On the right path

Achieving effective terminal exciton delivery in quantum dot antenna-sensitized multistep DNA photonic wires

Assembling DNA-based photonic wires around semiconductor quantum dots (QDs) creates optically active hybrid architectures that exploit the unique properties of both components. DNA hybridization allows positioning of multiple, carefully arranged fluorophores that can engage in sequential energy transfer steps while the QDs provide a superior energy harvesting antenna capacity that drives a Förster resonance energy transfer (FRET) cascade through the structures. Although the first generation of these composites demonstrated four-sequential energy transfer steps across a distance >150 Å, the exciton transfer efficiency reaching the final, terminal dye was estimated to be only ~0.7% with no concomitant sensitized emission observed. Had the terminal Cy7 dye utilized in that construct provided a sensitized emission, we estimate that this would have equated to an overall end-to-end ET efficiency of ≤ 0.1%. In this report, we demonstrate that overall energy flow through a second generation hybrid architecture can be significantly improved by reengineering four key aspects of the composite structure: (1) making the initial DNA modification chemistry smaller and more facile to implement, (2) optimizing donor-acceptor dye pairings, (3) varying donor-acceptor dye spacing as a function of the Förster distance R0, and (4) increasing the number of DNA wires displayed around each central QD donor. These cumulative changes lead to a 2 orders of magnitude improvement in the exciton transfer efficiency to the final terminal dye in comparison to the first-generation construct. The overall end-to-end efficiency through the optimized, five-fluorophore/four-step cascaded energy transfer system now approaches 10%. The results are analyzed using Förster theory with various sources of randomness accounted for by averaging over ensembles of modeled constructs. Fits to the spectra suggest near-ideal behavior when the photonic wires have two sequential acceptor dyes (Cy3 and Cy3.5) and exciton transfer efficiencies approaching 100% are seen when the dye spacings are 0.5 × R0. However, as additional dyes are included in each wire, strong nonidealities appear that are suspected to arise predominantly from the poor photophysical performance of the last two acceptor dyes (Cy5 and Cy5.5). The results are discussed in the context of improving exciton transfer efficiency along photonic wires and the contributions these architectures can make to understanding multistep FRET processes.

Fluorescence in nanobiotechnology: sophisticated fluorophores for novel applications

Nanobiotechnology is one of the fastest growing and broadest-ranged interdisciplinary subfields of the nanosciences. Countless hybrid bio-inorganic composites are currently being pursued for various uses, including sensors for medical and diagnostic applications, light- and energy-harvesting devices, along with multifunctional architectures for electronics and advanced drug-delivery. Although many disparate biological and nanoscale materials will ultimately be utilized as the functional building blocks to create these devices, a common element found among a large proportion is that they exert or interact with light. Clearly continuing development will rely heavily on incorporating many different types of fluorophores into these composite materials. This review covers the growing utility of different classes of fluorophores in nanobiotechnology, from both a photophysical and a chemical perspective. For each major structural or functional class of fluorescent probe, several representative applications are provided, and the necessary technological background for acquiring the desired nano-bioanalytical information are presented.