A Nile red-based near-infrared fluorescent probe for the detection of superoxide radical anion in living cells

Reactive oxygen species (ROS) are important products of aerobic metabolism and play important roles in many physiological and pathological processes. Among the ROS, superoxide radical anion (O₂^•‒) is a primary ROS and precursor of a few other kinds of ROS. O₂^•‒ is widely involved in regulating cell functions and is a critical biomarker for monitoring cellular stress responses. Thus, a real-time quantitative detection of O₂^•‒ is of great significance in ascertaining the pathological roles of O₂^•‒ in related diseases. Herein we report a near-infrared (NIR) fluorescent probe NR1 for the sensitive detection and imaging of O₂^•‒, which is rationally designed and synthesized through an esterification reaction of a Nile red dye and the diphenyl phosphinyl group. The NR1 probe exploits the strong nucleophilic mechanism of O₂^•‒ as a novel detection strategy, which leads to the cleavage of NR1 to yield Nile red as the fluorescence reporter and diphenyl phosphinyl functional group as the responsive unit. The developed NR1 probe exhibits highly sensitive, selective and rapid fluorescence turn-on response on addition of O₂^•‒ in the presence of potential ROS interfering species. Furthermore, the probe NR1 is successfully applied in imaging of various cancer cell lines (4T1 and Hela) and excellent imaging ability of O₂^•‒ in vitro is achieved. Biocompability of NR1 is also evaluated through MTS assay. The present study provides a novel strategy for molecular design of NIR fluorescent probe and the resulting probe shows great potential for practical application in complex bio-systems to study the development and progression of related diseases.