ARTICLE Electrochemical studies of ascorbic acid, dopamine, and uric acid at a DL-norvaline-deposited glassy carbon electrode Mahmoud Mohamed Kamel, Ehab Mahmoud Abdalla, Mohamed Sayed Ibrahim, and Yassin Mohamed Temerk Abstract: The electrochemical behavior of ascorbic acid (AA), dopamine (DA), and uric acid (UA) on a DL-norvaline-modiﬁed glassy carbon electrode (GCE) was studied by cyclic voltammetry. The bare GCE failed to distinguish the oxidation peaks of AA, DA, and UA in phosphate-buffered solution (pH 5.0), while the DL-norvaline-modiﬁed GCE could separate them efﬁciently. In differential pulse voltammetric (DPV) measurements, the modiﬁed electrode resolved the overlapped voltammetric responses of AA, DA, and UA into three well-deﬁned voltammetric peaks. Under optimum conditions, the anodic peak currents of DPV for AA, DA, and UA were proportional to the concentration in the range of 20–400, 1–40, and 15–180 mol/L, respectively, with a correlation coefﬁcient (r) of around 0.998. The detection limits were 5, 0.3, and 10 mol/L (S/N = 3) for AA, DA, and UA, respectively. Satisfactory results were achieved for the determination of AA in vitamin C tablets, DA in a dopamine ampoule sample, and UA in human blood serum samples. Key words: DL-norvaline-modiﬁed GCE, cyclic voltammetry, differential pulse voltammetry, ascorbic acid, dopamine, uric acid. Résumé : Le comportement électrochimique de l’acide ascorbique (AA), de la dopamine (DA) et de l’acide urique (AU) sur une électrode de carbone vitreux (« glassy carbon electrode » ou CGE) modiﬁée par la DL-norvaline a été étudié a ` l’aide de la voltampérométrie cyclique. La CGE nue n’a pas permis de séparer les pics d’oxydation de l’AA, de la DA et de l’AU dans une solution tampon de phosphate (pH = 5) alors que la CGE modiﬁée par la DL-norvaline s’est avérée efﬁcace pour les faire apparaître distinctement. Les mesures effectuées par voltampérométrie pulsée différentielle (VPD) ont montré que l’électrode modiﬁée a permis de résoudre le problème du chevauchement des pics voltampérométriques de l’AA, de la DA et de l’AU, en faisant apparaître trois pics bien distincts. Dans des conditions optimales, les courants de pic anodiques de VPD correspondant a ` l’AA, a ` la DA et a ` l’AU étaient proportionnels a ` des concentrations comprises respectivement entre 20 et 400, entre 1 et 40 et entre 15 et 180 mol/L, le coefﬁcient de corrélation (r) étant approximativement égal a ` 0,998. Les limites de détections étaient respective- ment de 5, 0,3 et 10 mol/L (S/N = 3) pour l’AA, la DA et l’AU. Des résultats satisfaisants ont été obtenus lors des mesures de concentration de l’AA dans des comprimés de vitamine C, de la DA dans des échantillons d’ampoule de dopamine et de l’AU dans des prélèvements de sérum sanguin humain. [Traduit par la Rédaction] Mots-clés : CGE modiﬁée par la DL-norvaline, voltampérométrie cyclique, voltampérométrie pulsée différentielle, acide ascor- bique, dopamine, acide urique. Introduction Ascorbic acid (AA) (vitamin C), dopamine (DA), and uric acid (UA) are compounds of great biological and chemical interest and play a signiﬁcant role in the metabolic system of human bodies. DA is a monoamine neurotransmitter and it plays a substantial role in the functioning of the central nervous and renal and hor- monal systems. 1 Its abnormal level in brain results in many neu- rological and psychiatric disorders including Parkinson’s disease, schizophrenia, and Alzheimer’s disease. 2–4 AA is important in nu- trition and health care of human beings. It is an essential additive as an antioxidant agent in foods, beverages, and pharmaceutical applications due to its participation in several human metabolic reactions. 5 Moreover, AA is necessary for the skin, connective tissues, and immune system. 6 UA is the end product of purine metabolism in the human body. 7 Its high concentration levels in body ﬂuids (e.g., serum and urine) are a sign of gout, hyperuricemia, or Lesch−Nyhan syndrome. 8 Study and determination of such im- portant compounds clinically and industrially is currently an area of very active investigation. Electrochemical methods have more advantages over other methods because of their quick response, high sensitivity, low detection limit, and simple use. The major problem in oxidation and detection of DA, AA, and UA at tradi- tional bare electrodes is an overlapping voltammetric response due to exhibiting oxidation peaks at potentials very close to each other. 9–11 Hence, several researches have been focused on im- provement of their selectivity, simultaneous detection, and coex- istence. The use of chemically modiﬁed electrodes has been one promising approach for minimizing these effects. Certain surface- bound redox mediators enhance the electron transfer kinetics and thus decrease the operating potential. The voltammetric dif- ferentiation between AA−DA and AA−UA at various mediators 12–20 has been reported much more extensively than their simultane- ous discrimination. Of such few attempts for their simultane- ous determination, different mediators have been investigated such as polymers, 21–25 carbon-based materials, 26–28 cetyltrimethyl ammonium bromide, 29 chitosan-functionalized graphene, 30 and acetone−water. 31 The use of amino acids on the electrode surface Received 22 January 2014. Accepted 23 February 2014. M.M. Kamel. Faculty of Science (Assiut), Al Azhar University, Cairo, Egypt. E.M. Abdalla. Gohina Girls institute, Sohag, Egypt. M.S. Ibrahim and Y.M. Temerk. Faculty of Science, Assiut University, Assiut, Egypt. Corresponding author: Mahmoud Mohamed Kamel (e-mail: mmk.electro@yahoo.com). 329 Can. J. Chem. 92: 329–336 (2014) dx.doi.org/10.1139/cjc-2014-0024 Published at www.nrcresearchpress.com/cjc on 25 February 2014. Can. J. Chem. Downloaded from www.nrcresearchpress.com by Dr. Ehab Abdalla on 04/01/14 For personal use only.