Bones play vital roles in human health. Noninvasive visualization of the full extent of bones is highly demanded to evaluate many bone-related diseases. Herein, we report poly (acrylic acid) (PAA)-modified NaLuF4:Yb/Er/Gd/Ce@NaYF4 nanoparticles (PAA-Er) with second near-infrared emission beyond 1500 nm (also referred as NIR-IIb) for high-resolution bone/bone marrow imaging and bone fracture diagnosis. The NIR-IIb optical-guided bone marrow imaging presents a high signal to noise ratio, which is superior to that for imaging in the NIR-II window (1000–1400 nm, NIR-IIa). Importantly, we also investigated the size-dependent accumulation of the nanoparticles and the possible accumulation mechanism of the designed PAA-Er nanoprobes in bone marrow. Due to the high affinity capability of the PAA-Er nanoprobes, a highly sensitive NIR-IIb optical-guided bone fracture diagnosis was successfully achieved. This novel technology paves the way to design lanthanide nanoprobes for NIR-IIb optical-guided high-resolution bone marrow imaging and bone-related disease diagnosis.

Lanthanide based upconversion (UC) nanoprobes have emerged as promising agents for biological applications. Extending the excitation light to the second near‐infrared (NIR‐II), instead of the traditional 980/808 nm light, and realizing NIR‐II responsive single‐band red UC emission is highly demanded for bioimaging application, which has not yet been explored. Here, a new type of NIR‐II (1532 nm) light responsive UC nanoparticles (UCNPs) with enhanced single‐band red UC emission and controllable phase and size is designed by introducing Er3+ as sensitizer and utilizing Mn2+ as energy manipulator. Through tuning the content of Mn2+ in NaLnF4:Er/Mn, the crystal phase, size, and emitting color are readily controlled, and the red‐to‐green (R/G) ratio is significantly increased from ≈20 to ≈300, leading to NIR‐II responsive single band red emission via efficient energy transfer between Er3+ and Mn2+. In addition, the single band red emitting intensity can be further improved by coating shell to avoid the surface quenching effect. More importantly, NIR‐II light activated red UC bioimaging and photodynamic therapy through loading photosensitizer of zinc phthalocyanine are successfully achieved for the first time. These findings provide a new strategy of designing NIR‐II light responsive single‐band red emissive UCNPs for biomedical applications.

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Excessive production of hydrogen sulfide (H2S) plays a crucial role in the progress of colon cancer. Construction of tumor‐specific H2S‐activated smart nanoplatform with controllable biodegradation is of great significance for precise and sustainable treatment of colon cancer. Herein, an endogenous H2S triggered Co‐doped polyoxometalate (POM‐Co) cluster with self‐adjustable size, controlled biodegradation, and sustainable cyclic depletion of H2S/glutathione (GSH) is designed for synergistic enhanced tumor‐specific photothermal and chemodynamic therapy. The designed POM‐Co nanocluster holds H2S responsive “turn‐on” photothermal property in colon cancer via self‐assembling to form large‐sized POM‐CoS, enhancing the accumulation at tumor sites. Furthermore, the formed POM‐CoS can gradually biodegrade, resulting in release of Co2+ and Mo6+ for Co(II)‐catalyzed •OH production and Russell mechanism‐enabled 1O2 generation with GSH consumption, respectively. More importantly, the degraded POM‐CoS is reactivated by endogenous H2S for recyclable and sustainable consumption of H2S and GSH, resulting in tumor‐specific photothermal/chemodynamic continuous therapy. Therefore, this study provides an opportunity of designing tumor microenvironment‐driven nanoprobes with controllable biodegradation for precise and sustainable anti‐tumor therapy.