An innovative bis-allyl rhodanine robust red fluorophore for wide-range optical pH sensing with reversible and durable acidic-alkaline OFF/ON fluorescence.

A sustainable, ultrasound-assisted approach for synthesizing a bis-rhodanine derivative (BR) was established using N-allylrhodanine and 5-bromo-2-hydroxyisophthalaldehyde, employing a recyclable glycerol/proline (2:1) deep eutectic solvent. Structural characterization was unequivocally confirmed by NMR, FT-IR, HRMS, and elemental analysis. The red donor-π-acceptor fluorophore (BR) was evaluated as a wide-range spectrofluorometric pH probe. At 5.0 µM, BR exhibits dual absorption maxima at 357 and 568 nm with high molar absorptivity, and a red emission band centered at 718 nm with a 149 nm Stokes shift. Fluorescence is reversibly governed by a protonation/deprotonation equilibrium: acidification (pH 6.5-1.0) quenches emission with strong linearity (r ≈ 0.998), whereas alkalinity (pH 8.5-12.0) stabilizes an ICT-favored emissive state and linearly sensitizes fluorescence (r ≈ 0.995). The signal stabilizes within ~ 30 s, remains durable under reversible cycling between the fluorescence OFF mode at pH 2.0 and the fluorescence ON mode at pH 10.5. Therefore, BR interestingly functions as a dual-regime fluorescent pH probe, exhibiting acid-induced quenching at low pH and base-induced sensitization at high pH, with an explicitly characterized near-neutral transition (pH 6.5-8.5) that bridges the two operating domains. Moreover, BR is highly selective to pH as it resists common ionic interferents. Finally, the developed BR-derived spectrofluorometric pH probe is validated for the assay of acidic and alkaline pH in real samples of tap and lake waters, as well as in fresh lemon and orange juices, with high accuracy (98.9-102.3% recovery) and precision (% RSD: 0.93-1.13%).