Valorization of Copaifera langsdorffii Leaf Litter as a Source of Reducing Compounds for the Green Synthesis of Silver Nanoparticles

Leaf litter, consisting of senescent or dead leaves that detach from plants and accumulate on the soil surface, plays an important role in nutrient cycling and may represent a source of bioactive compounds with biotechnological potential. Copaifera langsdorffii Desf., commonly known as copaíba, has a history of use in traditional medicine and recognized biological potential. Its leaf litter also exhibits biological activity and potential for agricultural applications. In this context, polyphenols and flavonoids present in plant material may act as reducing and stabilizing agents in the green synthesis of silver nanoparticles (AgNPs), nanomaterials with potential applications in various fields. This study aimed to develop a green synthesis method for AgNPs using an aqueous extract of C. langsdorffii leaf litter, characterize the resulting nanoparticles, and determine total phenolic and flavonoid contents. Different extract and AgNO₃ concentrations, as well as different pH values, were evaluated, and nanoparticle formation was monitored by UV–Vis spectrophotometry. The selected conditions were 35% extract, 5 mmol L⁻¹ AgNO₃, pH 8, a temperature of 30 °C, and a reaction time of 60 minutes, resulting in an absorption band between 420 and 450 nm. The AgNPs exhibited an average size of 18.49 ± 0.57 nm, a polydispersity index of 0.43 ± 0.02, and a zeta potential of −35.31 ± 1.92 mV. Total phenolic contents were 822.63 in the aqueous extract, 290.04 at the concentration used for synthesis, and 202.26 mg gallic acid equivalents per gram of dry weight in the AgNPs. Total flavonoid contents were 856.53, 151.87, and 605.20 mg rutin equivalents per gram of dry weight, respectively. The zeta potential obtained indicates characteristics associated with colloidal stability. The reduction in phenolic content after synthesis may be related to the involvement of these compounds in the reduction of Ag⁺ ions and nanoparticle stabilization. The increase in flavonoid content in the AgNPs compared with the extract used suggests a possible association of these compounds or their functional groups with the nanoparticle surface. These findings indicate that C. langsdorffii leaf litter represents a promising plant source of reducing compounds for the green synthesis of AgNPs, promoting the valorization of residual plant material and encouraging further investigations into their biotechnological and agricultural properties and applications.

Acknowledgment: National Council for Scientific and Technological Development (CNPq/MAI) – N°168434/2023-7