Tunable Defect Engineering in NaYF₄ Nanoparticles:Tailoring Trap States for Enhanced NIR-II Bioimaging

Fluorescent imaging in the second near-infrared window (NIR-II, 1000-1700 nm) enables tissue detection up to a centimeter in depth and achieves micrometer-level resolution at millimeter depths. However, NIR-II luminescent materials face several challenges, such as high synthesis costs, poor photostability, and insufficient emission intensity. We propose a defect engineering strategy using inexpensive metal ion doping to enhance the light absorption capacity of NaYF₄ nanocrystals for boosted NIR-II emission. Under 808/980 nm laser excitation, the metal-ion-doped NaYF₄ nanomaterials exhibit three well-resolved NIR-II emission peaks centered at 1057, 1333, and 1523 nm. Density functional theory (DFT) calculations reveal that metal ion doping induces local defect states within the conduction band and facilitates NIR-II emission through trap-mediated pathways. Furthermore, the defect-rich NaYF₄ nanomaterials demonstrate superior performance in NIR-II bioimaging. This defect-engineering strategy offers a novel paradigm for developing cost-effective, high-quantum-yield NIR-II luminescent materials.