An activatable ratiometric near-infrared fluorescent probe for hydrogen sulfide imaging in vivo

Ratiometric fluorescent probes hold great promise for in vivo imaging; however, stimuli-activatable ratiometric probes with fluorescence emissions in near-infrared（NIR） region are still very few. Herein, we report a hydrogen sulfide（H2S）-activatable ratiometric NIR fluorescent probe（1-SPN） by integrating a H2S-responsive NIR fluorescent probe 1 into a H2S-inert poly[2,6-(4,4-bis-（2-ethylhexyl）-4 H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7（2,1,3-benzothiadiazole）]（PCPDTBT）-based NIR semiconducting polymer nanoparticle（SPN）. 1-SPN shows "always on" PCPDTBT fluorescence at 830 nm and weak probe 1 fluorescence at 725 nm under excitation at 680 nm. The ratio of NIR fluorescence intensities between 725 and 830 nm(I725/I830)is small. Upon interaction with H2S, the fluorescence at 725 nm is rapidly switched on, resulting in a large enhancement of I725/I830, which is allowed for sensitive visualization and quantification of H2S concentrations in living cells. Taking advantage of enhanced tissue penetration depth of NIR fluorescence, 1-SPN is also applied for real-time ratiometric fluorescence imaging of hepatic and tumor H2S in living mice. This study demonstrates that activatable ratiometric NIR fluorescent probes hold great potential for in vivo imaging.