Quantum Yield Limits for the Detection of Single-Molecule Fluorescence Enhancement by a Gold Nanorod

Abstract

Fluorescence-based single-molecule optical detection techniques are widely chosen over other methods, owing to the ease of background screening and better signal-to-noise throughput. Nonetheless, the methodology still suffers from limitations imposed by weak emitting properties of most molecules. Plasmonic nanostructures, such as gold nanorods, can significantly enhance the fluorescence signal of a weak emitter, extending the application of these techniques to a wider range of species. In this work, we explore the lower limit of fluorescence quantum yield for single-molecule detection, using a single gold nanorod to enhance molecular fluorescence. We specifically designed an infrared dye with the extremely low quantum yield of 10 −4 and a comparatively large Stokes shift of 3000 cm −1 to demonstrate single-molecule detection by fluorescence enhancement. This example allows us to discuss more general cases. We estimate theoretically the optimal excitation wavelength and the plasmon resonance of the rod that maximize the fluorescence signals. We then confirm experimentally the detection of single-molecule fluorescence with an enhancement factor of 3 orders of magnitude for the quantum yield 10 −4 . Theoretical simulations indicate that single-molecule signals should be detectable for molecules with quantum yield as low as 10 −6 , provided the dwell time of the molecules in the plasmonic hot spot is long enough.

Plasmonic Enhancement of Two-Photon Excited Luminescence of Gold Nanoclusters

“…Short duration of single bursts in the order of 10 ms confirms the presence of a single nanocluster in the hotspot during local enhancement. Similar duration of emission enhancement of freely diffusing single molecule in the area of a gold nanorod was previously noted [ 42 ]. There were two solutions to preserve molecules for longer in the vicinity of near-field of nanorods: raise the viscosity of solvent, or use additional linkers [ 37 , 42 ].…”

“…Similar duration of emission enhancement of freely diffusing single molecule in the area of a gold nanorod was previously noted [ 42 ]. There were two solutions to preserve molecules for longer in the vicinity of near-field of nanorods: raise the viscosity of solvent, or use additional linkers [ 37 , 42 ]. We examined plasmonic enhancement of nanoclusters in solvents of the range of different densities, however, except of slower diffusion of fluorophores, high glycerine content solvents peel off immobilised nanorods from a glass surface.…”

A scanning planar Yagi-Uda antenna for fluorescence detection

“…For example, in the vicinity of plasmonic nanoparticles illuminated by laser light, the excitation intensity can increase by orders of magnitude [9,10]. In addition, plasmonic particles spectrally matched with fluorescent molecules can also increase the emission rate as well as the quantum efficiency, making them significantly brighter [11][12][13][14]. It is also well known that some of these structures operate like antennas to change the radiation pattern of emitters and make them radiate into smaller angles [15][16][17][18].…”

Enhancing Single-Molecule Fluorescence Spectroscopy with Simple and Robust Hybrid Nanoapertures

“…For example, a metal nanosphere when excited at surface plasmon resonance provides a strong near-field enhancement, which is more than five times larger than that provided by its “equivalent”, i.e., a circular nanoaperture in a metal film. − It should be noted that we do not classify nanoapertures as “nanoantennas”, which refer to conventional “nanoantenna” structures (e.g., plasmonic nanopillars and nanoparticle dimers), , simply because of their poor antenna characteristics of coupling far-field light into localized regions or vice versa. The small field enhancement leads to weak light–matter interactions within the nanoaperture, limiting the enhancement of spectroscopic signals (e.g., fluorescence and Raman) from molecules interacting with the nanoaperture. ,,− In comparison, conventional “nanoantenna” structures have shown stronger enhancement in the spectroscopic signals from the nearby molecules. − Such “nanoantenna” structures range from a single nanosphere ,− or nanorod , to the more complex engineered nanostructures such as dimers, ,− bowtie, , and Yagi-Uda . However, for many of the “nanoantenna” structures, the strong field enhancements rely on small nanogaps with dimensions of <10 nm in the structures, which often require expensive and low-throughput nanofabrication tools such as electron-beam lithography.…”

“…8,21,36−43 In comparison, conventional "nanoantenna" structures have shown stronger enhancement in the spectroscopic signals from the nearby molecules. 44−48 Such "nanoantenna" structures range from a single nanosphere 31,49−51 or nanorod 52,53 to the more complex engineered nanostructures such as dimers, 49,54−56 bowtie, 57,58 and Yagi-Uda. 59 However, for many of the "nanoantenna" structures, the strong field enhancements rely on small nanogaps with dimensions of <10 nm in the structures, which often require expensive and low-throughput nanofabrication tools such as electron-beam lithography.…”