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 polyerystalline 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, sigma(HA) (C/m(2)), as a function of ionic strength, position within a variety of grains, and across grain boundaries. The average sigma(HA) was found to be similar to-0.02C/m(2) and to vary from -0.0037 to -0.072C/m(2) with nanoscale position in relation to grain boundaries and crystal planes up to -0.19C/m(2)/mum. Positional measurement of nanoscale surface properties holds great promise in elucidating the molecular origins of physicochemical processes occurring at the biomaterial interface. (C) 2004 Elsevier Ltd. All rights reserved.