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⁻/SO₃²⁻ 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⁻/SO₃²⁻ 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⁻/SO₃²⁻ 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⁻/SO₃²⁻ 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.