Construction of self-sensitized LiErF 4 : 0.5% Tm 3+ @LiYF 4 upconversion nanoprobe for trace water sensing

LiErF 4 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 LiErF 4 simply by an inert shell coating, i.e., LiErF 4 @LiYF 4 . Owing to the unique and intrinsic ladder-like energy levels of Er 3+ ions, this LiErF 4 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% Tm 3+ into the LiErF 4 core, i.e., LiErF 4 : 0.5% Tm 3+ @LiYF 4 . The enriched Er 3+ ions and strong monochromic red emission natures make LiErF 4 : 0.5% Tm 3+ @LiYF 4 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.