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.
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