Molecular recognition meets microcontroller: calixarene-based fluorescent sensors for selective detection of 2-nitrobenzaldehyde.

Fluorescent supramolecular materials have emerged as versatile tools for detecting nitroaromatic compounds (NACs), which are key components of explosives and persistent environmental pollutants. Among these analytes, 2-nitrobenzaldehyde (2-NBZ) warrants particular attention due to its high toxicity, environmental persistence, and industrial relevance. 2-NBZ exhibits ecotoxicity (GHS H412 Harmful to aquatic life with long-lasting effects) and is classified by the US EPA as a priority pollutant because of its persistence and aquatic toxicity. However, selective and sensitive detection of 2-NBZ in complex environmental matrices remains challenging. This study reports a series of 2,6-dihydroxyacetophenone[4]arene based derivatives (DAP-AAQ: S3.1-S3.4). All sensors operate efficiently in aqueous solution at neutral pH and exhibit a selective "turn-on" fluorescence response toward 2-NBZ; among them, S3.1 demonstrated the strongest binding constant (Ka = 2.0 × 105 M-1) and the lowest detection limit (LOD = 0.25 µM), while showing negligible response to other common nitroaromatics. HR-MS titration and density functional theory (DFT) calculations reveal the formation of a 1 : 1 host-guest complex stabilized by hydrogen bonding and dipole-dipole interactions, driving an interamolecular charge-transfer quenching mechanism. Furthermore, the optimized fluorescent sensor was successfully integrated with a microcontroller (Arduino) RGB detection platform. The developed device demonstrated good analytical performance in water samples, with recovery values of 93.5-96.5%, % RSD below 1.5%, while maintaining relative errors below 6% compared to a standard fluorescence spectrometer. Overall, the proposed approach presents a simple, cost-effective, and reliable platform for on-site 2-NBZ detection.