Photoenhancement of quantum dots and conjugates measured by time-resolved spectroscopy

The response of solubilized quantum dot solutions to visible or UV irradiation is highly variable, and contradictory reports exist in the literature. Using several different preparations of core CdSe, core-shell CdSe/ZnS, and CdTe quantum dots (QDs), we investigated the time-resolved photoluminescence as a function of 400 nm irradiation. We found that photoenhancement and photodegradation were highly dependent upon irradiation power, with the QDs being highly stable at fluences of < 2 mW. However, great variability was seen among independent preparations of QDs, with fresher dots showing greater photostability than those that had been aged in organic solvent. Conjugation of dopamine to the QDs also led to variable effects, with some batches showing lifetime enhancement upon conjugation and others suppression. In all cases, QD-dopamine conjugates showed increased lifetimes upon irradiation, up to a maximum effect at ~ 5 min post irradiation at 2.4 mW. The antioxidant beta-mercaptoethanol also affected di ferent batches of QDs differently; it prevented photoenhancement with certain batches but not others. We propose a mechanism of photoenhancement and surface oxidation that relates the variability to the number of solubilising groups on the QD surface. The potential of photoenhancement as a sensing mechanism in cells is proposed.