In this work we report on the direct label-free detection and imaging of individual proteins via Rayleigh scattering without the need for field enhancement by microcavities or plasmonic particles . The detection of the scattered light takes place through its interference with the reflection of the incident laser beam (iSCAT – interferometric detection of scattering) . The resulting signal from each single protein is directly related to its polarizability and, thus, to its volume. This, in turn, allows for an estimate of the particle mass.
By combining iSCAT with surface functionalization of a substrate, we demonstrate a powerful biosensing platform capable of counting biomolecules in a direct and label-free fashion. In particular, we demonstrate detection of cancer marker proteins in buffer solution as well as in the presence of other abundant proteins. Furthermore, we report on super-resolution imaging of individual protein binding sites as well as their association and dissociation kinetics. The ability to detect single analyte molecules without the use of any labels improves the performance of the state-of-the-art biosensors by orders of magnitude to the ultimate graininess of biological matter. Detection, localization, and imaging of binding events of individual molecules also open new and exciting opportunities for studying protein interactions.
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