ORIGINAL PAPER Poly(thionine)-carbon nanotube modified carbon film electrodes and application to the simultaneous determination of acetaminophen and dipyrone M. Emilia Ghica 1 & Grasyelle M. Ferreira 1,2 & Christopher M. A. Brett 1 Received: 16 March 2015 /Revised: 8 June 2015 /Accepted: 10 June 2015 /Published online: 25 June 2015 # Springer-Verlag Berlin Heidelberg 2015 Abstract Multi-walled carbon nanotube (MWCNT)- poly(thionine) (PTH) nanostructures were formed on carbon film electrodes (CFE), the polymer film being formed beneath or on top of MWCNT, to give two different sensor architec- tures, PTH/MWCNT/CFE or MWCNT/PTH/CFE. Characterization of the novel nanostructures was carried out by cyclic voltammetry, electrochemical impedance spectros- copy and scanning electron microscopy. The modified elec- trodes were then used for sensing acetaminophen (ACOP) and dipyrone by differential pulse voltammetry and fixed- potential amperometry independently and simultaneously. Analytical parameters were determined and the results com- pared with similarly modified electrodes reported in the liter- ature. An interference study as well as recovery measurements in pharmaceutical samples was successfully performed. Keywords Carbon nanotubes . Poly(thionine) . Electrochemical sensors . Acetaminophen . Dipyrone Introduction Electroactive polymer-modified electrodes are currently find- ing extensive application in electrochemical sensing, particu- larly modified electrodes based on poly(phenazine) or poly(triphenylmethane) polymers together with carbon nanotubes (CNT) [1]. The combination of polymer with CNT leads to a large enhancement of the double layer capac- itance, as exemplified by poly(thionine) [2], and synergistic effects in terms of sensitivity, e.g. [3] using poly(methylene blue), and can improve the access of analyte to the electrode surface as well as leading to electrocatalytic effects. Electropolymerisation of the phenazine on a bare carbon elec- trode surface or on multi-walled CNT (MWCNT), previously deposited on the electrode, leads to different polymer growth rates and mechanisms. The combinations of polymer and CNT modifiers with polymer on top of or underneath the MWCNT can give different responses towards electroactive species, which depends on the size of the monomer molecule and ease of nucleation [1]. For example, polymerization of thionine, Scheme 1a, on MWCNT-modified glassy carbon electrodes, was found to increase the sensitivity and reduce the detection limit for nitrite determination [4]. Other compos- ite electrodes with two different polymers, poly(brilliant cresyl blue) and poly 5-amino-2-naphthalenesulfonic acid [5] show also improved response for the determination of hydrogen peroxide. Acetaminophen and dipyrone form the main active constit- uent of a number of widely used pharmaceutical preparations, and therefore, the development of fast, low-cost, sensitive and selective methods for routine analysis of these compounds is required. Modern multimodal analgesia should be based on a good combination of analgesics, and non-opioid agents are recommended wherever possible for the treatment of acute pain [6]. Acetaminophen (N-(4-hydroxyphenyl)ethanamide, paracetamol, Scheme 1b), and dipyrone (sodium [2,3- dihydro-1,5-dimethyl-3-oxo-2-phenyl-1H-pyrazol-4- yl)methylamino]methanesulfonate, metamizol, Scheme 1c), are analgesic and antipyretic drugs mainly used to reduce fe- ver and relieve moderate pain. Recent studies have shown that both medicines provide effective postoperative pain control, * Christopher M. A. Brett cbrett@ci.uc.pt 1 Department of Chemistry, Faculty of Sciences and Technology, University of Coimbra, 3004-535 Coimbra, Portugal 2 Instituto de Ciência e Tecnologia, Universidade Federal dos Vales do Jequitinhonha e Mucuri, Diamantina, MG, Brazil J Solid State Electrochem (2015) 19:2869–2881 DOI 10.1007/s10008-015-2926-4