Control of packing order of self-assembled monolayers of magnetite nanoparticles with and without SiO2 coating by microwave irradiation

A close packed layer of Fe3O4 and SiO2-coated Fe3O4 nanoparticles can be assembled on silicon and glass substrates modified with polyelectrolytes following the principle of the layer-by-layer self-assembly (Science 1997, 277, 1232). Microwave (MW) treatment of the poly(dimethyldiallylammonium chloride) layer prior to the nanoparticle adsorption results in substantial reduction of the surface roughness of the particulate films. This effect is attributed to reduction of the length of partially desorbed segments of macromolecules protruding into the aqueous phase at a distance of about 70 nm as estimated from force-distance curves. Aggregation of nanoparticles on these segments is responsible for a relatively high degree of disorder in layer-by-layer self-assembled films. A brief MW exposure results in cross-linking of polyelectrolyte chains. This substantially reduces the number of loose segments, and improves 2D packing of nanoparticles. For optimized conditions, the rms roughness, R, of magnetite self-assembled films can be as low as 1.5-3.5 nm. For silica-coated magnetite, initial R of adsorbed films is typically 10-14 nm. It can be reduced to 5.5 nm following the MW treatment of the polyelectrolyte; however, this does not completely prevent the formation of multiparticle 3D aggregates. Further reduction of R to 3.5 nm can be achieved by a brief ultrasonication of the nanoparticulate film, which removes weakly attached particles.