Fluorescence (or more generally photoluminescence, PL) imaging is one of the most important biomedical imaging techniques, providing cellular or molecular level information with almost single molecule sensitivity. Since availability of the endogenous fluorophores for various bioimaging applications is limited, there is a clear need in development of the exogenous PL imaging probes. Being able to convert excitation light with lower frequency into PL with higher one, upconversion (UC) nanoparticles (NPs) arise as excellent optical probes for PL bioimaging, due to their unique spectral properties, exceptional stability and low toxicity . Along with UC PL, rare-earth doped NPs can also produce downconversion (DC) PL, allowing for PL imaging in the second and third infrared windows (NIR-II,III, ~1000-1350 and ~ 1550-1700 nm). Benefiting from the reduced scattering and autofluorescence in these spectral regions, NIR-II,III PL imaging was shown to significantly improve a signal-to-noise ratio in the in vivo imaging, probing deeper tissue with higher resolution . Despite promising results achieved, UC and DC NPs have common drawback of low absorptivity of rare-earth ions. Combination of UC and DC NPs with NIR dyes, absorbing and transferring excitation energy to rare-earth ions enables new paradigm in the development of UC/DC PL bioimaging. The talk will present examples of the dye-UC/DC NPs combinations and discuss challenges in their design and applications.
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