Upconversion nanoparticles (UCNPs) have attracted enormous interest over the past few years because of their unique optical properties and potential for use in various applications such as bioimaging probes, biosensors, and light-harvesting materials for photovoltaics. The improvement of imaging resolution is one of the most important goals for UCNPs used in biological applications. Super-resolution imaging techniques that overcome the fundamental diffraction limit of light rely on the photochemistry of organic dyes or fluorescent proteins. Here we report our progress toward super-resolution microscopy with UCNPs. We found that the red emission (655 nm) of core/shell UCNPs with the structure NaYF:Yb,Er/NaYF could be modulated by emission depletion (ED) of the intermediate state that interacts resonantly with an infrared beam (1540 nm). In contrast, the green emission bands (525 and 545 nm) of the UCNPs were less affected by irradiation with the infrared beam. The origin of such distinct behaviors between the green and red emissions was attributed to their different photophysical pathways.

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Vesicle transport conducted by motor protein multiplexes (MPMs), which is ubiquitous among eukaryotes, shows anomalous and stochastic dynamics qualitatively different from the dynamics of thermal motion and artificial active matter; the relationship between in vivo vesicle-delivery dynamics and the underlying physicochemical processes is not yet quantitatively understood. Addressing this issue, we perform accurate tracking of individual vesicles, containing upconverting nanoparticles, transported by kinesin−dynein-multiplexes along axonal microtubules. The mean-square-displacement of vesicles along the microtubule exhibits unusual dynamic phase transitions that are seemingly inconsistent with the scaling behavior of the meanfirst-passage time over the travel length. These paradoxical results and the vesicle displacement distribution are quantitatively explained and predicted by a multimode MPM model, developed in the current work, where ATP-hydrolysis-coupled motion of MPM has both unidirectional and bidirectional modes.

Lanthanide-doped upconversion nanoparticles (UCNPs) are inorganic nanomaterials in which the lanthanide cations embedded in the host matrix can convert incident near-infrared light to visible or ultraviolet light. These particles are often used for long-term and real-time imaging because they are extremely stable even when subjected to continuous irradiation for a long time. It is now possible to image their movement at the single particle level with a scale of a few nanometers and track their trajectories as a function of time with a scale of a few microseconds. Such UCNP-based single-particle tracking (SPT) technology provides information about the intracellular structures and dynamics in living cells. Thus far, most imaging techniques have been built on fluorescence microscopic techniques (epifluorescence, total internal reflection, etc.). However, two-dimensional (2D) images obtained using these techniques are limited in only being able to visualize those on the focal planes of the objective lens. On the contrary, if three-dimensional (3D) structures and dynamics are known, deeper insights into the biology of the thick cells and tissues can be obtained. In this review, we introduce the status of the fluorescence imaging techniques, discuss the mathematical description of SPT, and outline the past few studies using UCNPs as imaging probes or biologically functionalized carriers.

Because of their well-known optical properties, upconversion nanoparticles (UCNPs) are regarded as some of the most promising nanomaterials for bioimaging, biosensors, and solar cells. The nonblinking nature of their upconversion emissions has been a particularly beneficial advantage for live-cell imaging. However, the origin of this unique property has never been seriously investigated. We report, for the first time, the observation of stochastic photon emission (SPEM) in core/shell UCNPs (NaYF 4 :Yb 3+ ,Er 3+ /NaYF 4 ) on the microsecond and nanosecond time scales, even under continuous irradiation at 980 nm. This SPEM was attributed to slow "upconversion cycles". We consider that the conventionally reported, nonblinking nature of UCNP emissions can be attributed to the averaging of SPEMs from multiple Er 3+ ions and the low temporal resolution of previous observation. The off-time distribution, which possesses kinetics information for the upconversion pathways, was well fitted to a single exponential indicating involvement of a single rate-determining step. The distinct behaviors of the green and red emissions confirm their different photophysical pathways.