UV fluorescence enhancement by aluminum and magnesium equilateral bowtie nanoantennas

2020 
The intrinsic fluorescence of biomolecules such as proteins and nucleic acids lies in the ultraviolet (UV) range of the spectrum. UV plasmonic nano-structures have been shown to enhance the fluorescence quantum yield and reduce the lifetimes of various biomolecules. Fluorescence enhancement is contributed to by both excitation rate and emission rate enhancement. Since biomolecules are prone to photon-degradation in the UV range, excitation rate enhancement should be minimized, while radiative rate enhancement should be maximized. Although numerous nano-structures have been proposed both numerically and experimentally to enhance the fluorescence of native biomolecules, very few studies have achieved more than 10x radiative rate enhancement. Here we report systematic studies of fluorescence enhancement by equilateral bowtie nano-antennas (BNA) made of aluminum (Al) or magnesium (Mg) in the ultraviolet region. We modeled the emission rate enhancement using the excitation and emission peak wavelength of tryptophan. The quantum yield of tryptophan is also taken into account. Our results show that with the optimal geometry, Al BNA with oxide yields an excitation enhancement of 21× at the excitation wavelength of tryptophan (270nm), a radiative enhancement of 37×, a quantum yield enhancement of 5×, and a net fluorescence count rate enhancement of 64× at the emission wavelength of tryptophan (340nm). Mg BNA with oxide sustains the highest Purcell factor enhancement, 14×. The effect of the native oxide layer on both metals is investigated. The studies reported here are meaningful in the design of better UV plasmonic nano-structures for label-free sensing of biomolecules.
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