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

Positively charged Hap nano-crystals were prepared by using beta-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 beta-alanine by changing molar ratio of beta-alanine/Ca (beta/Ca ratio) in the solution. The rod-like nano-crystals were lengthened with addition of beta-alanine though their width did not vary; carboxyl groups of beta-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 beta-alanine is incorporated on the Hap nano-crystal surface up to the beta/Ca ratio of 1.0, though they are absent in the nano-crystals synthesized at beta/Ca ratio > or = 2.0. The zeta potential (zp) of beta-alanine-Hap nano-crystals prepared at beta/Ca = 0.4 and 1.0 of those incorporating beta-alanine exhibited positive charge at pH < or = 5.9. The saturated amounts of adsorbed BSA for the positively charged beta-alanine-Hap nano-crystals were increased 2.3-2.4-fold by their electrostatic attraction force between positively charged beta-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.