SiO2 nanoparticles were firstly prepared following the well known modified St ¨ober method [25].The core–shell mesoporous
silica was prepared according to the previous report with some modifications [26]. 0.5 g SiO2 was well dispersed in the mixture solution of 0.15 g cetyltrimethylammonium bromide (CTAB), 40 mL deioned water, 0.5 g concentrated ammonia aqueous solu-tion (28wt%), and 30 mL ethanol. After the solution was stirred for 0.5 h, 0.40 g TEOS was added dropwise to the solution with vigorous stirring. After reaction for 6 h, the product was collected by centrifugation and washed several times with ethanol and water. The above coating process was repeated twice. The structure-directing agent (CTAB) was removed by acid/solvent extraction, using a solution of 1M HCl in ethanol. This mixture was refluxed for 72 h, then filtered and washed with EtOH to remove the residual HCl.The product was dried at 60 1 C for 12 h in vacuo to obtain the synthesized core–shell mesoporous silica.