Tracking polystyrene particles

Polystyrene particles of a given diameter are 1000 less bright than gold particles because of the refractive index being closer to that of water. However, it is possible to image 19nm PS particles inside a hollow optical fiber coupling ~10mW laser power to the fiber.

In [@Faez2015Fast, Label-Free Tracking of Single Viruses and Weakly Scattering Nanoparticles in a Nanofluidic Optical Fiber], two different measurements are done: calculating the size from the diffusion coefficient and grouping particles based on scattering intensity. The first method is an absolute method (it does not require an a-priori knowledge of the particles), but shows larger fluctuations due to poor localization accuracy (see: limitations of nanoparticle tracking analysis). The intensity distribution is narrower. Moreover, the intensity fluctuation of a single particle (due to the inhomogeinity of the illumination within the channel itself) has a narrower distribution than the overall intensity fluctuation distribution of the sample.