Comparative Study of Natural Terpenoid Precursors in Reactive Plasmas for Thin Film Deposition

Abstract

If plasma polymer thin films are to be synthesised from sustainable and natural precursors of chemically heterogeneous composition, it is important to understand the extent to which this composition influences the mechanism of polymerisation. To this end, a well-studied monoterpene alcohol, terpinen-4-ol, has been targeted for a comparative study with the naturally occurring mix of terpenes (viz. Melaleuca alternifolia oil) from which it is commonly distilled. Positive ion mode mass spectra of both terpinen-4-ol and M. alternifolia oil showed a decrease in disparities between the type and abundance of cationic species formed in their respective plasma environments as applied plasma power was increased. Supplementary biological assay revealed the antibacterial action of both terpinen-4-ol and M. alternifolia derived coatings with respect to S. aureus bacteria, whilst cytocompatibility was demonstrated by comparable eukaryotic cell adhesion to both coatings. Elucidating the processes occurring within the reactive plasmas can enhance the economics of plasma polymer deposition by permitting use of the minimum power, time and precursor pre-processing required to control the extent of monomer fragmentation and fabricate a film of the desired thickness and functionality.

Plasma polymerization of biogenic precursors

“…Both techniques showed precursor fragmentation to increase with RF power, with a significant amount of precursor being retained at relatively low plasma power up to 10 W (Figure 6). [97] Accordingly, the optical, physical, and chemical properties of terpinen-4-ol thin films, such as optical band gap, refractive index, extinction coefficient, hardness, chemical stability and contact angle, are tunable with plasma power. Films deposited at higher RF power showed a surface with more, narrower and sharper peaks, while there was no significant change in surface roughness parameters R a , R q , and R max .…”

“…The authors identified a F I G U R E 6 Electron impact ionization residual gas analysis mass spectra of (a) neutral terpinen-4-ol species and the plasma phase at (b) 5 W, (c) 25 W, and (d) 50 W show increased fragmentation for higher plasma powers, the peak at m/z = 154 representing the terpinen-4ol monomer. [97] Reproduced from Grant et al [97] (CC BY 4.0). Copyright 2021 by the authors.…”

“…Both techniques showed precursor fragmentation to increase with RF power, with a significant amount of precursor being retained at relatively low plasma power up to 10 W (Figure 6). [ 97 ]…”

Decontamination-Induced Modification of Bioactivity in Essential Oil-Based Plasma Polymer Coatings

“… Schematic depiction of the experimental process. ( i ) plasma polymers are fabricated from the vapors of the Melaleuca alternifolia essential oil that deposit as a thin solid coating over virtually any substrata, in this case polyethylene terephthalate (PET), at 10–30 W input radio frequency (RF) power (for mechanism of polymerization please refer to [ 19 ]); ( ii ) a subset of these samples was exposed to atmospheric pressure argon plasma for up to 60 s, and their properties characterized; separately, aqueous media were activated using a similar atmospheric pressure argon plasma treatment; ( iii ) cells were seeded onto the surfaces of treated and untreated (control) polymer coatings, and their attachment and viability were characterized; cells were also grown on the surfaces of control polymer coatings in the presence of plasma-activated culture media. …”

“…The aim of this study was to investigate how a brief exposure to therapeutic doses of atmospheric pressure argon plasma may change the bioactivity and antibacterial performance of biocide-eluting polymer coatings fabricated using plasma polymerization [ 19 ]. As the delivery of plasma-generated species to bacterial targets typically takes place across a layer of fluid that encapsulates surfaces in vivo, materials may become exposed to chemically reactive species that are generated and remain in the plasma-treated liquid for a considerable period of time.…”