Enhancement of the fluorescence quenching efficiency of DPPH(•) on colloidal nanocrystalline quantum dots in aqueous micelles.

Luminescent CdS quantum dots capped with thioglycolic acid (CdS-TGA QDs) were demonstrated to serve as a fluorescence probe for a model organic radical, 2,2-diphenyl-1-picrylhydrazyl radical (DPPH(•)), employing the quenching of the CdS-TGA QDs emission signal by the radical. Under the optimum conditions, the quenching efficiency of DPPH(•) on CdS-TGA QDs was proportional to the concentration of DPPH(•), following Stern-Volmer relationship. Different types of surfactants (cationic, anionic and neutral surfactants) were introduced to CdS-TGA QDs in order to increase the detection sensitivity. The fluorescence intensity of CdS-TGA QDs was greatly enhanced by cationic and neutral surfactants. Moreover, the quenching efficiency of DPPH(•) on the QDs in the presence of micelles was remarkably ca. 13 times higher than that in the system without micelles. Effects of pH and concentration of surfactants on the fluorescence quenching of CdS-TGA QDs were investigated. Electron spin resonance (ESR) spectroscopy was also used to monitor the DPPH radical species in CdS-TGA QDs mixtures with and without micelles. Fluorescence quenching mechanisms of CdS-TGA QDs by DPPH(•) in the presence and in the absence of CTAB were proposed.