Direct optical sensing of single unlabeled small proteins and super-resolution microscopy of their binding sites

Abstract: More than twenty years ago, scientists succeeded in pushing the limits of optical detection to single molecules using fluorescence. This breakthrough has revolutionized biophysical measurements, but restrictions in photophysics and labeling protocols have motivated many efforts to achieve fluorescence-free single-molecule sensitivity in biological studies. Although several interesting mechanisms using vibrational spectroscopy, photothermal detection, plasmonics or microcavities have been proposed for biosensing at the single-protein level, no method has succeeded in direct label-free detection of single proteins. Here, we present the first results using interferometric detection of scattering (iSCAT) from single proteins without the need for any label, optical nanostructure or microcavity. Furthermore, we demonstrate super-resolution imaging of protein binding with nanometer localization precision. The ease of iSCAT instrumentation promises a breakthrough for industrial usage as well as fundamental laboratory experiments.