Electrochemical determination of hydrogen sulfide at carbon nanotube modified electrodes

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Abstract

Carbon nanotube (CNT) modified glassy carbon electrodes exhibiting a strong and stable electrocatalytic response towards sulfide are described. A substantial (400 mV) decrease in the overvoltage of the sulfide oxidation reaction (compared to ordinary carbon electrodes) is observed using the CNT-modified electrode, with oxidation starting at ca. −0.3 V (versus Ag/AgCl; pH 7.4). The CNT-coated electrodes thus allow highly sensitive, low potential (+0.1 V), stable amperometric sensing. A wide linear dynamic range (1.25–112.5 μM) was achieved with a detection limit of 0.3 μM (9 ppb). The enhance sensitivity is coupled to an improved stability. Such ability of carbon nanotubes to promote the sulfide electron-transfer reaction suggests great promise for miniaturized sensors for hydrogen sulfide.

Introduction

There is widespread awareness of the toxicity of sulfide in its liberated hydrogen sulfide (H2S) form. Even at a low concentration, H2S can lead to personal distress whilst at a higher concentration it can result in loss of consciousness, permanent brain damage or even death due to the neurotoxic effect of the gas [1]. Hydrogen sulfide is a by-product of many industrial processes and it is produced in nature primarily by decomposition of organic materials by bacteria whilst it is also a constituent of natural gas, sulfur deposits, volcanic gases and sulfur springs [2]. The unmediated release of high concentrations of sulfide can effectively overwhelm the checks and balances that operate under natural conditions and can considerably threaten surrounding ecosystems. Hence, there has been an increasing need for highly sensitive, fast-responding, portable devices for monitoring trace levels of sulfide in a variety of environmental and industrial applications.

There are several avenues for the detection of both sulfide and hydrogen sulfide; these embrace most instrumental methodologies [3]. Electrochemical techniques have provided several procedures as realized in the plethora of available potentiometric [4], [5], [6], galvanostatic [7], [8] and amperometric [9], [10], [11], [12], [13], [14], [15], [16], [17], [18] devices.

This paper reports on the greatly enhanced electrochemical reactivity and detection of sulfide at carbon nanotube (CNT) modified glassy carbon and carbon fiber electrodes. Carbon nanotubes have recently come to the forefront of electrochemical research because of their unique mechanical, electrical and chemical properties [19]. The electronic properties of these materials have been exploited as means of promoting the electron-transfer reaction for a wide range of biologically significant species including, proteins [20], NADH [21], neurotransmitters [22], [23], [24], cytochrome c [25] and cysteine [26], [27]. Similarly, in the following sections we report on the electrochemical oxidation of sulfide and its enhanced voltammetric detection at glassy carbon electrodes modified with CNT (dispersed in dimethylformamide [28], [29]).

Section snippets

Electrochemical apparatus and chemicals

All reagents were obtained from Sigma, of the highest grade available and used without further purification. All solutions and subsequent dilutions were prepared daily, using doubly distilled water. Cyclic voltammetric measurements were conducted using a PGSTAT12 computer controlled potentiostat (Eco-Chemie BV, Utrecht, Netherlands) whilst a bioanalytical system (BAS) CV-27 voltammograph, in connection with a BAS XY recorder were used for the amperometric experiments. A standard

Results and discussion

Fig. 1 displays cyclic voltammograms (CV) at CVD-CNT (A) and ARC-CNT (B) modified glassy carbon electrodes, as well as at the bare carbon surface (C) in the absence (dashed line) and presence of 500 μM sulfide. With both the unmodified and ARC-CNT electrodes a broad oxidation wave is observed in the presence of sulfide with a peak potential of +0.20 V. In contrast, the CVD-CNT-modified electrode exhibits remarkably different voltammetric behavior with a substantial negative shift in the

Conclusions

The voltammetric response of hydrogen sulfide at a glassy carbon electrode modified with both CVD- and ARC-prepared carbon nanotubes was examined and compared with those obtained at the bare glassy carbon electrode. The CVD-CNT-modified electrode exhibited an attractive low potential response towards sulfide in a manner analogous to the improvements observed previously for the oxidation of NADH [21] and hydrogen peroxide [34]. The sulfide oxidative response has been examined over a wide pH

Acknowledgements

Financial support from the US EPA STAR Program (grant number RD-83090001-0) is gratefully acknowledged.

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