A Highly Sensitive Colorimetric and Fluorescent Sensor for Bisulfate in Aqueous Media via pH-Responsive 2‑(1H‑Pyrrol-2-yl)pyridine.

We report 2-(1H-pyrrol-2-yl)-pyridine as a small-molecule, pH-responsive optical probe for highly sensitive bisulfate detection in aqueous media. The compound was prepared via a Suzuki coupling of 2-bromopyridine and N-Boc-2-pyrroleboronic acid with a high resultant yield. Because of its protonation-dependent photophysical behavior, the molecule exhibits distinct colorimetric and fluorescence responses to pH changes. Bisulfate, acting as a strong proton donor among common anions, efficiently protonates the probe, resulting in a red shift in absorption and significant emission quenching. This response enables rapid and sensitive detection of bisulfate with a limit of detection of 19.9 parts per billion (ppb) via colorimetric and 23.1 ppb via fluorescence. The photophysical changes are fully reversible upon addition of a strong base, demonstrating the pH-responsive sensing mechanism. Importantly, the sensor molecule displays a high selectivity for bisulfate when high concentrations of competitive anions are present, such as fluoride, chloride, bromide, iodide, nitrate, sulfate, bisulfite, thiocyanide, and dihydrogen phosphate. Consequently, this new sensor provides a workable procedure to detect bisulfate in an effective, low-cost, metal-free, and environmentally friendly manner.