Surface binding between the carboxylic group and the TiO2 nanoparticle is important in the potentially applicable dye-sensitized mesoporous TiO2 crystalline solar cell. However, the nature of the surface binding is still not clear enough. We propose a new method to study the surface binding between the carboxylic group and the TiO2 surface atoms by using an interface-sensitive molecular probe. The principle underlying this method is based on the triplet-triplet absorption of the probe molecule all-trans-retinoic acid (ATRA) formed by the photoinduced charge recombination between the TiO2 nanoparticle and the probe molecule. By characterizing the individual triplet absorption spectra of ATRA in different forms, including the synthesized ester-like linkage model compound tri-butoxyl titanium (IV) all-trans-retinoate (Ti(IV)(OBu)(3)RCOO; RCOO = retinoate), the composition of different surface binding forms of carboxylic groups on TiO2 nanoparticle surfaces in hexanol has been determined. The result shows that the simple adsorption form (anion, hydrogen bonding) comprises about 3% of the total adsorbed form, the ester-like form accounts for 63%, and chelating together with bridging form accounts for 34%.