Smartphone-based analytical platform for real-time, reversible detection of Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺ ions

This study explores the new Schiff base ligand, (Z)-N'-(1-(2-hydroxy-5-methoxyphenyl)ethylidene)-3-methoxybenzohydrazide (L), as a portable and selective sensor for the detection of Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺ ions. We hypothesized that L could enable real-time, reversible metal ion sensing via optical changes and suitable for smartphone-based analysis. The metal-ligand interactions were studied using UV-Vis, fluorescence, cyclic voltammetry, and NMR techniques. The ligand formed a 1:1 complex with good binding affinities and low detection limits. A colorimetric naked eye change and fluorescence responses allowed visual detection and quantitative analysis via RGB values evaluated by a smartphone application. The sensing process was reversible with Na₂EDTA and effective in the neutral pH range. The sensor was successfully applied to spiked water samples and germinated seeds. These results confirm the initial hypothesis and highlight the potential of L as a low-cost, effective tool for monitoring these ions for food quality assessment, environmental safety, and medical applications.