Electrochemical detection of l-cysteine using a boron-doped carbon nanotube-modified electrode
Introduction
The study of thiols (e.g., homocysteine, l-cysteine, and glutathione) provides critical insight into the proper physiological function and diagnosis of disease states. This is because the thiol compounds are of special significance in biochemistry and environmental chemistry [1], [2], [3]. l-Cysteine (L-CySH) is a sulfur-containing α-amino acid. It is one of about 20 amino acids commonly found in natural proteins [4]. It plays an important role in biological systems and has been used widely in the medicine and food industries [5]. Moreover, the coupling of l-cystine/l-cysteine is used as a model of the disulfide bond. It also serves as a model for the thiol group of proteins in a variety of biological media [4], [6]. Considering this, effort has been taken to develop sensitive methods for the detection of L-CySH in bodily fluids, pharmaceuticals, and food samples [7]. In the past, electrochemical methods have been the most sensitive/favored methods for the determination of thiols (and of cysteine in particular) [6], [8]. This is because they are easily automated, have a high sensitivity, and are capable of being readily integrated with other techniques for multi-analysis [9], [10].
However, the electrochemical method for analysis of thiols remains very challenging. For example, at the conventional electrodes (e.g., glassy carbon and gold electrodes), the electrochemical response of thiols is not satisfactory due to their sluggish electrochemical processes [8], [11], [12], [13]. High overpotentials of electrochemical oxidation are needed for thiols at the conventional electrodes. This results in surface oxide formation and fouling of these electrodes [7], [14]. In order to overcome these drawbacks, many strategies have been employed. These strategies include the use of mercury electrodes, diamond electrodes [4], [15], [16], enzyme-based biosensors [3], [9], [17] and chemically modified electrodes [2], [18], [19]. Although the analysis of thiols has been improved greatly, there are still some disadvantages that hinder thiol detection. These challenges include low sensitivity, complexity and expense.
Considering all this, it is critical to find a suitable material for the effective and simple determination of thiols. In recent years, novel nanomaterial (foreign atoms, B or N atom) doped carbon nanotubes have been prepared and used for many applications [20], [21], [22], [23]. This interest has stemmed from the finding that the electrochemical properties of CNTs can be improved by doping with foreign atoms [22], [23], [24], [25], [26]. In this paper, the electroanalysis of thiols (l-cysteine as a model) at a boron-doped carbon nanotube (BCNT)-modified glassy carbon (GC) electrode (BCNT/GC) was investigated. The boron-doping of CNTs introduces more edge plane sites on the surface of CNTs and may thus lead to more facile electron transfer [27], [28] because edge plane sites are the predominant sites of electron transfer [27], [28], [29], [30]. It is expected that this system will be particularly attractive for electrochemical applications. In particular, it may be useful in electrocatalysis and electrochemical determinations [31], [32]. On the other hand, more functional groups were produced at the surface of BCNTs. This finding is useful in resisting the fouling of the modified electrodes. The functional groups at the defective sites of CNTs may block the adsorption of species onto the CNTs [32], [33]. Using the BCNT/GC electrode for electroanalysis of L-CySH eliminates fouling of the electrodes. The intrinsic properties of the BCNT allow this resolution. Moreover, the BCNT/GC electrode shows excellent performance in that it is sensitive, exhibits a rapid response, and has good long-term stability. These features should prove favorable in the practical analysis of thiols.
Section snippets
Apparatus and reagents
Electrochemical measurements were performed on a CHI660A electrochemical workstation (Chenhua Instrument Company of Shanghai, China) with a conventional three-electrode cell. A BCNT/GC electrode (3 mm in diameter) was used as the working electrode. A saturated calomel electrode (SCE) and a platinum wire were used as the reference and auxiliary electrodes, respectively. All measurements were carried out in a phosphate buffer solution (PBS, 0.067 M, pH 7.4) at room temperature (25 ± 2 °C). All the
Electrooxidation of L-CySH at the BCNT/GC electrode
Fig. 1 presents the cyclic voltammograms of the bare GC (A), CNT/GC (B), and BCNT/GC (C) electrodes in 0.067 M phosphate buffer solution (PBS, pH 7.4) with and without 2 mM L-CySH at a scan rate of 50 mV s−1. It is clear that the background current of the BCNT/GC electrode is higher than that of the CNT/GC and GC electrodes. This is due to the rough surface of the BCNT caused by boron doping [35]. On the other hand, the oxidation peak of L-CySH can be observed at ca. +0.50 V using the bare GC
Conclusions
The electrochemical determination of L-CySH using a BCNT/GC electrode was investigated. The BCNT/GC electrode shows excellent electrochemical properties. These properties are due to more edge plane sites and functional groups on the surface of BCNTs. The BCNT/GC electrode shows striking analytical properties, such as high sensitivity, a low detection limit, good stability and a resistance to interference. These characteristics demonstrate that the BCNT/GC electrode should be suitable for the
Acknowledgments
This work was financially supported by NSFC (20675027, 20575019, 20335020), “973” Program of China (2006CB600903) and the SRF for ROCS, SEM, China (2001-498).
References (40)
- et al.
Curr. Appl. Phys.
(2007) - et al.
J. Electroanal. Chem.
(2008) - et al.
Anal. Biochem.
(2005) - et al.
Anal. Biochem.
(2007) - et al.
Anal. Biochem.
(1997) - et al.
Anal. Biochem.
(1989) - et al.
J. Chromatogr. B
(1998) - et al.
J. Electroanal. Chem.
(1994) - et al.
J. Electroanal. Chem.
(1977) - et al.
Electrochim. Acta
(2002)