Conjugation-regulated responsive performance: rapidly urine-excreted fluorescent probes for dual-response detection of SO2 and viscosity and their bioimaging applications

Sulfur dioxide (SO₂) concentration and viscosity levels within living organisms are closely associated with various physiological and pathological processes. The development of novel fluorescent probes enabling simultaneous detection of SO₂ and viscosity is crucial for advancing disease research. Herein, we synthesized two novel fluorescent probes (CyAN-1 and CyAN-2) featuring dual SO₂/viscosity responsiveness. These probes integrate an electron-withdrawing benzindolizine hemicyanine moiety and an electron-donating N, N-dimethylaniline unit, connected through carbon-carbon double bonds of differential conjugation lengths. This molecular architecture facilitates rapid SO₂ detection via Michael addition while enabling sensitive viscosity response through a restricted rotation mechanism. CyAN-1 exhibited a remarkable 300-fold fluorescence enhancement at 475 nm toward SO₂, while maintaining high sensitivity (86 nM) and rapid response (180 s) in aqueous media. Moreover, its fluorescence signal at 628 nm displays sensitive viscosity-dependent behavior across a broad range from 0.6 cP to 623.4 cP. Confocal fluorescence imaging indicated that CyAN-1 exhibits excellent lysosomal targeting capacity, enabling real-time visualization of exogenous/endogenous SO₂ and dynamic monitoring of lipopolysaccharide (LPS)-induced viscosity changes in HeLa cells. Moreover, CyAN-1 was employed to image viscosity abnormalities in inflammation, tumor and non-alcoholic steatohepatitis (NASH) model mice. Notably, it exhibits rapid urinary excretion following intravenous administration, thereby minimizing potential toxicity from prolonged retention of metabolites in organs. We envision CyAN-1 as a promising molecular probe for elucidating pathophysiological processes through simultaneous tracking of SO₂ and viscosity.