A NIR ratiometric probe for detecting hypochlorite and bisulfite/sulfite in aqueous media

We have designed and synthesized a hemicyanine-based near-infrared (NIR) ratiometric and colorimetric fluorescent probe (AHS) to detect both HSO₃^-/SO₃^2- and OCl^-. AHS probe contains a conjugated -C=C- bond and a -SMe group and exhibits NIR fluorescence emission at 666 nm (λ_ex = 520 nm) because of the intramolecular charge transfer (ICT) mechanism. The probe undergoes a Michael addition reaction with HSO₃^-/SO₃^2- at the conjugated -C=C- bond, resulting in the formation of an adduct that breaks the π conjugation in the probe and shifts the fluorescence to 488 nm (λ_ex = 420 nm). Similarly, upon reaction with hypochlorite (OCl^-), a strong oxidizing agent, the -SMe group is oxidized to -S(O)Me, resulting in a fluorescence shift to 580 nm (λ_ex = 520 nm). AHS demonstrated high selectivity and sensitivity for HSO₃^-/SO₃^2- and OCl^-, with detection limits of 1.96 nM and 164.67 nM, respectively. Additionally, the probe exhibits excellent water solubility and strong anti-interference ability and can detect both the analytes at pH 7-10. AHS was successfully applied for the detection of HSO₃^-/SO₃^2- and OCl^- on paper strips and in real and water samples with a high degree of recovery, demonstrating its potential for practical applications in environmental and biological analysis.