Initial nucleation process of hydroxyapatite on organosilane self-assembled monolayers

Initial nucleation process of hydroxyapatite (HAp) formed from simulated body fluid (SBF) was investigated on Si substrates on which self-assembled monolayers (SAM) of 3-aminopropyltriethoxysilane (APTS) and trimethylchlorosilane (TMCS) were formed. The synthesized HAp appeared hemispherical shape composed of tiny plate-like particles. Nucleation of HAp was formed just in the early stage, and then the nucleated HAp proceeded to growth process. Saturated number density of HAp particles was ca. 10(5) to 10(6)/cm(2). Saturated number density increased with increasing calcium ion and phosphate ion concentrations in SBF, and showed slight dependence on the kind of SAM, ordered as OH is approximately equal to CH3 > NN2. These results indicate that depletion of calcium and phosphate ions in the vicinity of substrates should limit nucleation while growth of particles continues. Depletion factor in the modified nucleation rate formula gives the saturated nucleation density. It is indicated that the minimum ionic concentration for HAp nucleation depends on the kind of SAR I.