Abstract
The vast majority of proteins are intrinsically fluorescent in the ultraviolet, thanks to the emission from their tryptophan and tyrosine amino-acid constituents. However, the protein autofluorescence quantum yields are generally very low due to the prevailing quenching mechanisms by other amino acids inside the protein. This motivates the interest to enhance the radiative emission rate of proteins using nanophotonic structures. Although there have been numerous reports of Purcell effect and local density of optical states control in the visible range using single dipole quantum emitters, the question remains open to apply these concepts in the UV on real proteins containing several tryptophan and tyrosine amino acids arranged in a highly complex manner. Here, we report the first complete characterization of the Purcell effect and radiative rate enhancement for the UV intrinsic fluorescence of label-free β-galactosidase and streptavidin proteins in plasmonic aluminum nanoapertures. We find an excellent agreement with a calibration performed using a high quantum yield UV fluorescent dye. Demonstrating and intensifying the Purcell effect is essential for the applications of UV plasmonics and the label-free detection of single proteins.
Highlights
The spontaneous emission depends on the photonic environment and is not an inherent process
While the spontaneous emission rate enhancement has been widely observed for single quantum emitters in the visible range, its extension into the UV domain remains essentially limited to simple fluorescent dyes such as p-terphenyl or isolated tryptophan amino acids [13,14,15,16]
We aim at measuring experimentally these different decay rate constants for two different proteins (β-galactosidase and streptavidin) to quantify the influence of a plasmonic nanoaperture on the local density of optical states (LDOS) and explore if Purcell enhancement can be demonstrated for label-free proteins
Summary
The spontaneous emission depends on the photonic environment and is not an inherent process. This phenomenon, called the Purcell effect [1], has been observed in photonic crystal microcavities [2, 3] or plasmonic optical nanoantennas [4,5,6,7,8]. While the spontaneous emission rate enhancement has been widely observed for single quantum emitters in the visible range, its extension into the UV domain remains essentially limited to simple fluorescent dyes such as p-terphenyl or isolated tryptophan amino acids [13,14,15,16]. The modifications of emission rates for specific proteins have been revealed in
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