Triple-mode speciation of Fe²⁺ and Fe³⁺ using optical and paper-based digital imaging

Herein, we report the synthesis and application of a Schiff base probe, (Z)-2-(((2-hydroxynaphthalen-1-yl)methylene)amino)benzoic acid (HNABA), for selective speciation of Fe²⁺ and Fe³⁺ using colorimetric, fluorometric, and smartphone-assisted paper-based digital sensing. The probe exhibited a distinct optical response towards both iron species through coordination involving phenolic oxygen and imine nitrogen donor atoms. The complex formation and structural features were confirmed by UV-Vis absorption, fluorescence spectroscopy, Fourier transform infrared (FTIR) spectroscopy, nuclear magnetic resonance spectroscopy (NMR), and electrospray ionization-mass spectrometry (ESI-MS) analyses, which also supported the metal-ligand binding mechanism. Experimental parameters, including pH and reaction time, were optimized to achieve the maximum sensitivity and stability. The linearity range for the determination of Fe²⁺ was 0.05-0.3 mg L^-1 with LOD values of 0.015, 0.02, and 0.03 mg L^-1 using colorimetry, fluorometry, and digital paper-based sensing, respectively. Similarly, the linearity range for the determination of Fe³⁺ was 0.05-0.3 mg L^-1 with LOD values of 0.02, 0.018, and 0.023 mg L^-1 using colorimetry, fluorometry, and digital paper-based sensing, respectively. The methods demonstrated good precision (RSD ≤ 3.4%) and satisfactory recoveries of 97.6% and 99.0% for Fe²⁺ and Fe³⁺, respectively in spiked water samples. The integrated optical and digital imaging platform offers a rapid, economical, and reliable strategy for iron speciation in environmental monitoring applications.