Hydrothermal and anion exchange synthesis of Mn(V)-doped Ba5(PO4)3Cl nano-apatite toward NIR-II temperature sensing.

The second near-infrared window (NIR-II) in the range of 1000-1400 nm is ideal for in vivo imaging and sensing through reduced scattering, absorption, and autofluorescence. However, there are only a few nanophosphor systems with emission in the NIR-II region. Here, we report on Mn5+-doped Ba5(PO4)3Cl nanoparticles (BPCl:Mn5+ NPs, d < 50 nm) toward NIR-II temperature sensing. BPCl:Mn5+ NPs are made by a two-step (hydrothermal and anion exchange) method. XRD, SEM, and TEM results showed that the as-prepared BPCl:Mn5+ NPs show high crystallinity, uniform size, and sphere-like morphology. The nanoparticles exhibit a broad excitation band of 500-850 nm and a temperature-sensitive peak emission at 1175 nm in the NIR-II range. NIR-II temperature sensing by 1E emission intensity is demonstrated with good linear fitting (R2 = 0.9895), high sensitivity (2.30% at 373 K), and good repeatability (99.0%). Thus, our study provides a path to develop a new NIR-II thermometer based on tetrahedral Mn(V) coordination.