A New Class of Bond-Length-Engineered Transition-Metal-Doped Nanocrystals with Tunable NIR-II Absorption and Emission

Abstract

Developing luminescent probes with tunable absorption and emission in the second near-infrared (NIR-II) or shortwave infrared (SWIR) region (1000–2500 nm) is essential for advanced photonic technologies, as this region offers dramatically reduced photon scattering in complex media and intrinsic compatibility with optical communication bands. Here, we report a new class of NIR-II nanoprobes based on transition-metal Ni2+-doped fluoride nanocrystals, in which Ni2+ ions serve simultaneously as both sensitizer and activator. Theoretical analysis and experimental validation reveal that varying the matrix from NaMgF3 to MgF2, KMgF3, and NaMnF3 systematically modulates the local Ni–F bond length and the coordination environment of Ni2+. This bond-length engineering enables broadband and finely tunable NIR-II absorption (1318–1414 nm) and emission (1620–1780 nm) from d–d electronic transitions, with the optimized NaMgF3:9.6%Ni@NaMgF3 nanocrystals achieving a quantum yield of 20.7% under 1320 nm excitation. Using these distinctive excitation–emission fingerprints, we demonstrate low-crosstalk in vivo multiplexed imaging with high signal-to-noise ratios. This work establishes transition-metal-doped nanocrystals as a broadly tunable platform and expands the scope of inorganic luminescent materials for next-generation photonic applications.