Azo-Dye Based Molecular Switches for Visual Detection of Metal Ions: Advances and Perspectives

The development of simple, efficient, and cost-effective sensing systems for metal ion detection has attracted significant interest due to the vital roles of metal ions in environmental, biological, and industrial processes. Essential ions such as Fe³⁺, Cu²⁺, and Zn²⁺ are crucial for physiological functions but can cause severe disorders when imbalanced, whereas toxic ions like Hg²⁺, Pb²⁺, and Cd²⁺ pose serious risks even at trace levels. Conventional analytical techniques, though sensitive, are often costly, time-consuming, and unsuitable for rapid or on-site detection. Azo dyes have recently gained prominence as promising sensing platforms owing to their structural versatility, strong chromogenic properties, and additional biological activities. Their photochromic and solvatochromic behaviors allow distinct and reversible color changes upon metal ion binding, enabling naked-eye detection without sophisticated instruments. This short review outlines recent advances in azo dye-based molecular switches for visual metal ion detection, emphasizing design strategies, sensing mechanisms, and practical applications, while also addressing current challenges and future prospects for developing reliable, sustainable, and user-friendly sensing systems.