Assembling Alternate Dye−Polyion Molecular Films by Electrostatic Layer-by-Layer Adsorption

Electrostatic alternate adsorption was successfully employed for low-molecular-weight dyes, leading to a large variety of dye−polyion layer-by-layer assemblies. The assembling process of individual dye−polyion layers of Congo Red (CR)−poly(diallyldimethylammonium chloride) (PDDA) was investigated by using a quartz crystal microbalance (QCM). The in-situ QCM measurement revealed that the dye adsorption occurred at a rate similar to that of conventional polyion adsorption. Successful assembly of the CR layer was confirmed also by increasing intensities in the UV adsorption. The molecular aggregation within the dye layer was inferred from the observed absorption shift in the case of CR−PDDA. Apparently, stable film formation is promoted by dye aggregation in the adsorbed layer, although the extent of film growth was independent of conceivable aggregation in the solution. Successive frequency decreases due to film growth were observed for other representative dyes under optimized conditions. Comparison of the film thickness estimated from QCM frequency shift with the molecular dimension of individual dyes suggests the formation of well-packed monomolecular dye layers that is affected by size, number of charge, and spatial orientation of dye molecules. The alternate layer-by-layer assembly is now extended from linear polyions to other macroions (proteins and inorganic plates) and charged small molecules. Advantages and disadvantages of the alternate assembly were discussed relative to those of the Langmuir−Blodgett technique.