Quenching mechanism FRET

The duplex probe configuration provides for a close proximity of the donor/acceptor pair and may lead to two mechanisms of quenching FRET and direct transfer (contact-mediated) quenching [78]. Placement of the fluorophore and quencher toward the centre of the duplex probe sequence may further add to the quenching efficiency as the ends of duplexes are known to breathe and are not as tightly bound as internal base pairs. Duplex probes are relatively easy to synthesize as the fluorophore and quencher moieties do not have to be incorporated into the same strand. 

Quenching of the luminescence of the QDs by an electron transfer (ET) mechanism, or by a fluorescence resonance energy transfer (FRET) route, could reflect the formation of a recognition pair, or may be used to follow the dynamics of chemical processes that occur on the modified QDs. In these systems, the sensing events or the respective biocatalytic transformations involve the association/ dissociation of quencher/FRETacceptor units that electronically couple with the QDs. 

Similarly, semiconductor QDs were integrated with proteins, such that the hybrid systems would permit the real-time analysis of catalytic transformations stimulated by the proteins [102, 192-194]. For example, the hydrolytic functions of a series of proteolytic enzymes were followed by the application of QD reporter units, using the FRET process as a readout mechanism. In this case, the QlSe QDs were modified with peptide sequences that were specific for different proteases, where the quencher units were tethered to the peptide termini. Within the QDs/fluorophore-modified hybrid assembly, the fluorescence of the QDs was quenched. Subsequent hydrolytic... 

The different biocatalytic transformations probed with QDs have led to the implementation of a FRET process to follow the reaction progress. The ET quenching of QDs represents an alternative mechanism for probing biocatalytic transformations a typical example was the biocatalytic function of two enzymes, tyrosinase and thrombin, which were probed by using CdSe/ZnS QDs [196]. In this case, the CdSe/ ZnS QDs were capped with a monolayer of methyl ester tyrosine (34). A subsequent tyrosinase-induced oxidation of tyrosine to dopa-quinone led to the generation of ET quencher units that suppressed the luminescence of the QDs (Figure 6.28a). The... 

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