A study of photocatalytic activity of bare as prepared, bare aged and dopamine surface modified colloidal TiO2 nanoparticles was obtained following degradation reaction of herbicide RS-2-(4-chloro-o-tolyloxy)propionic acid under UV light irradiation. Results showed that the most active photocatalyst is bare aged TiO2 and the least active are dopamine modified nanoparticles. Results are discussed in the light of surface structure of TiO2 nanoparticles. The study of surface modification of TiO2 nanoparticles (4.5 nm, TiO2 NPs) with dopamine was also performed. The formation of inner-sphere charge-transfer complexes results in red shift of semiconductor absorption threshold (600 nm), compared to bare TiO2 NPs (380 nm). Effective band gap energy of 3.2 eV for bare TiO2 NPs is reduced to 2.1 eV for TiO2/dopamine charge transfer complexes. The binding structure was investigated by UV-vis and FTIR spectroscopy. The obtained optimal geometry for binding of dopamine to surface Ti atoms was binuclear bidentate-bridging. From the Benesi-Hildebrand plot, stability constant of the order 10(3) M-1 has been determined. (C) 2013 Elsevier B.V. All rights reserved.