Time-Gated DNA Photonic Wires with Forster Resonance Energy Transfer Cascades Initiated by a Luminescent Terbium Donor

Functional DNA nanotechnology is a rapidly growing area of research with many prospective photonic applications, including roles as wires and switches, logic operators, and smart biological probes and delivery vectors. Photonic wire constructs are one such example and comprise a Forster resonance energy transfer (FRET) cascade between fluorescent dyes arranged periodically along a DNA scaffold. To date, the majority of research on photonic wires has focused on setting new benchmarks for efficient energy transfer over more steps and across longer distances, using almost exclusively organic fluorescent dyes and strictly DNA structures. Here, we expand the range of materials utilized with DNA photonic wires by demonstrating the use of a luminescent terbium complex (Tb) as an initial donor for a four-step FRET cascade along a similar to 15 nm long DNA/locked nucleic acid (LNA) photonic wire. The inclusion of LNA nucleotides increases the thermal stability of the photonic wires while the Tb affords time-gated emission measurements and other optical benefits. Time-gating minimizes unwanted background emission, whether from direct excitation of fluorescent dyes along the length of the photonic wire, from excess dye-labeled DNA strands in the sample, or from a biological sample matrix. Observed efficiencies for Tb-to-dye energy transfer are also closer to the predicted values than those for dye-to-dye energy transfer, and the Tb can be used as an initial FRET donor for a variety of next-in-line acceptors at different spectral positions. We show that the key to using the Tb as an effective initial donor is to optimally position the next-in-line acceptor dye in a so-called sweet spot where the FRET efficiency is sufficiently high for practicality, but not so high as to suppress time-gated emission by shortening the Tb emission lifetime to within the instrument lag or delay time necessary for measurements. Overall, the initiation of a time-gated FRET cascade with a Tb donor is a very promising strategy for the design, characterization, and application of DNA-based photonic wires and other functional DNA nanostructures.