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Article

Atmospheric Microplasma Treatment Based on Magnetically Controlled Fe–Al Dynamic Platform for Organic and Biomaterials Surface Modification

by
Ivan Shorstkii
* and
Emad Hussein Ali Mounassar
Advanced Technologies and New Materials Laboratory, Kuban State Technological University, 2, Moskovskaya Str., Krasnodar 350072, Russia
*
Author to whom correspondence should be addressed.
Coatings 2023, 13(8), 1362; https://doi.org/10.3390/coatings13081362
Submission received: 14 July 2023 / Revised: 1 August 2023 / Accepted: 2 August 2023 / Published: 3 August 2023
(This article belongs to the Special Issue Powder Composite Surfaces, Functional Coatings and Films)
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Abstract

:
By exploiting the physical effect of the electron emission from a thermionic source in combination with a magnetically controlled Fe–Al dynamic platform to assist electrical discharge, we generated atmospheric microplasma (AM). The electrical characteristics of microplasma discharge-induced cold atmospheric plasma in combination with a magnetically controlled Fe–Al dynamic platform in open air were evaluated. The surface analysis of organic wheat seeds was investigated at two steps: (1) the coating effect of an Al nanoparticle with an electrons drift toward the seed surface along the direction of an electric field and (2) inelastic collision with secondary electrons moving from the cathode in the presence of an electric field. Using SEM microscopy analysis, it was found that plasma affects seed surface topography and apparent contact angle (ACA). The effect of atmospheric microplasma treatment leads to seed surface modification by the manifestation of fine mesh structures on the seed surface. Well-established AM technology will garner interest in agriculture and biomaterials coatings applications.

1. Introduction

Cold atmospheric plasma (CAP) has recently been found to have an effective application in processes for coating deposition and modification as an alternative to traditional chemical, mechanical and thermo-mechanical treatments [1]. Cold atmospheric microplasma assisted by an external source of electrons is regarded as emerging technology, which appears to be a viable alternative to low-pressure plasma treatment for biomaterials surface modification.
Cold plasma application has a number of advantages over well-known types of electrical technologies: uniformity of processing, seed integrity preservation and absence of chemical reagents. As a consequence, the use of plasma technologies in applied surface science represents a transition to environmentally friendly and safe technologies [2,3,4]. The CAP technique was recently applied to stimulate seed germination and plant growth, based on electrical discharges in corona mode [5,6], radio frequency discharge [7], arc discharge [8] and others [9,10].
Recently, several authors described a high efficiency of CAP for seed surface modification and further water uptake [11,12,13,14]. Randeniya et al. [11] applied CAP for wheat seeds. The results demonstrate a slight effect on germination of wheat caryopses. In comparison with CAP-treated samples, the seeds held in vacuum had about 10% higher germination. Burducea et al. [15] applied a helium atmospheric pressure plasma jet to evaluate the effect of cold plasma treatment on the morphology of wheat seeds. Evaluation of germination and plant growth for 10 days highlighted a specific trend for each cultivar. Ling et al. [12] studied the effects of CAP on soybean seedling growth and seed germination. It was found that seedling growth and water uptake parameters were significantly increased. De Groot et al. [13] applied CAP for cotton seed germination improvement. The results demonstrated that CAP treatment enhanced water absorption characteristics up to 25% in cotton seed. Atmospheric cold plasma treatment was studied by Chaple et al. [16]. It was reported that CAP has a great potential to modify the functional properties of foods and grains. Plasma treatment with an electric field of 80 kV enhances the flour hydration properties via structural changes. Los et al. [17] reported the characteristics modification caused by cold plasma. It was reported that water contact angle was significantly modified by CAP treatment after 60 s using a direct exposure method.
One of the possible explanations for the effects of CAP on characteristics modification is that plasma treatment causes structural changes [18,19,20]. Sholtz et al. [9] noted that seed germination is strongly influenced by reactive spikes of plasma exposure. Additionally, Randeniya et al. reported [11] that plasma seed treatment improves wettability. In this way, the main mechanism of cold plasma treatment is based on water absorption improvements via seed surface modification.
Since microplasma flow is operated at atmospheric pressure, the treatment process can be performed in normal conditions [21]. In addition, the microplasma flow temperature is close to room temperature because of the low energetic ions. It therefore make sense to use CAP for temperature-sensitive biological objects [21].
Given the significant advantages of CAP technology, it was interesting to check whether the CAP technique can be employed as a pretreatment method for biomaterials surface modification and to enhance the functional properties of a surface.
However, most of the reported research is based on surface modification of CAP technology. Hence, the present work was focused on internal structure modification as well as surface modification of wheat seed.

2. Materials and Methods

2.1. Materials and Samples Preparation

A hulled wheat (Triticum aestivum L.) was used as an object in this study. The initial moisture content of the grain samples was 10.1 ± 0.2%, measured by oven method [22]. The equivalent diameter of a single wheat kernel ranged between 4.07 and 4.23 mm.
Wheat seeds were treated by atmospheric microplasma according to a suggested scheme (Figure 1). Wheat samples were separated into two groups: untreated samples and samples with electrical treatment (protocol A: CAP with I = 5 mA, protocol B: CAP with I = 10 mA and protocol C: streamer discharge) with further water apparent contact angle, water absorption and surface analysis.

2.2. Experimental Setup

The experimental setup of cold atmospheric plasma assisted by thermionic emission is shown in Figure 2. Positive high-voltage pulses were generated by a system consisting of an Agilent 33220A function generator (Agilent Technologies, Santa Clara, CA, USA) and a high-voltage amplifier: a Matsusada 20-B-20 (Matsusada Precision Inc, Otsu, Japan). The amplifier maximum output voltage was 20 kV. Atmospheric microplasma generation was carried out with the support of a thermionic emission source in the form of a filament (active resistance value Ra = 2 ohms, V = 1.0 V). Amplitude, pulse duration and shape characteristics of the current and discharge voltage were monitored by a digital oscilloscope (Tektronix TDS 220) via the monitor output for current and voltage on a high-voltage amplifier. The pulse duration of the supplied pulse to the high-voltage amplifier was set at 100 microseconds. The characteristics of the supplied pulses were selected based on the conditions of gentle wheat seed microplasma treatment, which occurs at high values of current density.
A single wheat seed was set in a dielectric cell between electrodes without direct contact with the electrodes. Each sample was treated for 5 s. The difference in temperature between the CAP-treated and control wheat seeds was less than 2 °C (on the surface), which has been measured using an infrared thermometer. The electrode gap was set to ≈10 mm. The microplasma flow channel was perpendicular to the seed surface as shown in Figure 2.
To avoid the mechanosensitive ion channels formation during microplasma treatment, a multipoint anode was used. Fe3O4 microparticles with an average diameter of 60 microns were deposited on the anode surface in accordance with the technology [23].

2.3. CAP Fundamental Aspects

Electrons play a major and fundamental role in biological object surface modification. In the current study, a low-current cold plasma was applied for seed grain treatment. The typical characteristics of a wide range of plasma types is presented in Figure 3.
Authors will not analyze the curve in detail but the area selected and highlighted in blue color corresponds to the atmospheric discharges for the experiments. This kind of plasma assisted by thermionic emission is more relevant for grains, when it comes to large capacities of industrial processing. Such CAPs are characterized by very low current densities. The discharge is sustained by the emission of electrons via ion bombardment of the cathode. Thermionic emission sources can be found in SEM technologies, spectral technique and other fields of coating technologies [24,25].
However, it is still unclear whether mass transfer is driven by current density or by electric field strength. Moreover, the effect of electrode configuration on charge density and mass transfer has never been studied.

2.4. Water Uptake

A water bath (Grant OLS 200, Wiltshire, UK) was used to keep treated seed samples after atmospheric microplasma treatment at 20 °C. The water absorption kinetic curves were obtained by weight measurement each 5 min until 2 h. For each measurement, the hydrated samples were removed and placed on 2 layers of paper towels to remove the surface water by gently rolling the grains on the towel. After weight measurement, the samples were returned to the beaker.

2.5. Characterizations

A scanning electron microscope EVO HD 15 (Zeiss, Cambridge, UK) was used to observe the morphology features and surface changes after microplasma treatment. SEM was operated at two acceleration voltages: 10.0 kV and 15.0 kV.
Water apparent contact angles θ were applied to characterize the changes in surface properties. The lying drop method [26] was applied using a surface microscope. The values of the wetting angles were determined using the DropSnake—LB-ADSA software package.
To analyze the generation of reactive species (RS) during non-thermal plasma treatment, optical emission spectroscopy (OES) was performed to determine the relative RS levels using optical fiber located at 7 cm from the plasma area with a 10 s integration time and a CCD spectrometer (VISION2GO). The wavelength range of 200–1000 nm and spectral accuracy of 1 nm was set for CCD.

3. Results and Discussion

3.1. Atmospheric Microplasma Characteristics

Electrons flow starting from the cathode was accelerated by an applied external electric field. Figure 4a presents the experimental results of volt–ampere characteristics for electrode configurations with experimental protocol indication (□) and comparison with CAP mode without TE (∆). The resulting I–V characteristic demonstrates a microplasma discharge mode at low current values. It was obtained that the current values for protocols A, B, and C were 2 mA, 10 mA and 15 mA, respectively.
Figure 4a shows a current–voltage characteristic which demonstrates two sections: a linear I–V dependence, namely, in the pulse-periodic mode of the microplasma (up to protocol A) and in the streamer discharge mode (protocol C). In this case, the intersection of two extrapolated lines is identified with the beginning of the transition of the microplasma discharge mode to the streamer discharge mode [27]. As can be seen, the thermionic source leads to a steeper decrease in current with voltage in the microplasma mode of atmospheric pressure compared to the discharge mode without a thermionic emission source [28]. Finally, the applied voltage value was significantly decreased down to E ≈ 3–5 kV/cm.
Atmospheric microplasma flow on a seed surface in the configuration of “multipoint cathode–plate anode”, in accordance with protocol B, is shown in Figure 4b. A thin filamentary channel of a bright white glow is formed in the interelectrode gap when a high voltage pulse is applied to the anode. Visual observations of the filamentary plasma confirm the one-sided flow around the seed sample.
Figure 4c shows an oscillogram for Protocol B, demonstrating the current and voltage curves of a laboratory installation at atmospheric air pressure. The current waveform clearly shows the delay in increasing the electronic current relative to the moment of voltage supply to be 80 μs. Then, after 80 μs, the current reaches the level of 10 mA in the form of a bright filament (Figure 4b). The duration of the microplasma current significantly depends on the applied voltage. In particular, the duration is 520 MHz at U = 13.5 kV for Protocol A and 650 MHz at U = 14.9 kV for Protocol B. The electrical power over one pulse, estimated from the area of the oscillogram (Figure 4c), for protocols A, B and C was around 8 W, 40 W and 60 W, respectively. The corresponding energy of each pulse for protocols A, B and C was up to 16 mJ, 80 mJ and 120 mJ, respectively. The difference in temperature between the CAP-treated and control wheat seeds was less than 2 °C (on the surface), which has been measured using an infrared thermometer.
The used plasma source (OES) could reveal the factors contributing to the microstructural changes of the surfaces of the treated wheat seeds. A minor peak of nitric oxide radicals (NO▪) was detected at 234.67 nm and shown in Figure 5. Meanwhile, some peaks of excited N2 were observed in the range of 337–380 nm. The emission peak of hydroxyl radicals (OH▪) was also detected at 310 nm. The physical and chemical interactions between these active species and the surface of the wheat seed reduced the strength of the cell wall network and had an etching effect on the surface structure.

3.2. Surface Modification

Figure 6 shows the characterization of the properties of the CAP-treated wheat seeds’ surfaces in comparison with the control sample without CAP treatment.
The initial surface of wheat seeds is characterized by relatively high values of wetting angle, θ = 106° (water-based), and low surface energy γ. As a result of cold atmospheric plasma wheat seed treatment, the surface becomes more hydrophilic and is characterized by low wetting angles (θ = 58°).
The analysis of the topography of the seed surface before and after CAP treatment is shown in Figure 7. The shell of the wheat seeds in the control group (Figure 7a) is a fairly flat and smooth surface. As a result of the impact of CAP treatment on the seed shell (Figure 7b), structural changes were observed on the surface of seeds, consisting of the manifestation of a fine meshed structure on the surface of the seeds with sharply defined cell boundaries. With a further increase in the duration of exposure or the energy of the discharge, the effect of etching on the surface of the seeds increased.
Wheat seeds are an extremely complex biological object, and the effect of microplasma treatment can manifest itself through several channels: by modifying the surface layer of seeds; due to reactions involving electrons, ions and active radicals; ultraviolet plasma radiation, etc. [8]. On the other hand, plasma treatment parameters, such as plasma properties, energy and working gas composition, also have a significant impact on the reaction of seeds when exposed to plasma [29,30,31,32]. One of the most important factors of plasma treatment is the energy of the microplasma. Thus, with low-energy exposure to plasma, a slight effect on the modification of the surface of the seeds is observed; however, at the same time, a significant increase in the exposure energy has a negative effect on the seeds [11,33]. Another important factor to take into account with plasma seed treatment is the type of seed, since seeds of various plant species may react differently to plasma treatment.
Several studies have noted that CAP induces changes on the surface of seeds [29,30,31]. Sera et al. [30] reported significant surface changes as a result of CAP treatment of the surface of wheat and oat seeds. Microphotographs obtained from SEM demonstrated that the surface of the seeds had an etched character after cold plasma treatment. The same results were obtained during CAP treatment of rice seeds [6]. Surface properties modification after CAP treatment can potentially enhance the water transport through the seed structure [29,30,32]. In [32], it was found that an increase in the amount of water in the seeds of cumin plants (compared with control samples) occurs after plasma treatment.

3.3. Influence of CAP Treatment on Water Uptake

Figure 8 shows the water uptake curves for wheat grains of untreated (control) and CAP-treated samples. During the first 20 s of immersion, the water uptake curve demonstrates a positive linear behavior for CAP-treated samples. In general, the water uptake for samples treated with CAP increased to a value of 31.1 db% for protocol A. As can be seen in Figure 8, the water uptake by wheat grains increased in the order Protocol B> Protocol A > Protocol C. Wheat grain samples treated by protocol B show maximum water absorption.
The findings suggest an enhanced water uptake parameter of wheat seeds treated by cold atmospheric microplasma. Higher and more intensive water uptake has been shown to correlate positively with increased CAP treatment energy. In the case of the 10 mA CAP application mode, the most marked difference was visible after the first 20 s, when the seeds absorbed 25.1% more water compared to the control (Figure 8). In contrast, a higher energy level (>10 mA) causes a decrease in the dynamics of water uptake in microplasma-treated seeds and seed damage due to the joule heat. These findings suggest that CAP-treated seeds reached water saturation significantly faster compared to untreated seeds. For comparison, the well-known data of a low-temperature plasma treatment indicate increased water uptake of up to 23.3% after the first 2 h [19]. The effects of cold plasma treatment on soybean seed demonstrated a slight improvement of 14.03% [12].

4. Discussion

The current study on the effect of atmospheric microplasma treatment on wheat seed surface modification led to both similar and different conclusions to that of other authors’ results that reported on cold plasma application on seed modification. As a feedstock of this research, an authors-based methodology of microplasma generation assisted by thermionic emission was used. A brief theoretical part of the microplasma flow explanation was provided. For characterization, standard methods such as SEM analysis, water contact angle, and the effect of different CAP treatment protocols on the water uptake of wheat seeds were also investigated. Wheat seed surface modifications were observed under non-thermal pulsed filamentary atmospheric microplasma treatment at a medium energy level, an effect that is enhanced by the increasing discharge current value of plasma. The results showed an etching effect on a seed surface treated by CAP, and it was therefore supposed that a microplasma flow passes through the seed structure and forms an additional continuum (channel), which contributes to a more efficient and uniform process of moisture transfer. Such results corresponded to the results of Baldanov et al. [10] and Li et al. [12] for wheat seeds treated by glow discharge at atmospheric pressure.
Burduceaet et al. [15] also observed a similar etching of the wheat seed after exposure to plasma for 5 min. In another study, the SEM images of the surface of grain seeds showed that micro-fissures were formed at points where active plasma particles collided with the surface [10]. As mentioned by Burduceaet et al. [15], the importance of OES is its usage in the estimation of gas temperature and the vibrational temperature. The obtained plasma OES results can provide the energetic species information that will be used during the wheat seed treatment procedure.
In this research, a fast water uptake at the initial stage was observed for the atmospheric microplasma-treated seeds. After 20 s, water uptake was around 18%, compared to 2% for the untreated sample. Li et al. [12], who modified soybean seeds by cold plasma, show significant structure modification and water uptake results. In addition, de Groot et al. [13], who applied cold plasma to cotton seeds, achieved a significant increase in water absorption and warm germination improvement after 27 min of cold plasma treatment.
It is also possible that microplasma flow is involved in the non-preferential disruption of crystallites, resulting in cracks and channels in a wheat kernel structure, as reported in one previous study on wheat seed modification [10]. The appearance of the cracks and the spaces between granules were observed by scanning electron microscopy and reported by Yodpitak S. et al. [33] for rice grains. From the author’s point of view, in comparison with rice, which has a solid oval structure at its cross-section, wheat grains could demonstrate different behavior. The cross-section of a wheat kernel has a crease [34], which might decrease the stress factor during microplasma flow penetration.
Based on the findings, it is also supposed that the significant modifications and disruptions of wheat seed surfaces are directly related to faster imbibition processes.
Furthermore, an additional force of moisture movement can be based on the effect of electroosmosis, in which moisture saturated with ions under the action of electrotechnicians moves to the corresponding electrode [35]. After treatment, significant changes in the surface structure and coating of the wheat seeds increased the absorption and permeability of water in the seed. Since the hydrophilicity of the seeds was improved by atmospheric treatment with microplasma, the ability of the seeds or plants to absorb water and nutrition was relatively improved, leading to better growth, corresponding to the results of [16]. Several authors [15,36] also report the possibility of surface wetting for rice, bean and wheat seeds by application of cold plasma treatment. The change in the plant seeds’ wetting properties is due, at least in part, to the oxidation of their surface under plasma treatment. Secondarily, the change in wettability enhances the plant seeds’ water absorption (soaking) procedure.

5. Conclusions

Application of atmospheric microplasma assisted by thermionic emission can be used for seed surface modification and to facilitate moisture uptake in wheat grains. The increase in applied energy can lead to significant enhancement in water uptake; however, excessive energy, resulting in a streamer discharge formation, does not necessarily result in further improvements. Additionally, streamer discharge application caused thermal degradation of the wheat seed. Atmospheric microplasma treatment demonstrated an effect on seed surface modification, imprinting a fine mesh structure on the seed surface. This may be due to the additional continuum (channel) formation, which contributes to a more efficient and uniform process of moisture transfer.
Knowledge of reactive species formed and changes in seed surface structure is necessary to determine the exact reactions that result in CAP-induced changes in wettability and water absorption characteristics. This information will make it easier to adjust the plasma conditions to achieve the desired outcome.
Due to its advantages (homogeneous processing, no seed destruction and no chemical requirements), microplasma treatment can become an effective alternative to traditional coating deposition and modification technologies currently used in biofilm industries and agriculture.

6. Patents

Atmospheric microplasma generation was achieved according to the patent (RU #2727915).

Author Contributions

Conceptualization, I.S.; methodology, I.S.; software, E.H.A.M.; validation, E.H.A.M.; writing—original draft preparation, I.S and E.H.A.M.; writing—review and editing, I.S and E.H.A.M.; visualization, E.H.A.M.; supervision, I.S.; project administration, I.S.; funding acquisition, I.S. All authors have read and agreed to the published version of the manuscript.

Funding

The research was carried out with the financial support of the Kuban Science Foundation in the framework of the scientific project No. MFI-20.1/42.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data that support the finding of this study are available from the corresponding author upon reasonable request.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Experimental scheme for CAP treatment of wheat seeds.
Figure 1. Experimental scheme for CAP treatment of wheat seeds.
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Figure 2. Experimental setup scheme.
Figure 2. Experimental setup scheme.
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Figure 3. Typical current density-voltage curves with indication of cold atmospheric plasma assisted by thermionic emission for our experiments.
Figure 3. Typical current density-voltage curves with indication of cold atmospheric plasma assisted by thermionic emission for our experiments.
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Figure 4. I–V characteristics for protocol 1–3 at an interelectrode gap of 10 mm (a), CAP treatment procedure visualization (b), voltage (line-1) and current (line-2) oscillogram for air as the working gas: 4 kV/cell; 10 mA/cell; 100 μs/cell (c).
Figure 4. I–V characteristics for protocol 1–3 at an interelectrode gap of 10 mm (a), CAP treatment procedure visualization (b), voltage (line-1) and current (line-2) oscillogram for air as the working gas: 4 kV/cell; 10 mA/cell; 100 μs/cell (c).
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Figure 5. The optical emission spectrum of the cold atmospheric plasma. NO▪ (nitric oxide), OH▪ (hydroxyl radical), N2 (molecular nitrogen), N2+ (nitrogen cation), H2 (molecular hydrogen) and O (atomic oxygen).
Figure 5. The optical emission spectrum of the cold atmospheric plasma. NO▪ (nitric oxide), OH▪ (hydroxyl radical), N2 (molecular nitrogen), N2+ (nitrogen cation), H2 (molecular hydrogen) and O (atomic oxygen).
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Figure 6. Water droplets deposited on the wheat seed surface: (a)—control; (b)—after CAP treatment (t = 60 s).
Figure 6. Water droplets deposited on the wheat seed surface: (a)—control; (b)—after CAP treatment (t = 60 s).
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Figure 7. SEM photographs of wheat seed surface in the CAP treatment zone. (a) Control seed, without CAP treatment, (b) seed treated by CAP (protocol B), (c) seed treated in streamer discharge mode (protocol C). The selected area in (b) indicates cracks in the seed surface.
Figure 7. SEM photographs of wheat seed surface in the CAP treatment zone. (a) Control seed, without CAP treatment, (b) seed treated by CAP (protocol B), (c) seed treated in streamer discharge mode (protocol C). The selected area in (b) indicates cracks in the seed surface.
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Figure 8. Wheat grains moisture content during hydration at different CAP treatment protocols.
Figure 8. Wheat grains moisture content during hydration at different CAP treatment protocols.
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MDPI and ACS Style

Shorstkii, I.; Mounassar, E.H.A. Atmospheric Microplasma Treatment Based on Magnetically Controlled Fe–Al Dynamic Platform for Organic and Biomaterials Surface Modification. Coatings 2023, 13, 1362. https://doi.org/10.3390/coatings13081362

AMA Style

Shorstkii I, Mounassar EHA. Atmospheric Microplasma Treatment Based on Magnetically Controlled Fe–Al Dynamic Platform for Organic and Biomaterials Surface Modification. Coatings. 2023; 13(8):1362. https://doi.org/10.3390/coatings13081362

Chicago/Turabian Style

Shorstkii, Ivan, and Emad Hussein Ali Mounassar. 2023. "Atmospheric Microplasma Treatment Based on Magnetically Controlled Fe–Al Dynamic Platform for Organic and Biomaterials Surface Modification" Coatings 13, no. 8: 1362. https://doi.org/10.3390/coatings13081362

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