Biofunctionalized Gold Nanoparticles Obtained from Two Different Plant Extracts and its Chemical and Structural Comparison

Gold nanoparticles (GNPs) are attractive for multiple applications in biological environments due to their biocompatible nature, which can vary depending on their synthesis method [1]. In this regard, green methods are the most advantageous. Plant extracts provide an environment-friendly approach to obtain GNPs because contain abundant metabolites with the potential to act as reducing and capping agents in the presence of a gold precursor [2, 3]. Moreover, some of these metabolites have been evaluated for their antibacterial, antifungal, antitumoral or antiviral properties [4], which makes it possible to provide properties to GNPs that can be exploited in the healthcare area. In a nutshell, the type of plant and the route of synthesis, determine the final characteristics of the particles such as shape, size distribution and colloidal stability, and its possible applications are diversified depending on the biofunctionalizing layer resulted from the plant extract-mediated biosynthesis [5]. Using two different plant extracts, in this work we compare more specific features of the biosynthesized GNPs such as surface chemistry and microstructure. To obtain the GNPs, 1 mL aqueous extract of Jatropha dioica was added to a gold standard solution previously adjusted to pH 6.3, and 2 mL Coffea arabica aqueous extract were first mixed with the gold precursor, followed by pH 7.0 adjustment. The biosynthesis concluded after 4 hours of constant stirring at 25 and 70 °C, respectively. The two different routes of synthesis were the ones that showed the best results in colloidal stability for each of the extracts. X-Ray diffraction (XRD) allowed the identification of the face-centered cubic structure in both types of GNP. Diffraction peaks are presented in 2-theta scale at 38.2°, 44.2°, 64.6° and 77.6° and correspond to the Bragg´s reflection of (111), (200), (220), (311), respectively (Figure 1 a). Both diffractograms exhibited the intense peak at 38.3°, due to preferential growth in the (111) direction [6]. Figure 1 b presents scanning transmission electron micrographs (STEM), hemispherical particles were identified in both cases, an organic layer was found covering the nanoparticles even after separated and washed it from the synthesis extracts. Insets in STEM images correspond to ultraviolet-visible (UV-Vis) spectra of nanoparticles in aqueous medium and show the surface plasmon band at 528 nm, characteristic of nanometric gold. GNPs obtained with C. arabica extract presented homogenic size around 30nm; on the other hand, J. dioica extract GNPs showed larger and polydisperse sizes. The extracts and GNPs were analyzed by Fourier-transform infrared spectroscopy (FTIR), technique that confirmed the presence of characteristic functional groups of antioxidant compounds in J. dioica and C. arabica extracts. According to the spectra comparison (Figure 2 a) hydroxyl groups (-OH) of phenolic compounds were oxidized to C=O while reducing gold precursor to GNPs. Furthermore, both nanoparticles IR spectra showed common bands compared with their respective extracts. This suggests the extracts contain biomolecules that act as reducing and stabilizing agents, and some others were just involved in covering and biofunctionalize the GNPs surface. The morphology and size of the nanoparticles was confirmed by transmission electron microscopy (TEM) (Figure 2 b) The wash, resuspension in water and subsequent dilution in ethanol allowed us to observe the GNPs isolated from the organic layer remaining from the biosynthesis. In this work gold nanoparticles were obtained by a biological method in one step and using aqueous extracts of C. arabica and J. dioica. The nanoparticles had the characteristic crystalline structure and the surface plasmon band of nanometric gold. The inherent surface properties of these GNPs could give them functionality by self in biological environments. In both cases, particles functionalized with the molecules of the extracts were obtained with hemispherical morphology. The essential difference between both types of nanoparticles was their size distribution. Our short-term aim is to test the effect of these biofunctionalized nanoparticles in drug delivery and optical biosensing applications. a) XRD analysis. b) STEM images and UV spectra of biosynthesized GNPs using C. arabica (left) and J. dioica (right). a) FTIR spectra and b) TEM micrographs of the obtained GNPs.