Insulin-containing layer-by-layer (LbL) thin films were prepared by an alternate deposition of insulin and anionic polysaccharides (heparin, j-carrageenan, and fucoidan) through an electrostatic force of attraction between positively charged insulin and anionic polysaccharides at pH 3.0. The loading of insulin in the LbL films increased with the increasing number of layers (or the film thickness), depending on the polysaccharide type. LbL films composed of j-carrageenan contained higher amount of insulin than in heparin-and fucoidan-based films. The LbL films were fairly stable in acidic media, while insulin was released from the films in weakly acidic and neutral solutions as a result of loss of net positive charge in insulin. The released insulin retained its original structure.
Gold (Au) electrodes coated with layer-by-layer (LbL) thin films composed of chitosan (CHI) were prepared to evaluate the redox properties of hexaammine ruthenium ions, Ru(NH3)63+, and ferricyanide ions, Fe(CN)63− LbL films were prepared on an Au electrode by electrostatic LbL deposition using polycationic CHI and poly(vinyl sulfate) (PVS) or poly(acrylic acid) (PAA) as anionic component. Redox peak current in cyclic voltammetry of Ru(NH3)63+ on the CHI/PVS and CHI/PAA film-coated electrodes increased with increasing thickness of the films. Interestingly, the cyclic voltammograms showed two pair of redox peaks, originating from Ru(NH3)63+ diffusing across the LbL layers and from those confined in the film. The results were rationalized in terms of the electrostatic interactions between Ru(NH3)63+ and excess negative charges in the LbL films originating from PVS and PAA. In contrast, Fe(CN)63− was not confined in the LbL films due to electrostatic repulsion of Fe(CN)63− and excess negative charges. Significant amounts of Ru(NH3)63+ were confined in the films at pH 7.0, whereas few ions were bound at pH 3.0 due to the reduced net negative charge in the films. The results suggest a potential use of the CHI-containing LbL films as scaffold for immobilizing positively charged ionic species on the electrode surface.
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