The precursor (1 g) and spherical SiO2 (1 g) are well mixed and placed in an alumina crucible and then heated at 300 8C for 2 h with a heating rate of 10 8C min1. The unattached C-dots are removed by filtering with 100 mm meshes and repeated washing with DI water. The prepared SiO2 supported C-dots (CD@SiO2 for short) is dried at 85 8C before use.