Ratiometric near-infrared fluorescence nanothermometry in the OTN-NIR (NIR II/III) biological window based on rare-earth doped β-NaYF nanoparticles.

A novel nanothermometer based on over-1000 nm (OTN) near-infrared (NIR) emission of rare-earth doped ceramic nanophosphors (RED-CNPs) was developed for temperature measurement in deep tissue. Hexagonal-phase β-NaYF nanoparticles co-doped with Yb, Ho, and Er (NaYF:Yb,Ho,Er NPs) were synthesized and used as a nanothermometer. The NaYF:Yb,Ho,Er NPs displayed two OTN-NIR emission peaks in the second (NIR-II) (at 1150 nm of Ho) and third (NIR-III) (at 1550 nm of Er) biological window regions under NIR (980 nm) excitation in the first (NIR-I) biological window region. Oleic acid (OA) capped NaYF:Yb,Ho,Er NPs were dispersed in non-polar media, i.e., cyclohexane, and showed a temperature-dependent intensity ratio of the emission peaks of Ho and Er (I/I). The temperature-dependent I/I of the OA-NaYF:Yb,Ho,Er NPs was also evident through imitation tissue. The surfaces of the NaYF:Yb,Ho,Er NPs were modified with a poly(ethylene glycol) (PEG)-based block copolymer. The PEGylated NaYF:Yb,Ho,Er NPs were dispersed in water and emitted strong NIR-II and III emissions under NIR-I excitation. The PEGylated NaYF:Yb,Ho,Er NPs were injected into mice via the tail vein, and the OTN-NIR emissions of the PEGylated NaYF:Yb,Ho,Er NPs from the mouse blood vessels were clearly observed using an OTN-NIR fluorescence in vivo imaging system. In a polar media, water, the I/I of PEGylated NaYF:Yb,Ho,Er NPs was inversely related to the temperature. In both non-polar and polar media, the I/I values of the NaYF:Yb,Ho,Er NPs were almost linearly dependent on the temperature. The obtained NaYF:Yb,Ho,Er NPs are promising as a novel fluorescent nanothermometer for deep tissue.