Stability of synthesized PQDs in various pH values and different ionic strengths. (a) Effect of pH on the photoluminescence of 623-nm-emitting PQDs. PQD colloids were dispersed in varied buffers, pH 2 ~ 13, PQDs/buffer = 1:1 (v/v). (b) Influence of increasing ionic strength on the photoluminescence of PQDs. The final sodium chloride concentrations varied from 0 to 300 mM (pH = 7.4).