Luminescence properties of hydrophilic hybrid associates of colloidal CdS quantum dots and methylene blue

Luminescence properties of hybrid associates of semiconductor colloidal CdS quantum dots (QDs) with an average diameter of 2.5 nm and methylene blue (MB+) cations were investigated. Their photoluminescence spectra, excitation spectra and luminescence decay spectra recorded using the time-correlated single photon counting techniques were analyzed. The increase of efficiency of MB+ fluorescence at 405 nm excitation was found. It corresponds to the absorption region of CdS QDs. When [nCdSQDs]:[nMB+] concentrations ratio changes from 100:1 to 1:3 for associates, then a decrease in the intensity of CdS QDs luminescence band (580 nm) and its increase for MB+ band (674 nm) were observed. At the same time it was found that the average lifetime of CdS QDs luminescence is shortened and the lifetime of MB+ luminescence becomes significantly longer. It was concluded that dynamic quenching due to nonradiative resonance energy transfer takes place. The efficiency of this process was estimated and the corresponding rate constant was calculated. It is 5.6×107to8.6×107s−1.