Nanoparticles have become new tools for cell biology imaging1,sub-cellular sensing2,super-resolution imaging3,4 and drug delivery5.Long-term 3D tracking of nanopartides and their intracellular motions have advanced the understanding of endocytosis and exocytosis as well as of active transport processes6-8.The sophisticated operation of correlative optical-electron microscopy9,10 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 nanopartide from a cluster of nanoparticles or their aggregates8,11,12.Here we report that our eyes can track a single fluorescent nanopartide that emits over 4000 photons per 100 milliseconds under a simple microscope setup.By tracking a single nanoparfide 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 nanopartides.