Síntese verde de nanopartículas de prata via rota hidrotérmica utilizando bagaço de uva como biorredutor

Abstract

Green synthesis enables the mutual benefit of using agro-industrial residues and nanomaterials production. The ability of plant extracts to reduce metal ions is attributed to their antioxidant activities, and for this reason they have been used in this type of synthesis. Grape pomace is a by-product derived from the grape juice production, and due to its chemical composition, it has been underestimated when discarded as waste. In this study, the physicochemical properties of the pomace from grape pure juice (Vitis labrusca, L.) were investigated, and it was used as mediator in the hydrothermal synthesis of silver nanoparticles (AgNPs). The pomace proximate composition, total phenolic compounds (TPC) and antioxidant activity were evaluated. The characterization analyses were carried out for fresh and dried pomace extract. The TPC ranged from 55.02 to 161.54 mg of gallic acid equivalents (GAE) per 100 g of sample for the fresh and dried residue. The antioxidant activity, determined by the β-carotene/linoleic acid assay, is a key factor for this application and was categorized as high (>80%). The thermogravimetry showed that the grape residue has good stability and the occurrence of mass loss phases related to the volatile compounds of the sample. In the AgNPs hydrothermal synthesis, the performance in the reduction of 0.375 mM AgNO3 was analyzed through changes in the variables time, temperature and grape pomace extract concentration. The samples were chemically and structurally characterized by spectroscopy in the visible ultraviolet region (UV-vis), Zeta potential, particle size determination by Dynamic Light Scattering (DLS), X-ray diffraction (XRD) and morphological evaluation by Scanning Electron Microscopy with Field Emission Gun (SEM-FEG). The antibacterial potential of the nanoparticles was determined by the disk diffusion test and the microplate test, in order to evaluate the influence of extract concentrations against Gram-positive and Gram-negative representatives. The optimal results of the synthesis were observed at 100 °C, 1 h and extract concentration at 0.05 mg/L. In this condition, the AgNPs synthesized by the hydrothermal method exhibited a mean size of 37 nm, determined

by DLS, Zeta potential of -20 mV and monodisperse spherical morphology, observed by SEM- FEG. The UV-vis spectrum of AgNPs exhibited a peak at 460 nm, consistent with the silver

source consumption and the formation of metallic silver, in addition, it was exhibited a peak at 280 nm in the extract analysis, representing the typical pattern of anthocyanins and flavanoids absorption. XRD attributed the crystalline nature of AgNPs by detecting the diffractometric pattern with peaks and fcc (face centered cubic) geometric arrangement conforming to the metal identity. Antibacterial potential was successfully proved against Escherichia coli and Staphylococcus aureus, with MIC value at 12.5 and 50 μL, respectively, assigning validation to the grape pomace application in the reduction of silver ions (Ag+ to Ag0

) in an easy single- step protocol avoiding harsh or toxic chemicals generation. Thus, the grape residue showed

potential in the synthesis of AgNPs with good stability and satisfactory antimicrobial activity.