Electrochemical studies of tamsulosin hydrochloride using multiwalled carbon nanotube-modified glassy carbon sensor L. Lonappan, S. Issac, R. Joseph, D. Thomas, K.Girish Kumar Department of Applied Chemistry, Cochin University of Science and Technology, Kochi 682022, India E-mail: giri@cusat.ac.in Published in Micro & Nano Letters; Received on 22nd July 2011; Revised on 10th October 2011 A differential pulse voltammetric sensor for the determination of tamsulosin hydrochloride (TAM) using multiwalled carbon nanotubes (MWNTs)–Nafion-modified glassy carbon electrode (GCE) has been developed. MWNTs were dispersed in water with the help of Nafion and were used to modify the surface of GCE via solvent evaporation. At MWNT-modified electrode, TAM gave a well-defined oxidation peak at a potential of 1084 mV in 0.1 M acetate buffer solution of pH 5. Compared to the bare electrode, the peak current of TAM showed a marked increase and the peak potential showed a negative deviation. The determination conditions, such as the amount of MWNT–Nafion suspension, pH of the supporting electrolyte and scan rate, were optimised. Under optimum conditions, the oxidation peak current was proportional to the concentration of TAM in the range 1 × 10 23 M–3 × 10 27 M with a detection limit of 9.8 × 10 28 M. The developed sensor showed good stability, selectivity and was successfully used for the determination of TAM in pharma- ceutical formulations and urine samples. 1. Introduction: Recent years have witnessed significant interest in nanomaterials, such as nanocrystals, nanowires and nanotubes for their wide applications. Carbon nanotubes (CNTs) have been at the forefront of novel nano-scale investigations owing to their unique structure-dependent electronic and mechanical properties. Their small dimensions, closed topology and lattice helicity have enabled nanotubes to influence broad areas of science and technol- ogy, ranging from super strong nanocomposites to nanoelectronics [1]. Certainly, in the studies of electrochemical properties of biologically relevant molecules such as hydrogen peroxide [2], nicotinamide adenine dinucleotide [3], dopamine [4] and so on, multiwalled carbon nanotube (MWNT)-modified electrodes have shown superior performances compared to other carbon electrodes. Also, their subtle electronic properties suggest that MWNTs have the stability to promote the direct electron transfer reaction of some important biomolecules, such as microperoxidase [5], horseradish peroxidase [6], haemoglobin [7], glucose oxidase [8] and so on. Tamsulosin hydrochloride (TAM) is a potent and selective a 1 - adrenoceptor antagonist. This drug is used as an oral medication to ameliorate the bladder-outlet obstruction associated with prosta- tichypertrophy. Prostatic and urethral smooth muscle tone is main- tained by stimulation of postjunctional a 1 -adrenoceptors through the release of noradrenaline from the adrenergic nerves in both animals and humans [9]. Benign prostatic hyperplasia is associated with a bladder-outlet obstruction that has been postulated to occur via both mechanical compression exerted by the increased bulk of the prostate and alterations in the neural control of the prostatic smooth muscle. In recent years, a 1 -adrenoceptor antagonists have been increasingly used for the symptomatic treatment of lower urinary tract symptoms, suggestive of benign prostatic obstruction [10]. Several analytical methods have been reported for the analysis of TAM. These include liquid chromatography-mass spectrometry [11], liquid chromatography with fluorescence detection [12] and radio receptor assay [13]. The major drawbacks of these reported methods include potential loss of drug in the re-extraction pro- cedure, lengthy, tedious and time-consuming plasma sample prep- aration and extraction processes and interference of endogenous substances. Above all, these methods also require sophisticated and expensive instrumentation. In this context electrochemical detection methods are often advantageous as they offer high sensitivity, ease of use, portability and relatively inexpensive equipment. The main electrochemical methods involve reaction at different solid electrodes such as glassy carbon electrode (GCE), platinum, diamond, gold and transition metal-oxide electrode. However, the application of these electrodes is limited, because several species can poison the electrode surface and decrease the sensitivity and accuracy. The performance characteristics of these electrodes can be enhanced by using chemically modified electrodes. In continuation to our work on electroanalysis [14–17], an attempt has been made to study the electrochemical oxidation of TAM at an MWNT-modified GCE and to develop a convenient and sensitive method for the determination of TAM by differential pulse voltammetry (DPV) using the developed sensor. 2. Experimental 2.1. Apparatus: Voltammetric measurements were performed with a PC-controlled BAS Epsilon electrochemical analyser (Bio-analyti- cal System, USA). A three-electrode cell system consisting of MWNT – Nafion-modified GCE as working electrode, an Ag/ AgCl reference electrode and a platinum wire auxiliary electrode were used. Fourier transform infrared (FTIR) spectra of the MWNT were recorded on JASCO-4100 FTIR spectrometer using KBr discs. Scanning electron microscopic (SEM) images were obtained on a JOEL 6390 LV. Pure TAM was obtained as a gift sample and was used as received. Tablets of TAM, Veltam capsules (Intas Pharmaceuticals, India) were purchased from local drug stores. MWNTs (6–20 nm diameter and 1–5 mm length) and Nafion were purchased from Aldrich. MWNT was refluxed in concentrated HNO 3 for 48 h to cause segmentation and carboxylation [18]. All the chemicals used were of analytical reagent grade quality (Merck or Sigma) and they were employed without further purifi- cation and all solutions were prepared with Millipore water. 2.2. Preparation of MWNT-modified GCE: Prior to modification, the GCE was mechanically polished with alumina slurry down to 0.05 mm on a polishing cloth. Then the electrode was sonicated in methanol, water, HNO 3 solution and acetone, respectively. Acid treatment of MWNT was carried out as described in the litera- ture [18]. As a result of this treatment, length of MWNT got shor- tened and they gained functional groups which made them hydrophilic. The peaks at 1703 and 1564 cm 21 of the FTIR spec- trum demonstrated that carboxy groups and carboxylate groups are present on the surface of treated MWNTs. Micro & Nano Letters, 2011, Vol. 6, Iss. 10, pp. 867–870 867 doi: 10.1049/mnl.2011.0384 & The Institution of Engineering and Technology 2011