LAUSR

Abstract Bright, stable, and biocompatible fluorescent contrast agents operating in the second biological window (1000–1350 nm) are attractive for imaging of deep‐lying structures (e.g., tumors) within tissues. Ideally, these contrast agents also provide functional insights, such as information on local temperature. Here, water‐dispersible barium phosphate nanoparticles doped with Mn5+ are made by scalable, continuous, and sterile flame aerosol technology and explored as fluorescent contrast agents with temperature‐sensitive peak emission in the NIR‐II (1190 nm). Detailed assessment of their stability, toxicity with three representative cell lines (HeLa, THP‐1, NHDF), and deep‐tissue imaging down to about 3 cm are presented. In addition, their high quantum yield (up to 34%) combined with excellent temperature sensitivity paves the way for concurrent deep‐tissue imaging and nanothermometry, with biologically well‐tolerated nanoparticles.