Dual-emissive ratiometric "turn-on" probe for real-time sensing of HSO in environmental samples and live cells.

The optical detection of hydrogen sulfate anion (HSO) in the aqueous phase is particularly challenging and relatively rare, mainly due to their extremely high Gibbs free energy of hydration, which stabilizes the ions and hinders their interaction with the sensors. A novel ratiometric "turn-on" fluorometric probe, N-hydroxy-6-((2-(4-methoxyphenyl)quinolin-6-yl)oxy)hexanamide (5C), was rationally designed and synthesized to enable highly selective and ultrasensitive detection of hydrogen sulfate ion (HSO) in aqueous phase. The probe 5C exhibits bathochromic shift of 52 nm in its emission spectrum upon HSO addition, resulting into fluorometric color change from non-fluorescent to a strong blue fluorescence under UV light. The limit of detection (LOD) and quantification (LOQ) for 5C was determined to be 0.55 and 1.85 μM, respectively. Binding analysis through Job's plot and the Benesi-Hildebrand method indicated a 1:1 complexation between 5C and HSO, with an association constant (K) of 5.43 × 10 M. The spectral response of the probe was observed within 15 s of HSO addition. Additionally, the reversibility and stability of the synthesized compound 5C were investigated, along with its solid-state behavior, which was further validated through solid-state absorbance and fluorescence spectra. A sensing mechanism involves intermolecular hydrogen-bonding interactions between 5C and HSO, as confirmed by H NMR titration, IR spectroscopy and DFT studies. The probe also demonstrates excellent performance in real water samples and on test strips, achieving high accuracy and precision. Additionally, cell imaging studies confirmed the potential of 5C as a bio-imaging probe for tracking HSO in human breast cancer cells (MCF-7) with high efficiency, underscoring its potential in biological applications.