QDsThe CdS QDs were synthesized according to the previous reports with some modifications [29]. Hexametaphosphate-capped CdSQDs were prepared as following: CdCl2 solution (0.2 mL of 1 mol/L)was added into 20 mL of Na6P6O18(20 mmol/L) aqueous solu-tion with vigorous stirring at room temperature. After 10 min,sodium sulfide solution (120 L of 1 mol/L) was injected slowlyinto the mixed solution. The reaction mixture was allowed toproceed for another 1 h under vigorous stirring. The obtainedhexametaphosphate-capped CdS QDs solution was dialyzed inmembrane tubing with a molecular weight cut-off of 3 KDa againstultrapure water to remove small molecules and ions. The purifiedCdS QDs solution were freeze-dried into powder and measured, andthen the CdS QDs powder was dissolved in aqueous solution againwith 16.3 mg/mL mass concentration.