Using scattering instead of fluorescence for nanoparticle tracking analysis has the added advantage of a virtually unlimited photon budget. It is not time-limited (particles scatter light forever), and increasing the excitation power increases the scattered power linearly.
However scattering lacks the specificity of fluorescence, which means that having lower backgrounds is fundamental to be able to detect small objects. This can be achieved through hollow core fibers, hollow optical fibers, or interferometric detection schemes.
These are the other notes that link to this one.