Enhancing NIR‐II Upconversion Monochromatic Emission for Temperature Sensing

The upconversion luminescence (UCL) in the second near‐infrared window (NIR‐II) is highly attractive due to its excellent performance in high‐resolution bioimaging, anticounterfeiting, and temperature sensing. However, upconvertion nanoparticles (UCNPs) are normally emitted in visible light, potentially impacting the imaging quality. Here, a monochromatic Er3+‐rich (NaErF4:x%Yb@NaYF4) nanoparticles with excitation at 1532 nm and emission at 978 nm is proposed, both situated in the NIR‐II region. The proper proportion of Yb3+ ions doping has a positive effect on the NIR‐II emission, by enhancing the cross relaxation efficiency and accelerating the energy transfer rate. Owing to the interaction between the Er3+ and Yb3+ is inhibited at low temperatures, the UCL emission intensities at visible and NIR‐II regions show opposite trend with temperature changing, which establishes a fitting formula to derive temperature from the luminous intensity ratio, promoting the potential application of UCL in NIR‐II regions for the temperature sensing. Yb3+ doped Er3+‐rich nanoparticles are used for the monochromatic NIR‐II UC emission. Considering the energy exchange between Er3+ and Yb3+ at different temperature, this system is used to perform the UC emission of NIR‐II in potential applications for temperature sensing.