Nanoscale variation in surface charge of synthetic hydroxyapatite detected by chemically and spatially specific high-resolution force spectroscopy

The normal intersurface forces between nanosized probe tips functionalized with COO−- and NH3+-terminated alkanethiol self-assembling monolayers and dense polycrystalline phase pure synthetic hydroxyapatite (HA) were measured via a powerful nanomechanical technique called chemically specific high-resolution force spectroscopy. The data taken on approach of the probe tip to the HA surface was compared to the nonlinear Poisson-Boltzmann-based electrostatic double layer theory to predict the surface charge per unit area of the HA, σHA (C/m2), as a function of ionic strength, position within a variety of grains, and across grain boundaries. The average σHA was found to be ∼−0.02 C/m2 and to vary from −0.0037 to −0.072 C/m2 with nanoscale position in relation to grain boundaries and crystal planes up to −0.19 C/m2/μm. Positional measurement of nanoscale surface properties holds great promise in elucidating the molecular origins of physicochemical processes occurring at the biomaterial interface.