Fluorophore-gold nanoparticle FRET/plasmonic lasers with the streptavidin-biotin complex as the acceptor–donor linkage

Several Förster resonance energy transfer (FRET) lasers have been realized by employing the robust and versatile streptavidin-biotin (SPB) biocomplex as the acceptor–donor linkage. SPB offers a fixed acceptor–donor separation (“ruler”) of <6 nm, which lies within the Förster radius for a broad range of donors and acceptors. A Cy3-SPB-Cy5 conjugate laser (where Cy3 and Cy5 are cyanine dyes) peaking at λ ∼ 708 nm has been observed, and its bandwidth and threshold pump energy (at 532 nm) have been measured to be ∼4.5 nm and 118 µJ (corresponding to a pump energy density of 179 ± 5 µJ/mm2), respectively. Depolarization of the linearly polarized pump optical field by this FRET process is found to be <12%. To tether the acceptor and donor, the SPB complex requires only that either be conjugated, thereby allowing FRET processes to be examined among an extensive set of biomolecules, inorganics, and nanoantenna acceptors, for example. As a result, fluorophore-nanoparticle lasers having characteristics of both FRET lasers and plasmonic emitters have been demonstrated. Laser spectra and the phase shift induced by a 10 or 100 nm gold nanoparticle tethered to the Cy3-SPB complex suggest that both the fluorescent protein and nanoparticle are able to act as an acceptor. The brightness associated with this new class of fluorophore/nanostructure FRET lasers will broaden the scope of accessible biomedical diagnostics, including cellular imaging and the detection of DNA and proteins.