Laser scanning up-conversion luminescence microscopy for imaging cells labeled with rare-earth nanophosphors

Because of the ability to selectively reveal the objects of interest with subcellular resolution, fluorescence microscopy provides widespread applications from basic biological research to clinical diagnosis. However, challenges still remain in reducing the degree of photobleaching and increasing the contrast between signal and noise. Herein, we found that rare-earth nanophosphors exhibit a unique up-conversion luminescence mechanism and imaging modality and developed a new three-dimensional visualization method of laser scanning up-conversion luminescence microscopy (LSUCLM) with little photobleaching and no background fluorescence, by introducing a reverse excitation dichroic mirror and the confocal pinhole technique. Moreover, we demonstrated the up-conversion emission imaging of thin films containing embedded rare-earth nanophosphors and cells multilabeled with the nanophosphors and organic dyes. These data show that LSUCLM not only shares noninvasive benefits and deep penetration of two-photon microscopy but also offers some distinct advantages, such as little photobleaching of both organic dyes and rare-earth nanophosphors, no background fluorescence from either endogenous fluorophores or colabeled fluorescent probes, and excellent compatibility with conventional confocal microscopy.