Fluoride Hafnium/Zirconium-Softened\nNanoprobes for\nNear-Infrared-IIb and CT Dual-Mode Bioimaging

Fluoride-based lanthanide-doped nanoparticles (LDNPs)\nfeaturing\nsecond near-infrared (NIR-II, 1000–1700 nm) downconversion\nemission for bioimaging have attracted extensive attention. However,\nconventional LDNPs cannot be degraded and eliminated from organisms\nbecause of an inert lattice, which obstructs bioimaging applications.\nHerein, the core–shell LDNPs of Na<sub>3</sub>HfF<sub>7</sub>:Yb,Er@CaF<sub>2</sub>:Ce,Zr(Hf) [labeled as Zr(Hf)Ce-HC] with pH-selective\nand tunable degradability were synthesized for dual-modal bioimaging.\nNotably, the “softening” lattice of the Na<sub>3</sub>HfF<sub>7</sub> matrix and different Zr<sup>4+</sup>(Hf<sup>4+</sup>) doping amounts in the shell enable Zr(Hf)Ce-HC with acidity-dependent\nand tunable degradability. After coating of an optimized Ce<sup>3+</sup>-doped CaF<sub>2</sub>:Zr shell, the near-infrared-IIb (NIR-IIb,\n1500–1700 nm) luminescence intensity of ZrCe-HC is enhanced\nby 5.2 times compared with that of Na<sub>3</sub>HfF<sub>7</sub>:Yb,Er.\nThe Hf element with high X-ray attenuation allows ZrCe-HC as the contrast\nagent for computed tomography (CT) bioimaging. The modification of\noxidized sodium alginate endows ZrCe-HC with satisfying biocompatibility\nfor NIR-IIb/CT dual-modal bioimaging. These findings would benefit\nthe bioimaging applications of degradable fluoride-based LDNPs.