Electrochemical Conversion of Unreactive Pyrene to Highly Redox

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Letter pubs.acs.org/Langmuir

Electrochemical Conversion of Unreactive Pyrene to Highly RedoxActive 1,2-Quinone Derivatives on a Carbon Nanotube-Modified Gold Electrode Surface and Its Selective Hydrogen Peroxide Sensing Palani Barathi and Annamalai Senthil Kumar* Environmental and Analytical Chemistry Division, School of Advanced Sciences, Vellore Institute of Technology University, Vellore-632 014, India S Supporting Information *

ABSTRACT: Pyrene (PYR) is a rigid, carcinogenic, unreactive, and nonelectrooxidizable compound. A multiwalled carbon nanotube (MWCNT)-modified gold electrode surface-bound electrochemical oxidation of PYR to a highly redoxactive surface-confined quinone derivative (PYRO) at an applied potential of 1 V versus Ag/AgCl in pH 7 phosphate buffer solution has been demonstrated in this work. Among various carbon nanomaterials examined, the pristine MWCNTmodified gold electrode showed effective electrochemical oxidation of the PYR. The MWCNT’s graphite impurity promotes the electrochemical oxidation reaction. Physicochemical and electrochemical characterizations of MWCNT@PYRO by Raman spectroscopy, FT-IR, X-ray photoelectron spectroscopy, and GC-MS reveal the presence of PYRO as pyrene−tetrone within the modified electrode. The quinone position of PYRO was identified as ortho-directing by an elegantly designed ortho-isomer-selective complexation reaction with copper ion as an MWCNT@PYRO-Cu2+/1+-modified electrode. Finally, a cytochrome c enzyme-modified Au/MWCNT@PYRO (i.e., Au/ MWCNT@PYRO-Cyt c) was also developed and further demonstrated for the selective biosensing of hydrogen peroxide. terpyridyl- and porphyrin-linked15 PYRs, and ferrocene-derived PYR16 were glued to CNTs by simple chemical combination methods and extended to various applications. Interestingly, in this work we observed the electrooxidation of MWCNT-glued PYR. At the end of this Letter, we also demonstrate copper ion and Cytochrome c protein (Cyt c)-immobilized MWCNT@ PYRO systems (i.e., Au/MWCNT@PYRO-Cu2+/1+ and Au/ MWCNT@PYRO-Cyt c as selective chemical and biochemical sensors for H2O2 in pH 7 phosphate buffer solution (PBS)). The Au/MWCNT@PYRO-Cu2+/1+ study was helpful in assigning the exact position of the PYRO’s oxygen atoms.

1. INTRODUCTION Pyrene (PYR), one of the carcinogenic polyaromatic hydrocarbons (PAHs), has been considered to be a priority pollutant by the United States Environmental Protection Agency (U.S. EPA).1,2 It is highly unreactive and rigid in structure. Several stringent biological and chemical oxidation methods were reported for the decarcinogenation of PYR (e.g., bacteria and fungi extracellular peroxidase,3,4 the Fenton reagent (Fe2+ + H2O2),5 photocatalysis,6 electroenzymatic reaction,7 ozone reaction,8 and permanganate treatment9). The violent chemical oxidation reactions often resulted in the formation of various products such as CO2,5 monohydroxypyrene,8 and 1,2-, 1,6-, or 1,8-pyrenedione.6,7 Meanwhile, the electrooxidation of various pyrene derivative self-assembled monolayers (SAMs) on gold surfaces, which have been prepared via 11-mercaptoundecanoic acid and adipoyl chloride linkers to 1,6- or 1,8-pyrenedione derivatives in a strong-acid medium, was also reported.10−13 Note that it is highly intricate to oxidize the rigid π-bonded PAH structure without any enzymes or a derivatization approach. In this work, we report an elegant and direct electrochemical oxidation of PYR on a pristine multiwalled carbon nanotube (MWCNT)-modified gold electrode surface to highly redox-active quinones (PYRO) (designated as Au/ MWCNT@PYRO) as a surface confined species at an applied potential (Eapp) of 1 V versus Ag/AgCl at neutral pH. Owing to the strong hydrophobic and π-electron characteristics, PYR has often been used as a glue material for the CNT.14−18 For instance, catechol-functionalized PYR,14 cobalt © 2013 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Reagents and Materials. Pyrene, MWCNTs (>90% carbon basis, outer diameter 10−15 nm, inner diameter 2−6 nm, length 0.1− 10 μm), and single-walled carbon nanotubes (SWCNTs, 50−70% carbon basis, outer diameter 1−1.5 nm) were purchased from SigmaAldrich. Other chemicals used were all ACS-certified reagent grade and used without further purification. Screen-printed gold electrodes (SPEAu) were purchased from Zensor R&D, Taiwan. Aqueous solutions were prepared using deionized and alkaline potassium permanganate distilled water (designated as DD water). The pH 7 phosphate buffer solution (PBS) supporting electrolyte with ionic strength I = 0.1 mol L−1 was used throughout this work. (Caution! Because PYR is highly carcinogenic, proper care must be taken during handling.) Received: June 1, 2013 Revised: July 21, 2013 Published: August 9, 2013 10617

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Scheme 1. Schematic Representation of the Electrochemical Conversion of Au/MWCNT@PYR to Au/MWCNT@PYRO (A-C) and the Electrocatalytic Reduction of H2O2 on Au/MWCNT@PYRO-Cu2+/1+ (D) and Au/MWCNT@PYRO-Cyt c (E, F), along with a Control Experiment Relating to the Electrochemical Conversion of Au/PYR to Au/PYRO (G)

Figure 1. Twenty continuous CV responses of (A) PYR modified (physically adsorbed) on Au (a, inset) and Au/MWCNT (b) in pH 7 PBS and (B) the PYR-exposed Au/MWCNT medium transferred to a blank consisting of pH 7 PBS (curve a) and a control consisting of Au/MWCNT conditioned at +1 V vs Ag/AgCl for 120 s (without PYR) (Au/MWCNT*, where * indicates conditioning at 1 V, curve b). Plots of (C) the anodic peak current (A1, ipa) vs Eapp for the Au/MWCNT@PYRO system and (D) the surface excess (ΓPYRO) vs various Au/carbon@PYRO-modified electrodes. Scan rate = 50 mV·s−1. The inset figure is a cartoon for the electrochemical conversion of PYR → PYRO. 2.2. Instrumentation. Voltammetric measurements were all carried out with a CHI model 660C electrochemical workstation (USA). The three-electrode system consists of Au and its chemically modified electrodes (CMEs) as the working electrode (0.0414 cm2), Ag/AgCl as a reference electrode, and platinum wire as the auxiliary electrode. FTIR analysis was carried out with a Jasco 4100 spectrophotometer using the KBr method. A Jeol GCMATE II (Japan) instrument was used for GC-MS characterization. XPS

analyses were performed using an Omicron ESCA spectrometer (Germany) with a monochromatic Al Kα X-ray source. The C 1s binding-energy (BE) peak at 284.6 eV was taken as an internal reference to correct the XPS spectrum. XPSPEAK41 software was used for the deconvolution of the XPS peaks. 2.3. Procedures. Functionalized MWCNT (f-MWCNT, where f indicates functionalized) and purified MWCNT (p-MWCNT, where p indicates purified) samples were prepared by treating the pristine 10618

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Figure 2. Comparative responses of (A) Raman spectroscopy of (a) MWCNT, (b) MWCNT@PYR, and (c) MWCNT@PYRO, (B) FTIR/KBr of (a) MWCNT, (b) MWCNT@PYR, and (c) MWCNT@PYRO, and (C) XPS responses of (a) SPE-Au/MWCNT@PYRO and (b) SPE-Au/ MWCNT@PYR. (D) Mass spectrometry result for the ethanolic extract of MWCNT@PYRO. (E) Comparative CV patterns of the Au/MWCNT@ PYRO-Cu2+/1+-modified electrode (a) without and (b) with 500 μM H2O2 in pH 7 PBS and a control experiment relating to (c) Au/MWCNT@ PYRO (without Cu) and (d) Au/MWCNT@Cu (without PYRO) in the presence of 500 μM H2O2 in pH 7 PBS. (F) Schematic representation of the electrocatalytic reduction of H2O2 on a Au/MWCNT@PYRO-Cu2+/1+-modified electrode. SPE-Au indicates the screen-printed gold electrode. MWCNT with concentrated 13 M HNO3 and dilute HNO3 per our previous report.19 Physiochemical characterizations by thermogravimetric analysis (TGA) and powder X-ray diffraction (XRD) showed the presence of 6.88, 2.2, and 31.27% impurities resulting from metal oxides (Fe2O3, NiO, and Co2O3), graphitic traces, and amorphous carbon within the pristine MWCNT, f-MWCNT and SWCNT samples, respectively.19 A CNT-modified Au electrode (Au/CNT) was prepared by drop casting 5 μL of an aliquot from 1 mg of CNT dispersed in 500 μL of ethanol (10 min sonicated stock solution) on a cleaned Au electrode and drying the electrode in air for 3 ± 1 min at room temperature followed by the drop casting of 5 μL of 1 mg of PYR dissolved in 500 μL of ethanol on the surface of Au/CNT. Then the Au/CNT@PYRads electrode was potential cycled in a window from −1.0 to +1.0 V versus Ag/AgCl at v = 50 mV·s−1 for 20 continuous cycles (n = 20, the number of cycles) or subjected to Eapp = +1.0 V versus Ag/AgCl (anodic oxidation method) in pH 7 phosphate buffer solution (PBS) (Scheme 1). The copper ion immobilization on the different modified electrodes was carried out per our previous report.20 In brief, the potentiostatic polarization of the working electrode was carried out in freshly prepared 2 mM CuSO4 dissolved in pH 7 PBS at an applied potential of −1 V versus Ag/AgCl for 240 s (Scheme 1D). The Au/ MWCNT@PYRO-Cyt c modified electrode was prepared by an immersion method in which Au/MWCNT@PYRO was dipped in a Cyt c stock (10 mg of Cyt c enzyme dissolved in 500 μL pH 7 PBS) for 20(±1) min, washed, and dried in air (Scheme 1E,F). 2.4. Sample Preparations for the Physicochemical Characterizations. For the FTIR analysis, an as-prepared SPE-Au/ MWCNT@PYRO film, dried in a desiccator overnight, was carefully separated out from the surface using a doctor’s needle (0.5 mm × 3.5

cm) and subjected to further examination. XPS and Raman spectra experiments were carried out using screen-printed SPE-Au/ MWCNT@PYRO-modified electrodes. For the GC-MS analysis, an ethanolic extract of Au/MWCNT@PYRO, filtered using a syringe filter (Nupore, 0.22 μM), was used.

3. RESULTS AND DISCUSSION Initially, a PYR-modified Au electrode designated as Au/PYR was prepared by the simple drop casting of 5 μL of a stock solution containing 1 mg of pyrene dissolved in 500 μL of ethanol on a cleaned Au electrode surface. When the above modified electrode was subjected to continuous potential cycling in a potential window of −1 to +1 V versus Ag/AgCl in pH 7 PBS, it failed to show any faradic response (inset of Figure 1A, Scheme 1G). This observation indicates the unreactive character of PYR on a conventional solid electrode in aqueous solution. This is the reason that SAM and covalently bonded PYR derivative-modified electrodes were used for the electrooxidation.10−13 Interestingly, if the above PYR experiment is repeated on an MWCNT-modified Au electrode designated as Au/MWCNT@PYR, which has been prepared by the successive drop casting of 5 μL of 2 mg/mL MWCNT (Scheme 1A) suspended in ethanol followed by PYR as the first and second layers, respectively (Scheme 1B), then the profound growth of a redox peak centered at an equilibrium potential of E1/2 −0.1 ± 0.005 V versus Ag/AgCl (A1/C1) is seen. The calculated surface excess, ΓPYRO, for the A1 process is 10619

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18.94 nmol·cm−2 (Figure 1A, curve b). The redox peak increases its current magnitude upon increasing CV cycling number up to 20. Later on, saturation in the response was noticed. After the experiment, the above working electrode was gently washed with pH 7 PBS, the medium was transferred to a blank consisting of pH 7 PBS, and continuous CV cycle was again performed. As can be seen in Figure 1B, the A1/C1 redox peak was retained without any quantitative alteration of the shape of the peak. This observation denotes the formation of a highly redox-active surface-confined redox couple on the MWCNT surface-bound PYR electrochemical oxidation process (Scheme 1C). The inset the Figure 1 is a cartoon of the electrochemical oxidation of carcinogenic pyrene on an MWCNT to nonhazardous pyrene tetrone (PYRO) at an applied potential of 1 V, where the PYR → PYRO conversion process is illustrated by the concept of the conversion of a plant seed on soil under irrigation to a useful flower. To obtain precise information about the PYR’s oxidation potential, we have recorded the PYR electrochemical oxidation reaction at discrete applied potentials (Eapp) of 0−1.1 V versus Ag/AgCl under a potentiostatic polarization condition with a potential holding time of 120 s. As displayed in Figure 1C, at Eapp = 1.0 V versus Ag/AgCl, a maximum A1/C1 redox peak current response, beyond which a decreases in the potential for the redox peak currents were noticed, is seen (Supporting Information Figures S1 and S2). The redox peak has a surfaceconfined nature (the plot of ipa vs scan rate (v) is linear). As in Figure S3, the peak-to-peak separation is ΔEp(Epa − Epc) = 55 ± 2 mV and follows Nernstian-type pH-dependent behavior (∂Epa/∂pH = −57 mV vs Ag/AgCl, Epa = anodic peak potential, Figure S4). Note that the surface excess obtained with the Eapp method of preparation (ΓPYRO = 19.2 nmol·cm −2 ) is appreciably similar to that of the E-cycled prepared electrode value. For simplicity, the preparation method and E app mentioned in the second case are chosen as optimal for all of the studies. The as-prepared Au/MWCNT@PYRO was further characterized by various physicochemical techniques, and the data are summarized below: (i) Powder XRD analysis of MWCNT@ PYRO showed two discrete 2θ peaks at 11.65 and 28.52° apart from the characteristic 2θ peaks of MWCNT (graphitic response at 25.7°) and PYR (10.69, 16.52, 21.35, 23.72, 39.9, and 52.0°) (Figure S5). On the basis of our recently published report on a catechol-encapsulated MWCNT system with specific 2θ peaks at 12.15 and 26.27°,21 we initially speculate that the discrete XRD peaks observed in this work are due to surface-bound quinone or a quinol derivative (i.e., PYRO). (ii) Raman spectroscopy characterization of MWCNT@PYR before and after Eapp at 1 V versus Ag/AgCl treatment yielded a specific increment in the intensity ratio (I) of the disordered graphitic (D) over the regular graphitic (G) band (ID/IG) from 0.55 to 0.61. The D band increment after the electrochemical treatment suggests the possibility of oxygenation of the MWCNT surface-bound PYR to PYRO (Figure 2A). Meanwhile, as a control experiment, unmodified Au/MWCNT (i.e, Au/MWCNT*, * indicates coonditioning at 1 V) was also subjected to the 1 V versus Ag/AgCl electrooxidation treatment and calculated for the Raman intensity band ratios. An ID/IG value of 0.58, which is relatively smaller than for the PYRO case, was observed. The above results support PYR → PYRO conversion in this work. (iii) Extended characterization with FT-IR yielded an appreciable signal at 1672 cm−1 corresponding to a carbonyl (>CO) group, which also supports the

existence of PYRO (Figure 2B). (iv) Finally, the X-ray photoelectron spectroscopy (XPS) analysis of a screen-printed gold electrode modified with MWCNT@PYRO (SPEAu/ MWCNT@PYRO) in comparison to a control sample, SPEAu/MWCNT@PYR, showed a specific enhancement in the >CO peak response correspondingly at BE values of 285.1 (C 1s) and 532.2 (O 1s) (Figure 2C, Figure S6, and Table S1). This result authentically confirms the presence of oxygenated PYR (i.e., PYRO with the MWCNT@PYRO sample). However, the number of oxygen species added to PYR and their specific positions are unclear. To obtain this precise information, the ethanolic extract of Au/MWCNT@ PYRO was obtained, where the electrode sonicated in 250 μL of ethanol and filtered using a syringe filter (Nurope, 0.22 μm) was subjected to GC-MS analysis (Figure 2D). Among six GC fraction peaks (Figure S7), one of the major peak at a retention time of 19.97 s had a molecular weight (Mw) value of 264, which may corresponds to species {PYR + 4O−2H+}. The above results indicate the formation of a mixture of products, including four oxygenated PYRs (i.e., PYRO is the quinone derivative) for the MWCNT surface-bound PYR electrochemical oxidation process. At this stage, the identification of the exact oxygen position of PYRO is a challenging task. In this respect, we elegantly designed an electrochemical experiment where MWCNT@ PYRO is allowed to form a complex with a copper ion. It has been reported that a copper ion can form a selective complex with a 1,2-dihydroxy aromatic system and shows a specific redox peak at −0.2 to +0.2 V versus Ag/AgCl in pH 7 PBS.22,23 Note that other quinones such as 1,3-dihydroxy and 4,9dihydroxy derivatives would not form any such complex with the copper ion! The central idea of this work is that if the 1,2dihydroxy quinones/quinol derivative is formed, then it can be identified through the specific {PYRO-copper} complex through its specific redox peak. Figure S8 is the typical comparative CV responses of unmodified copper and Au/ MWCNT@PYRO-Cu2+/1+ systems, where the Cu-modified electrode was prepared by a potentiostatic polarization method at −1 V versus Ag/AgCl for 240 s with 1 mM CuSO4 containing pH 7 PBS.21 Unexpectedly, there is no marked alteration of the A1/C1 redox peak in the potential window of −0.2 to +0.2 V versus Ag/AgCl before and after copper modification except for a new minor peak at a more negative potential, −0.30 V (C2), which may be attributed to copper ion deposition on unmodified MWCNT (i.e., MWCNT@Cu (Figure S9)). Presumably, the amount of complex {PYROCu2+/1+} formed on the interface may be very small, and its response may be masked by the huge PYRO redox peak (A1/ C1). To resolve this problem, we have taken H2O2 as a specific probe, where it is reduced selectively at the {PYRO-Cu2+/1+} redox peak and in turn can show a significant irreversible signal from −0.2 to −0.2 V versus Ag/AgCl. Note that the unmodified system, Au/MWCNT@PYRO, would not mediate H2O2 (Figure 2E, curve c). Figure 2E, curves a and b, is the typical CV response of the Au/MWCNT@PYRO-Cu2+/1+ system without and with 500 μM H2O2. Interestingly, a specific H2O2 reduction signal appears at −0.1 V versus Ag/ AgCl, where the existence of A1/C1 was noticed (Figure 2F, Scheme 1D). A control electrode, Au/MWCNT@Cu, yielded the reduction signal at a more negative potential, −0.40 V versus Ag/AgCl, where a minute amount of C2 exists (Figure 2E, curve d). These results confirm the existence of 1,210620

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Figure 3. (A) CV responses (I−IV) of Au/MWCNT@Cyt c (a), Au/PYRO@Cyt c (b), Au/MWCNT@PYRO (c), and Au/MWCNT@PYRO-Cyt c (d) without and with 500 μM H2O2 in pH 7 PBS at v = 10 mV·s−1. (B) Amperometric i−t responses of Au/MWCNT@PYRO (a), Au/MWCNT (b), and Au/MWCNT@PYRO-Cyt c (c) for sensing 25 μM spikes of H2O2 and (C) Au/MWCNT@PYRO-Cyt c response for interference from cysteine (CySH), nitrite (NO2−), ascorbic acid (AA), and dopamine (DA) in stirred pH 7 PBS at an applied potential of −0.1 V vs Ag/AgCl.

dihydroxy or 1,2-diquinone derivatives on the Au/MWCNT@ PYRO-modified electrode. Different CNTs (viz., p-MWCNT, f-MWCNT, and SWCNT) and other carbon materials such as graphite nanopowder (GNP) and activated charcoal (AC) were also examined for surface-bound PYR electrochemical oxidation as studied above. Except for charcoal, other systems showed qualitatively similar A1/C1 redox peak features, but to different extents (Figure S10). With respect to the surface excess (ΓPYRO) value of the A1 peak, the order of CNT@PYRO formation is pristine MWCNT@PYRO (19.2 nmol·cm−2) > fMWCNT@PYRO (10.93 nmol·cm−2) > p-MWCNT@PYRO (7.79 nmol·cm−2) > SWCNT@PYRO (3.25 nmol·cm−2) > GNP@PYRO (1.34 nmol·cm−2) (Figure 1D). The following conclusions can be drawn from the above observations: (i) hexagonal carbon with graphitelike structure is necessary for PYR electrooxidation, (ii) dense multiwalled CNTs are responsible for higher electrochemical conversion, (iii) the hydrophilic nature of f-MWCNTs induces a repulsive interaction with hydrophobic PAH and in turn a reduction in the electrochemical oxidation, and (iv) metal impurities such as oxides of iron, cobalt, and nickel and nanographite19,24−26 may play a specific role in the enhanced surface-bound oxidation of PYR. To determine the case iv contribution, separate control PYR electrochemical oxidation experiments were performed with nano Fe2O3, NiO, and Co2O3 modified Au systems, but no such specific A1/C1 formations were noticed (Figure S11). On the basis of the GNP result (Figures 1D and S10), it can be concluded that the graphitic impurity within the pristine MWCNT is responsible for the enhanced electrochemical activity in this work.

Finally, a Cyt c enzyme-integrated MWCNT@PYRO was constructed and utilized for H2O2 biosensing in a physiological pH range (Scheme 1E). The biosensor, Au/MWCNT@PYROCyt c, was prepared by a simple immersion method. In brief, Au/MWCNT@PYRO was immersed in 10 mg of Cyt c dissolved in 500 μL of pH 7 PBS for 20 ± 2 min, followed by washing with a copious amount of water to remove unbound Cyt c from the surface. Figure 3A shows comparative CV responses of Au/MWCNT@PYRO-Cyt c (optimal) and Au/ MWCNT@Cyt c, Au/PYRO@Cyt c, Au/MWCNT@PYRO (control experiments) without and with 500 μM H2O2 in pH 7 PBS. Interestingly, the enhanced electrocatalytic reduction of H2O2 was observed only with the optimal modified electrode (Scheme 1F). Meanwhile, the optimal electrode was subjected to the amperometric i−t sensing of H2O2 at −0.1 V versus Ag/ AgCl in pH 7 PBS. A systematic increase in the current signal with increasing H2O2 concentration was noticed. The calibration plot was linear in the range of 25−250 μM with a regression and sensitivity of 0.99 and 0.44 A·M−1·cm−2, respectively. The analytical parameter observed in this work is better than in some literature reports27−29 (Figure 3B and Table S2). The modified electrode does not interfere with cysteine (CySH), nitrite (NO2−), ascorbic acid (AA), or dopamine (DA), which indicates the high selectivity toward H2O2 sensing (Figure 3C).

4. CONCLUSIONS MWCNT-modified electrodes showed effective surface-bound electrochemical oxidation of PYR to highly redox-active surfaceconfined quinone species at 1 V versus Ag/AgCl in pH 7 PBS, unlike the nonresponse with conventional electrodes, Au and GCE. Collective physicochemical characterizations of 10621

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carbons: Role of PAH−Surface Interaction. J. Am. Chem. Soc. 2010, 132, 15968−15975. (9) Brown, G. S.; Barton, L. L.; Thomson, B. M. Permanganate Oxidation of Sorbed Polycyclic Aromatic Hydrocarbons. Waste Manage. 2003, 23, 737−740. (10) Mazur, M.; Blanchard, G. J. Probing Intermolecular Communication with Surface-Attached Pyrene. J. Phys. Chem. B 2005, 109, 4076−4083. (11) Mazur, M.; Blanchard, G. J. Photochemical and Electrochemical Oxidation Reactions of Surface-Bound Polycyclic Aromatic Hydrocarbons. J. Phys. Chem. B 2004, 108, 1038−1045. (12) Mazur, M.; Blanchard, G. J. Surface Immobilized Optical Probes: Pyrene Molecules Covalently Attached to Silica and IndiumDoped Tin Oxide. J. Bioelectrochem. 2005, 66, 89−94. (13) Ding, L.; Fang, Y.; Blanchard, G. J. Probing the Effects of Cholesterol on Pyrene- Functionalized Interfacial Adlayers. Langmuir 2007, 23, 11042−11050. (14) Jaegfeldt, H.; Kuwana, T.; Johansson, G. Electrochemical Stability of Catechols with a Pyrene Side Chain Strongly Adsorbed on Graphite Electrodes for Catalytic Oxidation of Dihydronicotinamide Adenine Dinucleotide. J. Am. Chem. Soc. 1983, 105, 1805−1814. (15) McQueen, E. W.; Goldsmith, J. I. Electrochemical Analysis of Single-Walled Carbon Nanotubes Functionalized with Pyrene-Pendant Transition Metal Complexes. J. Am. Chem. Soc. 2009, 131, 17554− 17556. (16) Goff, A. L.; Moggia, F.; Debou, N.; Jegou, P.; Artero, V.; Fontecave, M.; Jousselme, B.; Palacin, S. Facile and Tunable Functionalization of Carbon Nanotube Electrodes with Ferrocene by Covalent Coupling and π-Stacking Interactions and Their Relevance to Glucose Bio-sensing. J. Electroanal. Chem. 2010, 641, 57−63. (17) Ehli, C.; Aminur Rahman, G. M.; Jux, N.; Balbinot, D.; Guldi, D. M.; Paolucci, F.; Marcaccio, M.; Paolucci, D.; Melle-Franco, M.; Zerbetto, F.; Campidelli, S.; Prat, M. Interactions in Single Wall Carbon Nanotubes/Pyrene/Porphyrin Nanohybrids. J. Am. Chem. Soc. 2006, 128, 11222−11231. (18) Haddad, R.; Holzinger, M.; Maaref, A.; Cosnier, S. Pyrene Functionalized Single-Walled Carbon Nanotubes as Precursors for High Performance Biosensors. Electrochim. Acta 2010, 55, 7800−7803. (19) Kumar, A. S.; Gayathri, P.; Barathi, P.; Vijayaraghavan, R. Improved Electric Wiring of Hemoglobin with Impure-Multiwalled Carbon Nanotube/Nafion Modified Glassy Carbon Electrode and Its Highly Selective Hydrogen Peroxide Biosensing. J. Phys. Chem. C 2012, 116, 23692−23703. (20) Kumar, A. S.; Barathi, P.; Chandrasekara Pillai, K. Highly Stable and Redox Active Nano Copper Species Stabilized FunctionalizedMultiwalled Carbon Nanotube/Chitosan Modified Electrode for Efficient Hydrogen Peroxide Detection. Colloids Surf., A 2012, 395, 207−216. (21) Kumar, A. S.; Swetha, P. Electrochemical-Assisted Encapsulation of Catechol on a Multiwalled Carbon Nanotube Modified Electrode. Langmuir 2010, 26, 6874−6877. (22) Zen, J. M.; Chung, H. H.; Kumar, A. S. Selective Detection of oDiphenols on Copper-Plated Screen-Printed Electrodes. Anal. Chem. 2002, 74, 1202−1206. (23) Kumar, A. S.; Ji, Y. M.; Sornambikai, S.; Chen, P. Y.; Shih, Y. Flow Injection Analysis of Ellagic Acid in Cosmetic Skin-Whitening Creams Using a Dendritic Nanostructured Copper-Gold Alloy Plated Screen-Printed Carbon Electrode. Int. J. Electrochem. Sci. 2011, 6, 5344−5356. (24) Kolodiazhnyi, T.; Pumera, M. Towards an Ultrasensitive Method for the Determination of Metal Impurities in Carbon Nanotubes. Small 2008, 4, 1476−1484. (25) Ambrosi, A.; Pumera, M. Regulatory Peptides Are Susceptible to Oxidation by Metallic Impurities within Carbon Nanotubes. Chem. Eur. J. 2010, 16, 1786−1792. (26) Banks, C. E.; Crossley, A.; Salter, C.; Wilkins, S. J.; Compton, R. G. Carbon Nanotubes Contain Metal Impurities Which Are Responsible for the “Electrocatalysis” Seen at Some NanotubeModified Electrodes. Angew. Chem., Int. Ed. 2006, 45, 2533−2537.

MWCNT@PYRO by XRD, Raman, FT-IR, XPS, and GC-MS revealed PYR-quinone-like species formation on MWCNT surfaces. The PYR-quinone position was identified as ortho by a specific copper complexation experiment. Overall, the electrooxidation methodology introduced in this work is significant in four respects: (i) It is a simple, easy way to degrade PAH, (ii) it enables the recycling of PAHs as a redox-active polyaromatic ortho-hydroquinone@MWCNT electrode, (iii) it allows us to understand the surface oxidation features of the graphene (PYR is the subunit of graphene), and (iv) it enables the development of a new H2O2 biosensor with the Cyt c protein.

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ASSOCIATED CONTENT

S Supporting Information *

XPS data, comparison of analytical parameters, effect of applied potential and time on Au/MWCNT@PYRO formation, effect of CV scan rate and solution pH, XRD responses, GC-MS chromatogram, control of CV responses of Au/MWCNT@ PYRO-Cu2+/1+ and Au/MWCNT@Cu, and effect of various carbons and metal oxides on the conversion of PYR → PYRO. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +91-416-2202754. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the Department of Science and Technology (DST), India, under the Science and Engineering Research Council scheme for financial support. P.B. thanks the Council of Scientific and Industrial Research (CSIR) for the award of her senior research fellowship.

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REFERENCES

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