Biogenic synthesis of silver nanoparticles for colorimetric detection of Fe3+ in environmental samples: DFT calculations and molecular docking studies.

Abstract

Olive leaf (OL) extract, rich in phenolic compounds, was employed as a green reductant and capping agent for the biogenic synthesis of silver nanoparticles (Ag NPs), providing an eco-friendly alternative to conventional chemical methods. The OL-Ag NPs demonstrated dual functionality as a colorimetric Fe³⁺ sensor and broad-spectrum antimicrobial agent, characterized by DLS, UV-vis spectroscopy, FT-IR, XRD, and FE-SEM. Optimization of the Fe³⁺ sensing parameters via CCD combined with RSM identified optimal conditions of pH 5.8, 211 μL probe volume, and 3 min complexation time, resulting in rapid detection with a visible color change from pale yellow to dark green. The interference study demonstrated that OL-Ag NPs selectively detect Fe³⁺ in aqueous samples through Fe³⁺-specific chelation-induced agglomeration, exhibiting no cross-reactivity with coexisting ions. DFT calculations elucidated the stable interaction mechanism between OL-Ag NPs and Fe³⁺ ions, supported by molecular electrostatic potential maps and binding energy analyses. The colorimetric nanoprobe exhibited excellent selectivity for Fe³⁺ over competing metal ions, with a low detection limit (LOD) of 0.81 μM and limit of quantification (LOQ) of 2.7 μM. Field-deployable test strips enabled rapid on-site detection of Fe³⁺ ions, exhibiting concentration-dependent color shifts from pale yellow to dark green. The sensor achieved recoveries of 86-92.5 % in real water samples, consistent with ICP-OES results. Biological evaluations of OL-Ag NPs revealed strong antibacterial activity, with inhibition zones of 1.6 mm against B. subtilis (highest growth inhibition), 1.2 mm against S. aureus and E. coli, and 1.0 mm against P. aeruginosa (lowest growth inhibition), comparable to gentamicin. Molecular docking simulations supported these findings, showing binding free energies of -8.41 kcal/mol with S. aureus and -4.65 kcal/mol with E. coli proteins. Cytotoxicity assays on Hu02 cells indicated low toxicity and effective cellular uptake, with intracellular imaging confirming Fe³⁺ detection capability. Overall, this study presents a simple, cost-effective, and environmentally benign synthesis of OL-Ag NPs with dual functionality as a highly sensitive colorimetric sensor for Fe³⁺ and an effective antimicrobial agent, promising broad applications in environmental monitoring and biomedicine.