Phase-Selective Electroprecipitation of Calcium Phosphate Thin Films at Physiological Temperatures

Abstract

A protein-compatible combination of temperature and pH conditions allows for phase-selective electrosynthesis of bonelike calcium phosphate materials. The procedure involves an acidic aqueous electrolyte at 20−40 °C for an electroprecipation reaction based on nitrate reduction. X-ray diffraction and Raman spectroscopic data confirm that hydroxyapatite (Ca10(PO4)6(OH)2) is the kinetically preferred phase under galvanostatic electroprecipitation conditions with lower phosphate concentrations (∼0.03 M) in the electrolyte, whereas brushite (CaHPO4·2H2O) is favored when electrolytes with higher phosphate concentrations are used. Titration data suggest that the origin of this concentration-dependent phase selectivity is related to differences in buffering capacity of the electrolytes in the immediate vicinity of the working electrode surface, which explains the variability in phases in previous reports of calcium phosphate electrosynthesis. Critical parameters for controlling film formation are galvanostatic control of the electroprecipitation (for local pH changes that trigger deposition), phosphate concentration (to regulate buffering effects), and the presence of ammonia and ammonium ions in the bulk electrolyte (for complexing to improve film coverage and uniformity).