Redox properties of the ferricyanide ion on electrodes coated with layer-by-layer thin films composed of polysaccharide and poly(allylamine)

Polyelectrolyte multilayer thin films were prepared by an alternate deposition of poly(allylamine hydrochloride) (PAH) and anionic polysaccharides {carboxymethylcellulose (CMC) and alginic acid (AGA)} on the surface of a gold (Au) disk electrode, and the binding of ferricyanide [Fe(CN)₆] ^3- and hexaammine ruthenium ions [Ru(NH₃) ₆]³⁺ to the films was evaluated. Poly(acrylic acid) (PAA) was also employed as a reference polyanion bearing carboxylate side chains. A quartz-crystal microbalance study showed that PAH-CMC and PAH-AGA multilayer films grow exponentially as the number of depositions increases. The thicknesses of five bilayers of (PAH-CMC)₅ and (PAH-AGA)₅ films were estimated to be 150 ± 20 and 90 ± 15 nm, respectively, in the dry state. The PAH/polysaccharide multilayer film-coated Au electrodes exhibited a redox response to the [Fe(CN)₆]^3- ion dissolved in solution, irrespective of the sign of the surface charge of the film, suggesting the high permeability of the films to the [Fe(CN)₆]^3- ion. In contrast, the PAH-PAA film-coated Au electrodes exhibited a redox response only when the outermost surface of the film was covered with a positively charged PAH layer. However, the permeation of the [Ru(NH ₃)₆]³⁺ cation was severely suppressed for all of the multilayer films. It was possible to confine the [Fe(CN) ₆]^3- ion in the films by immersing the film-coated electrodes in a 1 mM [Fe(CN)₆]^3- solution for 15 min. Thus, the [Fe(CN)₆]^3--confined electrodes exhibited a cyclic voltammetric response in the [Fe(CN)₆]^3- ion-free buffer solution. The loading of the [Fe(CN)₆]^3- ion in the films was higher when the surface charge of the film was positive and increased with increasing film thickness. It was also found that the [Fe(CN) ₆]^3- ion confined in the films serves as an electrocatalyst that oxidizes ascorbic acid in solution.