2.1. Surface Properties and Chemical Composition of As-deposited Plasma Polymer Coatings
Optically transparent (k
→0 over the 500–1000 nm wavelength range) plasma polymer coatings were deposited on the surfaces of polyethylene terephthalate (PET) substrata (Figure 1
) and their thickness estimated using a Cauchy model, as described in [21
]. Plasma polymer coatings with a thickness of ~500 nm were selected because previous studies had found that films of lower thickness deposited on the surface of lotus-like titanium substrata may possess a reduced ability to retard bacterial attachment and subsequent biofilm formation [12
]. This is likely because the antibacterial coatings based on essential oils, and synthesized using RF plasma polymerization, have a complex mechanism of activity that combines prevention of attachment due to a favorable combination of surface chemistry and surface topography, as well as killing of attached microorganisms through elution of biologically active molecules into the near-surface region [22
]. In the case of thinner films, their ability to elute sufficiently high doses of antimicrobial agents may be lower, leading to less effective retardation of microbial proliferation and biofilm growth.
AFM examination revealed substrate surfaces with a significantly different roughness profile (Figure 2
A–F). As the input energy increased, the root mean squared (RMS) roughness Rq
was first found to increase from 3.2 nm to 6.0 nm for samples fabricated at 10 W and 15 W, then reduced to 2.2 nm (20 W) and 1.7 nm (25 W), and once again began to increase for polymer coatings fabricated at 30 W (Table 1
). The maximum peak height Rmax
followed a similar trend, changing from 22.5 nm (10 W) to 7.6 nm (25 W) to 33.8 nm (30 W). It is possible that at lower RF power levels (10–15 W), the level of monomer fragmentation was low, and the deposition was a combination of chemical polymerisation-type reactions in the gas phase, with surface chemical reactions being initiated by mild ion bombardment. Further reactions could then be taking place with plasma-activated monomer species and their fragments, with the subsequent physical adsorption of neutral monomer species and their oligomers and their entrapment within the framework of the growing coating matrix.
Once the power was increased to 20–25 W, monomer fragmentation in the gas phase became more efficient, with smaller molecular fragments being delivered to the surface. More intense ion bombardment of the surface promotes chemical reactions at the surface, resulting in a more homogenous, smoother film that is dominated by cross-linked branched hydrocarbons. The subsequent increase in the RF power to 30 W further intensified the extent of ion bombardment, leading to the removal of polymer coating material from the surface via etching and desorption of fragments due to the higher temperature of the substrate. The combination of etching and desorption may be responsible for the increased roughness of the resultant surface, with a significantly larger depth of valleys (at 33.8 nm for 30 W, as opposed to 18.2 nm obtained using a power level of 15 W) supporting this possibility. Indeed, the maximum depth of pits and valleys (Rmin) for samples fabricated at 30 W was found to be more similar to that of samples subjected to pure Ar plasma, where surface etching was found to be the dominant process.
The XPS results presented in Figure 2
G and Table 1
confirm that as the RF power was increased, the dissociation of oil constituents, many of which contain alcohol groups, became more pronounced, leading to their transformation into hydrocarbon-rich polymer chains, and a gradual reduction in the oxygen content in the polymer matrix. The slight increase in the oxygen content for the coatings deposited at 30 W was most likely due to the post-synthesis interactions of the polymer coating with the ambient air. The location of the carbon and oxygen peaks that dominate the atomic composition of the coatings was found to be similar regardless of the deposition power, yet their magnitude differed, suggesting the formation of an increasingly hydrocarbon-rich material. Deconvolution of the peaks suggested that oxygen is present in the two binding states as C--O/C=O/Si—O (at ~530 eV) and *O--C=O (at ~531 eV), whereas carbon is in C-C/C-H, C-O, C=O and O-C=O (at 285, and ~286.5, 288 and 289 eV, respectively).
This was confirmed by analysing the FTIR spectra of the coatings (representative spectrum shown in Figure 2
H). The N1s peak that appeared in some of the spectra was fitted with two peaks at ~400.0 and 402 eV, corresponding to chemical states where nitrogen atoms are attached to carbon atoms in the polymer coating change in the form of either amine or amide, and where nitrogen atoms are bonded to a single oxygen atom (C-N-O, N-C+
]. Where N1s peak lacked fine structure to assist fitting, the width of the peak suggested contributions from more than one chemical state of the element. Nitrogen is believed to originate from post-deposition ageing reactions of the unreacted monomer fragments on the surface of the freshly deposited polymer coating with ambient air [24
For the deposition power range 10–30 W, the FTIR spectra show a similar shape, with difference in peak magnitude related to the reduction in the oxygen moieties (at ~3450 and 1708 cm−1
). The relationship between the magnitude of peaks associated with the stretching and bending of CH2
groups (e.g., 1450 and 1370 cm−1
) suggested an increase in the cross-linking of carbon chains for coatings fabricated at higher RF powers, which has previously been linked to the reduced elution of active species for this type of coating [22
] and, consequently, their reduced antibacterial activity [12
2.2. Effect of Atmospheric Pressure Ar Plasma Treatment on Plasma Polymer Surfaces
Exposure of the surfaces of polymer coatings to an Ar plasma jet resulted in a notable increase in the quantity of oxygen-bearing groups on the surface of the polymer coating. The FTIR spectra collected in the transmission mode enabled the probing of the bulk chemistry of the coating, whereas XPS was used to capture changes in the 10 nm-top-most surface layer of the coating. Examination of both spectra suggests that exposure of the substrate to the plasma jet resulted in a significant oxidation of the surface, with a significant quantity of oxygen atoms being added to the surface (Figure 3
). The atomic fraction of oxygen on the surface increased for all samples. For coatings fabricated at 15 W, the oxygen increased from 26 to 30%. For other coatings, the oxygen fraction increased from 25.1 to 29.7% (for 10 W), from 22.9 to 25% (for 20 W), from 22.8 to 25.3% (for 25 W), and from 21.4 to 24.5% (for 30 W). It should be noted that even prior to the treatment, the polymer coating surfaces were rich in oxygen groups.
These additional oxygen atoms can come from several sources. First, during plasma polymerisation, oxygen moieties present in the monomer molecules and their fragments are retained within the polymer matrix. In the case of the deposition process used here, polymer coatings fabricated at lower RF power tended to have a greater fraction of O atoms in their structure.
Second, immediately after deposition, exposure of the coatings to the ambient air leads to a change in surface chemistry, with polymer coatings possessing a significant number of radicals on their surface being more susceptible to reactions with oxygen species in air. Through this mechanism, hydrocarbon-rich coatings deposited at higher RF powers gain a significant quantity of -OH groups on their surfaces in the form of low molecular weight oxidized materials. This has been demonstrated previously using XPS depth profiling, where etching is used to remove layers of polymer coating in-between XPS scans [28
]. It should be noted that these low molecular weight oxidized materials can be easily removed by exposing the surface to a polar solvent, e.g., water, and thus cannot be relied upon as a reliable means of controlling cell-surface interactions.
Third, during argon plasma treatment, oxygen species that are produced as a result of interactions between Ar metastable species and ambient air have sufficiently long lives to allow them to reach and actively modify the surface, introducing -OH hydroxyl (the broad 3100–3600 cm−1
band on FTIR spectra, Figure 3
A), –COOH carboxylic acid (~1640 cm−1
), and C=O ketone (~1730 cm−1
) groups. While the magnitude of these peaks changed as a result of argon plasma treatment, there were no new peaks introduced. This was consistent with the findings from the XPS spectra, where it was found that while the atomic fraction of oxygen increased, the position of the peaks that made up the O 1s and C 1s peaks remained in approximately the same position.
While air is rich in ambient nitrogen and nitrogen molecules, and these molecules would be excited as a result of interactions with the plasma-generated Ar metastable species and photons (as evident from the optical emission spectrum of Ar plasma in air), and then carried towards the surface by the jet, the very short lifetime of these species means that the excited and ionized nitrogen species are unlikely to reach and interact with the polymer coating surface to a significant extent. For example, for coatings fabricated at 15 W, the argon plasma treatment resulted in an increase in the atomic fraction of oxygen, from 26.0 to 29.6 %. Deconvolution of the O 1s spectrum showed oxygen was in two bonding states, as *O--C=O (at ~531 eV) and C--O/C=O (at ~530 eV). When compared to the unmodified plasma polymer coating, the relative fraction of *O--C=O was greater, whereas the fraction of C--O/C=O was lower, at ~20 vs. 13%, and 80 vs. 86.6, respectively (Figure 3
, Table 2
). Deconvolution of the C 1s spectrum showed four peaks, corresponding to C-C/C-H (at 285 eV), C-O (at ~286.5 eV), C=O (at ~288 eV) and O-C=O (at ~289 eV). The relative fraction of C-C/C-H was lower in the treated polymer coating when compared to its untreated counterpart, at 58 vs. 55%, respectively, whereas the relative fraction of states corresponding to carbon bonded to oxygen increased. It should be noted the noise in the spectra may conceal low levels of nitrogen that would be present on all plasma polymer coatings due to interactions with ambient air.
In addition to chemical modification, it is not unusual for the surfaces exposed to plasma-generated oxygen species and photons to experience material removal (e.g.,
etching) and material redistribution across the surface. AFM examination of the surfaces revealed that while the RMS roughness of the samples increased, particularly when examining images with a scanning size of 1 µm × 1 µm, the increase was not significant. For example, for samples fabricated at 15 W (Table 2
), the RMS roughness increased from ~6.0 to ~7.3 nm (for 1 µm × 1 µm) and from ~5.2 to 5.5 nm (10 µm × 10 µm). The maximum peak height also increased slightly for 1 µm × 1 µm scans, decreasing substantially from ~54.8 to ~34.3 nm when surfaces were visualised using the scanning size of 10 µm × 10 µm. It should be noted that just as water (or any other polar solvent) can be used to remove low molecular weight oxidized materials that remain on the surface after the coating is first exposed to the ambient air post-synthesis, these low molecular weight fragments can be displaced by exposing the surface to heat or the stream of the plasma jet. Thus, it is possible that the removal of these loose molecular fragments is responsible for the reduction in the maximum peak height at that scale. The maximum depth of pits and valleys (Rmin
) did not change significantly, from ~18.2 nm to 22.0 nm (for 1 µm × 1 µm) and from ~20.3 nm to 18.7 nm (for 10 µm × 10 µm), confirming that the flow removal of loosely attached fragments and not necessarily etching was the primary mechanism behind the changes in surface morphology.
It is also possible that the initial removal of low molecular weight fragments is followed by the formation of a similar type of molecular fragments (as the polymer chains are attacked by the plasma-generated oxygen species). Nodule-like agglomerates of similar low molecular weight fragments have been shown to appear on the surfaces of polyethylene terephthalate, polyethylene and polypropylene treated with a similar atmospheric pressure plasma jet operated at 10 kV [29
]. The plasma generated in this study is less intense and has a lower temperature, which may provide an explanation for the less pronounced surface restructuring observed in our study when compared to the aforementioned example.
Collectively, the changes in the chemistry and topography of the polymer coatings have led to a substantial increase in the hydrophilicity of coatings, with water contact angles changing from > 70° to 56.8° after 5 s of treatment, and to < 40° after 60 s of treatment. It should be noted that the decrease in the contact angle as a function of plasma treatment time is not linear, with the contact angle decreasing rapidly within the first 5–10 s, and then remaining fairly stable after 30 s of treatment. This observation is consistent with surface oxidation being the main mechanism responsible for plasma-surface interactions.
It is possible that substantially extending the treatment time would lead to a more significant change in the morphology and bulk structure of the polymer coating, due to gradual degradation of polymer chains via oxidation further promoted by UV irradiation and mild heating. Prolonged exposure of plasma polymers made of γ-terpinene (one of key constituents of tea tree oil) to UV–A (λ = 350 ± 25 nm), UV–B (λ = 300 ± 25 nm) and UV–C (λ = 254 nm) light for 24, 48 and 672 h in ambient led to significant changes in the chemistry of the polymer, with UV-C inducing significant photochemical degradation and oxidation even under oxygen-poor conditions [30