  Optoplasmonic methods capable of single protein detection so far rely on
analyte immobilization in order to facilitate detection [1-6]. These detection
schemes, even if they facilitate transient single-molecule detection [7,8] via
consequent formation and cleavage of chemical bonds, typically exhibit time
resolutions on the order of milliseconds. The need for analyte immobilisation
is a direct consequence of the minuscule dimensions of plasmonic near fields
typically providing sub-attolitre-sized detection volumes which in turn demand
sub-microsecond temporal resolution for the direct detection of proteins in
motion. Here we show that such temporal resolution can indeed be achieved. We
demonstrate the observation of single proteins as small as Hemoglobin
(molecular weight: 64 kDa) as they traverse plasmonic near fields of gold
nanorods and interact with their surface, all while maintaining signal-to-noise
ratios larger than 5 and an unprecedented temporal resolution well below
microseconds. This method enables the label-free observation of single-molecule
dynamics on previously unaccessible timescales.
