Nano-sized hydroxyapatite (HAP) is of interest in biomaterials science due to its similarity to bone mineral. In this study, HAP modification using 3-aminopropyltriethoxysilane (APTES) was carried out in toluene and the effect of reaction time and curing temperature on the surface layers formed was investigated through X-ray photoelectron spectroscopy, Fourier transform infrared (FT-IR) and solid-state nuclear magnetic resonance (NMR) spectroscopy. It is shown that the chemical composition is strongly influenced by the curing temperature; with low temperatures of 50 and 100 °C resulting in a fully condensed APTES layer, an intermediate temperature of 150 °C causing partial oxidation of the surface layer with the conversion of some amine functionality to amides while curing at a temperature of 200 °C additionally leads to thermal decomposition of the silane layer and a loss of the pendent amine groups. However, the stability of these particles in aqueous solution indicated a loss of the silane layer for samples cured at 150 °C or less and it is concluded that there is a trade-off between the availability of functionality for further chemical grafting and the stability for these APTES-modified HAP materials. Subsequent attachment of the polyelectrolyte poly(acrylic acid) (PAA) via both ionic interaction and covalent bonding using carbodiimide chemistry resulted in particles with more negative zeta potentials (-27 to -18 mV) compared to pure HAP, which were stable to dispersion in aqueous solution, both with respect to their chemical composition at the particle surface and to aggregation.