Redox Reactions of Ferricyanide Ions in Layer-by-Layer Deposited Polysaccharide Films:  A Significant Effect of the Type of Polycation in the Films

Redox reactions of ferricyanide ions, [Fe(CN)₆]^3-, in polysaccharide thin films that were prepared by layer-by-layer (LbL) deposition on the surface of a gold electrode were studied electrochemically by cyclic voltammetry. LbL films composed of alginic acid (AGA) and carboxymethylcellulose (CMC) were successfully prepared using poly(ethyleneimine) (PEI) and poly(diallyldimethylammonium chloride) (PDDA) as the cationic counterparts in the electrostatic LbL deposition. The deposition behavior of the PEI-based films significantly depended on the pH of the solutions from which the LbL films were deposited, while the effects of pH were negligibly small for the PDDA-based films due to the pH-independent positive charges on the PDDA chains. The cyclic voltammogarms (CVs) of [Fe(CN)₆]^3- ions on the LbL film-coated electrodes revealed that all the LbL films tested are permeable to [Fe(CN)₆]^3- ions and that the redox reactions of [Fe(CN)₆]^3- ions proceed smoothly in the LbL polysaccharide films. It was found that [Fe(CN)₆]^3- ions are concentrated in the films from the bulk solution, depending on the pH of the medium and on the type of polycations in the film. The PEI-based films concentrated [Fe(CN)₆]^3- ions more effectively in an acidic solution than in neutral and basic media, while the pH effect was not observed for the PDDA-based films. In addition, we found that the [Fe(CN)₆]^3- ions are confined in the LbL films due to a strong binding of the ions to the positively charged sites arising from the protonated amino groups in the films. The confined [Fe(CN)₆]^3- ions exhibited redox reactions in the films, with the redox potentials being shifted to the positive or negative direction in the PEI- or PDDA-based film, respectively, as compared to the redox potential of diffusing [Fe(CN)₆]^3- ions. Thus, significant effects of the type of polycation in the LbL films on the redox reactions of [Fe(CN)₆]^3- ions were observed.