Alternate Assembly of Ordered Multilayers of SiO₂ and Other Nanoparticles and Polyions

Alternate layer-by-layer assembly of colloidal SiO 2 particles with polycations has been investigated by quartz crystal microbalance (QCM), scanning electron microscopy (SEM), and atomic force microscopy (AFM). QCM measurement confirmed the high regularity and reproducibility of the assembling process that depends on particle concentration, particle size, and ionic strength. The individual adsorption step was completed within 15 s. The thickness of adsorbed layers increased with increasing SiO 2 concentrations at the three particle sizes used (45, 25, and 78 nm in diameter), unlike the case for other polyion assemblies. It also increased with increasing ionic strength of aqueous SiO 2 dispersions. According to SEM observation, the assembled film possessed surprisingly flat surfaces at optimized ionic strengths. AFM observation revealed that SiO 2 particles were not closely packed. The neutralization ratio of SiO 2 and PDDA was estimated by turbidity measurement. Comparison of turbidity and QCM data indicated that the positive charge on PDDA was not completely neutralized by the negative charge on SiO 2 particles in the course of alternate adsorption. This is apparently caused by a large difference in rigidity and charge density between SiO 2 and PDDA. Since the charge density on PDDA is significantly larger than that on SiO 2, all of the former charges cannot form short-distance ion pairs with surface charges of rigid SiO 2 particles. The formation of long-distance charge pairs between SiO 2 and PDDA, unlike the case for oppositely charged pairs of linear polyions, appears to be the origin of the remarkable dependence of film thickness and surface morphology on ionic strength and particle concentration. The other nanoparticles (CeO 2 and TiO 2 ) were similarly assembled with oppositely-charged linear polyions. Further modifications of the film structure were demonstrated by assembly between particles with different sizes and that between SiO 2 and enzyme and by taking advantage of premixing components.