The effect of surface charge on hydroxyapatite nucleation

Both the charged surfaces and condition of supersaturated solutions are thought to be the important factors that greatly affect the nucleation of hydroxyapatite (HAp) onto the substrates. In the present study self-assembled monolayers (SAM) terminated with amino (NH2-SAM) and hydroxyl (OH-SAM) headgroups were employed to generate the positively and negatively charged surfaces, respectively. The soaking experiments for HAp nucleation were carried out in the supersaturated solutions (1.5 SBF) with Ca2+ and PO43− ions concentrations 1.5 times higher than in simulated body fluid (SBF). The stability of solutions deeply depends on pH values if other factors (temperature, ionic series and their concentration, etc.) are fixed. Consequently, the opposite micropatterns of HAp films were observed on the UV-patterned SAM with NH2 and OH terminals in a stable solution (1.5 SBF, pH=7.2) where no particles formed and in an unstable solution (1.5 SBF, pH=7.6) where homogeneous nucleation occurred. The preferential nucleation of HAp was found on negative surface (OH-SAM) in a stable supersaturated solution, while selective deposition of HAp was formed on positive surface (NH2-SAM) in the unstable solution. Considering the latter results, it is conceivable that the electrostatic adhesion of microparticles formed in the solution dominated the formation of HAp in the unstable solution.