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

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.