Ratiometric near-infrared fluorescence nanothermometry in the OTN-NIR (NIR II/III) biological window based on rare-earth doped β-NaYF 4 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.