Fermented juices as reducing and capping agents for the biosynthesis of size-defined spherical gold nanoparticles

Abstract

Gold nanoparticles (AuNPs) are of scientific and industrial significance; however, the traditional synthesis methods employ toxic compounds. Hence, non-toxic and environmentally friendly AuNPs synthesis methods are of special interest. Here, AuNPs were produced using four solutions of fermented grape juices. UV/Vis absorption spectrophotometry and transmission electron microscopy indicated that AuNPs synthesized with a solution based on semi-sweet red grapes were mostly spherical with narrow size distribution (average diameter of 82.1 ± 36.2 nm). AuNPs of similar spherical morphology but smaller size were obtained using a solution based on semi-dry red grapes (57.1 ± 16.4 nm). A large variety of AuNPs shapes and broader size distribution were produced when solutions based on semi-sweet or dry white grapes were applied. In this case, the average sizes of the AuNPs were 271.6 ± 130.2 nm and 76.0 ± 47.2 nm, respectively. Using energy dispersive X-ray spectroscopy, Au, C, and O were detected, confirming formation of biogenic AuNPs in all cases. Mie theory calculations for AuNPs synthesized with the aid of solutions based on red grapes suggest that their optical properties are different and best suited for distinct downstream applications. Attenuated total reflectance Fourier transform infrared spectroscopy, the Folin-Ciocalteu assay, and the Bertrand’s method were used to examine bioactive compounds present in the solutions applied for synthesis. Phenolics, and to a lesser extent reducing sugars, were identified as likely playing a significant role in reduction and stabilization of the AuNPs. These results display the great potential of these solutions for green synthesis of size defined AuNPs, and illustrate that different grape varieties may be used to obtain AuNPs with unique properties. Keywords: Nanostructures, Bioreduction process, Phenolics, Reducing sugars, Mie scattering