ORIGINAL PAPER Preparation, characterization, and electrocatalytic properties of hybrid coatings of hexacyanometalate-doped-cationic films Shen-Ming Chen & Wen-Yan Chzo & R. Thangamuthu Received: 30 September 2007 / Revised: 17 November 2007 / Accepted: 30 November 2007 / Published online: 15 January 2008 # Springer-Verlag 2007 Abstract Electrochemically active hybrid coatings based on cationic films, didodecyldimethylammonium bromide (DDAB), and poly(diallyldimethylammonium chloride) (PDDAC) are prepared on electrode surface by cycling the film-covered electrode repetitively in a pH 6.5 solution containing Fe(CN) 6 3- and Ru(CN) 6 4- anions. Modified electrodes exhibited stable and reversible voltammetric responses corresponding to characteristics of Fe(CN) 6 3-/4- and Ru(CN) 6 4-/3- redox couples. The cyclic voltammetric features of hybrid coatings resemble that of electron transfer process of surface-confined redox couple. Electrochemical quartz crystal microbalance results show that more amounts of electroactive anionic complexes partitioned into DDAB coating than those doped into PDDAC coating from the same doping solution. Peak potentials of hybrid film-bound redox couples showed a negative shift compared to those at bare electrode and this shift was more pronounced in the case of DDAB. Finally, the advantages of hybrid coatings in electrocatalysis are demonstrated with sulfur oxoanions. Keywords Hybrid coatings . Simultaneous doping . Modified electrode . DDAB . PDDAC . Electrocatalysis . Sulfur oxoanions Introduction Modification of electrode surfaces with electroactive films has been the active area of research during past three decades mainly due to their many potential applications in widely differing areas like energy conversion and storage, electrocatalysis, electroanalysis, electrochromism, molecular electronics, biosensor, and media for controlled drug release [1–4]. In particular, potential applications of such electro- active coatings in understanding the mechanism of electron transfer reactions in complex biological systems, such as enzymes and antibodies, and catalysis of electrochemical reactions have provided much of the incentive for this development [1–8]. In view of the enormous practical and fundamental interest of the subject, chemically modified electrodes is still one of the active areas of research. Among the materials used to fabricate electroactive coating on electrode surfaces, ionomer membranes, with ionizable groups attached to organic polymer backbones, is attractive in many aspects. The use of ionomers for surface modification has grown in popularity ever since Oyama and Anson [9] demonstrated the binding of counterionic reactants to poly(4- vinylpyridine) (PVP) film on the electrode surface. In acidic medium, the protonated PVP coating extracts the anionic complexes like ferrocyanide and hexachloroiridate from the contacting solution by ion-exchange process. Similarly, Du Pont’ s Nafion membrane has been used to immobilize the cationic reactants on the electrode surfaces [3]. Because this is simple means to immobilize electroactive species to the electrode surface, later this approach extended to similar materials. Kunitake et al. [10, 11] first incorporated iron heme proteins into vesicles and multilayer films of insoluble surfactants, and this work paved the way to explore electrochemistry of films of complex biological macro- molecules such as proteins and surfactants or lipids on electrodes [ 12–19]. Rusling et al. [ 13] has found that inclusion of myoglobin (Mb) in films of cationic surfactants, didodecyldimethyl ammonium bromide (DDAB), on pyrolytic graphite (PG), or glassy carbon J Solid State Electrochem (2008) 12:1487–1495 DOI 10.1007/s10008-007-0486-y S.-M. Chen (*) : W.-Y. Chzo : R. Thangamuthu Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, No. 1, Section 3, Chung-Hsiao East Road, Taipei, Taiwan 106, Republic of China e-mail: smchen78@ms15.hinet.net