Construction of self-sensitized LiErF4: 0.5% Tm3+@LiYF4 upconversion nanoprobe for trace water sensing

LiErF4 was commonly used as a dipolar-coupled antiferromagnet, and was rarely considered as a luminescent material. Herein, we achieved the strong red upconversion emission of LiErF4 simply by an inert shell coating, i.e., LiErF4@LiYF4. Owing to the unique and intrinsic ladder-like energy levels of Er3+ ions, this LiErF4 core-shell nanostructures present red emission (∼ 650 nm) under multi-band excitation in the near-infrared (NIR) region (∼ 808, ∼ 980, and ∼ 1,530 nm). A brighter and monochromic red emission can be further obtained via doping 0.5% Tm3+ into the LiErF4 core, i.e., LiErF4: 0.5% Tm3+@LiYF4. The enriched Er3+ ions and strong monochromic red emission natures make LiErF4: 0.5% Tm3+@LiYF4 nanocrystals very sensitive for trace water probing in organic solvents with detection limit of 30 ppm in acetonitrile, 50 ppm in dimethyl sulfoxide (DMSO), and 58 ppm in N, N-dimethylformamide (DMF) under excitation of 808 nm. Due to their superior chemical and physical stability, these nanoprobes exhibit excellent antijamming ability and recyclability, offering them suitable for real-time and long-term water monitoring.