NIR-II Ratiometric Imaging of SOD Activity with a Superoxide-Responsive Nanoprobe

Superoxide dismutase (SOD) maintains redox homeostasis, and its activity reflects oxidative stress in aging, inflammation, and diseases. However, real-time monitoring of SOD activity in vivo remains challenging. Here we constructed a superoxide anion radical (O2•-)-responsive NIR-II ratiometric probe based on a "substrate-monitoring and activity-back-calculation" strategy. The probe consists of an O2•--responsive unit (NKO, emission at 950 nm) and a stable internal reference unit (PbS quantum dots, emission at 1665 nm) conjugated via polyethylene glycol, affording robust aqueous dispersibility and biocompatibility. Ratiometry (F950/F1665) suppresses artifacts from probe concentration, biodistribution, and microenvironmental variability, enabling precise O2•- quantification and sensitive, indirect inference of SOD activity. Using this probe, we achieved dynamic visualization of SOD activity under different oxidative stress conditions in living cells, an acute liver injury mouse model, and a tumor-bearing mouse model. This work provides an intuitive and quantitative molecular imaging basis for elucidating mechanisms of oxidative stress in disease development.