A Molecular Logic Gate-Based Optical Sensor for Rapid Phosphate Detection in Environmental Waters

Abstract Phosphate (PO43–) is an essential component in nearly all biological and ecological systems, yet simple, affordable, and rapid detection methods remain limited. Here, we report a proton-transfer-driven dual-mode acridinedione molecular logic sensor for selective detection of PO43– in aqueous environments. The probe, 4-fluoro-9-phenyl-acridine-(1,8)-dione (FPAD), incorporates N–H functional sites that enable selective and reversible proton-transfer interactions with phosphate, resulting in distinct colorimetric and fluorescence “turn-on” responses. Mechanistic studies using nuclear magnetic resonance (1H NMR), Fourier transform infrared (FTIR), UV–vis, fluorescence spectroscopy, and density functional theory (DFT) calculations confirm proton-transfer-controlled electronic modulation and selective binding behavior. The sensor exhibits naked-eye detection with a limit of detection (LOD) of 4 ppm and a fluorescence response range of 4–20 ppm (LOD = 0.3 ppm). Integration into cellulose paper-based test strips enables portable and low-cost detection using simple imaging devices, including smartphones. The sensor demonstrates rapid and accurate phosphate quantification in real water samples with recovery values of 97–98% (tap water), 99–109% (river water), and 94–125% (lake water). In addition, reversible optical switching induced by methanol enables logic-gate-type behavior, highlighting molecular information processing capability. Overall, this work establishes a metal-free dual-mode acridinedione sensing platform with programmable switching and logic-gate information processing for rapid, selective, and portable detection of phosphate in environmental water systems.