Microscopic inspection and tracking of single upconversion nanoparticles in living cells

Nanoparticles have become new tools for cell biology imaging(1), sub-cellular sensing(2), super-resolution imaging(3,4) and drug delivery(5). Long-term 3D tracking of nanoparticles and their intracellular motions have advanced the understanding of endocytosis and exocytosis as well as of active transport processes(6-8). The sophisticated operation of correlative optical-electron microscopy(9,10) and scientific-grade cameras is often used to study intercellular processes. Nonetheless, most of these studies are still limited by the insufficient sensitivity for separating a single nanoparticle from a cluster of nanoparticles or their aggregates(8,11,12). Here we report that our eyes can track a single fluorescent nanoparticle that emits over 4000 photons per 100 milliseconds under a simple microscope setup. By tracking a single nanoparticle with high temporal, spectral and spatial resolution, we show the measurement of the local viscosity of the intracellular environment. Moreover, beyond the colour domain and 3D position, we introduce excitation power density as the fifth dimension for our eyes to simultaneously discriminate multiple sets of single nanoparticles.