Sustainable and integrated cascade valorization of the biomass generated by the invasive plant Flaveria bidentis: optimization of ultrasound-assisted phenols extraction coupled to the green synthesis of biogenic antimicrobial silver nanoparticles

Abstract

Introduction This study addresses the integrated valorization of residual biomass from Flaveria bidentis, an invasive species native to the Americas through the recovery of phenolic compounds (phenolic acids and flavonoids, mainly glycosylated and sulfated – the latter being rare in nature –) and their subsequent use in the green synthesis of silver nanoparticles. The optimization of ultrasound‐assisted aqueous extraction of phenolic compounds from F. bidentis biomass and the direct use of the resulting extracts as reducing and stabilizing agents for the green synthesis of silver nanoparticles remain largely unexplored. Methods The phenolic compounds were extracted by ultrasound‐assisted extraction (UAE) using water as extraction solvent. Extraction time, temperature, and solvent/sample ratio were optimized within the investigated experimental domain using response surface methodology based on a three‐variable Doehlert experimental design (R2 ≥ 0.96). The optimized aqueous extracts were then used directly, without purification, as reducing and stabilizing agents in the green synthesis of silver nanoparticles. These nanoparticles were characterized in terms of size and morphology, and their antibacterial activity against Gram‐ positive and Gram‐negative bacteria was subsequently evaluated using the diffusion assay. Results and Discussion Optimal conditions for phenols extraction were established from leaves (68 min, 59 °C, 20 mL/ g) and from flowers (65 min, 75 °C, 20 mL/g). Under these conditions, the results revealed a key finding: the high yield of phenolic compounds from F. bidentis, particularly from the leaves (250.86 mg/g), using water as the sole solvent. Spherical biogenic silver nanoparticles (∼30–31 nm; PDI: 0.26–0.46; ζ‐potential: −15.0 to −12.1 mV) were successfully synthesized and exhibited significant antimicrobial activity. The results demonstrate the feasibility of integrating phenolic extraction and nanoparticle synthesis into a single process, in which the aqueous extracts obtained under optimized conditions can be directly employed to produce functional nanomaterials without intermediate purification steps. Thus, the proposed cascade valorization strategy enabled the sustainable transformation of F. bidentis biomass into bioactive compounds and antimicrobial nanomaterials, adding value to this plant waste.