Graphite-Polyurethane and Graphite-Silicone Rubber Composite Electrodes for Electrochemical Characterization and Determination of Minoxidil Rogerio A. Sousa, a, c, d Sidney X. Dos-Santos , b Eder T. G. Cavalheiro, b Christopher M. A. Brett* c a Departamento de Química, Universidade Federal de S¼o Carlos, 13.565-905, S¼o Carlos, SP, Brazil b Instituto de Química de S¼o Carlos, Universidade de S¼o Paulo, 13.560-970, S¼o Carlos, SP, Brazil c Departamento de Química, Faculdade de CiÞncias e Tecnologia, Universidade de Coimbra, 3004-535 Coimbra, Portugal tel: 351-239-854470; fax: + 351-239-827703 d Current address: Centro Universitµrio das Faculdades Associadas (UNIFAE), Largo Eng. Paulo de Almeida Sandeville, 15, 13870-377, S¼o Jo¼o da Boa Vista, SP, Brazil *e-mail: brett@ci.uc.pt Received: November 7, 2012 Accepted: January 9, 2013 Published online: February 22, 2013 Abstract Graphite-polyurethane (GPUE) and graphite-silicone rubber (GSRE) electrodes have been used for oxidative de- termination of the vasodilator minoxidil in pharmaceutical samples and in plasma and urine electrolyte solutions. The electrooxidation process at 0.840 V (GPUE) and 0.860 V (GSRE) vs. SCE was characterized over a wide pH range by cyclic voltammetry and electrochemical impedance spectroscopy. Quantification using cyclic, differential pulse, and square-wave voltammetry, gave results agreeing with high pressure liquid chromatography, that has simi- lar micromolar detection limits, but with a simpler analytical procedure. Advantages of these composite electrodes are non-adsorption of analyte or oxidation products, a long useful life and robustness. Keywords: Composite electrode, Graphite-polyurethane electrode, Graphite-silicone rubber electrode, Minoxidil DOI: 10.1002/elan.201200606 1 Introduction Minoxidil (2,4-diamino-6-piperidinopyrimidine-3-oxide, MX, Scheme 1) is an odorless white crystalline powder, insoluble in water, acetone or alkaline solutions, slightly soluble in alcohols, and freely soluble in acidic solutions [1], with pK a = 4.6 [2]. MX has been used as an orally ad- ministrated peripheral vasodilator drug, applied in the treatment of refractory hypertension patients [3]. Exces- sive oral administration of this drug causes liquid reten- tion and hirsutism [4]. Initially described as an antihyper- tensive drug, MX has also had new applications in derma- tology, especially in the treatment of androgenic alopecia [4, 5]. In this case, this drug has been topically applied in order to stimulate hair growth by inducing vasodilation with consequent increase in the local irrigation and blood flow [5]. Commercially available topical use formulations typically contain 2 % MX (20 mg mL 1 ), in ethanol/propy- leneglycol or their mixture with 2-n-nonyl-1,3-dioxolane as a vehicle [6,7]. The literature describes MX determination using liquid chromatography [8, 9], photometric titrations [10], UV-vis spectrophotometry [11] and flow injection with both pho- tometric [4, 12] and amperometric [13] detection proce- dures. Electroanalytical procedures based on reduction using differential pulse polarography [3] and polarogra- phy plus spectrophotometry [14] have been reported. The use of composite electrodes, which are defined by Tallman and Petersen [15] as “a material consisting of at least one conducting phase commingled with at least one insulating phase”, as detectors in electrochemical analysis is becoming common. In such materials the insulator phase acts as an agglutinant. These electrodes can be clas- sified according to the way in which the conductor and the insulator are distributed within the material. In partic- ular, disperse composite materials in which graphite is ag- glutinated by polymers present good properties for use as electrochemical sensors [16–19]. The purpose of this work is to investigate the voltam- metric behaviour of MX using composite electrodes Scheme 1. Structural formula of MX. 706  2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Electroanalysis 2013, 25, No. 3, 706 – 715 Full Paper