Electrochemical Behavior of the 1,10-Phenanthroline Ligand on a

Aug 13, 2014 - Elsye Agustina , Jeungchoon Goak , Suntae Lee , Youngho Seo , Jun-Young Park , Naesung Lee. ChemistryOpen 2015 4 (10.1002/open.v4.5), ...
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Electrochemical Behavior of the 1,10-Phenanthroline Ligand on a Multiwalled Carbon Nanotube Surface and Its Relevant Electrochemistry for Selective Recognition of Copper Ion and Hydrogen Peroxide Sensing Prakasam Gayathri 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: 1,10-Phenanthroline (Phen) is a well-known benchmark ligand and has often been used in the coordination chemistry for the complexation of transition metal ions, such as Fe2+, Ni2+, and Co2+. Because the electro-oxidation potential of Phen is much higher (>2 V versus Ag/AgCl) than the water decomposition potential, i.e., ∼1.5 V versus Ag/AgCl, in pH 7, it is practically difficult to electro-oxidize Phen in aqueous medium using any conventional electrodes, such as glassy carbon electrode (GCE), gold, and platinum. Interestingly, herein, we report an unexpected oxidation of Phen to a highly redox active 1,10-phenanthroline-5,6dione (Phen-dione) and its confinement on a multiwalled carbon nanotube (MWCNT)-modified glassy carbon electrode (GCE/ MWCNT@Phen-dione) surface by potential cycling of Phen-adsorbed GCE/MWCNT (GCE/MWCNT@Phen) from −1 to 1 V versus Ag/AgCl in pH 7 phosphate buffer solution. GCE/MWCNT@Phen-dione showed selective recognition of copper ion (GCE/MWCNT@Phen-dione−Cu2+) by catalyzing the hydrogen peroxide reduction reaction in a neutral pH solution. The precise structure of the Phen electro-oxidized product has been identified after characterizing the electrode and/or ethanolic extract of the product by various techniques, such as Raman, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) (for copper complex), liquid chromatography−mass spectrometry (LC−MS), electrospray ionization−mass spectrometry (ESI−MS) (for copper complex), cyclic voltammetry (CV), and in situ electrochemical quartz crystal microbalance (EQCM) and comparing electrochemical behavior of several control compounds, such as phenanthrene and 9,10-phenanthrenequinone. It is concluded that the product formed is 1,10-phenanthroline-5,6-dione, wherein the dione position is ortho to each other and the copper ion is complexed with nitrogen of the phenanthroline ring. With extended electrochemical oxidation of a structurally similar ligand, 2,2′-bipyridine failed to show any such electrochemical dynamics. Finally, applicability of GCE/MWCNT@Phen-dione−Cu2+ for electrochemical sensing of hydrogen peroxide in a couple of real samples is successfully demonstrated. acid and sulfuric acid mixture.8,9 Phen-dione is an excellent redox system (Eo′ ∼ −0.l V versus Ag/AgCl) and is found to mediate oxidation of several biological compounds, such as βnicotinamide adenine nucleotide (NADH) and ascorbic acid (AA) at low potentials (∼0 V versus Ag/AgCl).10,11 In 2004, Mirifico et al. have reported electrosynthesis of Phen-dione from Phen by constant potential electrolysis at 2.4 V versus saturated calomel electrode (SCE) in CH3CN/H2O (4:1).12 Because the Phen oxidation reaction occurred at potential relatively higher than the water decomposition potential (1.5 V versus Ag/AgCl in pH 7), it cannot be oxidized solely in aqueous medium. Electrochemical oxidation of Phen in aqueous solution is unprecedented in the literature. Herein,

1. INTRODUCTION Tailoring the electrode surface with a highly redox active molecular architecture is cutting-edge research in electroanalytical and surface chemistry. Such tailored electrodes have fascinating applications in photoelectrochemical, energy storage, chemical, and biochemical sensing. The use of 1,10phenanthroline (Phen) as one of the most versatile chelating ligands in the coordination chemistry has been widely reported.1 Further, the metal complexes of Phen have potential interest in different areas, such as DNA interaction,2 nanotechnology,3,4 molecular recognition, photocatalysis,5 solar cell devices,6 colorimetric analysis, and self-assembly.7 Because the uncomplexed Phen has non-amenable electrochemical character, it does not involve in any oxidation/reduction reaction under normal conditions. It can be chemically oxidized to 1,10phenanthroline-5,6-dione (Phen-dione) upon stringent condition in the presence of potassium bromide in a fuming nitric © XXXX American Chemical Society

Received: March 26, 2014 Revised: August 13, 2014

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Figure 1. CV responses of (A and B) Phen adsorbed on (A) GCE and (B) GCE/MWCNT and (C) GCE/MWCNT@Phen-dione, (D) phenanthrene (GCE/MWCNT@Ph), (E) bipyridine (GCE/MWCNT@bpy), and (F) phenanthrenequinone (GCE/MWCNT@Ph-Qn) adsorbed GCE/MWCNTs (curve a). (Curve b) CV responses of 1 mM Cu2+ ion-exposed and medium-transferred electrodes as (A) GCE/Phen:Cu2+, (C) GCE/MWCNT@Phen-dione−Cu2+, (D) GCE/MWCNT@Ph:Cu2+, and (F) GCE/MWCNT@Ph-Qn:Cu2+. (E) Curve b is CV of GCE/ MWCNT. Medium, pH 7 PBS; scan rate, 50 mV s−1. Note that Phen-dione = Phen-Oxid.

Recently, MWCNTs are found to be an unique matrix for organic redox molecular transformation and in situ immobilization of some intermediate redox species.20−22 Nevertheless, understanding of the electrochemical reaction mechanism on the tiny carbon nanotube (CNT)-modified electrode surface and identification of products formed on the surface are highly challenging research.23,24 In our recent studies on the electrochemical oxidation of polyaromatic hydrocarbons, such as anthracene and napthene,25 and aromatic alcohols, such as phenol26 and α-naphthol,27 at MWCNT-modified glassy carbon electrode surface (GCE/MWCNT), we observed selective formation and immobilization of hydroquinone derivatives (1,4-dihydroxy derivative, dihydroxyl in para) on the MWCNT. The formation of hydroquinone derivatives on the MWCNT surface could be due to the π−π interaction between the aromatic ring, sp2 carbon of the CNT, and the restricted diffusion of the adsorbed organic moiety. Interestingly, in the present study with Phen, a catechol derivative (Phen-dione; 1,2-dihydroxy-Phen, dihydroxy in ortho) was formed unexpectedly and immobilized on the MWCNT surface, unlike the previous published reports with the polyaromatic hydrocarbons (PAHs) and phenols with the hydroquinone derivatives.25−27 Overall, the interesting features of the present work are (i) an unexpected electrochemical oxidation of non-amenable electrochemical compound, Phen, to highly redox-active Phen-dione molecule on a MWCNT surface in an aqueous medium, (ii) in situ immobilization of Phen-dione on the MWCNT (GCE/MWCNT@Phen-dione), (iii) selective electrochemical recognition of the Cu2+ ion by GCE/MWCNT@Phen-dione (GCE/MWCNT@Phen-dione− Cu2+), where the two nitrogen atoms of Phen-dione were selectively involved in the chelation, and (iv) electrochemical sensing of hydrogen peroxide by GCE/MWCNT@Phendione−Cu2+ in a neutral pH solution.

we report a facile electrochemical oxidation of non-amenable Phen to highly redox-active Phen-dione on a multiwalled carbon nanotube (MWCNT)-modified glassy carbon electrode surface in an aqueous medium of pH 7 phosphate buffer solution. Further, stable confinement of Phen-dione on MWCNT was achieved, and selective recognition of copper and electrochemical sensing of hydrogen peroxide in neutral pH solution was studied. Indeed, conventional electrodes, such as glassy carbon (GCE), platinum, and gold electrodes have failed to show any such electrochemical dynamics of Phen on its surface. Several {Cu−Phen} complex chemically modified graphite electrodes have been reported, where the {Cu−Phen} complexes were prepared by either direct adsorption or sequential method, in which adsorption of the Phen ligand followed Cu2+ complexation, on an activated pyrolytic graphitic electrode.13−17 The {Cu−Phen} complex-modified electrodes showed a redox peak at an equilibrium potential, E1/2 = −75 ± 20 mV, versus SCE with a surface excess value of 4 ± 1 × 10‑10 mol cm−2 in an aqueous solution of pH ∼5.13−17 Meanwhile, (i) {M−Phen-dione, where M = Ru2+ and Zn2+} complexes (Eo′ = −55 ± 5 mV versus SCE and Γ ∼ 0.1 × 10−9 mol cm−2), which were discretely synthesized by a chemical route and covalently immobilized on an activated glassy carbon electrode (GCE*, * = activated) using various combinations of linkers and (ii) Phen-dione metal complexes, bearing a electropolymerizable ligand vinylbipyridine, chemically modified on a GCE* (activated in 0.1 M NaOH at 1.2 V versus Ag/AgCl) were also reported in the literature.18,19 Preparation of such molecular architecture involves tedious synthetic procedures and is time-consuming. In this work, a {Cu−Phen-dione} complex-immobilized MWCNT-modified electrode (designated as GCE/MWCNT@Phen-dione−Cu2+) is prepared by a simple in situ electrochemical technique in short time (50 ± 2 min). B

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Scheme 1. (B−D) Schematic Representation of the Electrochemical Conversion of GCE/MWCNT@Phen to GCE/MWCNT@ Phen-dione, (F, H, and J) Different Ways for Copper Ion Complexation on GCE/MWCNT@Phen-dione and (G, I, and K) Its Hydrogen-Peroxide-Mediated Reduction Reactions along with Control Experiments Relating to the Electrochemical Oxidation of (A) GCE/Phen, (E) GCE/MWCNT@Ph, (L) GCE/MWCNT@bpy, and (M) GCE/MWCNT@Phen-Qn and GCE/ MWCNT@Phen-Qn:Cu2+

2. EXPERIMENTAL SECTION

surface (GCE/MWCNT@Phen) showed a unique electrochemical response (Figure 1B and panels B−D of Scheme 1), in which a continuous growth of an redox peak was observed up to 20 cycles, after that the response became saturated, at an equilibrium potential, E1/2 (Epa + Epc/2, where Epa and Epc are anodic and cathodic peak potentials) of −100 ± 3 mV versus Ag/AgCl (Figure 1B). After the above experiment, the electrode was gently washed with water and repeated the continuous CV in fresh pH 7 PBS. The electrochemically treated electrode is tentatively designated as GCE/MWCNT@ Phen-Oxid, where Oxid is the oxidized form. It is interesting to notice that the above redox peak was retained without any alteration in the peak current and potential on GCE/ MWCNT@Phen (curve a in Figure 1C). The redox peak was surface-confined (plot of anodic peak current, ipa, versus scan rate is linear starting from origin; data not enclosed) and pH-dependent Nernstian type [slope of the plot anodic (Epa) or cathodic peak (Epc) potential against pH, ∂Epa or ∂Epc/∂pH ∼ −62 mV decade−1; data not enclosed] in character. The strong π−π interaction between Phen and MWCNT and the restricted diffusion of Phen on the MWCNT surface are likely reasons for the unique electrochemical oxidation of Phen. To understand the influence of potential on the electrochemical oxidation reaction, Phen-adsorbed MWCNT was discretely subjected to various applied potentials (static) at −1, −0.5, 0, 0.4, 0.8, and 1 V versus Ag/AgCl for 240 s and then CV-cycled in a potential window from −0.8 to 0.6 versus Ag/ AgCl in pH 7 PBS (Figure 2A). Unexpectedly, there was no

Chemicals and reagents used were all of analytical grade and used without further purification. Voltammetric measurements were carried out with a CH Instruments, Inc. model 660C electrochemical workstation (Austin, TX). The three-electrode system consisted of glassy carbon (Bioanalytical System, West Lafayette, IN, 3 mm diameter, 0.0707 cm2 area) or its chemically modified form as the working electrode, Ag/AgCl as the reference electrode, and platinum wire as the auxiliary electrode. GCE/MWCNT was prepared by drop casting 5 μL of an aliquot from a MWCNT/ethanol suspension (8 mg/mL ethanol) on a cleaned GCE followed by drying in room temperature for about 5 ± 1 min. GCE/Phen, GCE/MWCNT@Phen, GCE/MWCNT@Phen-dione, GCE/MWCNT@Ph-Qn, and GCE/ Phen were prepared by drop casting 5 μL of respective organic compound−ethanol solution (8 mg/mL ethanol) on a cleaned GCE or GCE/MWCNT surface followed by 5 ± 1 min air drying at room temperature. Copper-ion-complexed GCE/MWCNT@Phen-dione (GCE/MWCNT@Phen-dione−Cu2+) and other CNT-modified electrodes were prepared by the immersion method, in which the respective chemically modified electrode is immersed in 1 mM CuSO4 dissolved pH 7 phosphate-buffered saline (PBS) for 150 ± 3 s at opencircuit potential conditions.

3. RESULTS AND DISCUSSION Initial cyclic voltammetry (CV) experiments with a Phenadsorbed unmodified GCE (GCE/Phen) in pH 7 PBS showed a featureless voltammetric response (curve a in Figure 1A and Scheme 1A), indicating non-amenable characteristics of uncomplexed Phen on the conventional electrode (GCE). Interestingly, CV of a Phen-ligand-adsorbed GCE/MWCNT C

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(e) −0.9, and (f) −1 V versus Ag/AgCl (Figure 2C). The response at the potential window from +0.6 to −1 V only showed a specific redox peak. It is obvious that keeping a negative potential at −1 V is essential for the Phen electrooxidation on the MWCNT. Overall, potential cycling at the window from −1.0 to 0.6 V is a minimum requirement for the Phen oxidation on the MWCNT surface. On the basis of the results, it can be speculated that nitrogen in Phen first becomes oxidized to radical species at 0.6 V and there is activation of the 5,6 carbon of Phen for subsequent reduction to the dihydroxy-Phen derivative at −1 V versus Ag/AgCl as in Scheme S2 of the Supporting Information. Note that, although the electrochemical conversion of graphite to graphitic oxide at positive potentials (1 V versus Ag/AgCl) is well-known,28 both materials are non-redox-active in character. In addition, as a control experiment, phenanthrene (Ph), which has a structure similar to Phen but without any nitrogen atoms was subjected to electrochemical studies (GCE/ MWCNT@Ph) (Figure 1D and Scheme 1E). Zero redox response was noticed, unlike the GCE/MWCNT@Phen-Oxid case (curve a in Figure 1C). Possibly the rigid structure and high oxidation potential of Ph (∼2 V versus Ag/AgCl; it is relatively higher than the water oxidation potential) are possible reasons for the zero electrochemical oxidation behavior. These preliminary results attributed to the unique electrochemical behavior of the heterocyclic organic compound, Phen, on the MWCNT-modified electrode surface. It is a difficult task to find out that the electrochemical reaction occurred and the product formed on the MWCNT surface (i.e., Phen-Oxid). Because the amount of electroactive organic compounds adsorbed on the MWCNT is very low (probably nanomolar concentration or microgram quantity) and coupled with the MWCNT matrix, it is difficult to identify the product formed on the surface using conventional characterization techniques, such as nuclear magnetic resonance (NMR), which requires a minimum of milligram quantity of test organic compound for the analysis. In this work, we elegantly designed several experiments (panels F−K of Scheme 1) and used several control samples (panels A,

Figure 2. Effect of potential on CV responses of GCE/MWCNT@ Phen in pH 7 PBS at v = 50 mV s−1. (A) CV responses of GCE/ MWCNT@Phen after conditioning at different applied potentials of −1, −0.5, 0, 0.4, 0.8, and 1 V versus Ag/AgCl for 240 s. (B) E-cycling experiment with a fixed starting potential of −1 V and varying end potentials of 0.5, 0.6, and 1 V versus Ag/AgCl. (C) E-cycling experiment with a fixed end potential of 0.6 V and varying starting potentials of −0.8, −0.9, and −1 V versus Ag/AgCl.

redox peak formation in all of the cases! This observation indicated the absence of any static-applied potential effect on the electrochemical oxidation of Phen. The Phen-adsorbed MWCNT electrode was subjected to various potential cycling (E-cycling) experiments (cases I and II), where in the first case I, the starting potential was fixed at −1 V and varied the final potentials as (a) +0.5, (b) +0.6, and (c) 1 V versus Ag/AgCl. As seen in Figure 2B, the final potential at 0.6 V or above only showed specific redox peak. In case II, the final potential was fixed at +0.6 V and varied the starting potentials as (d) −0.8,

Figure 3. SEM images of (A) SPCE/MWCNT@Phen, (B) SPCE/MWCNT@Phen-dione, and (C) SPCE/MWCNT@Phen-dione−Cu2+. TEM images of (D) MWCNT (unmodified) and (E and F) MWCNT@Phen-dione−Cu2+ (at different magnifications). D

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Figure 4. (A) Raman and (B) infrared (IR) spectroscopic responses of (a) MWCNT, (b) MWCNT@Phen, (c) MWCNT@Phen-dione, and (d) MWCNT@Phen-dione−Cu2+. (C) XPS of screen-printed carbon electrode (SPCE)−MWCNT@Phen-dione−Cu2+. The insets are the core-level XPS responses of N 1s and Cu 2p.

Figure 5. In situ CV (5th cycle) and EQCM responses of Au/MWCNT@Phen-dione-modified electrode in pH 7 PBS at v = 50 mV s−1.

and MWCNT@Phen-Oxid samples further supports the presence of oxygen functional groups, such as quinone (>CO) at 1613 cm−1 an graphitic carbon (CC) at 1633 cm−1 on GCE/MWCNT@Phen-Oxid (Figure 4B). These physicochemical characterization results support the oxidation of surface-bound Phen to oxygen-rich product (Phen-Oxid) upon the electrochemical treatment procedure. The in situ electrochemical quartz crystal microbalance (EQCM) technique is further used to probe the electrochemical oxidation reaction and mechanism against the potential. Electrochemical oxidation of organic molecules, such as aromatic hydrocarbons and phenols, often followed a radical pathway reaction in aqueous medium, where the electrogenerated radical molecule involved in either polymerization, in which several monomeric radical compounds combined together to form long-chain molecules (case I), or oxidation to a quinone-type molecular structure, where water and/or dioxygen molecules were taken by the radical species (case II).25,26 The electropolymerized organic molecules, such as polyaromatic hydrocarbon25 and polyphenols,26 were electroinactive, whereas the quinone-type molecules were highly redox-active in nature.26,27 Figure 5A is a 5th cycle in situ CV− EQCM response of a Phen-adsorbed Au/MWCNT electrode (EQCM−Au/MWCNT@Phen) in pH 7 PBS (initial EQCM patterns are noisy in response and, hence, omitted). The molecular mass of the electroactive species involved in the electrochemical oxidation process can be calculated using the following formula: Mw = Fm/Q, where Q is the charge passed, Mw is the molar mass per electron (g mol−1 = molecular

E, L, and M of Scheme 1) to identify the structure of the product formed on the MWCNT surface. These data are summarized in the following section. Scanning electron microscopy (SEM) photographs of the screen-printed electrode (SPE)/MWCNT@Phen and SPE/ MWCNT@Phen-Oxid samples placed in panels A−C of Figure 3 showed agglomerated white spots on SPE/MWCNT@Phen and the absence of any such mark on SPE/MWCNT@PhenOxid. It is expected that the drop casting of the Phen ligand on the MWCNT might lead to the agglomerated structure on the MWCNT, and upon the electrochemical treatment, some of the adsorbed compounds might be oxidized and leached out from the surface. Further characterization is necessary to identify the details of the adsorbed compounds. Figure 4A is a comparative Raman spectroscopy characterization of SPE-Au/ MWCNT (curve a), SPE-Au/MWNCT@Phen (curve b), SPEAu/MWCNT@Phen-Oxid (curve c) systems. All of these samples showed qualitatively similar peaks at about 1330 ± 5 and 1570 ± 5 cm−1 corresponding to the disordered (D band; possibly because of oxygen functional groups and sp3 carbon) and ordered (G band; sp2 carbon) graphitic structures of the respective samples. The peak intensity ratio, ID/IG, can be taken as a measure to quantify the graphitic structure.27,29 Calculated intensity ratio values for the samples are 0.58, 0.57, and 0.70. The specific increment in the ratio from 0.57 to 0.70 after the electrochemical treatment of GCE/MWCNT@Phen may be attributed to the generation of oxygen-rich organic compound on the surface (i.e., Phen-Oxid).27,29 Fourier transform infrared spectroscopy (FTIR) characterization of the MWCNT@Phen E

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weight), and F is the Faraday constant.26 On the basis of the equation, Mw values involved in the potential window from −1 to +1 V versus Ag/AgCl are calculated as 32 ± 0.5, 19 ± 2, 32 ± 0.8, 32 ± 0.4, 32 ± 2, and 17 ± 2 g mol−1, which correspond to the molecular species O2, H2O, O2, O2, O2, and H2O, respectively, indicating operation of the case II type of reaction mechanism, i.e., addition of water and oxygen species on the electrochemical oxidation process. These observations and the specific redox peak at −100 ± 3 mV versus Ag/AgCl evidenced the formation of quinone-like molecular species on the electrochemical treatment of GCE/MWCNT@Phen. Beside, the nature of the quinone molecule (how many oxygen species in the molecule) and the position of the oxygen species (either catechol or hydroquinone type) are to be identified. For that, an ethanolic extract of the chemically modified electrode and a control Phen sample were subjected to liquid chromatography−mass spectrometry (LC−MS) analyses as in panels A and B of Figure 6. Feeble molecular peaks at m/z 181.28 and

1,2-dihydroxy molecule (pyrenequinone derivative) was selectively complexed with copper ion and mediated hydrogen peroxide reduction reaction at ∼ −0.1 ± 0.05 V versus Ag/ AgCl (because of the copper ion), whereas the 1,4-dihydroxy compound has failed to show any such complexation and hydrogen peroxide electrocatalysis. The copper ion complexation experiment was carried out by immersing the respective chemically modified electrode in 1 mM CuSO4 at pH 7 PBS for 150 ± 3 s. Curve b in Figure 1C is a typical CV response of copper-ion-complexed GCE/MWCNT@Phen-dione (GCE/ MWCNT@Phen-dione−Cu2+; Scheme 1F). No difference in the CV was noticed before and after the Cu2+ exposure (curves a and b in Figure 1C and panels D and F of Scheme 1). Presumably, the amount of copper ion complexed on the surface is very low, and the apparent redox potential of the copper (II/I) complex, Eo′ ∼ −0.1 ± 0.03 V versus Ag/AgCl, is similar to the redox peak of GCE/MWCNT@Phen-dione (−0.1 ± 0.03 V); hence, the copper immobilization response could not be identified in the above experiment. Nevertheless, a clear hydrogen peroxide electrocatalytic signal was noticed with GCE/MWCNT@Phen-dione−Cu2+ and no electrocatalytic response with the unmodified GCE/MWCNT@Phen-dione system, as in curves b and c of Figure 7A, evidencing the involvement of copper in the GCE/MWCNT@Phen-dioneCu2+ system indirectly. To ensure the observation, the GCE/ MWCNT@Phen-dione−Cu2+ system was also subjected to SEM, transmission electron microscopy (TEM), Raman, FTIR, and X-ray photoelectron spectroscopy (XPS) analyses. A group of white-colored patches on the CNT surface (Figure 3C) and a Cu 2P3/2 signal at the binding energy of 932.3 eV along with a N 1s signal at 400 eV (Figure 4C) were observed with SEM and XPS, respectively, supporting the presence of the complexed copper on the Phen-dione matrix. Similar to the SEM, TEM of MWCNT@Phen-dione−Cu2+ also showed finite black-colored spots on the inner walls of the MWCNT (panels D−F of Figure 3), which might be due to the complexed copper ions.31,32 Raman analysis of SPE−Au/MWCNT@Phendione showed marked decrement in the ID/IG ratio from 0.7 to 0.61 after the Cu2+ complexation process (curves c and d in Figure 4A). Presumably, the quinone form of the structure is turned to the dihydroxyl form and complexed with Cu2+. This structural change might be the reason for the decrement in the ratio. What position Cu2+ is complexed with Phen-dione is unclear: in either dioxygen (case III; Scheme 1J), N,N′ (case IV; Scheme 1H), or both sites of the Phen-dione system (case V; Scheme 1F). To find out the Cu2+ ion complexing position in Phen-dione, a control compound, 9,10-phenanthrenequinone (Ph-Qn, where there is no nitrogen atoms; Scheme 1M; case VI), was subjected to the CV study as in Figure 1C. The case VI system showed a redox peak at E1/2 = −225 ± 5 mV versus Ag/AgCl, which is closer to that of GCE/MWCNT-Phendione−Cu2+ (E1/2 = −100 ± 3 mV) but without any electrocatalysis for hydrogen peroxide reduction (Figure 7B). Meanwhile, comparative FTIR characterization of MWCNT@ Phen-dione and MWCNT@Phen-dione−Cu2+ samples showed a specific shift in the ν(CN) signal from 1633 to 1742 cm−1 [the FTIR signal of ν(CO) at 1613 was unaltered] (curve d of Figure 4B). The following conclusions can be drawn from the above observations: (i) the position of dioxygen in Phendione should be 5,6 carbons, and (ii) copper ion complexed with nitrogen atoms of Phen-dion but not on the dione position. The nature of the Cu2+ complex formed was further

Figure 6. (A and B) LC−MS and (C) ESI−MS results for an ethanolic extract of (b) MWCNT@Phen-dione and (c) MWCNT@Phendione−Cu2+. (a) Control response for Phen.

210.82, which may correspond to {Phen + 1H} and two oxygenated Phen ({Phen + 2O}) molecular species, were identified. This result confirmed the presence of two oxygen atoms (dione) in the electroactive product. Thus, here, PhenOxid is redesignated as Phen-dione for convenience. To obtain precise information about the position of dione, whether it is in 1,2 (catechol type) or 1,4 (hydroquinone type) position, GCE/MWCNT@Phen-dione is subjected to copper ion complexation studies like our previous work,29,30 where a F

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Figure 7. CV responses of (A) GCE/MWCNT@Phen-dione−Cu2+ (a) without and (b) with 500 μM H2O2 and (c) GCE/MWCNT@Phen-dione with 500 μM H2O2. CV responses of (B) GCE/MWCNT@Ph-Qn:Cu2+ (a) without and (b) with 500 μM H2O2. (C) Effect of the H2O2 concentration on the CV of the GCE/MWCNT@Phen-dione−Cu2+-modified electrode in pH 7 PBS, and the inset plot is [H2O2] versus ipc. (D) Cartoon for the electrocatalytic effect of H2O2 on GCE/MWCNT@Phen-dione−Cu2+. CV scan rate = 10 mV s−1.

Figure 8. Amperometric i−t responses of GCE/MWCNT@Phen-dione−Cu2+ for continuous spikes (50 μM) of (A) H2O2, (B) H2O2 and other interfering chemicals, and (C) real samples (R) with standards (R + S1−4). The inset plot is the current versus [H2O2] in pH 7 PBS at an applied potential of 0 V versus Ag/AgCl. Curves b and c in panel A are control responses of GCE/MWCNT:Cu2+ and GCE/MWCNT.

of coordination property, was also tested for electrochemical oxidation on MWCNT, but non-response behavior was noticed! (Figure 1E and Scheme 1L). This observation was attributed selective identification of the Phen ligand through the electrooxidation of the 5,6 carbon position to the redoxactive dione on the MWCNT-modified electrode surface. The possible reason for the selective oxidation of Phen on MWCNT may be due to electronic charge on the 5,6 carbon atom, interaction between the hexagon carbon (sp2) and graphite carbon (sp2), and structural factors. The efficient electrocatalytic behavior of GCE/MWCNT@ Phen-dione−Cu2+ further tuned to electrochemical sensing of hydrogen peroxide in a neutral pH. GCE/MWCNT@Phendione−Cu2+ has adsorption-controlled electron transfer and

identified by subjecting the clear ethanolic extract of MWCNT@Phen-dione−Cu2+ to electrospray ionization− mass spectrometry (ESI−MS) as {[(Phendihydroxyl)2(CH3CH2OH)(H2O)Cu(II)]−1H}, with a m/z peak at 552.11. The value is closely matching with the expected m/z value of 552.01 (inset compound in Figure 6C). Extended experiments with other metals, such as Ni2+, Fe2+, and Zn2+, for the complexation with the GCE/MWCNT@ Phen-dione electrode and their respective hydrogen peroxide electrocatalyses showed poor responses, unlike the Cu2+ case (see Figure S1 of the Supporting Information). These results attributed selective recognition of Cu2+ by GCE/MWCNT@ Phen-dione in this work. Further, another heterocyclic organic ligand, 2,2′-bipyridine (bpy), which is similar to Phen in terms G

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ation on nitrogen atoms. GCE/MWCNT@Phen-dione−Cu2+ showed an excellent electrocatalytic reduction signal to hydrogen peroxide. Applicability of the sensor was further demonstrated to detect hydrogen peroxide in real samples, such as cosmetic hair dye and fruit juice samples, with appreciable recovery values.

Nernstian-type pH-dependent behavior (see Figures S2 and S3 of the Supporting Information). The effect of the H2O2 concentration on the CV of GCE/MWCNT@Phen-dione− Cu2+ yielded a linear increase in the reduction peak current (ipc) in a range from 100 μM to 4 mM with a current sensitivity of 48 nA/μM and regression of 0.998; after that concentration, the peak current became saturated (Figure 6C). With concern about the mechanism, it is proposed that the reduced form of the copper complex-modified electrode (GCE/MWCNT@ Phen-dione−Cu1+) mediated the reduction of H2O2 to the hydroxide ion. To support the observation, a copper− phenanthroline complex [Cu(phen)32+] prepared by the solution phase method33 and modified on the MWCNT was subjected to the H2O2 reduction reaction (see Figure S4 of the Supporting Information). An efficient H2O2 reduction by mediated reduction of MWCNT@Phen−Cu2+/1+ similar to the present case was noticed (see Figure S4 of the Supporting Information). Amperometric i−t sensing of hydrogen peroxide was also tested with GCE/MWCNT@Phen-dione−Cu2+ at an applied potential of 0 V versus Ag/AgCl as in Figure 8A. The electrode showed a regular increase in the current signals against 50 μM H2O2 spiking in pH 7 PBS. The plot of current versus the H2O2 concentration is linear up to 350 μM with current sensitivity and regression coefficient values of 3 nA/μM (0.0424 AM−1 cm−2) and 0.9854. The obtained sensitivity in this work is higher than the some of the recently reported values.34,35 Six repeated detections of 50 μM H2O2 showed a relative standard deviation value of 4.5%, and the calculated detection limit value (signal-to-noise ratio = 3) is 5.4 μM. The effect of interfering chemicals, such as cysteine (CySH), ascorbic acid (AA), uric acid (UA), nitrate (NO3−), and nitrite (NO2−), on the detection of H2O2 was tested as in Figure 8B. Except for CySH and AA, which showed about 30% interferences, no other chemicals interfered with the present system. Applicability of the present sensor is further demonstrated by the detection of hydrogen peroxide in two real samples, cosmetic hair dye (sample 1; Figure 8C) and grape juice (sample 2; Figure S5 of the Supporting Information), by a standard addition method. Table S1 of the Supporting Information provides detailed information about the detected hydrogen peroxide. After the dilution factor corrections, the corresponding hydrogen peroxide concentrations detected in these real samples are 5.9 (0.06%) and 0.34 mM, respectively. Calculated recovery values are ∼100%.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details, structures of various organic compounds used in the paper (Scheme S1), proposed mechanism for the electrochemical oxidation of Phen to Phen-dione on a GCE/ MWCNT-modified electrode (Scheme S2), CV responses of GCE/MWCNT@Phen-dione with various metals (Fe2+, Ni2+, and Zn2+) and its electrochemical responses to H2O2 (Figure S1), effect of the scan rate and pH on CV responses of GCE/ MWCNT@Phen-dione−Cu 2+ (Figure S2 and S3), CV responses of Cu(phen)32+ prepared by the solution phase method (Figure S4), and H2O2-containing real sample analysis data (Figure S5 and Table S1). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +91-416-2202754. E-mail: askumarchem@yahoo. com. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the Department of Science and Technology (DST), India, under the Science and Engineering Research Council scheme, for financial support. The authors also thank Professor K. Sriraghavan, Organic Chemistry Division, Vellore Institute of Technology (VIT) University, for his valuable discussion on this work.

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REFERENCES

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4. CONCLUSION An uncomplexed Phen ligand, which is a non-amenable character, became easily electrooxidized to highly redox-active Phen-dione and confined on a MWCNT-modified GCE surface (GCE/MWCNT@Phen-dione) unusually in aqueous solution, unlike the non-response behavior on conventional carbon electrodes, such as a GCE. This GCE/MWCNT@Phen-dionemodified electrode showed selective recognition of the copper ion in a neutral pH solution by formation of a specific complex and catalyzing electrochemical reduction of hydrogen peroxide. This Cu complexation study, in addition to physicochemical and electrochemical characterizations of GCE/MWCNT@ Phen-dione by SEM, Raman, FTIR, in situ EQCM, LC−MS, ESI−MS, and examination with several control compounds, phenanthrene and phenanthrenequinone, revealed that the electro-oxidized product of Phen is Phen-dione and the position of dione in Phen-dione is ortho, with Cu2+ complexH

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dx.doi.org/10.1021/la502651w | Langmuir XXXX, XXX, XXX−XXX