The overall deF% after 4-h light irradiation exhibited a nonlinear dependence on the excitation intensity, even with LED light (Fig. , with a slope of 2.5). This observation indicates multiphoton processes are involved in the PFOS decomposition. The reduction potential of the conduction band of CdS (-0.9 V vs. SHE) is lower than that of PFOS (-1.3 V vs. SHE) even taking the pH and quantum size effect into account (-1.19 V vs. SHE). Moreover, the emission decay of CdS NCs was not affected at all by the presence of PFOS (Supplementary Fig. ). These results strongly suggest that hydrated electrons and higher excited states generated by Auger recombination are involved in PFOS decomposition. The defluorination efficiency increased with decreasing NC diameter (Fig. ), and the defluorination efficiency substantially decreased in commercial bulk CdS although the primary particle size estimated by the linewidth of X-ray diffractions was 7.4 nm. The efficient decomposition reaction in smaller NCs is consistent with the fact that Auger recombination is enhanced in a strong confinement regime and the rate of Auger recombination increases with smaller NC size in addition to the larger surface-tovolume ratio in smaller NCs. Moreover, the slope of the power dependence in this study exceeded 2, whereas the experimentally observed slope of the power dependence of two-photon processes is typically below 2 owing to the involvement of other optical processes and reabsorption by photogenerated species. It suggests that another photochemical process is involved in PFOS decomposition, as discussed later.