Radio‐Activated Selenium‐Doped Janus Ag/Ag 2 Se x S y Nanoparticles for Precise Cancer NIR‐II Fluorescence Imaging and Radiosensitization Therapy

Abstract The efficacy of radiotherapy (RT) is often limited by insufficient tumor selectivity and suboptimal therapeutic responses. To overcome these problems, a new kind of selenium‐doped Ag/Ag 2 S Janus nanoparticles (Ag/Ag 2 Se x S y JNPs) is presented as radio‐responsive molecular probes for precise tumor imaging and enhanced radiosensitization. By adjusting the selenium precursor input, heterojunction nanoparticles with tunable doping ratios are synthesized, optimizing X‐ray absorption and energy storage properties. Upon X‐ray irradiation, the Ag/Ag 2 Se x S y JNPs interact with overexpressed hydrogen peroxide (H 2 O 2 ) in tumor cells, generating highly toxic hydroxyl radicals (·OH), which effectively induce tumor cell apoptosis. Additionally, Selenium incorporation improves electron–hole pair separation efficiency and enhances the photocurrent response, promoting increased electron transfer and ·OH generation, thus amplifying reactive oxygen species (ROS) production and enhancing radiosensitization. Furthermore, the fluorescence “OFF‐ON” mechanism, triggered by H 2 O 2 ‐induced etching of silver allows real‐time monitoring of H 2 O 2 levels via the second near‐infrared window (NIR‐II) fluorescence (FL) imaging “Turn On”, which delineates tumor boundaries for precise RT and reduce side effects to normal tissue. This dual‐functional platform not only enables real‐time tracking but also enhances therapeutic outcomes, offering a promising approach to precision cancer treatment.