High-Performance Taste Sensor Made from Langmuir -Blodgett Films of Conducting Polymers and a Ruthenium Complex Marystela Ferreira,* ,† Antonio Riul, Jr., ‡ Karen Wohnrath, † Fernando J. Fonseca, § Osvaldo N. Oliveira, Jr., † and Luiz H. C. Mattoso* ,‡ Instituto de Fı ´ sica de Sa ˜ o Carlos, Universidade de Sa ˜o Paulo, CP 369, 13560-970, Sa ˜ o Carlos/SP, Brazil, EMBRAPA Instrumentac ¸ a ˜ o Agropecua ´ ria, CP 741, 13560-970, Sa ˜ o Carlos/SP, Brazil, and Escola Polite ´ cnica da Universidade de Sa ˜o Paulo, USP, CEP 05508-900, Sa ˜ o Paulo, SP, Brazil A sensor array made up of nanostructured Langmuir- Blodgett (LB) films is used as an electronic tongue capable of identifying sucrose, quinine, NaCl, and HCl at the parts- per-billion (ppb) level, being in some cases 3 orders of magnitude below the human threshold. The sensing units comprise LB films from conducting polymers and a ruthenium complex transferred onto gold interdigitated electrodes. Impedance spectroscopy is used as the prin- ciple of detection, and the importance of using nanostruc- tured films is confirmed by comparing results from LB films with those obtained from cast films. The development of artificial taste sensors is motivated by their possible impact on the food and beverage industry as well as on the quality control of any liquid substance. Among the envisaged advantages of an artificial sensor are the possibility of use in toxic and unpleasant substances, the preservation of sensitivity even after prolonged use, and the higher sensitivity compared to the human counterpart. An artificial sensor generally mimics the human tongue in that it distinguishes tastes based on the so-called global selectivity concept, according to which the biological system groups all information received by the gustatory cells into distinct patterns of response. 1,2 In previous papers, 3,4 we showed that the combination of thin films of different materials could be used as an electronic tongue, in which impedance spectroscopy proved to be advantageous in comparison with potentiometry 5-11 and voltammetry. 12-14 In this paper, we extend the previous work 3,4 to show the importance of using ultrathin films in obtaining high sensitivity, detecting sucrose, quinine, NaCl, and HCl at the parts- per-billion (ppb) level. For that we compare sensor arrays made with nanostructured Langmuir-Blodgett (LB) films and cast films from conducting polymers, a metallic complex, mer- [RuCl 3 (dppb)- (py)] (dppb ) PPh 2 (CH 2 ) 4 PPh 2 ; py ) pyridine) (Rupy), and mixtures of conducting polymers and Rupy. We also indicate that the manipulation of composite Rupy/ conducting polymer films may be tailored to specific applications, particularly because ruthenium complexes might complex with heavy metals. EXPERIMENTAL METHODS AND THEORETICAL BACKGROUND The sensing units were obtained from two types of film: cast films and LB films from Rupy, polypyrrole, and polyaniline. The syntheses of these materials were described in refs 15-17. Cast films 1 μm thick were produced by casting a chloroform solution of pure polyaniline (PANI), 18 pure Rupy, and the mixture with 10% in moles of Rupy in PANI. For the sensing units with LB films, 11 layers (i.e., total thickness of ∼11 nm) were deposited from pure Rupy, PANI, polypyrrole (PPy), and composite LB films of PANI-Rupy (10%and 30%Rupy in moles) and PPy-Rupy (30% of Rupy in moles) onto gold interdigitated electrodes. A detailed description of the LB film fabrication process can be found in refs * Corresponding authors. E-mail: mattoso@ cnpdia.embrapa.br. Fax: 55 16 272 5958. E-mail: mstela@ if.sc.usp.br. † Universidade de Sa ˜ o Paulo. ‡ EMBRAPA Instrumentac ¸ a ˜ o Agropecua ´ ria. § Escola Polite ´cnica da Universidade de Sa ˜ o Paulo. (1) Schiffman, S. S. Physiol. Behav. 2000 , 69, 147-159. ( 2) Pfaffman, C. Handbook of Physiology; J. Field American Physiological Society: Washington, DC, 1959; Vol 1. (3) Riul, A., Jr.; dos Santos, D. S., Jr.; Wohnrath, K.; Di Tommazo, R.; Carvalho, A. C. P. L. F.; Fonseca, F. J.; Oliveira, O. N., Jr.; Taylor, D. M.; Mattoso, L. H. C. Langmuir 2002 , 18, 239-245. 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