According to our proposed mechanism in Figure , CdS QD gel should form the H • -doped CdS (CdS • -H) after abstracting a hydrogen atom from C-H substrates during d-HAT. To observe the formation of CdS • -H, we mixed CdS QD gel with a stable aprotic C-H partner, toluene, under waterand air-free condition. After blue light irradiation for 10 mins, we observed a color change of the reaction mixture from yellow (CdS QD gel's original color) to brown (Figure ). After exposing the solution to air, the brown hue returned to yellow (Figure ). Similar reversible color changes were also previously observed when TiO2 and ZnO nanoparticles were irradiated by UV light and then exposed to oxidants such as TEMPO and air. The altered color was attributed to the formation and consumption of the protonated reduced TiO2 and ZnO nanoparticles (H + -TiO2/e -and H + -ZnO/e -). Note, H + /e is equivalent to a hydrogen atom (H • ) so H + -TiO2/e -and H + -ZnO/e can be considered as H •doped TiO2 and ZnO. Also, the observed brown color in Figure is similar to the color of reduced CdS QDs prepared by chemical reduction using Na/biphenyl or photochemical reduction in the presence of reductants such as alkyl amines. The above evidence supports that CdS • -H was formed from CdS QD gel during the d-HAT process. However, we want to point out that the H • -doped photocatalysts in the previous studies were formed via an electron transfer process, but, in our case, it should result from the d-HAT process between excited CdS QD gel and C-H substrates.