Investigation of Antibacterial 1,8-Cineole-Derived Thin Films Formed via Plasma-Enhanced Chemical Vapor Deposition

The need for low-fouling coatings for biomedical devices has prompted considerable interest in antibacterial compounds from natural and sustainable sources, such as essential oils. Herein, a tea tree oil-based precursor, 1,8-cineole, is used to fabricate antimicrobial films (denoted ppCin) by plasma-enhanced chemical vapor deposition. Film properties were comprehensively characterized using a variety of surface and bulk analytical tools, and the plasma gas phase is assessed using optical emission spectroscopy, which can be correlated to ppCin film properties. Notably, film wettability increases linearly with plasma pressure, yielding water contact angles ranging from ∼50° to ∼90°. X-ray photoelectron spectroscopy reveals less oxygen is incorporated at higher pressures, likely arising from the lower density of OH_(g) species. Further, we utilized H₂O_(v) plasma surface modification of the ppCin films to improve wettability and find this results in a substantial increase in surface oxygen content. To elucidate the role of film wettability and antibacterial properties, both as-deposited and H₂O_(v) plasma-modified films were exposed to Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus using glass slides and hydrocarbon films deposited from 1,7-octadiene as positive controls. Overall, bacteria attach to a similar extent on all films, including controls, yet only essential oil-based films significantly prevent biofilm formation (4-7% coverage compared to ∼40% for controls).