Phosphonopropionic acid coating as platform for the efficient grafting of (bio)molecules to hydroxyapatite nanoparticles

Abstract

A new synthesis strategy intended to increase apatite available surface groups for the covalent functionalization with (bio)molecules was developed. To that purpose, hydroxyapatite nanoparticles (Ap) were synthesized and terminated with 3-phosphonopropionic acid (PPA) yielding Ap with surface bound propionic acid through P-C bonds (ApCOO– ). A thorough characterization of ApCOO– by IR spectroscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, thermal gravimetric analysis, and specific surface area determination (BET model), strongly support an efficient surface grafting of the organophosphonates and a hydroxyapatite core of similar morphology and phase composition to that of synthetic Ap. ApCOO– was able to adsorb bovine serum albumin (BSA) up to 0.20 mg m–2 and showed no cytotoxic effects towards Balb/C 3T3 cells. ApCOO– abundant carboxyl surface groups facilitated the particles grafting through stable amide bonds with basic fuchsin (Fu) and tyrosine (Ty) yielding well covered surfaces. Fu-bound hydroxyapatite powders showed ten times stronger fluorescence than free Fu in ethanol, thus expanding the potential uses of Ap as fluorescent sensors in the red region of the visible spectrum. On the other hand, Ty-bound hydroxyapatite powders showed negative surface charges and high stability of the coating in aqueous suspensions. In contrast, Ty-physically adsorbed hydroxyapatite powders rendered positively charged surfaces and an unstable coating.