AVoltammetric Sensor Based on Modified Multiwall Carbon Nanotubes for Cysteamine Determination in the Presence of Tryptophan Using p-Aminophenol as a Mediator Ali A. Ensafi,* Hassan Karimi-Maleh Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111, Iran tel.: + 98 311 3912351; fax: + 98 311 3912350 *e-mail: Ensafi@cc.iut.ac.ir Received: April 22, 2010; & Accepted: June 16, 2010 Abstract Determination of cysteamine and tryptophan is described by electrochemical methods using p-aminophenol-multi- wall carbon nanotubes paste electrode. Cysteamine and tryptophan in mixture can each be measured independently from each other with a potential difference of 600 mV. The results showed that the electrocatalytic currents in- creased linearly with cysteamine and tryptophan concentrations over the ranges 0.5–300 mmol L 1 and 10.0– 650 mmol L 1 , respectively. The detection limits for cysteamine and tryptophan are found to be 0.14 and 5.9 mmol L 1 , respectively. The proposed method is successfully employed for the determination of cysteamine in both capsule and urine samples. Keywords: Cysteamine, Tryptophan, Carbon nanotubes, p-Aminophenol, Electrocatalytic effect, Voltammetry DOI: 10.1002/elan.201000270 1. Introduction Cysteamine, 2-mercaptoethylamine, is the simplest stable aminothiol and a degradation product of the amino acid cysteine. Under the trade name of Cystagon, cysteamine is used in the treatment of cysteine excretion disorders. Cysteamine cleaves the disulfide bond with cysteine to produce molecules that can escape the metabolic defect in cystinosis and cystinuria [1]. It is also used for the treatment of radiation sickness [2]. Cysteamine crosses the plasma and lysosomes and reacts with crystallized cys- tine within the lysosomes to form cysteine and cysteine– cysteamine mixed disulfides, leaving through the lysine porter [3]. The cysteamine and its disulfide, cystamine, have been shown to be neuroprotective in a number of cell culture and animal models [4]. Several methods have been reported for the determina- tion of CA in different samples including chromatography [5–7], electrophoresis [8], gas chromatography with flame photometric detection [9], ion exchange chromatography [10], and electrochemical methods [10, 12] using modified electrodes. However, chromatographic methods still have to cope with certain limitations such as selecting a suita- ble column or a mobile phase while liquid chromato- graphic methods are limited by finding a suitable reactant for post-column reactions (in order to increase sensitivi- ty). Long response time, expensive instruments, compli- cated procedure, and low detection capability are the shortcomings associated with most such methods report- ed. Tryptophan is one of the 20 amino acids and an essen- tial one in the human diet. In the standard genetic code, it is encoded as codon UGG. While the d-stereoisomer is occasionally found in naturally produced peptides (for ex- ample, the marine venom peptide contryphan), only the l-stereoisomer of tryptophan is used in structural or enzyme proteins [13]. Tryptophan is commonly synthe- sized in plants and microorganisms from shikimic acid or anthranilate [14]. Health food stores have for some time been supplying tryptophan as a dietary supplement. Many people have reported it as a safe and reasonably effective sleep aid, which might be due to its ability to increase brain levels of serotonin (a calming neurotransmitter when present in moderate levels) [15] and/or melatonin (a sleep-inducing hormone secreted by the pineal gland in response to darkness or low light levels) [16]. Some users of MDMA (street name “ecstasy”) will eat trypto- phan-containing foods to shorten the ’come down’ effect of having lower levels of serotonin than usual (due to an extra large release caused by the drug). However, trypto- phans effectiveness as a sleep aid, especially in normal patients, has not been fully confirmed by the mixed re- sults obtained from clinical research [17]. Furthermore, tryptophan has shown some effectiveness in the treatment of a variety of other conditions typically associated with low serotonin levels in the brain such as seasonal affective disorder [18] and premenstrual dysphonic disorder [19]. 2558  2010 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim Electroanalysis 2010, 22, No. 21, 2558 – 2568 Full Paper