Label free imaging and deep tracking of single biological nanoparticles in free solution by reflection enhanced dark field scattering microscopy

Label free optical imaging of single biological nanoparticles and deep tracking of their behaviors in free solution are very important in cell biology, but quite challenging due to the weak optical signals produced from small biological nanoparticles and the lack of deep tracking method in Z axis. Here, by simply introducing a reflective surface, we are able to significantly enhance the sensitivity of dark field scattering microscopy (DFSM). The significant enhancement on the sensitivity facilities the location of nanoparticles in Z axis over several hundred micrometers, enabling the deep tracking of single exosomes in free solution, as well as the clearly observation and identification of organelles at single cells with least exogenous interferences. The reflection enhanced dark field scattering microscopy (REDFSM) can be performed on a reflected dark field microscope without the reform of the equipment or the use of high power laser, implying its potential wide application in cell research, due to its improved sensitivity and simplicity. Compared with the widely used fluorescence microscopy which only sees what is labeled, the label free REDFSM can see most of the organelles in cell with high imaging quality, providing additional opportunity to discover new cell biological phenomena.