Carbon nanotubes paste electrode

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Abstract

The performance of carbon nanotubes paste electrodes (CNTPE) prepared by dispersion of multi-wall carbon nanotubes (MWNT) within mineral oil is described. The resulting electrode shows an excellent electrocatalytic activity toward ascorbic acid, uric acid, dopamine, 3,4-dihydroxyphenylacetic acid (dopac) and hydrogen peroxide. These properties permit an important decrease in the overvoltage for the oxidation of ascorbic acid (230 mV), uric acid (160 mV) and hydrogen peroxide (300 mV) as well as a dramatic improvement in the reversibility of the redox behavior of dopamine and dopac, in comparison with the classical carbon (graphite) paste electrodes (CPE). The substantial decrease in the overvoltage of the hydrogen peroxide reduction (400 mV) associated with a successful incorporation of glucose oxidase (GOx) into the composite material, allow the development of a highly selective and sensitive glucose biosensor without using any metal, redox mediator or anti-interference membrane. No interference was observed at −0.100 V even for large excess of ascorbic acid, uric acid and acetaminophen. A linear response up to 30 mM (5.40 g l−1) glucose with a detection limit of 0.6 mM (0.11 g l−1) were obtained with the CNTPE modified with 10% w/w GOx. Such an excellent performance of CNTPE toward hydrogen peroxide, represents a very good alternative for developing other enzymatic biosensors.

Introduction

Since the discovery of carbon nanotubes in 1991 [1], they have been the target of numerous investigations due to their unique properties [1], [2], [3], [4]. Carbon nanotubes are built from sp2 carbon units and they present a seamless structure with hexagonal honeycomb lattices, being several nanometers in diameter and many microns in length [2], [3]. There are two groups of carbon nanotubes, multi-wall carbon nanotubes (MWNTs) and single-wall carbon nanotubes (SWNTs) [2]. MWNTs can be visualized as concentric and closed graphite tubules with multiple layers of graphite sheet defining a hole typically from 2 to 25 nm separated by a distance of approximately 0.34 nm [1], [2], [3]. A SWNTs consist of a single graphite sheet rolled seamlessly, defining a cylinder of 1–2 nm diameter. Carbon nanotubes can behave as metals or semiconductors depending on the structure, mainly on the diameter and helicity [2], [3].

Several authors [5], [6], [7], [8], [9], [10], [11], [12] have reported the excellent electrocatalytic properties of nanotubes in the redox behavior of different biomolecules. Britto et al. [5] have reported a dramatic improvement in the electrochemical behavior of dopamine with ΔEp of 30 mV at nonactivated carbon nanotube electrodes constructed by using bromoform as binder. Li et al. [6] have reported the advantages of using glassy carbon modified with single wall carbon nanotubes on the voltammetric behavior of norepinephrine, dopamine and ascorbic acid. The ability of a SWNT-glassy carbon modified electrode for the highly sensitive and selective detection of dopac in the presence of 5-hydroxytryptamine was also demonstrated [7]. Li et al. [8] have described the catalytic properties of activated SWNT film-modified glassy carbon electrodes toward the reduction/oxidation of cytochrome c. Wang et al. [9] demonstrated that carbon-nanotubes-modified glassy carbon electrodes show a very significant electrocatalytic activity toward NADH with an important decrease in the oxidation overpotential. Recent studies demonstrated improved electrochemical behavior of hydrogen peroxide and catecholamine neurotransmitters on glassy carbon electrodes modified with SWNTs and MWNTs solubilized in nafion [10]. Hill et al. [11] have reported a preliminary work demonstrating the feasibility to use carbon nanotubes as an electrode material, packing the tubes into a glass capillary in mineral oil, deionised water, nujol or bromoform. They evaluated the electrochemical properties of the resulting electrode by using cytochrome c and azurin and found that these proteins can be immobilized on and within opened nanotubes without denaturation. Recently, Wang and Musameh [12] described the attractive performance of a composite material prepared by dispersion of MWNTs within Teflon. The resulting electrode keeps the excellent electroactivity of MWNTs even in presence of the hydrophobic material and allows the incorporation of enzymes like alcohol dehydrogenase and glucose oxidase for the development of enzymatic biosensors.

This article reports on the advantages of carbon nanotubes paste electrodes (CNTPE) prepared in an easy, fast and very effective way by using mineral oil as binder. The resulting CNTPE retains the properties of the classical carbon paste electrode (CPE) such as the feasibility to incorporate different substances, the low background currents, the easy renewal and composite nature. Therefore, this new composite electrode combines the ability of carbon nanotubes to promote electron-transfer reactions with the attractive advantages of composite materials. The electrochemical behavior of different biomolecules such as dopamine, ascorbic acid, dopac and uric acid as well as hydrogen peroxide, compound widely involved in enzymatic reactions of interest at this new composite material is described. The suitability of CNTPE for developing highly sensitive glucose enzymatic biosensors by incorporation of glucose oxidase (GOx) within the composite matrix is also illustrated in the following sections.

Section snippets

Apparatus

The measurements were performed with an EPSILON potentiostat (BAS). The electrodes were inserted into the cell (BAS, Model MF-1084) through holes in its Teflon cover. A platinum wire and Ag/AgCl, 3 M NaCl (BAS, Model RE-5B) were used as counter and reference electrode, respectively. All potentials are referred to the latter. A magnetic stirrer provided the convective transport during the amperometric and stripping measurements.

The carbon nanotubes paste electrode (CNTPE) was prepared by mixing

Electrochemical behavior

Since the work reported by Adams in 1958 [13], carbon composite electrodes have received enormous attention. Among them, the composites made of graphite powder and mineral oil are the most widely known [14]. As its analogous CPE, carbon nanotubes paste electrodes display a broad potential window and a low background current. Fig. 1 shows linear-sweep voltammograms obtained at 0.100 V s−1 in different deoxygenated solutions: 0.10 M sulfuric acid (a), 0.050 M phosphate buffer, pH 7.40 (b) and

Conclusions

The properties of CNTPEs based on the dispersion of carbon nanotubes within mineral oil have been demonstrated. The new material combines the advantages of composite materials with the electrochemical properties of carbon nanotubes. CNTPEs offer a dramatic improvement in the electrochemical behavior of dopamine, ascorbic acid, uric acid, dopac and hydrogen peroxide. The feasibility of incorporating GOx into the matrix was also illustrated. The important decrease in the hydrogen peroxide

Acknowledgements

The authors thank Fundación Antorchas, Consejo Nacional de Investigaciones Cientı́ficas y Técnicas de Argentina (CONICET), Secretarı́a de Ciencia y Tecnologı́a de la Universidad Nacional de Córdoba (SECyT), Agencia Nacional de Promoción Cientı́fica y Tecnológica and Asociación de Bioquı́micos de la Provincia de Córdoba (ABC) for the financial support. M.D.R. thanks Agencia Nacional de Promoción Cientı́fica y Tecnológica, ABC and CONICET for the fellowships received.

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