Size-Tunable
and Monodisperse Tm<sup>3+</sup>/Gd<sup>3+</sup>-Doped Hexagonal NaYbF<sub>4</sub> Nanoparticles with Engineered Efficient Near Infrared-to-Near
Infrared Upconversion for In Vivo Imaging
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Abstract
Hexagonal NaYbF<sub>4</sub>:Tm<sup>3+</sup> upconversion nanoparticles hold promise for use in high
contrast near-infrared-to-near-infrared (NIR-to-NIR) in vitro and
in vivo bioimaging. However, significant hurdles remain in their preparation
and control of their morphology and size, as well as in enhancement
of their upconversion efficiency. Here, we describe a systematic approach
to produce highly controlled hexagonal NaYbF<sub>4</sub>:Tm<sup>3+</sup> nanoparticles with superior upconversion. We found that doping appropriate
concentrations of trivalent gadolinium (Gd<sup>3+</sup>) can convert
NaYbF<sub>4</sub>:Tm<sup>3+</sup> 0.5% nanoparticles with cubic phase
and irregular shape into highly monodisperse NaYbF<sub>4</sub>:Tm<sup>3+</sup> 0.5% nanoplates or nanospheres in a pure hexagonal-phase
and of tunable size. The intensity and the lifetime of the upconverted
NIR luminescence at 800 nm exhibit a direct dependence on the size
distribution of the resulting nanoparticles, being ascribed to the
varied surface-to-volume ratios determined by the different nanoparticle
size. Epitaxial growth of a thin NaYF<sub>4</sub> shell layer of ∼2
nm on the ∼22 nm core of hexagonal NaYbF<sub>4</sub>:Gd<sup>3+</sup> 30%/Tm<sup>3+</sup> 0.5% nanoparticles resulted in a dramatic
350 fold NIR upconversion efficiency enhancement, because of effective
suppression of surface-related quenching mechanisms. In vivo NIR-to-NIR
upconversion imaging was demonstrated using a dispersion of phospholipid-polyethylene
glycol (DSPE-PEG)-coated core/shell nanoparticles in phosphate buffered
saline