Biosurfactant-mediated biosynthesis of CuO nanoparticles and their antimicrobial activity

In the design and synthesis of novel functional materials, a few important challenges that have to be addressed by the material research community are (1) developing robust, reproducible, and environmentally friendly green methods for the synthesis of various nanomaterials and (2) exploring new functional nanomaterials to combat with multi-drug-resistant pathogens. This paper addresses the above-mentioned challenges by (1) demonstrating a green synthesis for the CuO nanoparticles using environmentally benign biosurfactant rhamnolipid (RL) and (2) exploring the antimicrobial activity of RL functionalized/capped CuO nanoparticles (RL-CuO NPs) against various bacterial and fungal species. CuO nanoparticles (CuO NPs) were synthesized using RL by hydrothermal method at pH 9–10. Capping of RL on CuO NPs was probed by characterizing the sample with Fourier-Transform Infrared Spectroscopy (FT-IR) and Thermo Gravimetric Analysis (TGA). Whereas X-Ray Diffraction (XRD), Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM) were used to understand the phase, crystallinity, and morphology of CuO NPs. The antimicrobial activity of RL-CuO NPs was examined using resazurin microtitre assay, against Gram-positive bacteria, such as Streptococcus mutans, Staphylococcus aureus, Enterococcus faecalis, for which the minimum inhibition concentration (MIC) values were observed at low concentrations, i.e., 7.8 µg/L of RL-CuO NPs. For Gram-negative bacteria viz. Shigella dysentriae and Salmonella typhi (except Pseudomonas aeruginosa) MIC values were observed at little higher concentrations of RL-CuO NPs, i.e., 250 µg/L. Anti-fungal activity of RL-CuO NPs was examined against Candida albicans, Aspergillus niger and MIC values for these fungi are observed at 125 µg/L and 62.5 µg/L, respectively.