A dual-mode colorimetric and ratiometric near-infrared fluorescent probe for on-spot quantification of SO2 derivatives.

SO2 derivatives (SO32-/HSO3-) are essential intermediates in plant sulfur metabolism and are extensively used as preservatives and antioxidants in the food industry, highlighting the urgent need for efficient, on-spot methods for their accurate quantification. Herein, we present GL, a rationally designed dual-mode probe that combines colorimetric and ratiometric near-infrared (NIR) fluorescence responses for SO32- detection. GL exhibits strong intrinsic NIR fluorescence and, upon reaction with SO32- via a specific nucleophilic addition mechanism, produces both a visible color change and a ratiometric fluorescence shift. This dual-signal strategy enhances specificity, reduces background interference, and enables reliable cross-validation. The versatility of GL was demonstrated through diverse applications, including bioimaging SO32- in living cells and zebrafish, monitoring SO32- accumulation in Brassica chinensis leaves and stems under varying sulfur conditions, and, when integrated with a self-made portable sensing system, achieving on-spot quantitative detection of SO32- in real food samples.