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(a) Confocal upconverted luminescent image of individual UCNPs. (b) Live-cell imaging of UCNPs in NIH 3T3 murine fibroblasts, showing virtually zero autofluorescence background. (c,d) Zoom-in time trace and histogram of emission intensity, showing no on/off behavior-nonblinking. (Reprinted with permission from ref (20). Copyright 2009 Highwire press PNAS.)
