Nature Commun. 5, 4495 (2014)

The optical detection of single proteins is possible without requiring fluorescent labelling, report scientists in Germany. Marek Piliarik and Vahid Sandoghdar from the Max Planck Institute for the Science of Light in Erlangen have used their interferometric detection of scattering (iSCAT) system to sense cancer marker proteins in a buffer solution. It has previously been thought that the amount of light scattered by a single protein far from any optical resonance would be too small to detect. However, the use of iSCAT provides sufficient sensitivity and contrast to perform such a task. The researchers performed their measurements at a rate of 3,000 frames per second with 15 μW of incident 450 nm laser light and used a CMOS camera to detect both the scattered light from the sample and a reference beam. The optical intensity involved was well below the typical damage threshold of a biological sample. A variety of biologically relevant proteins — including fibrinogen, mouse immunoglobulin, bovine serum albumin and carcinoem-bryonic antigen — were successfully detected using the scheme.