Atmospheric Pressure Plasma Jet Treatment of Human Hair Fibers

3.1 Virgin Hair

The FE-SEM images of virgin hair, virgin hair treated with atmospheric pressure helium, helium/oxygen, argon, and argon/oxygen plasma jets for 10 min are presented in Fig. 1. For comparison, virgin hair treated with basic hydrogen peroxide solution is also presented. The virgin hair (Fig. 1a) has the overlapping cuticle structure with organic residues on and in between the layers of the cuticle. The organic residues are most likely 18-MEA and any other matter gathered from the environment (scalp) and hair care products.

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Treatment of virgin hair with helium plasma jet for 10 min (Fig. 1b) removed the residues on the cuticles. After the treatment, only the residues in between the layers remained since the plasma jet could not reach these locations. The surface morphology of fiber remained unchanged. Introduction of oxygen into the helium plasma (Fig. 1c) not only cleaned the surface but also created a slightly rough surface covering the entire epicuticle. This new formation is most likely due to the oxidation of the organic/inorganic matter on the surface. The argon plasma jet treatment of the virgin hair surface (Fig. 1d) leaved a clean surface without organic residues; however, there was minor surface damage in terms of discoloration and pitting. The lack of organic coating on the surface is also deduced from the quality of the SEM image recorded under the same conditions as others. Argon plasma seems to have a greater cleaning effect than helium; even the spaces between the layers were cleaned and crisp SEM images could be taken. Mixing oxygen with argon (Fig. 1e) removed the residual organics effectively and high-quality SEM images could be taken. However, it also generated small humps all over the surface causing increased roughness. This effect is much more significant compared to the He/O2 plasma jet case. The virgin hair sample treated with basic hydrogen peroxide (Fig. 1f) on the other hand did not show signs of severe cleaning or etching on the surface. The residual matter on and in between the layers was still highly visible.

The detailed XPS analysis of plasma jet-treated virgin hair is presented in Fig. 2. The bottom spectra belong to untreated virgin hair. The surface carbon content is about 80 % mostly due to the aliphatic carbon from 18-MEA registered at a binding energy of 285.0 eV. A weak shoulder on the higher binding energy side in the C 1 s region reveals the presence of carbonyl and carboxyl species. These species, and the nitrogen-, oxygen-, and sulfur-containing moieties, are due to the protein layers under the 18-MEA layer. This is a valid assumption since XPS is sensitive to the top 3–5 nm of the surface due to the inelastic electron mean free path [29], and the length of a C18 carbon chain is about 2.1 nm [30]. The symmetric N 1 s peak, centered at 400.3 eV, is indicating the presence of amine groups due to the amino acids present in the epicuticle layer [29]. The nitrogen content is about 3.5 %. The O 1 s peak is positioned at 532.4 eV for carboxyl moiety of the peptides with an abundance of 13.3 %. The S 2p region shows the presence of two different sulfur moieties; the organic sulfur, i.e., S–C or S–S, at 163.9 eV from the cystine groups and an oxidized sulfur moiety at 168.7 eV [29]. The sulfur peak at 168.7 eV is assigned to the cysteic acid (–SO3 −) group, which arises due to the stripping of the 18-MEA layer [19]. This reveals that the virgin hair fiber has regions where the 18-MEA layer is already stripped. The intensity of the oxidized sulfur moiety is higher than the organic sulfur groups. However, the unexpected intensity pattern can be explained by the attenuation of the photoelectrons from the organic sulfur moieties by the long carbon chain lipid layer, since the virgin hair is expected to have extensive 18-MEA coverage. The amount of total surface sulfur is low, ca. 0.8 %, which yields a non-stoichiometric C/S ratio for pure 18-MEA layer. This observation also supports the fact that the observed XPS spectra have contributions from the underlying protein layer. Finally, Si 2p region reveals the presence of 2.3 % silicon due to the use of hair care products. The binding energy of 102.3 eV confirms the siloxane structure [29].

Figure 2 also shows the XPS results of virgin hair treated with helium, helium/oxygen, argon, and argon/oxygen plasma jets for 10 min as well as hair treated in basic hydrogen peroxide for 10 min. For the hair treated with helium plasma jet, the clear increase in the intensity of the high-binding energy side of C 1 s reveals the oxidation of surface carbon species. At the same time, the carbon amount decreased to 60.5 % which is consistent with the SEM observations. The increase in the oxidized carbon species was also accompanied with the increase in the oxygen content of the surface from 13.3 to 26.2 %. This increase is most likely due to the reaction between oxygen and/or water vapor with the radical groups on the surface generated during the plasma jet treatment. At the same time, the nitrogen amount increased slightly to 6.6 % and oxidized nitrogen species arose in the higher binding energy side of the N 1 s spectrum. A significant change in the S 2p spectrum was observed; the oxidized sulfur amount increased several folds and the total sulfur content increased to 2.7 %. The silicon 2p peak showed a slight shift in the binding energy to 103.2 eV indicating the partial oxidation of the silicon species. The amount of silicon also increased to 4.0 %. One can argue that upon removal of the organic residue, nitrogen, sulfur, and silicon species are more exposed on the surface and slight increases in the quantities were detected. For these species, the oxidation of moieties is also evident due to the increasing intensity of peaks on the high binding energy side. However, alternative explanations are also possible for the increased quantities. It is well known that nitrogen in air can be excited and ionized by the metastable helium atoms present in the helium plasma jet [31, 32]. Therefore, nitrogen addition to the organic structure is also a possibility. The increase in the oxidized sulfur moiety signifies that the helium plasma jet application strips the 18-MEA layer, and the cysteic acid groups are now exposed on the top surface.

Introduction of 1 % oxygen into the helium plasma modified the surface similar to the helium plasma jet case. The C 1 s region showed a small increase in the high binding energy side, while the total carbon amount decreased to 61.4 %. The N 1 s peak broadened toward the higher binding energy side, the O 1 s peak slightly shifted to higher binding energy, the intensity of the S 2p peak for cysteic acid groups increased, and the Si 2p peak shifted toward higher binding energy side. Comparable to the helium plasma jet case, the nitrogen, sulfur, and silicon species previously located at the subsurface moved toward the top surface upon the removal of the residual organic layer.

Argon plasma jet treatment of virgin hair was quite effective in cleaning the surface hydrocarbons. Although the amount of carbon (64.4 %) is comparable to previous cases, the SEM images corroborate to the better cleaning performance of argon. The reason of similar C 1 s intensity is the underlying organic content, i.e., cuticle, which is common for all hair samples. In addition to the better cleaning performance, the argon seems to have higher oxidation efficiency as deduced by the nitrogen and oxygen 1 s signals. The oxidized nitrogen species at 402.2 eV gained significant intensity with argon plasma jet treatment compared to other treatments. The increase in the nitrogen content is not as much as the previous cases, i.e., 4.9 %; however, this may be an indication of a mild etching process, as well. The binding energy shift of the O 1 s peak to 532.7 eV also supports the higher oxidation efficiency of the argon plasma jet. The oxygen content at 23.7 % is comparable to previous cases. The S 2p region was dominated by the cysteic acid groups at 168.6 eV with 2.6 % abundance. The organic sulfur content around 164 eV is quite weak indicating a near-complete stripping of the 18-MEA layer. This is probably the reason of much crispier SEM image since charging of surface is less likely to happen when it is almost completely covered with anionic cysteic acid groups. The Si 2p region showed the presence of Si4+ species at 103.3 eV as well as less oxidized moieties with an overall abundance of 4.4 %.

Introduction of 1 % oxygen to argon plasma jet altered the surface composition slightly compared to argon only jet, despite the morphological changes observed in SEM images. The carbon content decreased to one of the lowest values observed, i.e., 60.9 %, with contributions from oxidized carbon species. The nitrogen signal was a little lower than the argon only jet, i.e., 4.1 %, with significantly less contribution of the oxidized nitrogen species. The O 1 s peak was shifted to slightly higher binding energies and reached the maximum amount, i.e., 25.8 % among all the samples investigated. The sulfur species were almost completely the cysteic acid moieties at 167.7 eV; however, the amount, 1.7 %, was the lowest among all the plasma jet treatment cases. The silicon amount also presented another extreme; the quantity of 7.4 % was the highest among all samples. The majority of silicon species had +4 oxidation state with binding energy at 103.3 eV in addition to the less oxidized moieties at lower binding energy values. These results demonstrate the corrosive nature of the argon/oxygen plasma jet. The argon/oxygen jet treatment clearly has started to remove the cysteic acid groups from the surface causing a decrease in the sulfur amount. The increase in the silicon amount is related to the small humps observed with SEM covering the entire surface. The oxidized silicon species (SiO2) bind to the surface and enrich while the organic material is removed, probably in the form of volatile oxides, during the mild etching process.

For comparison, the virgin hair was also treated with 3 % H2O2 solution as described in the experimental part. Peroxide treatment decreased the carbon amount from 80.1 to 68.0 % while generating significant amounts of oxidized carbon species. The nitrogen amount increased significantly to 8.8 % due to the presence of ammonia in the solution. The O 1 s peak, contrary to other cases, was positioned at 531.5 eV and its amount was registered as 18.2 %. The opposite shift in the oxygen position signifies the differences in the chemical/physico-chemical processes involved in removing the 18-MEA layer. This effect is also visible in the unshifted binding energy position of the silicon species. The sulfur 2p region showed the presence of two different sulfur species, organic sulfur at 163.5 eV, and the cysteic acid moieties at 167.7 eV with comparable intensity. The total amount of surface sulfur was greater than that of virgin hair. Contrary to the deduction from the SEM images, the H2O2 treatment did generate chemical changes on the surface by removing the 18-MEA layer partially. However, the extent of this chemical change was not as significant as plasma jet treatment, nor it shared the same chemical processes.

3.2 Dyed Hair

The FE-SEM images of the dyed hair, dyed hair treated with helium, helium/oxygen, argon, and argon/oxygen plasma jets and H2O2 solution are presented in Fig. 3. The dyed hair surface looks quite different than the virgin hair (Fig. 3a). A thick layer coats the epicuticle layer and fills the gaps between the layers. Residual organic matter, most likely from the scalp lipids, is visible on the step edges of the cuticle layers and on the sides of the image. Helium and argon plasma jet treatments were successful in removing the organic residue, but they did not induce any noticeable morphology changes on the surface (Figs. 3b and d, respectively). Introduction of oxygen into the helium plasma jet not only cleaned the organic matter, but also generated pitting corrosion on the coating (dye) layer as indicated with arrows in Fig. 3c. Argon/oxygen plasma jet treatment (Fig. 3e), on the other hand, lead to an overall coverage of the surface with small lumps, most likely oxidation products. These lumps increased the surface roughness of the fiber significantly. Hydrogen peroxide treatment of dyed hair (Fig. 3f) did not induce noticeable morphology change within 10 min.

The XPS analysis of the dyed hair in “as is”, plasma jet- and hydrogen peroxide-treated forms are presented in Fig. 4. The dyed “as is” hair has C 1 s peak at 285.0 eV with a small shoulder on the high binding energy side indicating the small amount of carbonyl and carboxyl species on the surface. The surface carbon amount was 61.5 %. The N 1 s region showed the presence of 3.6 % nitrogen in the form of amine and nitro groups at 400.1 and 402.8 eV, respectively. The surface oxygen amounted to 21.9 % with a binding energy of 532.5 eV. The sulfur 2p region shows the presence of two different kinds: the organic sulfur at 164.0 eV and the oxidized sulfur at 168.0 eV. The total amount of sulfur was 1.1 % with 2/3 of them in oxidized form. The amount of silicon was quite high due to the dye, 11.9 %, and the binding energy of 102.2 eV indicated the presence of siloxane species.

Helium plasma jet treatment of the dyed hair fiber created minor changes on the surface chemistry. The C 1 s region seems to have lower intensity around 287 eV, i.e., carbonyl region, whereas the intensity around 289 eV, i.e., carboxyl region, increased. The oxidation of surface carbon was also accompanied by the increase in the oxygen amount to 25.0 %. The helium plasma jet treatment diminished the nitrogen and sulfur amounts and left the surface with the oxidized moieties of these elements. The silicon amount remained essentially the same; however, the oxidation of the silicon species with helium jet treatment was evident by the shift in the Si 2p binding energy to 103.2 eV, i.e. Si4+ form. These chemical changes could be attributed to effective cleaning of the helium plasma jet with very mild surface erosion.

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He/O2 plasma jet treatment of the surface led to slightly less carbon amount (57.8 %), while the higher binding energy side of the C 1 s spectrum also slightly lost intensity. The nitrogen, sulfur, and silicon amounts were slightly higher than helium only plasma jet, and all shifted to higher binding energy values indicating the oxidation potential of the plasma jet. The slight decrease in the carbon amount and the slight increase of the other elements in the surface composition can be attributed to the light corrosive effect of the plasma jet. Since the SEM results also showed the minor pitting erosion, one can argue that these small changes in the elemental composition were due to exposure of the subsurface.

The argon plasma jet treatment had a discernable oxidation effect on the surface; it generated oxidized carbon, nitrogen, sulfur, and silicon species. The total carbon amount decreased to 53.2 % while the carboxyl species increased. Nitrogen amount increased to 6.3 % with the nitro groups (402.5 eV) as intense as the amine groups (400.1 eV). A similar trend was also generated with the sulfur content; the amount increased to 2.9 % and was dominated by the oxidized sulfur species, i.e., cysteic acid moieties. The oxygen amount increased to 29.2 % and the peak broadened in parallel to the multitude of the oxidized species on the surface. Due to the increase in other species, a decrease to 8.4 % in the silicon amount was detected. The silicon oxidation state was determined as +4. The increases in the nitrogen and sulfur species with respect to the “as is dyed fiber” stems from the slight corrosion effect of the argon jet plasma such that the subsurface species, i.e., protein cysteine moieties, can be exposed afterward.

Introduction of oxygen to the argon plasma on the other hand significantly altered the surface chemistry as well as the morphology. While the total carbon amount decreased to 41.0 %, a significant intensity was registered for the oxidized carbonyl and carboxyl species. The nitrogen content increased to 9.1 % comparable intensity in the amine and nitro groups present. The oxygen amount shot up to 36.0 % and the surface sulfur amount increased to the highest value of 3.6 % in the oxidized form. The silicon was converted to Si4+ with a binding energy of 103.4 eV and the amount was 10.3 %. The structures observed in the SEM image are most likely the silicon oxidation products bound to the surface. However, the enrichment effect was not as significant as the He/O2 plasma jet case due to already high silicon amount and thick overcoat. The Ar/O2 plasma jet has a significant oxidation effect on the dyed hair as well as a moderate corrosive effect.

The hydrogen peroxide treatment of dyed hair resulted in the selective cleaning of the oxidized carbon species. The carbon amount remained essentially unchanged with respect to the “as is” dyed hair, while the amount of oxidized carbon decreased. Slight decreases in the nitrogen, oxygen, and sulfur amount were detected. The silicon amount increases slightly with respect to the “as is” dyed hair, which is due to the overall decrease in the other species. However, the oxidation state of silicon, as deduced from the unchanged binding energy, remained the same. Therefore, the hydrogen peroxide treatment on the dyed hair could not induce as significant chemical and morphological changes as the treatment of the virgin hair with the used formulation and duration.

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