Abstract
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.
Highlights
The synthesis and characterization of lanthanide-doped upconversion nanoparticles (UCNPs) are well established [1,2,3,4,5,6]
UCNPs are excited by near-infrared (NIR) lasers, whose wavelength is well matched to the absorption wavelength of Yb3+ (980 nm) and Nd3+ (808 nm) [1,2,3,4,5]
UCNPs can be monitored for a long time [13,14]
Summary
The synthesis and characterization of lanthanide-doped upconversion nanoparticles (UCNPs) are well established [1,2,3,4,5,6]. UCNPs are excited by near-infrared (NIR) lasers, whose wavelength is well matched to the absorption wavelength of Yb3+ (980 nm) and Nd3+ (808 nm) [1,2,3,4,5]. In this NIR range, biomolecules show minimum absorption in vitro and in vivo [7,8,9,10,11,12]. It is important to note that the spectra of UCNPs do not depend on their size, which indicates that any variations in the particle dimensions (usually from sub-10 nm to several hundred nm) do not affect their photophysical properties [37,38]
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