Electrode coating with ultrathin films containing electroactive molecules for biosensor applications

Redox molecules, i.e., ferrocene (Fc), ferrocene carboxylic acid (Fc-COOH), and 3-3', 5-5'-tetramethylbenzidine (TMB), are good mediators for typical coenzymes of several dehydrogenases. Such mediators were immobilized in monolayers of dipalmitoylphosphatidic acid (DPPA) by means of the cospreading technique at the water-air interface. TMB was found to interact more strongly than Fc and Fc-COOH with the DPPA matrix; moreover, we observed that DPPA/TMB monolayers could be transferred on different solid supports with the Langmuir-Blodgett (LB) technique. LB films, containing TMB immobilized by means of cospreading and incubation procedure, were characterized recording UV-Vis absorption spectra. Such investigation evidenced the presence of TMB with different oxidation states. Hybrid films containing the mediators were prepared combining two techniques: LB and self-assembly of octadecylmercaptane and octylmercaptane on gold. The resulting structures were characterized by means of ellipsometry and contact angle measurements. Information on the influence of the multilayer structure on the electrode currents and on the redox activity of the immobilized mediators was obtained by cyclic voltammetry. A significant electrochemical signal was observed for TMB included in LB films. The sets of results independently evidenced that TMB can be quantitatively embedded in the ultrathin films of a lipid matrix maintaining its electroactive behavior.