Ultrafast ET with a characteristic time constant of approximately 70 fs between CdSe QDs (mean radii of 1.4 nm) photoexcited in the lowest 1S electron state (lambda(exc) = 539 nm), and the molecular electron acceptor MV(2+) adsorbed on the QD surface was observed. The photophysics of such a system was investigated by time-resolved transient absorbance spectroscopy in the UV-visible spectral region. Our studies for the coupled system as a function of excitation intensity at lambda(exc) = 387 nm show that the ET processes compete efficiently with Auger recombination in CdSe QDs and at least 4 e-h pairs can be separated by ET to the electron acceptor MV(2+).
Applications of water-soluble quantum dots (QDs) in the life sciences are limited by their poor colloidal stability in physiological media and nonspecific interaction with biomatter, particularly cell membranes. We have studied colloidal stability and nonspecific interactions with living cells for zwitterionic d-penicillamine-coated QDs (DPA-QDs) and the traditionally used carboxylated 11-mercaptoundecanoic acid-coated QDs (MUA-QDs) and found clear advantages of DPA-QDs. In single molecule fluorescence experiments, DPA-QDs showed no aggregation over the physiologically relevant pH range of 5-9, whereas MUA-QDs showed significant aggregation below pH 9. Upon exposure to living Mono Mac 6 cells, DPA-QDs, which possess overall charge-neutral surfaces, exhibited weak interactions with the cell membrane and were easily removed by flushing with buffer. By contrast, the highly charged MUA-QDs strongly associated with the cells and could not be removed even by extensive rinsing with buffer solution. DPA-QDs exhibit a high chemical stability even in strongly oxidizing conditions, in contrast to cysteine-coated QDs reported earlier. This beneficial property may arise from reduced interactions between DPA ligands due to steric effects of the methyl groups on their beta-carbon atoms.
Water-soluble thiolated molecules are among the most commonly used ligands to render quantum dots (QDs) water-soluble and biocompatible. These ligands maintain a relatively small total QD size, which makes them useful for many biological applications, but often cause a reduction of the quantum yield. The resulting quantum yields vary significantly between different reports and a complete understanding of the mechanism underlying the quenching of luminescence is lacking. We have studied the effect of ligand-exchange reaction time with 11-mercaptoundecanoic acid (MUA) and find an exponential decrease of quantum yield with time. We have also investigated the quenching effect of various commonly used thiolated ligands. A strong dependence on size and charge of the quenching molecule was observed; the Stern-Volmer plots were nonlinear and represent multiple quenching pathways. By comparison with nonthiolated analogues, the relative contributions of the thiol group, carboxyl group, and alcohol group to the quenching behavior were assessed. Furthermore, luminescence lifetime analysis revealed that quenching is static rather than diffusive in origin, indicating that it only arises from ligands coordinated to the QD surface. Our results will be helpful for QD chemists and biophysicists to select the optimal ligands for their particular application.
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