NIR‐II Responsive Upconversion Nanoprobe with Simultaneously Enhanced Single‐Band Red Luminescence and Phase/Size Control for Bioimaging and Photodynamic Therapy

Abstract 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 Er 3+ as sensitizer and utilizing Mn 2+ as energy manipulator. Through tuning the content of Mn 2+ in NaLnF 4 :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 Er 3+ and Mn 2+ . 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.