UV fluorescence enhancement by aluminum and magnesium equilateral bowtie nanoantennas

Abstract

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.

Resonance mechanisms of coupled-particle plasmonic nano-antennas to maximize UV fluorescence enhancement of biological molecules

“…In this study, we chose BNAs to perform our analysis but the method employed here can be applied to other plasmonic nano-antennas coupled through gap modes. In the visible wavelength range, significant enhancement factors have been achieved by gold and silver BNAs [29,34,35]. A 1300 × enhancement factor was observed experimentally on an organic dye molecule excited by a gold BNA [30].…”

“…Various designs of plasmonic nanostructures have been studied for the applications of plasmonic enhanced near-fields or fluorescence emission, such as nanoholes [5,10,15,23,26], nanopillars [8], Yagi-Uda nanoantennas [27], and bowtie nanoantennas [28][29][30][31]. The triangle-shaped bowtie nanoantennas (BNAs) have attracted enormous attention due to their high electric-field enhancement factors (EF) owing to their small gaps confining the electric-fields through in-plane near-field coupling [32][33][34]; their response spectrum can be tuned by adjusting the vertex angle, size and gap [31]. In this study, we chose BNAs to perform our analysis but the method employed here can be applied to other plasmonic nano-antennas coupled through gap modes.…”

“…Despite of efforts in the design of aluminum (Al) BNAs, the EF factors remain significantly smaller than in the visible range [29,31,34,[36][37][38]. The challenges of achieving high EF factors in the UV range are caused by the inherent characteristics of BNA resonances.…”

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.…”