Influence of surface charge and protein intermediary layer on the formation of biomimetic calcium phosphate on silica nanoparticles

Abstract

The biomineralization process in bone involves the assembly of calcium phosphate (CP) as a bio-inorganic nanocomposite in association with extracellular matrix proteins. Towards developing synthetic systems that can promote the formation of biomimetic hydroxyapatite, the formation of CP from solution onto the surfaces of a model system composed of silica nanoparticles (SNPs) of differing physiochemical characteristics was studied. In addition to varying charge, two protein mediators collagen and phosvitin were adsorbed onto the surface of SNPs to form protein-coated SNPs, and the precipitation of CP onto the SNP from solutions containing physiologically relevant concentrations of calcium and phosphate ions was studied. A comparison of zeta potential (ζ) versus pH isotherms between the protein-coated SNP and native protein reveals deviation in ζ that can be attributed to perturbation in the protein structure. Analysis of the ζ versus pH isotherms for CP–protein–silica nanocomposites and the native CP indicates that CP deposition on SNPs occurs in a heterogeneous manner with segregated regions of CP. Interestingly, SNP composites containing phosvitin exhibit lower component segregation relative to composites containing collagen, and that the difference in segregation originates from the difference in ζ of each adsorbed protein. This suggests that protein-mediated biomineralization might be governed by surface energetics as much as biology.