  Single molecule detection provides detailed information about molecular
structures and functions, but it generally requires the presence of a
fluorescent marker which can interfere with the activity of the target molecule
or complicate the sample production. Detecting a single protein with its
natural UV autofluorescence is an attractive approach to avoid all the issues
related to fluorescence labelling. However, the UV autofluorescence signal from
a single protein is generally extremely weak. Here, we use aluminum plasmonics
to enhance the tryptophan autofluorescence emission of single proteins in the
UV range. Zero-mode waveguides nanoapertures enable observing the UV
fluorescence of single label-free beta-galactosidase proteins with increased
brightness, microsecond transit times and operation at micromolar
concentrations. We demonstrate quantitative measurements of the local
concentration, diffusion coefficient and hydrodynamic radius of the label-free
protein over a broad range of zero-mode waveguide diameters. While the
plasmonic fluorescence enhancement has generated a tremendous interest in the
visible and near-infrared parts of the spectrum, this work pushes further the
limits of plasmonic-enhanced single molecule detection into the UV range and
constitutes a major step forward in our ability to interrogate single proteins
in their native state at physiological concentrations.
