Wang, F. et al. Light Sci. Appl. 7, e18007 (2018).

Upconverting nanoparticles convert near-infrared light into visible light. The particles are typically very stable and bleach-resistant and, from this perspective, are good labels for tracking experiments. Wang et al. now show that single nanoparticles can be detected within cells by eye, through the microscope eyepiece. They determined the threshold for this detection and show that single nanoparticles can be distinguished from clusters. In post-acquisition analysis, the researchers monitored the diffusion of multiple individual nanoparticles over 21 seconds and inferred the viscosity of the cellular environment on the basis of this movement. Finally, they made use of the excitation power density dependence of upconverting nanoparticle clusters to introduce a new 'channel' for multiplex biological imaging.