  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 (LDOS) 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 \b{eta}-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.
