Quantum Dots as Förster Resonance Energy Transfer Acceptors of Lanthanides in Time-Resolved Bioassays

We report a flexible and modular design for biosensors based on exploiting semiconductor quantum dots (QDs) and their excellent Förster resonance energy transfer (FRET) acceptor properties along with the long-lived fluorescent lifetimes of lanthanide donors. We demonstrate the format's wide application by developing a broad adenosine diphosphate (ADP) sensor with quantitative and high-throughput capabilities as a kinase/ATPase assay method. The sensor is based on a Terbium (Tb)-labeled antibody (Ab) that selectively recognizes ADP versus ATP. The Tb-labeled Ab (Ab-Tb) acts as a FRET donor to a QD, which has an ADP modified His6-peptide conjugated to its surface via metal-affinity coordination. This strategy of using self-assembly, modified peptides to present antibody epitopes on QD surfaces is readily transferable to other assays of interest. We utilize time-resolved FRET (TR-FRET) to measure the amounts of Ab-Tb bound to the QD by looking at the emission ratio of the QD and Tb in a time-gated manner, minimizing background signal. With the addition of free ADP the antibody is competitively separated from the QD and a change in the ratiometric emission signal correlates with the free ADP concentration. The sensor obtained a detection limit below 10 nM of free ADP and quantitation limit of 35 nM ADP using 8 nM of sensor. Quantitative values were obtained for a model enzyme (glucokinase) kinetics, as well as demonstrations of the assays capability to distinguish enzyme inhibitors. We discuss future outlooks and note areas for improvement in similar design strategies.