Modified multiwall carbon nanotubes paste electrode as a sensor for simultaneous determination of 6-thioguanine and folic acid using ferrocenedicarboxylic acid as a mediator Ali A. Ensafi * , H. Karimi-Maleh Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111, Iran article info Article history: Received 16 November 2009 Received in revised form 8 January 2010 Accepted 13 January 2010 Available online 18 January 2010 Keywords: Simultaneous determination 6-Thioguanine and folic acid Carbon nanotubes paste electrode Electrocatalytic effect Voltammetry abstract A multiwall carbon nanotubes modified electrode was prepared using ferrocenedicarboxylic acid as a mediator to be used as an amperometric sensor for the simultaneous determination of 6-thioguanine and folic acid. The electrochemical behaviors of the compounds at this modified electrode were studied using cyclic voltammetry, differential pulse voltammetry, and electrochemical impedance spectroscopy. The results indicated that the chemically modified electrode exhibited an efficient electrocatalytic activ- ity towards the oxidation of 6-thioguanine and folic acid at pH 9.0. Using chronoamperometry, the cat- alytic reaction rate constant was calculated at 8.2  10 2 (mol L 1 ) 1 s 1 . The catalytic peak current was linearly dependent on 6-thioguanine and folic acid concentrations in the range of 0.01–100 lmol L 1 6-thioguanine and 4.6–152 lmol L 1 folic acid. The detection limits for 6-thioguanine and folic acid were 0.0085 lmol L 1 and 1.1 lmol L 1 , respectively. The RSD% for 10 replicates determination of 0.70 and 11.0 lmol L 1 of 6-thioguanine were 1.3% and 1.1%, respectively, whereas for 15.0 and 30.0 lmol L 1 folic acid, they were 0.9% and 1.1%, respectively. The modified electrode showed good sensitivity, selectivity, and stability. It was successfully applied for the determination of both 6-thioguanine and folic acid in real samples. Ó 2010 Elsevier B.V. All rights reserved. 1. Introduction 6-Thioguanine (6-TG), 2-thiouracil, 4-thiouracil, and 2,4-dithio- uracil have been studied frequently for its numerous pharmacolog- ical, biochemical, and biological applications [1]. Belonging to the family of anti-metabolites drugs, 6-TG is a drug used in the treat- ment of cancer [2]. Its principal use is for the treatment of acute leukaemias and chronic myeloid leukemia [3]. As a guanine ana- logue, it is transformed inside the cell into 6-thioguanilyic acid, which, by seudofeedback interference with purine biosynthesis, interferes with the synthesis of guanine nucleotides [4]. Some of its activities may also be due to the incorporation of thioguanine nucleotides into both RNA and DNA, but the end-result is inducing cell cycle arrest and apoptosis. It is metabolized via methylation by thiopurine methyltransferase [5]. Several methods have been proposed for the determination of 6-TG in tablets or urine including chromatography [6–8], spectro- fluorimetric [9], and electrochemical methods [10,11] using modi- fied electrodes. However, there are still certain limitations in spectrofluorimetric and chromatographic methods such as select- ing suitable column or mobile phase, finding a suitable reactant for post-column reaction (in order to increase sensitivity) in liquid chromatography, and facing many interfering substances in spec- trofluorimetric methods. Most of the methods reported suffer from disadvantages such as complicated procedure, long response time, requirement of expensive instruments, and low detection capability. Folic acid (N-[p-{[(2-amino-4-hydroxy-6-pteridinyl) methyl] amino}benzoyl]-l-glutamic acid), also known as vitamin M and folacin or folate (the anionic form), is a form of the water-soluble vitamin B 9 . Folate is also found in vegemite or marmite, with an average serving (5 g) containing 100 lg. Folate is also synthesized in bacteria. Several important nutrients serve as building blocks of a healthy pregnancy. Folic acid is one such nutrient of great impor- tance, especially for women planning for pregnancy. Adequate fo- late intake during the preconception period helps protect against a number of congenital malformations [12]. The most notable birth defects that occur from folate deficiency are neural tube defects, which result in malformations of the spine (spina bifida), skull, and brain (anencephaly [13,14]. Women planning for pregnancy are advised to eat foods fortified with folic acid or to take supple- ments in addition to eating folate-rich foods to reduce the risk of some serious birth defects. Taking 400 lg of synthetic folic acid 1572-6657/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2010.01.010 * Corresponding author. Tel.: +98 311 3912351; fax: +98 311 3912350. E-mail address: Ensafi@cc.iut.ac.ir (A.A. Ensafi). Journal of Electroanalytical Chemistry 640 (2010) 75–83 Contents lists available at ScienceDirect Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem