The synthesis of copper nanoparticles (CuNPs) was accomplished by using a rapid, green, and versatile argon plasma reduction method that involves solvent extraction. With this method, a plasma-solid state interaction forms and CuNPs can be synthesized from copper(II) sulfate using a low-pressure, low-temperature argon plasma. Characterization studies of the CuNPs revealed that when a metal precursor is treated under optimal experimental conditions of 80 W of argon plasma for 300 s, brown CuNPs are synthesized. However, when those same brown CuNPs are placed in Milli-Q water for a period of 10 days, oxidation occurs and green CuNPs are formed. Confirmation of the chemical identity of the CuNPs was performed by using X-ray photoelectron spectroscopy. The results reveal that the brown CuNPs are predominantly Cu0 or what we refer to as CuNPs, while the green CuNPs are a mixture of Cu0 and Cu(OH)2 NPs. Upon further characterization of both brown and green CuNPs with scanning electron microscopy (SEM), the results depict brown CuNPs with a rod-like shape and approximate dimensions of 40 nm × 160 nm, while the green CuNPs were smaller in size, with dimensions of 40-80 nm, and more of a round shape. When testing the antibacterial activity of both brown and green CuNPs, our findings demonstrate the effectiveness of both CuNPs against Escherichia coli and Staphylococcus aureus bacteria at a concentration of 17 μg/mL. The inactivation of S. aureus and E. coli 7-day-old biofilms required CuNP concentrations of 99 μg/mL. SEM images of treated 7-day-old S. aureus and E. coli biofilms depict cell membranes that are completely damaged, suggesting a physical killing mechanism. In addition, when the same concentration of CuNPs used to inactivate biofilms were tested with human fibroblasts, both brown and green CuNPs were found to be biocompatible.
Keywords: antibacterial; biocompatible; copper; green; nanoparticles; plasma reduction.