Design Strategies, Mechanistic Insights, and Applications of <i>N</i> ‑Acylhydrazone‑Based Fluorometric and Colorimetric Chemosensors for Anion Detection

Abstract Anions play essential roles in environmental, biological, and industrial systems, and their accurate detection is critical for public health and ecological balance. For example, excess fluoride in drinking water or cyanide in industrial waste poses serious health hazards. N ‐acylhydrazone‐based chemosensors have recently gained attention as versatile tools for selective and sensitive anion detection, owing to their structural flexibility, ease of synthesis, and tunable optical properties. This review highlights recent advances in fluorometric and colorimetric N ‐acylhydrazone sensors, focusing on key recognition mechanisms such as hydrogen bonding, deprotonation, and nucleophilic addition. We examine how molecular structure influences sensing behavior, discussing structure–property relationships and photophysical mechanisms such as PET, ICT, and ESIPT. Environmental factors such as solvent composition, pH, and binding stoichiometry are also considered. Emphasis is placed on practical applications in biological analysis, food safety, and environmental monitoring. Comparisons with other sensor platforms are provided, alongside a critical assessment of limitations such as signal interference and aqueous instability. Finally, we outline rational design strategies to improve selectivity, sensitivity, and real‐world applicability in complex media.