Abstract

In this study, a green and sensitive spectrophotometric method was developed for Fe³⁺ detection using silver nanoparticles (AgNPs) synthesized from Phyllanthus amarus extract. Comprehensive characterization using UV–Vis, TEM, EDX, XRD, FT-IR, and zeta potential analysis confirmed the formation of well-dispersed, crystalline, and bio-functionalized colloidal AgNPs with an average size of 8.41 ± 2.79 nm and a zeta potential of −24.86 ± 1.26 mV. The synthesized AgNPs exhibited a distinct surface plasmon resonance peak at 410 nm, which was quenched upon interaction with Fe³⁺ owing to oxidative etching, resulting in a visible color change from yellow to colorless. Thermochemical analysis based on density functional theory confirmed that the etching process was both energetically favorable (ΔH < 0) and thermodynamically spontaneous (ΔG < 0). The sensor demonstrated high selectivity for Fe³⁺, with optimal sensing at pH 7, 1:2 AgNPs-to-Fe³⁺ ratio, and 5-min reaction time. The method showed a wide linear range (0.1–15 mg L⁻¹, R² = 0.9989), low detection (0.058 mg L⁻¹) and quantification (0.18 mg L⁻¹) limits, and excellent reproducibility (RSD < 2.0 %). Actual water sample validation using atomic absorption spectroscopy confirmed its accuracy and practical utility. This eco-friendly nanosensor provides a rapid, low-cost, and reliable approach for Fe³⁺ detection, demonstrating strong potential for environmental monitoring and surface-based sensing applications.