Green synthesis of stable gold and silver nanoparticles using aqueous extract of Sargassum spp. and their application in catalysis.

Abstract

This study reports an eco-friendly synthesis of stable gold (Au) and silver (Ag) nanoparticles (NPs) using aqueous extract of Sargassum spp. collected from Caribbean beaches in Quintana Roo, Mexico. The biomolecules present in the extract acted as both reducing and stabilizing agent for nanoparticle formation. UV-Vis spectroscopy confirmed the formation of Au and Ag NPs through localized surface plasmon resonance bands at 537 and 416 nm, respectively, with excellent colloidal stability over two weeks. XRD analysis revealed face-centered cubic crystalline structures, while TEM images showed predominantly quasi-spherical Au NPs (9 ± 2.2 nm) and more polydisperse Ag NPs (11.4 ± 4 nm). Discrete dipole approximation simulations indicated that Au colloids were mainly composed of cuboctahedral nanoparticles, whereas Ag colloids exhibited cuboctahedral, decahedral, and truncated octahedral morphologies. FTIR analysis confirmed the presence of hydroxyl and amine functional groups involved in nanoparticle stabilization. The catalytic activity of the nanocolloids was evaluated through the reduction of 4-nitrophenol (4-NP) and degradation of Rhodamine B (RhB) in the presence of NaBH4. Ag NPs showed significantly higher catalytic efficiency than Au NPs, achieving 99.3% reduction of 4-NP within 1 min with a rate constant, k = 8.44 × 10- 2 s- 1, while Au NPs required 7 min (k = 1.04 × 10- 2 s- 1). Similarly, complete RhB degradation was achieved within 2 min using Ag NPs and 4 min using Au NPs with corresponding rate constants of 3.78 × 10- 2 and 1.94 × 10- 2 s- 1, respectively. These findings highlight the strong potential of green-synthesized noble metal nanoparticles for wastewater remediation.

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