  Single-molecule fluorescence techniques have revolutionized our ability to
study proteins. However, the presence of a fluorescent label can alter the
protein structure and/or modify its reaction with other species. To avoid the
need for a fluorescent label, the intrinsic autofluorescence of proteins in the
ultraviolet offers the benefits of fluorescence techniques without introducing
the labelling drawbacks. Unfortunately, the low autofluorescence brightness of
proteins has greatly challenged single molecule detection so far. Here we
introduce optical horn antennas, a dedicated nanophotonic platform enabling the
label-free detection of single proteins in the UV. This design combines
fluorescence plasmonic enhancement, efficient collection up to 85{\textdegree}
angle and background screening. We detect the UV autofluorescence from
immobilized and diffusing single proteins, and monitor protein unfolding and
dissociation upon denaturation. Optical horn antennas open up a unique and
promising form of fluorescence spectroscopy to investigate single proteins in
their native states in real time.
