Analytical Potentialities of Carbon Nanotube/Silicone Rubber Composite Electrodes: Determination of Propranolol Sidney Xavier dos Santos , a, b Éder T. G. Cavalheiro, a Christopher M. A. Brett* b a Departamento de Química e Física Molecular, Instituto de Química de S¼o Carlos, Universidade de S¼o Paulo, C.P. 676, 13560-970 S¼o Carlos/SP, Brazil b Departamento de Química, Faculdade de CiÞncias e Tecnologia, Universidade de Coimbra, 3004-535 Coimbra, Portugal *e-mail: brett@ci.uc.pt Received: April 17, 2010; & Accepted: July 20, 2010 Abstract A new composite electrode based on multiwall carbon nanotubes (MWCNT) and silicone-rubber (SR) was devel- oped and applied to the determination of propranolol in pharmaceutical formulations. The effect of using MWCNT/ graphite mixtures in different proportions was also investigated. Cyclic voltammetry and electrochemical impedance spectroscopy were used for electrochemical characterization of different electrode compositions. Propranolol was determined using MWCNT/SR 70 % (m/m) electrodes with linear dynamic ranges up to 7.0 mmol L 1 by differential pulse and up to 5.4 mmol L 1 by square wave voltammetry, with LODs of 0.12 and 0.078 mmol L 1 , respectively. Analysis of commercial samples agreed with that obtained by the official spectrophotometric method. The electrode is mechanically robust and presented reproducible results and a long useful life. Keywords: Carbon nanotubes, Silicone rubber, Composite electrodes, Propranolol, Nanotubes DOI: 10.1002/elan.201000262 1. Introduction Since their discovery in 1991, by Ijima [1], carbon nano- tubes (CNT) have been of considerable interest due to their unique properties [2]. CNT are nanostructures con- sisting of graphene sheets with hexagonal sp 2 carbon atoms, arranged in the form of cylinders, with diameters of the order of nanometres and lengths of micrometers. They combine high surface area, conductivity, chemical stability and significant mechanical strength [3]. The CNT can behave like metals or semiconductors de- pending on the structure [4]. Their electronic properties suggest that they present the ability to promote electron transfer in electrochemical reactions with electroactive species in solution [5–9]. Moreover, CNT have attracted considerable attention due to the reported electrocatalyt- ic properties of carbon nanotube modified electrodes [10– 15]. The advantages of CNT as electrode material or as modifier of conventional working electrodes in voltam- metry has been extensively demonstrated in the large number of papers published, including in review articles that describe many advantages such as large active sur- face of small dimension electrodes, as well as enhanced electron transfer and electrocatalytic properties [2, 8, 16– 20]. The development of electrochemical sensors using carbon nanotube modification can be extended to the preparation of composite electrodes in which the CNT are agglutinated by polymers such as silicone rubber, which has been used for preparation of graphite silicone rubber composite electrodes (GSR), first described by Pungor and Szepesvµry [21] and recently used by us [22, 23]. The main advantages of composite electrodes are the ease of preparation and surface renewal, possibility of modifier incorporation and good reproducibility and re- peatability of active area. According to the definition of Tallman and Petersen [24], in which a composite electrode is a material consist- ing of at least one conductor phase and at least one insu- lator phase, the silicone rubber is the insulator phase, and the CNT is the conductor phase, with all the advantages of high conductivity and large surface area, allowing the production of highly sensitive electrodes [21–23]. Amongst the different methods to prepare and modify electrodes with CNT [4, 9, 25–27], Wildgoose et al. [28] describe different chemical and electrochemical modifica- tion strategies and electroanalytical and bioanalytical ap- plications. Some of these articles report the preparation of composite electrodes based on CNT and epoxy resin with characterization by cyclic voltammetry (CV) and electrochemical impedance spectroscopy [29, 30]. Propranolol (1-isopropylamino-3-(1-naphthyloxy)-2- propranolol), whose structure is presented in Scheme 1, is one of the drugs classified as b-adrenergic receptor block- ers, b-adrenergic antagonists or simply b-blockers [31, 32]. The b-adrenergic antagonists are widely used in the treat- ment of cardiovascular diseases, arterial hypertension, cardiac arrhythmias, and angina pectoris as well as for 2776  2010 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim Electroanalysis 2010, 22, No. 23, 2776 – 2783 Full Paper