expensive high-power, femtosecond pulsed laser [39] and can be excited by
continuous-wave (CW) diode lasers operating at low power (1 W cm
À2 ) [40] –
this can facilitate fast imaging using a wide-field microscopy. In summary, NIR
light-absorbing, real-time imaging UCNPs offer broad applications as real-time,
long-term tracker for cell imaging [41] as well as sensitive contrast agent with
high-contrast resolutions for whole-body optical imaging [42].
Fig. 5 (a) In vivo imaging of rat subcutaneously injected to abdominal skin with green-emitting
QDs (CdTe) under UV excitation (left) and green-emitting UCNPs of PEI/NaYF 4 :Yb,Er under NIR
excitation (right). Adapted from Chatterjee et al. [31] with permission. (b) Multicolor imaging of a
mouse using NIR-to-blue UCNPs of NaY 0.78 Yb 0.2 Tm 0.02 F 4 , NIR-to-green UCNPs of
NaY 0.78 Yb 0.2 Er 0.02 F 4 , and NIR-to-red UCNPs of NaY 0.78 Yb 0.3 Er 0.01 F 4 : three colors of UCNPs
were clearly differentiated after spectral unmixing. Adapted from Cheng et al. with permission [32]
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T. Kim and J. V. Jokerst