Hindawi Publishing Corporation Journal of Chemistry Volume 2013, Article ID 946230, 7 pages http://dx.doi.org/10.1155/2013/946230 Research Article A Voltammetric Sensor Based on NiO Nanoparticle-Modified Carbon-Paste Electrode for Determination of Cysteamine in the Presence of High Concentration of Tryptophan Hassan Karimi-Maleh, 1 Maryam Salimi-Amiri, 2 Fatemeh Karimi, 1 Mohammad A. Khalilzadeh, 1 and Mehdi Baghayeri 3 1 Department of Chemistry, Science and Research Branch, Islamic Azad University, Mazandaran, Iran 2 Department of Physics, Sari Branch, Islamic Azad University, Sari, Iran 3 Department of Chemistry, Faculty of Science, Hakim Sabzevari University, P.O. Box 397, Sabzevar, Iran Correspondence should be addressed to Hassan Karimi-Maleh; h.karimi.maleh@gmail.com and Fatemeh Karimi; fm024@gmail.com Received 25 May 2013; Accepted 8 September 2013 Academic Editor: Yehia Mechref Copyright © 2013 Hassan Karimi-Maleh et al. Tis is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. A carbon-paste electrode modifed with ferrocenecarboxaldehyde and NiO nanoparticle (NiO/NPs) was used for the sensitive and selective voltammetric determination of cysteamine in the presence of tryptophan. Te oxidation of cysteamine at the modifed electrode was investigated by cyclic voltammetry (CV), chronoamperometry, and square-wave voltammetry (SWV). Te values of the catalytic rate constant ( ℎ ) and difusion coefcient () for cysteamine were calculated. Te modifed electrode exhibits an efcient electron-mediating behavior together with well-separated oxidation peaks for cysteamine and tryptophan. At the optimum pH of 7.0 in a 0.1M phosphate bufer solution, the SWV anodic peak currents showed a linear relationship versus cysteamine concentrations in the range of 0.09–300.0 M and a detection limit of 0.06 M. Finally, the proposed method was also examined as a selective, simple, and precise electrochemical sensor for the determination of cysteamine in real samples such as urine and capsule. 1. Introduction Cysteamine is a signifcant thiol drug for the treatment of cystinosis [1]. Since cysteamine therapy has become available for patients with nephropathic cystinosis (MIM219800) in the early 1980s, quality of life for these patients has greatly improved [2, 3]. Te most frequent and most severe form, infantile cystinosis, is characterized by the development of renal Fanconi syndrome in the frst year of life and leads to end-stage renal disease in the frst decade of life when untreated [4]. Mutations in the CTNS gene, encoding for lysosomal cystine transporter cystinosin, are the causes of cystinosis [5]. Lysosomal accumulation of cystine, which is the hallmark of this autosomal recessive disorder, can be depleted by the aminothiol cysteamine [6]. Although treat- ment with cysteamine substantially decreases intracellular cystine accumulation, renal Fanconi syndrome is not cured, but end-stage renal disease can be postponed in the majority of the patients. Numerous chemical and instrumental tech- niques for detection of cysteamine have been reported [7–11]. Tryptophan (Trp), a vital constituent of proteins and a precursor for biologically important molecules such as the neurotransmitter serotonin and the neurohormone mela- tonin, is an amino acid essential to humans and animals due to its physiological roles [12]. It is also indispensable in human nutrition for establishing and maintaining a positive nitrogen balance [2]. In previous reports, spectroscopy [13], chemiluminescence [14], capillary electrophoresis [15], high- performance liquid chromatography [16], and electroanalysis [17–19] have been developed for tryptophan determination. Among these methods, electrochemical techniques, with high sensitivity, high accuracy, and simple operation, have