Fluorescence quenching caused by aggregation of water-soluble CdSe quantum dots

We investigate the fluorescence quenching phenomenon that water-soluble CdSe quantum dots exhibit upon aggregation. First, we synthesize the hydrophobic CdSe quantum dots and convert them water-soluble using surface-ligand exchange with mercaptoacetic acid. We then induce aggregation of CdSe quantum dots by adding electrolyte to suspension. To monitor the change in photochemical and physical properties upon aggregation, various experimental techniques such as quasi-elastic light scattering (QELS), transmission electron microscopy (TEM), zeta potential measurement, and UV–vis and photoluminescence spectroscopy are employed. We find that addition of electrolyte to water-soluble quantum dot suspension reduces the colloidal stability of quantum dots, causing aggregation and hence the fluorescence quenching of quantum dots. Increasing electrolyte concentration accelerates aggregation of quantum dots, causing faster fluorescence quenching phenomenon. We rationalize the experimental results with the classical DLVO model that accounts for the interparticle interaction and colloidal stability in suspension. We expect that the fluorescence quenching phenomenon caused by quantum dot aggregation can be applied in many fields including molecular sensing and imaging probes.