Supporting Information. This interaction is further confirmed by varying the

0.06, 0.63, and 2.08 mW for 514.5, 632, and 785 nm excitation wavelengths, respectively. Electrochemical Studies. All electrochemical experiments were carried out using CHI660C potentiostat, where GCE of 1 cm2 area was used as working electrode, saturated calomel electrode (SCE) as reference, and Pt wire as counter electrode. Electrochemical synthesis of PANI was carried out by potentiodynamic polymerization in the potential range of 0 to 0.85 V versus SCE at a scan rate of 50 mV/s. The iR drop (ohmic drop at the interface was found to be 3Ω)-corrected differential pulse voltammetry (DPV) for EGO/PANI composite electrode was carried out with increment of 4 mV, amplitude of 25 mV, sampling width of 0.0167 s, and pulse period of 0.2 s.

■

RESULTS AND DISCUSSION EGO possesses oxygen-containing ionizable functional groups, and hence it is hydrophilic in nature. However, a close examination of the EGO surface reveals that it can behave as an amphiphilic material, wherein both hydrophobic (unoxidized region) and hydrophilic (oxidized) regions coexist.2 Figure 2 shows the STM image of EGO, wherein unoxidized regions (sp2 domains, similar to highly oriented pyrolytic graphite, HOPG) and oxidized regions (sp3 domains, hydrophilic oxygen containing functional groups) are present on the surface. This is responsible for the stabilization of EGO at the air−water interface without any additional surfactant. The stability of EGO monolayer at the interface is confirmed by observing constant pressure−area isotherms and BAM images (Figure S1 13999

dx.doi.org/10.1021/jp302782w | J. Phys. Chem. C 2012, 116, 13997−14004

The Journal of Physical Chemistry C

Article

Figure 4. Cyclic voltammograms obtained during potentiodynamic polymerization of EGO/anilinium complex (a) in situ with applied surface pressure and (b) external polymerization. Concentration of anilinium chloride used in the subphase is 5 mM and that of EGO colloid is 0.1 mg/mL to spread on the water surface. Area of the trough used is 250 cm2. Area of glassy carbon electrode used for in situ polymerization is 1 cm2. Concentration of anilinium chloride used for ex situ polymerization is 5 mM. GCE, SCE, and Pt foil electrodes are used as working, reference, and counter electrodes, respectively.

Figure 5. iR-corrected (compensated for solution resistance) differential pulse voltammograms of EGO/PANI composite modified GCE prepared by (a) ex-situ polymerization and (b) in situ polymerization under applied surface pressure. External cell with 0.1 M HCl as the electrolyte is used for carrying out the voltammetric studies.

below. It is well known that EGO contains unoxidized regions (graphene domains) along with oxidized regions, as evidenced by transmission electron microscopy.32 Transfer of electrons to anilinium monomer can occur through these graphene domains. Electron transfer through hopping mechanism to the monomer can also result in polymerization. Similar reasons have been proposed to explain the electrochemical activity of polymer (Nafion)-modified electrodes.33,34 Figure 5 shows the differential pulse voltammograms of the polymerized EGO/PANI composites under two different conditions mentioned above. The ex situ polymerized sample shows three clear peaks at 0.093/0.057 V (A/A′), 0.425/0.423 V (B/B′), and 0.537/0.528 V (C/C′), which correspond to leucoemaraldine−emaraldine, quinone−hydroquinone, and emaraldine−pernigraniline redox states, respectively. This is similar to the reported redox activity of PANI synthesized by chemical, as well as electrochemical methods.31 However, the EGO/PANI composite prepared under applied surface pressure in situ shows only two peaks (with slight variation in peak potentials), and the peak at C/C′ vanishes completely. This implies that the oxidation of emaraldine to pernigraniline does not take place under the conditions employed. This is possibly due to increased stabilization of the polaronic structure that makes the oxidation of emaraldine to pernigraniline difficult.13 To probe the possibility of shift in the redox potential to more positive values, the scan range is extended to 1 V. However, no redox peak related to emaraldine to pernigraniline conversion is observed. The current increase in this region is found to be due

concentration of anilinium in the subphase with the same concentration of EGO at the interface (Figure S3 of the Supporting Information). With increase in anilinium concentration in the subphase from 0 to 5 mM, the lift-off area (area at which gas−liquid phase transition occurs) and final surface pressure increase from 140 to 240 cm2 and 30 to 52 mN m−1 respectively. This change typically corresponds to an increase in the density of the material at the air−water interface,30 or, in other words, increased interaction of anilinium with EGO resulting in large surface concentration of anilinium ions at the interface. EGO/anilinium monolayer at air−water interface is confirmed by performing expansion−compression isocycles, where surface pressure shows a small decrease initially and stabilizes after about five cycles and stays constant thereafter. Only anilinium in the subphase without any EGO at the interface shows surface pressures to be very low (Figure 3). Electropolymerization of EGO/anilinium monolayer is performed in situ on the GCE electrode kept touching the interface as in Schaefer mode, under applied surface pressure (Figure 1b). The counter and reference electrodes are in the subphase, and the contact leads are taken in such a way that they do not disturb the monolayer. The cyclic voltammograms obtained during in situ and ex situ polymerization are given in Figure 4, and the currents increase as a function of number of cycles and stay constant after awhile. The observations for the ex situ polymerization are similar to the reported literature on aniline oxidation.31 Electron-transfer mechanism from GCE to aniline through EGO as mediator may be explained as given 14000

dx.doi.org/10.1021/jp302782w | J. Phys. Chem. C 2012, 116, 13997−14004

The Journal of Physical Chemistry C

Article

Figure 6. Raman spectra of (I) ex-situ polymerized EGO/PANI and (II) in situ polymerized EGO/PANI under surface pressure (a) 200−1250 and (b) 200−950 cm−1. The regions are separately given to avoid interferences from high intense D and G bands of EGO.

to degradation of PANI, as previously reported35 (Figure S4 of Supporting Information). Composites based on CNT/PANI have been reported to show similar behavior.13 Raman spectroscopy is an excellent tool that will yield molecular level information on the orientation of the polymer. In-order to verify the possibility of laser induced damage of the sample during Raman measurements, spectra have been recorded continuously at the same spot for about 120 min (five continuous scans of 20 min each over 200−2000 cm−1 range). The spectra remain the same indicating that the sample does not degrade under the conditions employed. This is true for all the laser wavelengths studied. The spectra are represented in different regions since the intensities of G and D bands of EGO at 1600 and 1350 cm−1 mask the spectral lines of PANI that are relatively less intense. Confirmation of PANI in EGO/PANI composite through electrochemical polymerization can be made by comparing Raman spectra of EGO and EGO/PANI (figure S5 of Supporting Information). Observation of clear peaks corresponding to PANI in the region 200− 1300 cm−1 evidence PANI formation which is not observed in the spectrum of EGO. The data observed using 514, 632, and 785 nm excitation wavelengths are similar though the spectral intensities are higher for 632 nm laser than that of 514 and 785 nm laser wavelengths. This is expected based on the resonance enhancement in the case of 632 nm laser wavelength as has been reported earlier.13 Figure S6 of Supporting Information shows the Raman spectra of EGO/PANI prepared under two different conditions in the frequency range 1000−2000 cm−1. EGO shows bands at 1355 and 1610 cm−1 corresponding to D and G band of GO respectively.36 The band positions and the relative intensities remain constant before and after polymerization implying that the structure of EGO does not undergo

any change during polymerization and hence EGO remains as such in EGO/PANI composites. The emaraldine form of PANI is reported to show bands at 1620 (ν C−C of the benzene rings), 1586 (ν CC of the quinoid rings), 1516 (ν CNH+ of the quinoid protonated diimine units), 1485 (ν CN of the quinoid nonprotonated diimine units), 1319 and 1340 (ν C−N+, characteristic bands of polaron radical cation), 1252 (ν C−N benzene diamine units), 1192 and 1166 cm−1 (C−H bending of the benzenoid and quinoid rings).13,37,38 The region between 200 and 1250 cm−1 contains many conformational dependent Raman modes. The band observed at 1166 cm−1 corresponding to C−H bending of the benzenoid ring represents bipolaronic lattice, and the one at 1192 cm−1 corresponding to C−H bending of the benzenoid ring represents the polaronic lattice. In the present studies, it is very clear from Figures 6 I(a) and II(a) that a band at 1192 cm−1 corresponding to the polaronic form is observed when the EGO/PANI composite is prepared in situ under applied surface pressure, whereas a band at 1166 cm−1 corresponding to bipolaronic structure of PANI is observed for ex situ polymerized sample. The polaronic lattice is observed only when PANI orients more in planar structure than coiled form.12 The planar orientation of PANI in the in situ prepared composite is confirmed by close observations of the region, 200−950 cm−1, as given in Figures 6 I(b) and II(b). The bands at 420 and 527 cm−1 are attributed to the C−C out-of-plane deformation modes and are very sensitive to conformational changes of PANI.13 These bands shift to lower wave numbers if PANI chain has small torsion angles, as expected based on theoretical and experimental studies.37,38 The in situ prepared EGO/PANI shows bands at 410 and 515 cm−1, suggesting that PANI chain possesses small torsion angle, whereas the ex situ prepared EGO/PANI composite shows a band at 425 cm−1, 14001

dx.doi.org/10.1021/jp302782w | J. Phys. Chem. C 2012, 116, 13997−14004

The Journal of Physical Chemistry C

Article

instance where surface pressure plays a role in tuning the properties of PANI, although it is not directly related to the present experiments on electrochemical polymerization. Possible interactions of EGO and anilinium may be explained as follows. Without EGO at the interface, anilinium ions are randomly distributed in the subphase. With EGO present at the interface, anilinium interacts electrostatically to form EGO/ anilinium complex forming a monolayer. Under applied surface pressure, once polymerization is initiated, the PANI probably moves from oxidized regions to unoxidized hydrophobic regions of EGO. This step is possibly driven by applied surface pressure as well as π−π interactions between delocalized electrons of PANI and delocalized electrons of unoxidized region of EGO. This is purely speculative and is based on similar π−π interactions proposed between CNT and PANI to explain preferred orientation of the polymer on CNTs.13 The latter step, which involves transfer of PANI to hydrophobic regions, may not occur to a great extent in externally polymerized sample, thus resulting in coiled bipolaronic structure. Because only a monolayer of EGO is present at the interface, the possibility of sandwich structure involving PANI in between two EGO sheets may not be possible. Hence, surface pressure as well as presence of EGO is required to orient the PANI in linear form. One may argue that the polar functional groups on EGO may favor orientation of cationic bipolaronic structure of PANI preferentially, but, as previously explained, the interplay of amphiphilic nature of EGO2 seems to be responsible for the present observations. It is interesting to note that the previous reports on PANI-based composites prepared using 1-D carbon nanotubes reveal planar structure,13 whereas composites prepared using 2-D reduced GO results in coiled (bipolaronic) form14 when polymerization is carried out through chemical method. Hydrogen Peroxide Sensing. The as-prepared linear and coiled forms of PANI may have different activities toward electrochemical reactions. Hydrogen peroxide sensing is used to amplify this aspect. There have been several studies reported on hydrogen peroxide sensing39,40 and the importance of H2O2 sensing in various areas such as food, pharmaceutical, clinical, industrial, and environmental is well-known and documented. The analysis of H2O2 has been reported using spectrometry,41,42 chemiluminescence,43 and electrochemistry44 using various materials.45,46 Electrochemical determination involves either reduction or oxidation of H2O2. A number of electrode materials such as nanoparticles,47 nanotubes,48 conducting polymers,49 and various carbon-based materials50−53 have been reported for this purpose. Among the polymers, PANI has received significant attention due to its low cost, ease of synthesis, stability, and interesting redox activity.54 Figure 7 shows the cyclic voltammograms of different electrode materials toward electrochemical reduction of H2O2 in aqueous buffer solutions. It is clear that the linear form of PANI yields higher currents than those observed with others. Steady-state chronoamperometric measurements have been carried out at an applied potential of −0.6 V for different concentration ranges (Figures 8 and 9), and the calibration curve obtained for linear PANI is given in Figure 9. The observations clearly reveal that the linear form is more catalytically active than that of coiled form of PANI. It should be pointed out that under the negative DC bias potential applied, it is likely that the oxidized form may be reduced to some extent, but still the activity of the linear PANI is clearly well above the coiled form. The detection limit observed for

revealing that PANI structure is in coiled configuration. The bands observed at 812, 835, and 878 cm−1 correspond to outof-plane C−H motions, which are very sensitive to torsion angle.38 The modes are clearly observed for EGO/PANI composite obtained by in situ polymerization, whereas they are very broad and ill-defined for the ex situ polymerized samples suggesting that PANI attains coiled structure when polymerized ex situ. Salvatierra and coworkers13 have reported the formation of polaronic form of PANI when CNT/PANI composite is prepared by chemical polymerization. Dmitrieva and coworkers have reported the effect of phenazine structure in the stabilization of polaron pair11 based on structure similarity. However, in a recent paper, Domingues and coworkers obtained PANI, which shows bipolaronic structure in the case of graphene/PANI composite obtained by chemical polymerization at liquid−liquid interface.14 The random orientation of PANI on graphene is proposed to be responsible for this observation. The present studies of carrying out polymerization under surface pressure in Langmuir trough helps in orienting PANI as the polymerization proceeds. The orientation observed in the present studies may be due to π−π interactions between PANI and unoxidized regions of EGO. The delocalized electrons of PANI can interact with the delocalized electrons of unoxidized region of EGO. This interaction is highly random in the case of external polymerization. Several control experiments have been carried out to confirm the requirement of surface pressure in Langmuir trough to obtain preferential orientation of PANI. The following conditions do not result in the orientation of PANI in the polaronic form: (1) ex situ polymerization, as previously explained using EGO monolayer-modified GCE as the working electrode for polymerization of anilinium present in bulk of the electrolyte in an external electrochemical cell without any surface pressure, (2) ex situ polymerization on bare GCE using anilinium chloride present in the bulk of the electrolyte (without EGO) in an external electrochemical cell, (3) ex situ polymerization using monolayer of EGO/anilinium complex formed on GCE in HCl supporting electrolyte (without any anilinium and EGO present in the bulk), and (4) monolayer on GCE obtained with EGO at the interface and PANI in the subphase of HCl. The Raman spectra of PANI composites based on various control experiments are given in Figure S7 of the Supporting Information. It should be pointed out that surface pressure is the main requirement once anilinium is brought to the interface (using EGO). When another surfactant, namely, dodecylbenzenesulfonic acid, is used at the interface and anilinium is present in the subphase, the in situ polymerization does not result in the preferential orientation of PANI in polaronic form revealing the requirement of EGO for the preferential orientation. The unoxidized regions of the EGO possibly maximize π−π interactions, leading to planar, polaronic structure, whereas oxidized regions interact with anilinium ions present in the subphase. Similar arguments have been invoked to explain the orientation of PANI when carbon nanotubes are used as support for chemical polymerization of aniline.13 Hence, it is clear that EGO brings the anilinium to the interface and the surface pressure along with EGO induces orientation of PANI. In this context, it should be mentioned that Unwin and coworkers have used a surfactant to bring PANI present in the subphase to the air−water interface.24 Subsequent surface pressure in the Langmuir trough is reported to induce insulator-to-metal transition of PANI. This is another 14002

dx.doi.org/10.1021/jp302782w | J. Phys. Chem. C 2012, 116, 13997−14004

The Journal of Physical Chemistry C

Article

(Kmapp) constant is determined using Lineweaver−Burke plot based on the following equation, where Icat is the electrocatalytic current, Imax is the maximum current under saturated substrate conditions, and CH2O2 is the bulk concentration of H2O2. K mapp 1 1 = + Icat Imax IcatC H2O2

The Kmapp value is found to be 0.2025 and 1.071 mM for polaronic and bipolaronic form of PANI/EGO composite, respectively. The observed Kmapp value for the polaronic form of PANI/EGO composite electrode is close to the values observed on electrodes modified with myoglobin/titania nanotubes (140 μM),48 hemoglobin/graphene−chitosan (230 μM),55 and hemoglobin/CNT (675 μM).56

Figure 7. Cyclic voltammograms for GCE, EGO-modified GCE, PANI-modified GCE, and PANI/EGO composite in polaronic and bipolaronic forms in 30 mM H2O2 in 0.1 phosphate buffer of pH 6.9 at a scan rate of 50 mV/s.

■

CONCLUSIONS In summary, the present studies have initiated the possibility of 2-D, in situ electrochemical polymerization in a Langmuir trough. Preferential orientation of PANI in planar polaronic form has been observed and confirmed using Raman spectroscopy. Furthermore, the linear form has been shown to be more active toward peroxide sensing than that of the coiled form. The difference in structure may have consequences in other areas such as electrochemical capacitors, and one may be able to differentiate the contributions of the polaronic and bipolaronic structures.

■

ASSOCIATED CONTENT

S Supporting Information *

Brewster angle microscopy images at air−water interface in different regions, UV−visible spectra of EGO monolayers, pressure−area isotherm for different concentration of anilinium subphase, cyclic voltammogram for electrochemical polymerization, Raman spectra, and chronoamperometric measurements. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 8. Steady-state current versus time response of (a) linear, (b) coiled forms of PANI/EGO, (c) EGO/GCE, and (d) GCE at −0.6 V versus SCE upon successive addition of 10 μM (10 μL of 10 mM H2O2 to 10 mL of buffer) peroxide. Electrolyte used is 0.1 M phosphate buffer solution (pH 6.9).

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +91 80 23600085. Tel: +91 80 22933315. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS We wish to acknowledge DST, New Delhi for financial assistance. REFERENCES

(1) Dreyer, D. R.; Park, S.; Bielawski, C. W.; Ruoff, R. S. Chem. Soc. Rev. 2010, 39, 228−240. (2) Cote, L. J.; Kim, J.; Tung, V. C.; Luo, J.; Kim, F.; Huang., J. Pure Appl. Chem. 2011, 83, 95−110. (3) Lerf, A.; He, H.; Forster, M.; Klinowski, J. J. Phys. Chem. B 1998, 102, 4477−4482. (4) Kuilla, T.; Bhadra, S.; Yao, D.; Kim, N. H.; Bose, S.; Lee, J. H. Prog. Polym. Sci. 2010, 35, 1350−1375. (5) Kim, H.; Abdala, A. A.; Macosko, C. W. Macromolecules 2010, 43, 6515−6530. (6) de Albuquerque, J. E.; Mattoso, L. H. C.; Faria, R. M.; Masters, J. G.; MacDiarmid, A. G. Synth. Met. 2004, 146, 1−10.

Figure 9. Plot of catalytic current versus H2O2 concentration for polaronic form of PANI/EGO composite-modified GCE. Inset: Steady-state current versus time for successive additions of 50 nM H2O2 in 0.1 M PBS, pH 6.9, −0.6 V versus SCE.

PANI/EGO composite where PANI is in polaronic form is 10 nM, whereas with the coiled structure it is 10 mM (Figure S8 of the Supporting Information). Apparent Michaleis-Menten 14003

dx.doi.org/10.1021/jp302782w | J. Phys. Chem. C 2012, 116, 13997−14004

The Journal of Physical Chemistry C

Article

(7) Li, D.; Huang, J.; Kaner, R. B. Acc. Chem. Res. 2009, 42, 135−145. (8) Valles, C.; Jimenez, P.; Munoz, E.; Benito, A. M.; Maser, W. K. J. Phys. Chem. C 2011, 115, 10468−10474. (9) Bernard, M. C.; Goff, A. H. L. Electrochim. Acta 2006, 52, 595− 603. (10) Dmitrieva, E.; Dunsch, L. J. Phys. Chem. B 2011, 115, 6401− 6411. (11) Dmitrieva, E.; Harima, Y.; Dunsch, L. J. Phys. Chem. B 2009, 113, 16131−16141. (12) Tagowska, M.; Pałys, B.; Jackowska, K. Synth. Met. 2004, 142, 223−229. (13) Salvatierra, R. V.; Oliveira, M. M.; Zarbin, A. J. G. Chem. Mater. 2010, 22, 5222−5234. (14) Domingues, S. H.; Salvatierra, R. V.; Oliveira, M. M.; Zarbin, A. J. G. Chem. Commun. 2011, 47, 2592−2594. (15) Yan, X.; Chen, J.; Yang, J.; Xue, Q.; Miele, P. ACS Appl. Mater. Interfaces 2010, 2, 2521−2529. (16) Bai, H.; Xu, Y.; Zhao, L.; Li, C.; Shi, G. Chem. Commun. 2009, 1667−1669. (17) Zhang, J.; Mandler, D.; Unwin, P. R. Chem. Commun. 2004, 450−451. (18) Kimkes, P.; Sohling, U.; Oostergetel, G. T.; Schouten, A. J. Langmuir 1996, 12, 3945−3951. (19) Kloeppner, L. J.; Duran, R. S. Langmuir 1998, 14, 6734−6742. (20) Kloeppner, L. J.; Batten, J. H.; Duran, R. S. Macromolecules 2000, 33, 8006−8011. (21) Fichet, O.; Tran-Van, F.; Teyssie, D.; Chevrot, C. Thin Solid Films 2002, 411, 280−288. (22) Bruno, F. F.; Akkara, J. A.; Samuelson, L. A.; Kaplan, D. L.; Mandal, B. K.; Marx, K. A.; Kumar, J. K.; Tripathy, S. K. Langmuir 1995, 11, 889−892. (23) Quinn, B. M.; Prieto, I.; Haram, S. K.; Bard, A. J. J. Phys. Chem. B 2001, 105, 7474−7476. (24) Zhang, J.; Barker, A. L.; Mandler, D.; Unwin, P. R. J. Am. Chem. Soc. 2003, 125, 9312−9313. (25) Ramesh, P.; Bhagyalakshmi, S.; Sampath, S. J. Colloid Interface Sci. 2004, 274, 95−102. (26) Ramesha, G. K.; Sampath., S. J. Phys. Chem. C 2009, 113, 7985− 7989. (27) Ramesha, G. K.; Vijaya Kumara, A.; Muralidhara, H. B.; Sampath, S. J. Colloid Interface Sci. 2011, 361, 270−277. (28) Cote, L. J.; Kim, F.; Huang, J. J. Am. Chem. Soc. 2008, 131, 1043−1049. (29) Paredes, J. I.; Villar-Rodil, S.; Martnez-Alonso, A.; Tascon, J. M. D. Langmuir 2008, 24, 10560−10564. (30) Cote, L. J.; Kim, J.; Zhang, Z.; Sun, C.; Huang, J. Soft Matter 2010, 6, 6096−6101. (31) Eftekhari, A.; Yazdani, B. J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 2204−2213. (32) Erickson, K.; Erni, R.; Lee, Z.; Alem, N.; Gannett, W.; Zettl, A. Adv. Mater. 2010, 22, 4467−4472. (33) White, H. S.; Leddy, J.; Bard, A. J. J. Am. Chem. Soc. 1982, 104, 4811−4817. (34) Martin, C. R.; Rubinstein, I.; Bard, A. J. J. Am. Chem. Soc. 1982, 104, 4817−4824. (35) Vivier, V.; Cachet-Vivier, C.; Regis, A.; Sagon, G.; Nedelec, J.-Y.; Yu, L. T. J. Solid State Electrochem. 2002, 6, 522−527. (36) Kudin, K. N.; Ozbas, B.; Schniepp, H. C.; Prudhomme, R. K.; Aksay, I. A.; Car, R. Nano Lett. 2008, 8, 36−41. (37) Colomban, P.; Folch, S.; Gruger, A. Macromolecules 1999, 32, 3080−3092. (38) Cochet, M.; Louarn, G.; Quillard, G. S.; Buisson, J. P.; Lefrant, S. J. Raman Spectrosc. 2000, 31, 1041−1049. (39) Veal, E. A.; Day, A. M.; Morgan, B. A. Mol. Cell 2007, 26, 1−14. (40) Fan, L.; Zhang, Q.; Wang, K.; Li, F.; Niu, L. J. Mater. Chem. 2012, 22, 6165−6170. (41) Chang, Q.; Zhu, L. H.; Jiang, G. D.; Tang, H. Q. Anal. Bioanal. Chem. 2009, 395, 2377−2385.

(42) Dickinson, B. C.; Huynh, C.; Chang, C. J. J. Am. Chem. Soc. 2010, 132, 5906−5915. (43) Chen, W. W.; Li, B. X.; Xu, C. L.; Wang, L. Biosens. Bioelectron. 2009, 24, 2534−2540. (44) Delvauxa, M.; Walcarius, A.; Demoustier-Champagne, S. Anal. Chim. Acta 2004, 525, 221−230. (45) Peng, Y.; Jiang, D. L.; Su, L.; Zhang, L.; Yan, M.; Du, J. J.; Lu, Y. F.; Liu, Y. N.; Zhou, F. M. Anal. Chem. 2009, 81, 9985−9992. (46) Razmi, H.; Mohammad-Rezaei, R.; Heidari, H. Electroanalysis 2009, 21, 2355−2362. (47) Saha, K.; Agasti, S. S.; Kim, C.; Li, X.; Rotello, V. M. Chem. Rev. 2012, 112, 2739−2779. (48) Liu, A.; Wei, M.; Honma, I.; Zhou, H. Anal. Chem. 2005, 77, 8068−8074. (49) Narang, J.; Chauhan, N.; Pundir, C. S. Analyst 2011, 136, 4460− 4466. (50) Qian, L.; Yang, X. Talanta 2006, 68, 721−727. (51) Zhou, M.; Shang, L.; Li, B.; Huang, L.; Dong, S. Biosens. Bioelectron. 2008, 24, 442−447. (52) Li, Z.; Cui, X.; Zheng, J.; Wang, Q.; Lin, Y. Anal. Chim. Acta 2007, 597, 238−244. (53) Guo, S.; Wen, D.; Dong, S.; Wang, E. Talanta 2009, 77, 1510− 1517. (54) Dhand, C.; Das, M.; Datta, M.; Malhotra, B. D. Biosens. Bioelectron. 2011, 26, 2811−2821. (55) Xu, H.; Dai, H.; Chen, G. Talanta 2010, 81, 334−338. (56) Zhao, Y.-D.; Bi, Y.-H.; Zhang, W.-D.; Luo, Q.-M. Talanta 2005, 65, 489−494.

14004

dx.doi.org/10.1021/jp302782w | J. Phys. Chem. C 2012, 116, 13997−14004