pH‐Responsive Indicator Displacement Assay of Acetylcholine Based on Acridine– <i>p</i> ‐Sulfonatocalix[4]arene Supramolecular System: Fluorescence Off/On Switching and Reversible pK _a Shift

Abstract

Abstract Host‐guest complexes of protonated (AcH + ) and neutral (Ac) forms of biologically important acridine dye with p ‐sulfonatocalix[4]arene (SCX4) and their responses towards acetylcholine (AcCh) as a competitive binder has been investigated using photochemical studies. Unlike Ac, the AcH + undergoes strong binding with SCX4 and their differential binding results in a large upward pK a shift for the bound dye. Dye binding to SCX4 causes a drastic fluorescence quenching, witnessing a strong fluorescence “turn OFF”, which is switched to strong fluorescence “turn ON” by the presence of neurotransmitter, AcCh, acts as a stimulus cum competitive binder. This is a unique system showing controlled binding and release for both AcH + and Ac forms of the dye triggered by AcCh, convincingly established through absorption, pK a tuning, fluorescence modulation and NMR shift. Fluorescence “OFF/ON” switching observed in this study demonstrates an efficient indicator displacement methodology, achieving a control over the selectivity in binding and release of the dye/analyte, having prospective uses in designing new optical sensors and smart functional materials for analytical and biological applications.