Personally, I would soak my samples in a soap solution of Bodipy. This hydrophobic fluorescent dye will diffuse into the plastic and turn them fluorescent. In the rest of the sample it will adsorbe on the sand surface as non fluorecent oligomers. This yellow dye will disrupt the colour analysis of your microplastic but it will but it offers a high fluorescent yield at a wavelength that is well detedted by both detectors and eye. Shape and size will be preserved. The amount quantification will depend on the level of doping. At high doping the fluorescence signal will be proportionl the the area of the nano-plastic. At low doping, it will be proportional to the volume.

But I guess that that question has already been adressed in the litherature, since it is very promizing approach. Bodipy is cheaper if you pruchase it as laser dye "Pyromethene 546 ".

Flow cytometry

Normally, the use Nile red.

but generally, the sample need some preparation step:

- density separation to remove high density particles (soil, sand, etc)

- oxidation to remove some organic particles

- dye

- analysis by fluorescence microscopy

But, my acquaintance informed me that those dye colors also stain some organic particles in addition to microplastics, therefore this use is being carefully considered.