size reached to a maximum value at the PZC. TEM image and FT-IR spectroscopy analysis confirmed the formation of TiO 2 cluster aggregates and successive growth of the hydroxyl groups on different sites on TiO 2 surface, respectively, although no effect of irradiation on the phase transformation of TiO 2 nanoparticles was observed through the use of Raman spectroscopy. The fluorescence spectroscopy analysis illustrated the interactions between the surface hydroxyls formed during UV irradiation on TiO 2 surface and functional groups on HA molecules, which accelerated the aggregation process of TiO 2 nanoparticles. Widespread HA and UV irradiation in the natural environment significantly altered the surface properties and stability of the nanoparticles in aqueous environments.