NIR-NIR upconverting optical temperature sensing based on the thermally coupled levels of Yb3+-Tm3+ codoped Bi7F11O5 nanosheets

The upconversion of near-infrared (NIR) fluorescence, with a high tissue penetration depth, possesses a distinct advantage in biological temperature detection but is limited by the lack of rare earth-doped nanophosphors with intense and selective NIR UC emission. Here, we show that high-efficiency NIR temperature sensing can be achieved by appropriately doping Tm 3+ activators into a polarized layered host, utilizing the high environmental sensitivity of the 3 H 4 → 3 H 6 electric dipole transition (808 nm) of Tm 3+ . To confirm this concept, Yb 3+ /Tm 3+ codoped Bi 7 F 11 O 5 upconverting nanosheets were synthesized and characterized. Furthermore, the thermometric performance of the samples was analyzed based on the temperature-dependent fluorescence intensity ratio from the thermally coupled levels ( 3 F 3 / 3 H 4 ) of Tm 3+ . The results showed that, under 980 nm diode laser excitation, the prepared samples can produce a high NIR-to-visible UC emission ratio (I 800nm /I 475nm ) of over 2000 for Tm 3+ . The maximum sensitivity S a /S r was 1.4% K −1 /0.577 K −1  at 303 K based on the 3 F 3 and 3 H 4 levels, and the material exhibited a wide functional temperature range of 303–573 K. The results of our work not only improve the understanding of the optical properties of RE 3+ ions in layered materials but also show a potential application for physiological temperature sensing in biological tissues and cells.