Thin Solid Films 446 (2004) 301–306 0040-6090/04/$ - see front matter  2003 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2003.10.006 Langmuir–Blodgett films from polyaniline y ruthenium complexes as modified electrodes for detection of dopamine Marystela Ferreira *, Luıs R. Dinelli , Karen Wohnrath , Alzir A. Batista , Osvaldo N. Oliveira Jr. a, b c b a ´ Instituto de Fısica de Sao Carlos, Universidade de Sao Paulo, CP 369, 13560-970 Sao Carlos, SP, Brazil a ´ ˜ ˜ Departamento de Quımica, Universidade Federal de Sao Carlos, CP 676, 13565-905 Sao Carlos, SP, Brazil b ´ ˜ Departamento de Quımica, Universidade Estadual de Ponta Grossa, Campus de Uvaranas, 84030-900 Ponta Grossa, PR, Brazil c ´ Received 2 April 2003; received in revised form 10 September 2003; accepted 2 October 2003 Abstract Langmuir–Blodgett (LB) films from mixtures of polyaniline (PANI) and the ruthenium complex mer-wRuCl (dppb)(py)x 3 (dppbsPPh (CH ) PPh;pyspyridine)(Rupy) wereusedtomodifyelectrodesandtodetectdopamine (DA).Theelectrochemical 2 24 2 response has been used as the principle of detection in order to exploit the molecular-level interaction between PANI and Rupy in the LB films. Cyclic voltammograms were performed in a potential range where the Rupy complex is not electroactive and the response is dominated by the interconversion between the oxidation states leucoemeraldine and emeraldine of PANI. In the presence of DA a further redox pair appears at approximately 230 and 0 mV, associated with oxidationyreduction of DA to dopaminequinone. The current increased linearly in the range between 4.0=10 and 1.2=10 molyl. The detection limit for y5 y3 DA concentrations for the 21-layer LB film from PANIyRupy was 4.0=10 molyl, which is sufficient to detect DA in a y5 pharmaceutical product. The modified electrode based on PANIyRupy LB films is selective to DA, particularly because the oxidation potential for DA is lowered, in comparison to a bare indium-tin-oxide electrode. The presence of ascorbic acid, which is an important interferent for DA, could be detected when its concentration was three times that of DA.  2003 Elsevier B.V. All rights reserved. Keywords: Langmuir–Blodgett films; Electrochemistry; Polymers; Sensors 1. Introduction Dopamine (DA) is an important neurotransmitter in mammalian central nervous systems and the loss of DA- containingneuronsmayresultinseriousdiseasesinclud- ing Parkinson’s disease. Detection of DA has therefore been a subject of considerable interest, which can be done using electrochemical methods due to the elec- troactive nature of DA w1–10x. A review on chemically modified electrodes for detecting DA in vivo has been produced by O’Neill w11x. The limit of detection of DA varies from work to work depending on the electrodes used and the purpose of the sensing activity. Also relevant is the fact that the voltammetric response of DA suffers from the interference of ascorbic acid (AA) that usually coexists in vivo as anions w12x. Modified electrodes may be produced by coating with Langmuir– *Corresponding author. Tel.: q55-16-273-9825; fax: q55-16-271- 5365. E-mail address: mstela@if.sc.usp.br (M. Ferreira). Blodgett (LB) films w13–17x, which may be advanta- geous as higher sensitivity can be reached because the films are ultrathin. Another advantage offered by the LB technique is the control over molecular architecture, which allows combination of more than one material in a single sensing unit. The variety of materials that can be used is also immense. For example, LB films from the ruthenium complex, mer-wRuCl (dppb)(pic)x (pics 3 picoline) and from mixtures of Rupic and stearic acid displayed electroactivity, while cast films from these materials are not electroactive w14x. This is demonstra- tion that electroactivity depends on the film organization imparted by the LB technique. Conducting polymers are especially interesting as their electrical and optical properties vary within a wide range depending on the conditions under which they operate, such as pH and ionic strength. Following the development of several strategies to render the conduct- ing polymers solution-processible, the fabrication of LB films from various conducting polymers is now well