Self-assembly of gold nanoparticles induced by poly(oxypropylene)diamines

A series of poly(oxypropylene)diamines D230, D400, D2000, and D4000, having molecular weights of hydrophobic segments of 230, 400, 2000, and 4000, were used as ligands to synthesize self-organized gold nanocrystals. Ligand exchange significantly reduced the average particle size and the polydispersity of nanocrystals, and this effect was more remarkable as the molar ratio of amine groups to Au3+ ions ([N]/[Au3+] ratio) was increased. Under the same [N]/[Au3+] ratio of 100, D2000 generated an ordered 2D-monolayer; however, D230 and D400 colloids formed mainly a densely packed 3D structure with minor 2-D layers, and D4000 presented disordered 3D and 2D structures. The gap among the nanoparticles was found to be increased with the increasing molecular weight of the hydrophobic segment of ligands, accompanied by the decreasing wavelength of UV-vis absorption bands. This increased gap can be interpreted as the ligand thickness calculated from the equation of steric force increasing with increasing molecular weight of the hydrophobic segment. The potential energies obtained from the calculated ligand thickness according to the soft sphere model show more steep potential wells for D230 and D400 colloids than that for the D2000 colloid. This explains why the aggregation hardly occurred for the gold nanoparticles obtained under D2000, where the nanoparticles are single crystals having face center crystal structure with a lattice constant of 2.36 A and have grain sizes close to the average particle sizes, evidenced from the results of transmission electron microscopy and X-ray diffraction spectroscopy.