Investigation of Apatite Deposition onto Charged Surfaces in Aqueous Solutions Using a Quartz‐Crystal Microbalance

Hydroxyapatite (HAp) deposition onto positively charged surfaces (i.e., self‐assembled monolayers (SAMs) terminated with NH 2 head groups) and negatively charged surfaces (i.e., OH‐SAMs (weak) and COOH‐SAMs (strong)) soaked at 50°C in aqueous supersaturated solutions (1.5 SBF, pH 7.0–7.6; SBF = simulated body fluid) was investigated using a quartz‐crystal microbalance. The results revealed that the solution conditions greatly influenced the formation of HAp on the charged surfaces. In a stable supersaturated solution of simulated body fluid (1.5 SBF, pH <7.4), more strongly negative surfaces had a more powerful induction capability for the heterogeneous nucleation of HAp (COOH > OH), whereas nucleation was obviously prohibited on a positive surface (NH 2 ‐SAM). On the other hand, after the calcium phosphate particles had nucleated homogeneously in an unstable soaking solution (1.5 SBF, pH ≧7.4), adhesion of the HAp microparticles to the NH 2 ‐SAM was observed. A two‐step interaction is conceivable to describe the formation of HAp on the positive NH 2 ‐SAM: At the first stage, electrostatic interaction dominates the adhesion of HAp microparticles; at the second stage, hydrogen bonds possibly form between the HAp microparticles and the amino head groups of the NH 2 ‐SAM, for a firm bonding with the substrate, and the microparticles grow progressively into a thin film. The electrophoretic behaviors of the HAp microparticles confirmed this hypothesis.