Synthesis and antimicrobial activity of CuO nanoparticles stabilized by rhamnolipid biocomplex of microbial origin

Abstract

A wide range of both physical and chemical methods has been developed for the synthesis of copper-based nanoparticles. At the same time, especially promising from the ecological point of view methods of obtaining colloidal solutions are the so-called “green” methods of synthesis, the essence of which is the use of natural biologically active substances and can also enhance the antimicrobial activity of the inorganic component. A promising class of polyfunctional substances can be considered biogenic surfactants (biosurfactants), namely the products of microbial synthesis, which are now widely studied and considered an environmentally friendly alternative to synthetic surfactants. Such biosurfactants often also have antimicrobial and antiviral activities, and therefore their combination with copper-based nanoparticles will combine and enhance the properties of both components to create modern environmentally friendly drugs with antimicrobial action. Among the wide range of promising microorganisms-producers of surfactants, the genus Pseudomonas deserves a special attention, which synthesizes extracellular surfactant glycolipids with high surface, emulsifying and foaming activities. Therefore, the aim of this work was to obtain concentrated colloidal solutions of copper oxide in the presence of a biogenic surfactant, so-called rhamnolipid biocomplex synthesized by culture of Pseudomonas sp. PS -17, which simultaneously provides satisfactory stabilization of colloidal copper solutions and is non-toxic, to study the effect of rhamnolipid biocomplex concentration on the characteristics of the obtained copper oxide nanoparticles, as well as to investigate their antimicrobial action. Concentrated colloidal solutions of copper oxide, stabilized by rhamnolipid biocomplex (RBC), which is a low molecular weight secondary metabolite of bacteria of the genus Pseudo ­ monas sp. PS -17 and at the same time provides satisfactory stabilization of colloidal solutions of copper oxide and is non-toxic have been synthesized. Copper oxide nanoparticles (CuO–NPs) were obtained by the decomposition reaction of freshly prepared copper hydroxide in an alkaline medium in the presence of RBC. The obtained CuO–NPs were investigated using scanning electron microscopy, spectroscopy in the UV-visible range, as well as the method of X-ray diffraction. It was found that the obtained CuO–NPs form agglomerates of irregular shape with a size of 35–90 nm. The sizes of crystallites of synthesized CuO–NPs were calculated by X-ray diffraction patterns. It was found that the size of the crystallites decreases from 12 to 5 nm with increasing initial concentration of RBC. The antimicrobial activity of the obtained concentrated sol against two test microorganisms, namely Agrobacterium tumefaciens and Xanthomonas campestris was studied. It was found that the obtained CuO–NPs show a sufficiently high antimicrobial activity against these microorganisms, and the values of the minimum inhibitory concentration (MIC) and the minimum bactericidal concentration (MBC) are 20 and 40 μg/ml, respectively. Keywords: nanoparticles of copper oxides, green synthesis, antimicrobial activity.