A QD-aptamer beacon for the detection of thrombin was recently reported,155 and the principle of this detection system is schematically illustrated (Figure 16.7). Briefly, QDs were surface
functionalized with an aptamer known to bind to the thrombin protein to develop a thrombin-detecting QD-aptamer beacon. A second oligonucleotide, having complementary sequences to the thrombin aptamer and bearing a quencher, was synthesized and utilized to quench the system in the absence of thrombin. When the quencher oligonucleotide was incubated with the QD-aptamer conjugate in the absence of thrombin, the hybridization of the quencher oligonucleotide and the thrombin aptamer proximated the quencher molecules to the surface of the QD, resulting in quenching effect through fluorescence resonance energy transfer (FRET).156,157 In the presence of thrombin (1 mM), the quencher oligonucleotide is displaced, resulting in restoration of the fluorescence of the QD-aptamer conjugate. Using this QD-aptamer beacon system, a 1 mM concentration of thrombin could be detected specifically. We postulate that multiplexed detection of a panel of protein targets may be performed in parallel by formation of QD-aptamer conjugates from different aptamers, and QDs possessing different emitting fluorescence profile.158,159 In addition, considering a growing number of tumor-specific or tumor-associated aptamers that are available today, it may be increasingly possible to develop more specific and more efficient cancer diagnostic probes for a myriad of important cancers. We have recently developed a QD-aptamer conjugate using the A10 PSMA aptamer and have demonstrated the differential binding and uptake of these vehicles by cells that express the PSMA antigen (unpublished results).
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