We do alot of 3D particle localization of micro/nano fluorescent particles as well as the development of new techniques. It would be nice to have a small fluorescent particle (
I guess you've looked at the ThermoFisher Scientific (was Duke Scientific) Fluoro-Max Dyed Green, Red, and Blue Aqueous Particles (for contamination control and flow tracing). These are in the 30 - 200 nm range. See page 2 on:
Dimitar P. Mitev , I had never heard of nanodiamonds with NV centers. Looks perfect actually. I've ordered some and I'll update this question with the results.