Novel Graphene-Gold Nanoparticle Modified Electrodes for the High

Oct 18, 2011 - National Institute for Research and Development of Isotopic and Molecular Technologies, Donath Street, No. 65-103,. RO-400293 Cluj-Napo...
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Novel Graphene-Gold Nanoparticle Modified Electrodes for the High Sensitivity Electrochemical Spectroscopy Detection and Analysis of Carbamazepine Stela Pruneanu,*,† Florina Pogacean,† Alexandru R. Biris,† Stefania Ardelean,† Valentin Canpean,‡ Gabriela Blanita,† Enkeleda Dervishi,§ and Alexandru S. Biris*,§ †

National Institute for Research and Development of Isotopic and Molecular Technologies, Donath Street, No. 65-103, RO-400293 Cluj-Napoca, Romania ‡ Nanobiophoton Center, Interdisciplinary Research Institute in Bio-Nano-Science, Babes-Bolyai University, Cluj Napoca 400084, Romania § Nanotechnology Center, Systems Engineering Department, University Arkansas at Little Rock, 2801 S. University Avenue, Little Rock, Arkansas 72204, United States ABSTRACT: A novel graphene-gold nanoparticle composite deposited on gold electrode (Au-Gr-AuNPs) was employed to detect carbamazepine (CBZ), an antiepileptic drug, used here as a model system. The same approach can also be used to detect other additional organic compounds. The presence of gold nanoparticles (size between 10 and 20 nm) encased in graphene sheets was evidenced by TEM and HRTEM, while the AFM analysis was used to study the morphology of the graphene-gold nanoparticle films used for the electrochemical studies. Various electrochemical methods were employed to study carbamazepine oxidation, such as cyclic voltammetry, linear sweep voltammetry, and electrochemical impedance spectroscopy. The results clearly showed that the modified electrode exhibited excellent electrocatalytic effect toward oxidation of carbamazepine. The peak current intensity significantly increased (up to 2 times) while the peak potential shifted to lower oxidation potential (∼100 mV), compared with the unmodified electrode. A detection limit (DL) of 3.03  106 M was obtained with the graphene-gold nanoparticle-modified electrode (S/N = 3). In addition, an equivalent electrical circuit was developed and used to interpret the experimental EIS data. The circuit contains the solution resistance (Rs), the charge-transfer resistance (Rct), the Warburg impedance (ZWt, transmissive boundary), and the double-layer capacitance (Cdl).

1. INTRODUCTION Carbamazepine (Scheme 1) is a tricyclic compound used as an anticonvulsant drug for the treatment of epilepsy and bipolar disorder, as well as trigeminal neuralgia. It can also be administered to patients who have other illnesses including schizophrenia, neuromyotonia, attention-deficit hyperactivity disorder (ADHD), and post-traumatic stress disorder. As a result of its widespread use, carbamazepine is currently considered one of the emerging pollutants in ground and surface water; therefore, its accurate determination by fast and reliable methods is highly desirable. Recent studies have found that carbamazepine is persistent, and its removal efficiency by wastewater treatment plants (WWTPs) is below 10%.1 The employment of electrochemical techniques for the detection of carbamazepine is a valuable alternative to the more laborious methods generally used, like liquid chromatographic methods.2,3 Kalanur and Seetharamappa4 have recently reported the study of carbamazepine oxidation at glassy carbon electrode, in PBS solution (pH 7.4). Two oxidation peaks were noticed at r 2011 American Chemical Society

Scheme 1. Carbamazepine Chemical Structure

potentials of 1.183 and 1.401 V/Ag(AgCl), respectively, but no peak was observed in the reverse scan. Hence, the oxidation process was an irreversible one. The proposed oxidation mechanism occurs by formation of two radical cations which give rise to a dimer. The dimers subsequently undergo electrochemical Received: July 20, 2011 Revised: October 16, 2011 Published: October 18, 2011 23387

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The Journal of Physical Chemistry C oxidation to form diradical dimers. Veiga et al.5 have used a multiwalled carbon nanotube film-coated GCE for voltammetric determination of carbamazepine in PBS solution (pH 6.89). They reported a single oxidation peak at around +1.09 V/ Ag(AgCl) with no additional postpeak in the forward scan or reduction peak in the reverse scan. The authors suggest a two-step oxidation mechanism (electrochemicalchemical) which occurs simultaneously and therefore cannot be distinguished in the cyclic voltammogram. Moses et al.6 have studied the electrochemical reduction of carbamazepine in aqueous solutions using mercury electrodes. They reported a bielectronic reduction mechanism, which implies the saturation of the double bond present in the heterocycle of this molecule. Given the need to detect small amounts of chemical molecules, significant research has been devoted to the development of new high-sensitivity electrochemical sensors. Among these research efforts, a new approach was to develop materials with novel electrochemical properties that could further increase the electrochemical sensitivity of some of the commonly used methods for the detection of small concentrations of various organic and biological molecules. Some of the most promising of such methods included the use of carbonaceous nanomaterials as components of the electrochemical sensor. Lately, graphene with its unique properties (such as 2D morphology, graphitic structure, excellent electrical/thermal conductivity, and ability to be functionalized with various chemical groups) has been identified as one of the most suitable materials for various sensing technologies.712 Kang et al.13 have used a graphene-based electrochemical sensor which shows excellent electrocatalytic activity toward the reduction and oxidation of paracetamol. The method was applied to detect paracetamol in pharmaceutical preparation tablets (detection limit 3.2  108 M) without any interference from ascorbic acid or dopamine. Li et al.14 have reported the preparation of a nanocomposite film based on nafion and graphene which was subsequently used as enhanced sensing platform for ultrasensitive determination of cadmium. Wang et al.15 have used a graphene-chitosan modified electrode for selective determination of dopamine (DA) and compared its performance with that of a multiwalled carbon nanotube-modified electrode. The excellent performance of graphene to dopamine detection was attributed to ππ interaction between the phenyl structure of DA molecule and the two-dimensional hexagonal carbon structure of graphene, which makes the electron transfer feasible. In addition, the attachment of catalytic nanoparticles, such as gold, platinum, or silver, could further increase the electrocatalytic activity of these graphitic sheets.1621 Still, there is a significant challenge in controlling the uniformity of metal nanoparticle decoration of the graphene surface and especially the strength of the attachment between the organic and the inorganic nanostructure. We have developed a new method to grow in a single step few-layer graphene structures decorated with Au nanoparticles (graphene-AuNPs) by using a chemical vapor deposition process. This composite nanomaterial has been deposited onto the surface of gold electrodes and subsequently used in an electrochemical setup. Here, we present, for the very first time to our best knowledge, the oxidation of carbamazepine (a model organic molecule), taking advantage of the electrocatalytic properties of this novel graphene-AuNP modified gold electrode surface (further noted as Au-GR-AuNPs). Such nanostructured surfaces used as sensing electrode in an electrochemical setup represents an alternative to conventional analytical methods, being suitable for fast, sensitive,

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and inexpensive determinations. The morphological characteristics of graphene-AuNP composite and Au-GR-AuNP surface were investigated by high resolution transmission electron microscopy (HRTEM) and atomic force microscopy (AFM). In addition, a combination of analytical methods including cyclic voltammetry (CV), linear sweep voltammetry (LSV), and electrochemical impedance spectroscopy (EIS) was used to completely characterize the electrocatalytic properties of Au-GR-AuNPs nanostructured surface. This approach could be the foundation for platform technologies that could use organicinorganic nanostructural systems to enhance the detection sensitivity of various organic/biological compounds in the environment. This approach is the first, to our best knowledge, that presents the novel electrochemical properties of composite organicinorganic nanomaterial composed of Au nanoclusters embedded in graphitic sheets to enhance the electrochemical sensitivity of a detection reaction for carbamazepine. Furthermore, our findings have the potential to provide the ability to study the low intensity interactions between such organic molecules and biological molecules in the environment, medicine, pharmaceutical industry, or energy storage.

2. EXPERIMENTAL DETAILS Reagents and Solutions. All reagents used for the experiments were of analytical grade or better. Acetonitrile was purchased from Riede-deHa€en and dimethylformamide (DMF) from Sigma-Aldrich, Germany. Pure carbamazepine powder was purchased from Zhejiang Jiuzhou Pharmaceutical CO., Ltd., China. Tetra-butyl ammonium perchlorate (TBAP) was purchased from Fluka, Germany. A stock solution of 102 M carbamazepine was prepared in acetonitrile and 0.05 M TBAP and subsequently used for the preparation of lower concentration solutions (103106 M). Additionally, about 0.03 mg of graphene-AuNP composite was dispersed in DMF (1 mL) and kept at room temperature. Preparation of Graphene-Gold Nanoparticles Composite (Graphene-AuNPs). We prepared few-layer graphene-AuNPs composite by a simple chemical vapor deposition technique by using Au/MgO catalyst (1% wt Au) prepared by a deposition precipitation method with urea. For the synthesis of the grapheneAuNPs structures, we used the radio frequency (1.2 MHz, 5 kW) chemical vapor deposition (RF-CCVD) method with a watercooled reactor and methane as the carbon source (1000 °C, CH4 flow rate 80 mL/min, Ar 300 mL/min, synthesis time 30 min).2224 Preparation of Gold Electrode Modified with GrapheneAuNPs (Au-GR-AuNPs). Prior to modification, the gold electrode was polished with alumina suspension until a mirror-like surface was obtained. Next, the electrode was ultrasonically cleaned in ethanol and deionized water for about 3 min several times. After that, a 10 μL drop of colloidal suspension of grapheneAuNPs was deposited onto the gold substrate and dried at room temperature for about 10 h. In order to reduce the oxygen functionalities (hydroxyl, carbonyl, carboxyl, or epoxy) which are generally present on graphene surface, the electrode was polarized at a potential of 1.3 V versus Ag/AgCl for 600 s, in a solution of 0.1 M PBS (pH 7).17 Analytical Characterization of Graphene-AuNPs and AuGR-AuNPs Electrode. Morphological characterization of graphene-AuNPs composite and Au-GR-AuNPs electrode was performed by transmission electron microscopy (JEOL JEM-2100F) and by atomic force microscopy (Veeco Dimension 3100 AFM and Alpha 300A instrument (Witec)). For TEM analysis, the samples 23388

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Figure 1. Schematic representation of the synthesis of graphene decorated with Au nanoparticles by RF-CCVD over an Au/MgO catalyst (a); transmission electron analysis (TEM) (80 kV) of the resulting structures (b); schematic of the process used to deposit graphene-AuNPs composite over the top surface of a gold electrode to be further used in an electrochemical setup for the detection of carbamazepine (c); representative AFM image (tapping mode) of graphene-AuNPs composite deposited on gold substrate (d); Raman spectroscopy analysis of the graphene-AuNPs nanocomposite collected at 514 nm excitation wavelength (e).

were dispersed in 2-propanol and gently sonicated for 30 min. Several drops of the suspensions were placed on holey carboncoated grids and further used for the analysis. Cyclic voltammetry (CV), linear sweep voltammetry (LCV), and electrochemical impedance spectroscopy (EIS) measurements were performed using an Autolab Potentiostat (Nova 1.6 Software, EcoChemie, Utrecht, Netherlands) connected with a three-electrode cell. The surface area of the gold electrode was 0.031 cm2 (working electrode). A platinum electrode with a large surface area (∼2 cm2) was employed as the counter electrode while the reference was an Ag/AgCl electrode. CVs and LCVs were recorded between 0.6 and 1.65 V/SCE at a scan rate of 25 mV s1 (unless otherwise specified in the text). All impedance spectra were measured over a frequency range 0.1105 Hz by using a small sinusoidal excitation signal (10 mV amplitude). The applied potential was +1.4 V versus Ag/AgCl. All experiments were carried out in quiescent solutions of acetonitrile and

0.05 M TBAP. Data fitting was performed using Nova 1.6 software (EcoChemie-Netherlands). Raman spectra were collected with a JASCO NRS 3300 spectrophotometer in a backscattering geometry and equipped with a CCD detector (69 °C). The excitation laser beam Ar-ion with a wavelength of 514 nm and a power at the sample surface of 1.5 mW had an area of around 1 μm2 and was focused by using an Olympus microscope coupled to a 100 objective.

3. RESULTS AND DISCUSSION 3.1. Synthesis of Graphene-Au Nanoparticles Composites. The schematic of the process presented by this paper is shown in Figure 1a. The Au/MgO catalyst was found to synthesize graphene-AuNPs structures composed of 26 sheets and diameters of 600 ( 100 nm. An interesting observation was the fact that, during the growth process, the Au nanoparticles 23389

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initially supported on the MgO were lifted off by the graphene sheets during the growth process and became encased in their crystalline structure (Figure 1b). The size of these Au nanoparticles was found to be relatively uniform with diameters between 10 and 20 nm. The inset of Figure 1b shows the higher magnification of such a nanoparticle encased in the graphitic structure of the graphene sheets. The graphene-AuNPs composite was further solubilized and deposited onto the top surface of a gold electrode used for electrochemical studies (Figure 1c). Atomic force microscopy was additionally used to directly study the grapheneAuNPs composite assembled onto the top surface of the gold substrate. Figure 1d provides a representative AFM image (tapping mode) of the graphene-AuNP nanostructures. The AFM analysis of the structure clearly shows the vertical stacking of the graphitic layers. Raman analysis of the graphene-AuNPs composites indi-

cates the presence of all the graphene-specific bands (D band 1342 cm1, G band 1589.7 cm1, and the overtone 2D band 2678 cm1).25 Further analysis of the graphene-AuNPs composite deposited onto gold surface reveals a clear tendency of these nanostructures to form large agglomerates. Previous studies have confirmed that water molecules intercalated between the platelets form hydrogen bonds to the epoxy or hydroxyl functionalities, a key factor in maintaining the stacked structure of the graphene-like structures.26,27 After their deposition on the gold electrode, the clusters of graphene-AuNPs composite form agglomerations with an average height of up to 1 μm without the single-sheet morphology, however. In spite of this thickness value, our electrochemical measurements prove that the composite’s electrocatalytic properties are still preserved. 3.2. Electrochemical Characterization. Since carbamazepine has a very low solubility in water (17.7 mg L1 at 25 °C)28 in our studies we have chosen acetonitrile as solvent. Figure 2 shows successive cyclic voltammograms (3 cycles) recorded in the supporting electrolyte (acetonitrile +0.05 M TBAP), as well as in electrolyte containing 102 M carbamazepine (scan rate 25 mV s1). A two-wave oxidation peak can be seen at around +1.49 V/Ag (AgCl) accompanied by a small reduction peak at +1.16 V/ Ag (AgCl). The large separation between the oxidation and reduction peaks (≈330 mV) suggests that carbamazepine molecules undergo a quasireversible redox process. At a slow scan rate (between 5 and 50 mV s1), the redox process is diffusion-controlled as shown by Ipeak versus υ1/2 plot. (See inset of Figure 2.) This was further confirmed by the plot of log Ipeak versus log υ, which was linear within the same scan rate range and gave a slope of 0.6 (data not shown). This value is very close to the theoretical value of 0.5 reported by Laviron for a diffusion-controlled electrode process.29 The successive cyclic voltammograms show that the electrochemical signal of carbamazepine is almost unmodified, suggesting that the electrode surface is not blocked by the adsorption of

Figure 2. Successive cyclic voltammograms recorded with Au-GRAuNPs electrode in supporting electrolyte (acetonitrile +0.05 M TBAP, black line), as well as in electrolyte solution containing 10 2 M carbamazepine (three cycles, scan rate 25 mVs1, blue line). Inset shows variation of peak current intensity versus υ1/2 (diffusioncontrolled process).

Scheme 2. Proposed Mechanism for Oxidation of Carbamazepine

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Figure 4. LCVs recorded with Au (blue line) and Au-GR-AuNPs electrode (red line), respectively, in electrolyte containing various concentrations of carbamazepine (102 and 103 M), with scan rate 25 mV s1.

Figure 3. LCVs recorded with Au-GR-AuNPs electrode in electrolyte containing various concentrations of carbamazepine (105 to102 M); scan rate 25 mVs1 (a); variation of peak current intensity (Ipeak) with carbamazepine concentrations within 105 to 102 M range (b).

the oxidation products. However, in order to have reproducible results in our analytical determinations, the data obtained from the first scan (either CV or LCV) were always used. Atkins et al. have recently studied the electrochemical reduction of carbamazepine in acetonitrile (AC) and dimethylformamide (DMF), using GCE and microelectrodes.30 The reduction process took place at very high potential (2.27 V/SCE) and was accompanied by a well-defined oxidation peak at 2.2 V/SCE. A limit of detection of 3.89  106 mol L1 in AC, respectively, 3.21  106 mol L1 in DMF, was reported. Wang et al.31 have used two methods, differential pulse voltammetry (DPV) and fluorescence polarization immunoassay (FPIA), for accurately determining the carbamazepine level. They reported a detection limit of 1 μg mL1 (4.2  106 M) for DPV and 0.5 μg mL1 (2.1  106 M) for the FPIA technique. In our case, the electrocatalytic properties of the Au-GRAuNPs electrode have allowed the detection of the oxidation/ reduction peaks of carbamazepine at considerably lower potentials (+1.49 and, respectively, +1.16 V vs Ag(AgCl)). According to previous studies of Kalanur and Veiga,4,5 the oxidation proceeds at the nitrogen atom in the central ring of the molecule, which gives rise to radical cations; the cations subsequently form dimers that undergo further oxidation to dimer diradical (ECE reaction). In order to identify the oxidation products of carbamazepine, we have performed GC/MS experiments. Our results suggest a similar pathway for the electrochemical oxidation of carbamazepine, with the formation of dimers. The mechanism of carbamazepine oxidation is best described by an ECE type (see Scheme 2). The first step of the process is the electro-oxidation of carbamazepine

with the formation of radical cation, followed by formation of dimeric species by a very fast-coupling chemical reaction. The formation of dimeric species was confirmed by our mass spectral data (identified at m/z 470). The extended conjugation of carbamazepine allows more coupling modes.32 It cannot be established whether proton release from the cation radical occurs before or after dimerization. The unprotonated dimeric compounds are further oxidized by an overall two-electron process at potentials more positive than those required for oxidation of carbamazepine, and then, the resulting species are reduced in the reverse scan. Schematically, the reaction may proceed as shown in Scheme 2. The two-wave shape of the oxidation peak supports the ECE mechanism that carbamazepine molecules undergo during oxidation. This was observed by CV and LCV only at high concentrations (102 M); at lower concentrations, the two peaks overlap, generating a broad oxidation wave (see Figures 2 and 3a). LCV measurements show the increase of the peak current with carbamazepine concentration (Figure 3a). At low concentration (106 M), the recording overlapped with the background. A clear increase in the peak current was obtained at higher concentrations, which allowed the plotting of a calibration curve within 5  106 to 102 M range (see Figure 3b). A detection limit (DL) of 3.03  106 M was obtained in this case (S/N = 3). Our results differ from those obtained by Veiga et al.,5 who have used a multiwalled carbon nanotube film-coated GCE for voltammetric determination of carbamazepine in PBS solution (pH 6.89). In their case, the calibration plot was found to be linear in a very narrow range, from 1.3  107 to 1.6  106 M, but instead the detection limit was lower (4  108 M; S/N = 3). Although multiwalled carbon nanotubes and graphene are known to have excellent conducting properties, their electrocatalytic effect may depend on a variety of parameters, such as the morphology of the graphitic structure, the thickness of the nanostructured layer, or its surface absorption properties. In order to prove the electrocatalytic activity of the modified gold electrode, we also recorded LCVs using a bare gold surface (Figure.4, scan rate = 25 mV s1). A significant decrease in current (up to 2 times) was obtained with the bare electrode for all concentrations, along with a shift in the peak potential (≈100 mV to higher anodic potentials). For the purpose of clarity, only two concentrations are shown here. 23391

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Scheme 3. Gold Nanoparticles May Act as Small Antennas, Enhancing the Tunneling of Electrons towards the Nanostructured Surface

The electrocatalytic effect of a graphene-AuNP modified electrode may be attributed to ππ stacking interaction between carbamazepine and the sp2 hybridized carbon in graphene, which makes the electron transfer feasible. In addition, gold nanoparticles may act as nanoantennas, enhancing the tunneling of electrons toward the nanostructured surface (Scheme 3). Such findings clearly demonstrate the advantages of using a grapheneAuNP layer attached to a gold substrate. A further characterization of the nanostructured electrode was performed by measuring the electrochemical impedance spectra at a potential of +1.49 V/Ag (AgCl). (See the equivalent circuit and Nyquist plots represented in Figure 5 a,b.) At the lowest concentration (106 M), the spectrum overlapped with that obtained for the supporting electrolyte (background); therefore, only one curve was represented in this plot. The spectra are characterized by a single semicircle (high-medium frequency range) followed by a straight line at an angle of 45°, in the low frequency range. Such a line corresponds to the Warburg diffusion region and, in our case, appears only for concentrations higher than 104 M. The equivalent electrical circuit (Figure 5a) employed to fit the EIS experimental data contains the solution resistance (Rs), the chargetransfer resistance (Rct), the Warburg impedance (ZWt, transmissive boundary), and the double-layer capacitance (Cdl). The Warburg impedance is generally negligible at high frequency but becomes dominant at low frequency, due to the diffusion of redox species. When the diffusion layer has a semiinfinite thickness,33 the Warburg impedance (ZW) is expressed as follows: ZW ¼ σðωÞ1=2  jσðωÞ1=2

ð1Þ

where ω is the radial frequency and σ is the mass transfer coefficient equal to the sum of the contributions of the forms oxidized and reduced. In most cases, the diffusion layer has a finite thickness. If the interface is not permeable for electrons, then the 45° straight line will turn to a vertical line (in Nyquist plot) because the impedance is solely determined by the capacity with a constant resistance (so-called reflecting interface). If the interface is permeable for electron transfer, then the 45° straight line will bend to a semicircle (so-called transmissive interface).34 In this case, the Warburg impedance (ZWt) is given by the following: rffiffiffiffiffiffiffi! σ jω l ð1  jÞ ZWt ¼ pffiffiffiffi tanh ð2Þ D ω

Figure 5. Equivalent electrical circuit employed to fit the experimental EIS spectra (a); Nyquist diagrams obtained with Au-GR-AuNPs electrode in electrolyte containing various concentrations of carbamazepine (105102 M) at an applied potential +1.49 V/Ag/AgCl; the continue lines represent the fit based on the equivalent circuit (b); variation of Rct with carbamazepine concentration (c).

where D is the diffusion coefficient of ions and l is the thickness of the diffusion layer. The total impedance of the equivalent electrical circuit represented in Figure 5a is expressed by eq 3, which was used to fit the experimental results: Ztot ¼ Rs þ

ðRct þ ZWt Þ½1  jCdl ωðRct þ ZWt Þ 1 þ ½Cdl ωðRct þ ZWt Þ2

ð3Þ

The Nyquist plot (Figure 5b) shows that, with increasing carbamazepine concentrations, the large semicircle due to the coupling between Rct and Cdl gradually decreases. This can be attributed to a higher number of carbamazepine molecules that are oxidized at the electrode surface; consequently, the doublelayer capacitance increases, and the imaginary part of the impedance (Z00 ) decreases. Rct relates to surface modifications that enhance/hinder the transfer of electrons at the electrode/ solution interface. In our case, one can see that Rct has a linear variation with carbamazepine within a 105103 M concentration range (decreases from 110 to 5 kΩ); above 103 M, it exhibits a saturation tendency (∼890 Ω, Figure 5c). This saturation may be due to the accumulation of carbamazepine molecules 23392

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purely capacitive behavior (at low concentrations). This gradually decreases to 18° due to the coupling between Cdl and Rct (at the highest concentration).

Figure 6. Bode diagram obtained with Au-GR-AuNPs electrode in electrolyte containing various concentrations of carbamazepine (105102 M): absolute impedance versus frequency; the continue lines represent the fit based on the equivalent circuit (a); phase versus frequency (b); applied potential +1.49 V/Ag (AgCl).

within the graphene platelets, which in time leads to a poor electrical transfer between the graphene-AuNPs layer and gold substrate. Another representation of the EIS data is the Bode diagram which allows us to examine the absolute impedance, |Z|, and the phase angle as a function of frequency (Figure 6 a,b). In Figure 6a, one can see that all of the curves overlap in the medium-high frequency range (102105 Hz) and markedly vary in the low frequency range (0.110 Hz). This is an expected behavior, since the transfer of electrons takes place at high frequencies and this process is influenced by the state of the electrode surface and not by the carbamazepine concentration. In contrast, at low frequencies where the diffusion is the dominant process, the absolute impedance heavily depends upon the number of molecules that diffuse toward the electrode; therefore, its value significantly decreases with the increase of carbamazepine concentration (from 100 to 2 kΩ). (See also the inset of Figure 6a.) In addition, the phase angle also shows a marked variation with carbamazepine concentration (Figure 6b). It should be emphasized that, at high frequencies (103105 Hz), the electrode has a purely resistive behavior (the phase angle is close to zero) which indicates that the surface is highly conductive; therefore, the electrons can be easily exchanged with carbamazepine molecules that are in close proximity to the surface. At low frequencies (0.110 Hz), the charging of the double-layer becomes the predominant process; consequently, the phase angle is significantly larger and reaches a maximum at around 70°, which reflects an almost

4. CONCLUSIONS We have shown here the ability to synthesize, in a single-step RF-CCVD process, a novel nanocomposite based on graphene sheets decorated with gold nanoparticles (size between 10 and 20 nm). During the catalytic growth of graphene, the gold nanoparticles were lifted off the MgO support and encased in the graphitic structure. These nanostructures were used to coat the top surface of a gold electrode for the electrochemical detection of carbamazepine. The modified electrode exhibited an excellent electrocatalytic effect for oxidation of carbamazepine, reflected by a significant increase of the peak current (up to 2 times) and a shifting of the peak potential toward lower oxidation potential (∼100 mV), compared with the unmodified electrode. The detection limit for carbamazepine was found to be 3.03  106 M (S/N = 3). Additionally, an equivalent electrical circuit was developed to interpret and fit the experimental EIS data based on the solution resistance (Rs), the charge-transfer resistance (Rct), the Warburg impedance (ZWt, transmissive boundary), and the double-layer capacitance (Cdl). The charge-transfer resistance was found to linearly vary with carbamazepine within 105103 M concentration range (decreases from 110 to 5 kΩ). At higher concentrations, a saturation tendency was observed probably due to accumulation of carbamazepine molecules within the graphene platelets. The development of nanomorphologically controlled materials could result in highly sensitive processes for the highly sensitive electrochemical detection of various biochemical agents, down to single molecule level with important implications for a number of scientific areas that include nanotechnology, medicine, personal care, environment sensing, catalysis, or energy storage and generation.3538 ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (S.P.), [email protected] (A.S.B.).

’ ACKNOWLEDGMENT This work was supported by CNCSUEFISCDI, Project PNII-ID-PCE-2011-3-0125. The financial support of the Arkansas Science and Technology Authority, U.S. Department of Energy (DE-FG36-06GO86072), and U.S. National Science Foundation (NSF/EPS-1003970) is highly appreciated (A.S.B.). The editorial assistance of Dr. Marinelle Ringer is also acknowledged. The help of Dr. Fumiya Watanabe is highly appreciated for the TEM analysis. ’ REFERENCES (1) Jones, A.; Zabaczynski, S.; Gobel, A.; Hoffmann, B.; Loffer, D.; McArdell, C. S.; Ternes, T. S.; Thomsen, A.; Siegrist, H. Water Res. 2006, 40, 1686. (2) Mashayekhi, H. A.; Abroomand-Azar, P.; Saber-Tehrani, M.; Husain, S. W. Chromatographia 2010, 71, 517–521. (3) Subramanian, M.; Birnbaum, A. K.; Remmel, R. P. Ther. Drug. Monit. 2008, 30, 347–356. (4) Kalanur, S. S.; Seetharamappa, J. Anal. Lett. 2010, 43, 618–630. 23393

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