The CdS QDs were synthesized according to the previous reports
with some modifications [29]. Hexametaphosphate-capped CdS
QDs 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 slowly
into the mixed solution. The reaction mixture was allowed to
proceed for another 1 h under vigorous stirring. The obtained
hexametaphosphate-capped CdS QDs solution was dialyzed in
membrane tubing with a molecular weight cut-off of 3 KDa against
ultrapure water to remove small molecules and ions. The purified
CdS QDs solution were freeze-dried into powder and measured, and
then the CdS QDs powder was dissolved in aqueous solution again
with 16.3 mg/mL mass concentration.