The morphological structure of spherical SiO2, CD@SiO2 and MT-CD@SiO2 is compared by SEM. As shown in Fig. 3A, a rougher surface of CD@SiO2 compared to spherical SiO2 clearly indicates the successful loading of C-dots. As for MT-CD@SiO2, the loading layer on the sphere surface gets thicker, resulted from MT decoration.