Self-assembled supramolecular sensor for hydrogen sulfate via multipoint hydrogen bonding: real-sample analysis and...

Herein, a self-assembled rhodamine-indole based supramolecular probe (compound 1) has been reported for the selective and sensitive detection of hydrogen sulfate (HSO₄^-) ions in aqueous medium. The probe exhibited aggregation-assisted photophysical modulation, forming nanoscale assemblies as confirmed by DLS and FESEM analyses. In the absence of the analyte, compound 1 displayed dominant indole-centered emission at 465 nm, whereas interaction with HSO₄^- induced hydrogen-bond-assisted spirolactam ring opening, generating a conjugated xanthene framework. This structural transformation produced a distinct ratiometric fluorescence response, characterized by suppression of indole emission and enhancement of xanthene emission at 554 nm. The probe demonstrated high sensitivity with a detection limit of 0.03 µM and a clear linear ratiometric response (R² > 0.99) in the micromolar concentration range. Excellent selectivity (∼19-fold enhancement) toward HSO₄^- over competing anions and reversible sensing behavior (>85% recovery over five cycles) were observed. Mechanistic investigations revealed multipoint hydrogen-bonding interactions between the indole -NH unit and HSO₄^-. The practical applicability of the system was validated through analysis of commercial shampoo samples and portable paper-strip sensors, enabling smartphone-assisted detection of HSO₄^- ions. These results highlight a self-assembly-driven supramolecular strategy for developing portable and sustainable sensors for anionic species.