pH dependent fluorescence of thiol coated CdSe/CdS quantum dots in an aqueous phase.

The results presented in this paper show how the optical properties and colloidal stability of quantum dots (QDs) vary, depending on pH conditions. For this investigation, the as-synthesized hydrophobic CdSe/CdS QDs were transferred to the aqueous medium by surface modification with 3-mercaptopropionic acid (3-MPA). The ligand exchange procedure was applied under three different pH conditions: acidic, neutral and alkaline, to obtain three kinds of hydrophilic QDs dispersed in phosphate buffer. The efficiency of the functionalization of QDs was estimated based on the changes in absorbance spectra and the highest value was obtained under acidic conditions (45 %). The efficiency of photoluminescence (PL) was also best preserved under these conditions, nevertheless it was 30 times less than the PL of hydrophobic QDs. Then, all these three kinds of hydrophilic QDs were dispersed in solutions having wide range of pH (2-12) and investigated by absorbance and photoluminescence measurements. The results show that QDs subjected to a ligand exchange procedure are characterized by intensive photoluminescence at the selected pH values, which correspond to pKa of the ligand. This phenomenon is independent of the pH at which ligand exchange procedure was conducted. Moreover, it was found that the PL intensity preserves in the time of the experiment for QDs functionalized under neutral conditions, while it decreases for acidic and increases for alkaline conditions.