Pectin-functionalized silver nanoparticles derived from waste Robusta coffee husks: antibacterial activity and thermodynamic kinetic analysis of 4-nitrophenol reduction

Abstract

A sustainable route was developed to valorize discarded Robusta coffee husks into a functional biopolymer platform for nanocatalysis and antimicrobial applications. Pectin extracted from Robusta coffee husk waste was utilized as a dual-function reducing and stabilizing agent for the synthesis of silver nanoparticles (CHP-AgNPs) in an alkaline medium, eliminating the need for conventional toxic reducing agents and surfactants. The synthesis was systematically optimized by UV-vis monitoring of the Ag surface plasmon band, and the resulting CHP-AgNPs prepared at different Ag+/pectin ratios were comprehensively characterized by the physicochemical analysis. The CHP-AgNPs displayed broad-spectrum antibacterial activity against representative Gram-negative (E. coli) and Gram-positive (S. pyogenes) strains, with activity strongly dependent on Ag+/pectin formulation and dose. Beyond antimicrobial action, the catalytic capability of CHP-AgNPs was demonstrated using the NaBH4-assisted reduction of 4-nitrophenol where temperature-dependent kinetic analysis confirmed pseudo-first-order behavior and enabled the extraction of activation/transition-state thermodynamic descriptors. The optimized CHP-AgNPs catalyst also exhibited excellent reusability, retaining more than 91% of its initial catalytic activity after five consecutive reaction cycles. In general, this work presents a circular-bioeconomy approach that combines waste-derived pectin-based nanochemistry with dual antibacterial and catalytic functionality, supported by kinetic and thermodynamic analyses of the apparent catalytic behavior.