Dual-emissive ratiometric "turn-on" probe for real-time sensing of HSO4- in environmental samples and live cells

Abstract

The optical detection of hydrogen sulfate anion (HSO₄^-) 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 (HSO₄^-) in aqueous phase. The probe 5C exhibits bathochromic shift of 52 nm in its emission spectrum upon HSO₄^- 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 HSO₄^-, with an association constant (K_a) of 5.43 × 10³ M^-1. The spectral response of the probe was observed within 15 s of HSO₄^- 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 HSO₄^-, as confirmed by ¹H 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 HSO₄^- in human breast cancer cells (MCF-7) with high efficiency, underscoring its potential in biological applications.

Keywords: Cell imaging; DFT; HSO(4)(−) sensor; Real water samples.