Color change of toluidine blue induced by polyanionic quantum dots

The surface of 4.1 nm CdSe/ZnS, core/shell, quantum dots was stabilized with cysteine (Cys) or thioglycolic acid (TGA) ligands. Electrostatic interactions between the polyanionic quantum dots (QDs) and positively charged toluidine blue were studied by UV–vis absorption spectroscopy and Förster resonance energy transfer. The surface net charge on the QDs highly correlated with the dye's degree of metachromasia, which served as a direct and convenient surface electrostatic probe for negatively-charged nanoparticles.