Voltammetric determination of cysteine using carbon paste electrode modified with Co(II)-Y zeolite

Abstract

A novel zeolite modified electrode for use in voltammetric determination of l-cysteine (CySH) was described. The electrode comprises a Co(II)-exchanged zeolite Y as modifier in carbon paste matrix. First, the electrochemical behavior of Co(II) in modified carbon paste electrode was studied. The results demonstrated that diffusion can control the redox process of cobalt cations at the surface of the modified electrode. Then, the behavior of the electrode in the presence of CySH was studied by using cyclic voltammetry and a novel behavior was observed. In high concentration of CySH (above 10 mmol L⁻¹), one pair of semi-reversible electrochemical extra peak was observed which was assigned to the processes of oxidation–reduction of CySH at the unmodified and modified electrode. Acidic conditions with respect to the neutral one cause an increase in the electrode response. The modified electrode showed a suitable linear calibration graph in the concentration range of 1.0 × 10⁻⁹–1.0 × 10⁻³ mol L⁻¹ cysteine with a detection limit of 2.37 × 10⁻¹⁰ mol L⁻¹. The influence of potential interfering substances on the peak current was studied and the results showed that the method was highly selective for determination of CySH. Thus, the proposed electrode was used for the determination of CySH in real samples including human blood serum, urine, N-acetylcysteine tablet and powdered poultry feed and the satisfactory results were obtained. Typical features of the sensor can be summarized as: low cost, simple preparation, fast response, good stability and selectivity, wide linear range, low detection limit and high reproducibility.

Introduction

Cysteine ([(R)-2-amino-3-mercaptopropanoic acid]), one of about 20 amino acids commonly found in natural proteins, is a sulfur-containing amino acid. Cysteine (CySH) is critical for the proper metabolism of a number of essential biochemicals such as: heparin, biotin, lipoid acid, coenzyme A and glutathione. It has been used as a prospective radiation protector and cancer indicator in a number of pathological conditions including Parkinson's and Alzheimer's diseases as well as autoimmune deficiency syndrome (AIDS) [1], [2]. Moreover, it has also been widely used in the food and pharmaceutical industries as an antioxidant and biomarker, respectively [3]. Low level of CySH causes certain diseases including slow growth in children, depigmentation of hair, edema, lethargy, liver damage, loss of muscle and fat, skin lesions, and weakness [4]. Therefore, the detection of CySH in body fluids has become more important from biological and pharmacological stand points. Several methods for its detection have been reported including flow injection [5], [6], high performance liquid chromatography (HPLC) [7], [8], colorimetry [9], [10], chemiluminescence [11], spectrofluorimetry [12] and electrochemistry [13], [14], [15], [16], [17], [18]. Compared with the other methods, electrochemical techniques offer several advantages such as simplicity, ease of preparation and method, high sensitivity and selectivity, and relatively low cost [19], [20].

Chemically modified electrodes (CMEs) have recently attracted much attention due to their significant advantages [21], [22], [23], [24], [25], [26]. So, various chemically modified electrodes have been developed for the oxidation and detection of CySH [27], [28]. As a subcategory of CMEs, zeolite-modified electrodes (ZMEs) have been greatly studied [29], [30], [31]. ZMEs have some advantages over other CMEs due to their ion-exchange capacity and the molecular (size, shape and charge) selectivity properties of zeolites [32]. Particularly, substantial efforts have been dedicated to the development of sensors and biosensors using ZMEs [33], [34], [35], [36], [37].

In this work, we constructed a novel modified carbon paste electrode by incorporation of Co(II)-exchanged zeolite Y (Co²⁺Y/ZMCPE). The Co(II)-Y was obtained by ion exchanging of the parent Na-Y zeolite in a Co(II) solution as nitrate salt. Our preliminary experiments showed that the obtained Co²⁺Y/ZMCPE electrode exhibits good voltammetric response towards CySH. Hence, the fabricated electrode was used for the determination of CySH. Finally, the analytical performance of the electrode was evaluated for the determination of CySH in the patient blood serum, urine, N-acetylcysteine (ACC) tablet and powdered poultry feed (PPF) samples.