Tracking polystyrene particles
Polystyrene particles of a given diameter are 1000 less bright than gold particles of the same size. This is due to 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.
Results based on diffusion coefficient are the same as nanoparticle tracking analysis. However, the limitations of nanoparticle tracking analysis are linked to poor localization accuracy, and those are inherent to the optical microscope.
On the other hand, the intensity distribution of each particle in the channel is narrow. This is due to a relatively homogenous illumination profile. That may be the biggest advantage of nanoparticle tracking in hollow optical fibers, also of nanoparticle tracking in nano channels.
The crucial parameter to decide whether the method is worth pursuing, is that the intensity fluctuations of a single particle are narrower than the overall sample distribution. That may not be the case for the diffusion-based method, in which the uncertainty in size determination for a single particle is as broad (or more) than the overall sample.
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