Synthesis of positively charged calcium hydroxyapatite nano-crystals and their adsorption behavior of proteins

Positively charged Hap nano-crystals were prepared by using β-alanine and clarified the adsorption affinity of these surface amide functionalized Hap nano-crystals to proteins. Colloidal surface amide functionalized Hap nano-crystals were prepared by wet method in the presence of various amounts of β-alanine by changing molar ratio of β-alanine/Ca (β/Ca ratio) in the solution. The rod-like nano-crystals were lengthened with addition of β-alanine though their width did not vary; carboxyl groups of β-alanine are strongly coordinated to Ca2+ ions exposed on ac and/or bc faces to inhibit particle growth to a- and/or b-axis directions and enhance the particle growth along to the c-axis. No difference can be recognized on the crystal structure among the synthesized Hap nano-crystals by XRD measurements. However, the large difference was recognized by TG-DTA and FTIR measurements. Those measurements revealed that β-alanine is incorporated on the Hap nano-crystal surface up to the β/Ca ratio of 1.0, though they are absent in the nano-crystals synthesized at β/Ca ratio ≥ 2.0. The zeta potential (zp) of β-alanine-Hap nano-crystals prepared at β/Ca = 0.4 and 1.0 of those incorporating β-alanine exhibited positive charge at pH ≤ 5.9. The saturated amounts of adsorbed BSA for the positively charged β-alanine-Hap nano-crystals were increased 2.3–2.4-fold by their electrostatic attraction force between positively charged β-alanine-Hap nano-crystals and negatively charged BSA molecules. We were able to control the adsorption affinity of Hap nano-crystal by changing their surface charge.