*Review* **Insight on Solution Plasma in Aqueous Solution and Their Application in Modification of Chitin and Chitosan**

**Chayanaphat Chokradjaroen <sup>1</sup> , Jiangqi Niu <sup>1</sup> , Gasidit Panomsuwan <sup>2</sup> and Nagahiro Saito 1,3,4,5,\***


**Abstract:** Sustainability and environmental concerns have persuaded researchers to explore renewable materials, such as nature-derived polysaccharides, and add value by changing chemical structures with the aim to possess specific properties, like biological properties. Meanwhile, finding methods and strategies that can lower hazardous chemicals, simplify production steps, reduce time consumption, and acquire high-purified products is an important task that requires attention. To break through these issues, electrical discharging in aqueous solutions at atmospheric pressure and room temperature, referred to as the "solution plasma process", has been introduced as a novel process for modification of nature-derived polysaccharides like chitin and chitosan. This review reveals insight into the electrical discharge in aqueous solutions and scientific progress on their application in a modification of chitin and chitosan, including degradation and deacetylation. The influencing parameters in the plasma process are intensively explained in order to provide a guideline for the modification of not only chitin and chitosan but also other nature-derived polysaccharides, aiming to address economic aspects and environmental concerns.

**Keywords:** solution plasma process; aqueous solutions; chitin; chitosan; degradation; deacetylation

## **1. Introduction**

In physics and chemistry, plasma is fundamentally defined as one of the four states of matter. Solid, liquid, and gas states are more common on the earth due to the atmospheric condition, whereas the plasma state dominantly exists in the universe (>99%) (e.g., the sun, nebulae, etc.) [1]. Nevertheless, plasma can be artificially formed in the earth by giving sufficient thermal or electric energy supply, which ionizes the neutral gases to a quasi-neutral ionized gas [2]. The ionized gas is composed of negative and positive ions, free electrons, excited molecules, and excited atoms and molecules, as well as the emission of ultraviolet (UV) and high electric field [3]. Artificial plasma is classified as (i) thermal equilibrium plasma and (ii) non-thermal equilibrium plasma [4,5]. Thermal equilibrium plasma can be generated by a strong electrical power and, usually, under gas pressure of more than 5 kPa. Its gas and electron temperatures are nearly equal. On the other hand, in non-thermal equilibrium plasma, the gas temperature is lower than the electron temperature [4]. Non-thermal equilibrium plasma can be induced by giving sufficient energy under a vacuum system; however, it can also be generated under an atmospheric pressure environment by applying transient electrical or electrostatic discharges. Consequently, the non-thermal equilibrium plasma has offered several potential

**Citation:** Chokradjaroen, C.; Niu, J.; Panomsuwan, G.; Saito, N. Insight on Solution Plasma in Aqueous Solution and Their Application in Modification of Chitin and Chitosan. *Int. J. Mol. Sci.* **2021**, *22*, 4308. https://doi.org/ 10.3390/ijms22094308

Academic Editor: Akikazu Sakudo

Received: 30 March 2021 Accepted: 17 April 2021 Published: 21 April 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

applications. Artificial plasmas in the gas phase are predominantly investigated under a wide range of operating pressures, including both vacuum and atmospheric conditions, and temperatures. A variety of gases (e.g., Ar, N2, O2, H2, and mixtures) is applied to generate the plasmas depending on the purposes [6–8]. Even though plasma is theoretically mentioned as ionized gases, plasma can be in solid form (i.e., the formation of plasmon which is induced by the collective oscillation of free electrons moving around a lattice point) and liquid form (i.e., the generation of plasma in liquid, such as a pulsed electrical discharge in liquids) [4,9,10]. In the same manner, as the plasmas in the gas phase, plasma can be directly generated in the liquid phase by providing a sufficiently high electric field on the electrodes, leading to the electric breakdown of liquids. Over the past several years, the liquid-phase plasmas have been developed, along with the gas-phase plasmas. The liquid-phase plasmas have been mainly focused as a technology for wastewater treatment and water purification, owing to their ability to induce an effective production of reactive species, for example, hydroxyl radical (•OH), superoxide anion (O<sup>2</sup> – ), and hydrogen peroxide (H2O2) [4,11,12]. These reactive species can strongly oxidize and decompose organic pollutions and bacteria in water [12,13]. Recently, as the plasma technology being developed, the liquid-phase plasmas have been expanded and utilized for the synthesis and modification of various materials, such as noble-metal nanoparticles [13,14], metal oxides [15,16], carbon materials [17–20], and natural polysaccharides [21,22]. Articles on the modification of natural polysaccharides, including chitin, chitosan, cellulose, alginate, mushroom polysaccharide, and starch, by liquid-phase plasma have been rapidly published in the past decade (Figure 1), which can imply the growth of the liquid-phase plasma in this field. plasmas in the gas phase are predominantly investigated under a wide range of operating pressures, including both vacuum and atmospheric conditions, and temperatures. A variety of gases (e.g., Ar, N2, O2, H2, and mixtures) is applied to generate the plasmas depending on the purposes [6–8]. Even though plasma is theoretically mentioned as ionized gases, plasma can be in solid form (i.e., the formation of plasmon which is induced by the collective oscillation of free electrons moving around a lattice point) and liquid form (i.e., the generation of plasma in liquid, such as a pulsed electrical discharge in liquids) [4,9,10]. In the same manner, as the plasmas in the gas phase, plasma can be directly generated in the liquid phase by providing a sufficiently high electric field on the electrodes, leading to the electric breakdown of liquids. Over the past several years, the liquid-phase plasmas have been developed, along with the gas-phase plasmas. The liquid-phase plasmas have been mainly focused as a technology for wastewater treatment and water purification, owing to their ability to induce an effective production of reactive species, for example, hydroxyl radical (•OH), superoxide anion (O2–), and hydrogen peroxide (H2O2) [4,11,12]. These reactive species can strongly oxidize and decompose organic pollutions and bacteria in water [12,13]. Recently, as the plasma technology being developed, the liquid-phase plasmas have been expanded and utilized for the synthesis and modification of various materials, such as noble-metal nanoparticles [13,14], metal oxides [15,16], carbon materials [17–20], and natural polysaccharides [21,22]. Articles on the modification of natural polysaccharides, including chitin, chitosan, cellulose, alginate, mushroom polysaccharide, and starch, by liquid-phase plasma have been rapidly published in the past decade (Figure 1), which can imply the growth of the liquid-phase plasma in this field.

**Figure 1.** The number of articles relating to the modification of chitin and chitosan by liquid-phase plasma (data are shown in Supplementary Materials). **Figure 1.** The number of articles relating to the modification of chitin and chitosan by liquid-phaseplasma (data are shown in Supplementary Materials).

In this review, the insight into the plasmas in the liquid phase, mainly focused on an aqueous solution, is provided. The plasma technology called "solution plasma process (SPP)" will be dominantly explained. The discussion of recent development on the modification of chitin and chitosan, especially degradation, using the SPP will be given as examples. Modification techniques, reaction mechanisms, and changes in the properties of chitin and chitosan will be explained. In comparison with other existing methods, today, the modification of chitin and chitosan by the SPP is still at an early stage of development. The summary of relevant publications from the recent past to the present will provide benefits and a useful guideline for the researchers in the related fields to achieve the ecologically friendly and efficient method for the modification of chitin and chitosan. In addition, the remaining challenge and future trend of SPP technology in the field of not only chitin and chitosan but also other natural polysaccharides are also discussed to motivate future studies. In this review, the insight into the plasmas in the liquid phase, mainly focused on an aqueous solution, is provided. The plasma technology called "solution plasma process (SPP)" will be dominantly explained. The discussion of recent development on the modification of chitin and chitosan, especially degradation, using the SPP will be given as examples. Modification techniques, reaction mechanisms, and changes in the properties of chitin and chitosan will be explained. In comparison with other existing methods, today, the modification of chitin and chitosan by the SPP is still at an early stage of development. The summary of relevant publications from the recent past to the present will provide benefits and a useful guideline for the researchers in the related fields to achieve the ecologically friendly and efficient method for the modification of chitin and chitosan. In addition, the remaining challenge and future trend of SPP technology in the field of not only chitin and chitosan but also other natural polysaccharides are also discussed to motivate future studies.

#### **2. Solution Plasma Process (SPP): Chemistry and Influencing Parameters** Electrode geometric construction and phase patterns of liquid-phase plasmas can be

**2. Solution Plasma Process (SPP): Chemistry and Influencing Parameters**

*Int. J. Mol. Sci.* **2021**, *22*, x FOR PEER REVIEW 3 of 23

Electrode geometric construction and phase patterns of liquid-phase plasmas can be categorized into four groups: (1) direct electrical discharge between two electrodes [23], (2) contact electrical discharge between electrodes and the surface of the surrounding electrolyte, (3) miscible electrical discharge with external gas injection, and (4) special excitation electrical discharge (i.e., radio frequency, microwave irradiation or laser ablation). In this section, we will discuss on the direct discharge between two electrodes under the liquid-phase, which are presented by various terms. For example, submerged liquid plasma (wire-to-plate configuration using direct current) [24,25], pulsed plasma in liquid (rod-to-rod configuration, using pulsed voltage) [23], and solution plasma (pin-to-pin and wire-to-plate configurations, bipolar pulsed voltage) [26,27]. Apparently, these terms are assigned following their experimental setup, electrical power source, and electrodes configuration. Henceforward, the solution plasma process (SPP) with pin-to-pin electrode configuration will be described. SPP with pin-to-pin electrode configuration was firstly proposed by Takai and Saito's group [13,28]. In the SPP, the plasma is directly discharged between a pair of electrodes submerged under liquids at a short distance (0.2–1 mm), depending on its application, as shown in Figure 2. The power supplies are bipolar-pulsed high voltage supply. The use of pulsed voltage could reduce the current of the discharge. To a certain extent, it reduced the possibility of arc discharge due to thermal ion emission from the electrode and enhanced the number of carriers generated by secondary electron emission, thus enhancing the stability of discharge occurrence. Most studies using the SPP focus on the processing conditions and the properties and performance of the obtained products. The physical and chemical reactions occurring in the SPP are complex and rarely reported. Here, a brief explanation of the electrical discharge under an aqueous solution (e.g., electrical breakdown, formation of reactive species, and influencing factors) in the SPP is provided. categorized into four groups: (1) direct electrical discharge between two electrodes [23], (2) contact electrical discharge between electrodes and the surface of the surrounding electrolyte, (3) miscible electrical discharge with external gas injection, and (4) special excitation electrical discharge (i.e., radio frequency, microwave irradiation or laser ablation). In this section, we will discuss on the direct discharge between two electrodes under the liquid-phase, which are presented by various terms. For example, submerged liquid plasma (wire-to-plate configuration using direct current) [24,25], pulsed plasma in liquid (rod-to-rod configuration, using pulsed voltage) [23], and solution plasma (pin-to-pin and wire-to-plate configurations, bipolar pulsed voltage) [26,27]. Apparently, these terms are assigned following their experimental setup, electrical power source, and electrodes configuration. Henceforward, the solution plasma process (SPP) with pin-to-pin electrode configuration will be described. SPP with pin-to-pin electrode configuration was firstly proposed by Takai and Saito's group [13,28]. In the SPP, the plasma is directly discharged between a pair of electrodes submerged under liquids at a short distance (0.2–1 mm), depending on its application, as shown in Figure 2. The power supplies are bipolar-pulsed high voltage supply. The use of pulsed voltage could reduce the current of the discharge. To a certain extent, it reduced the possibility of arc discharge due to thermal ion emission from the electrode and enhanced the number of carriers generated by secondary electron emission, thus enhancing the stability of discharge occurrence. Most studies using the SPP focus on the processing conditions and the properties and performance of the obtained products. The physical and chemical reactions occurring in the SPP are complex and rarely reported. Here, a brief explanation of the electrical discharge under an aqueous solution (e.g., electrical breakdown, formation of reactive species, and influencing factors) in the SPP is provided.

**Figure 2.** Schematic illustration of the SPP with pin-to-pin electrode configuration and its application with different distances between electrodes: (**a**) [14,29,30], (**b**) [31,32], (**c**) [27,33–35], (**d**) [36–38], (**e**) [15,22], and (**f**) [39–42]. **Figure 2.** Schematic illustration of the SPP with pin-to-pin electrode configuration and its application with different distances between electrodes: (**a**) [14,29,30], (**b**) [31,32], (**c**) [27,33–35], (**d**) [36–38], (**e**) [15,22], and (**f**) [39–42].

#### *2.1. Electrical Breakdown*

In chemical physics, the difference between gas and liquid phases is the molecular density. The difference in molecular density causes different insulation to withstand capability, high collision frequency and energy dissipation rate, and low electron mobility

in liquids. Therefore, the plasma chemistry in gas and liquid phases is significantly different. The physical mechanism of gas-phase plasma can be explained based on the electron avalanche [43], which is a process that free electrons in the medium are strongly accelerated by an electric field, resulting in the collision with other atoms or molecules and then ionization. However, the electron avalanche can rarely occur in the liquid mediums because liquids usually have high molecular density, low mobility of charges, and recombination rate. However, as mentioned above, the plasmas in the liquid phase can be carried. The explanation was previously clearly given by Saito et al. in 2008 [44]. The liquid (e.g., aqueous solutions) near the electrode tips turn to gas or the formation of bubbles due to Joule heating, which causes solution vapor and electrolysis, resulting in gases such as H<sup>2</sup> and O2. When the bubble is formed, the electron avalanche is produced in bubbles. The electrical break down is developed inside the bubbles and then formed as the plasma channel, which is kind of like "unzipping". The electrical breakdown can be controlled by the injection of more electrons into the ionization field in the bubble. Heo et al. revealed the current-voltage waveform by low-pass filter circuits, which could reduce noise signals as compared to that of the conventional circuits when the bipolar pulsed voltage was applied [45]. The stages of the applied voltages, breakdown, and plasma generation in the solution plasma process are also proposed in Figure 3. in liquids. Therefore, the plasma chemistry in gas and liquid phases is significantly different. The physical mechanism of gas-phase plasma can be explained based on the electron avalanche [43], which is a process that free electrons in the medium are strongly accelerated by an electric field, resulting in the collision with other atoms or molecules and then ionization. However, the electron avalanche can rarely occur in the liquid mediums because liquids usually have high molecular density, low mobility of charges, and recombination rate. However, as mentioned above, the plasmas in the liquid phase can be carried. The explanation was previously clearly given by Saito et al. in 2008 [44]. The liquid (e.g., aqueous solutions) near the electrode tips turn to gas or the formation of bubbles due to Joule heating, which causes solution vapor and electrolysis, resulting in gases such as H<sup>2</sup> and O2. When the bubble is formed, the electron avalanche is produced in bubbles. The electrical break down is developed inside the bubbles and then formed as the plasma channel, which is kind of like "unzipping". The electrical breakdown can be controlled by the injection of more electrons into the ionization field in the bubble. Heo et al. revealed the current-voltage waveform by low-pass filter circuits, which could reduce noise signals as compared to that of the conventional circuits when the bipolar pulsed voltage was applied [45]. The stages of the applied voltages, breakdown, and plasma generation in the solution plasma process are also proposed in Figure 3.

In chemical physics, the difference between gas and liquid phases is the molecular density. The difference in molecular density causes different insulation to withstand capability, high collision frequency and energy dissipation rate, and low electron mobility

*Int. J. Mol. Sci.* **2021**, *22*, x FOR PEER REVIEW 4 of 23

*2.1. Electrical Breakdown*

**Figure 3.** Schematic diagrams of the applied voltage, breakdown, and plasma generation stages in the SPP with the background of the current and voltage waveforms obtained by low-pass filter circuits [45]. **Figure 3.** Schematic diagrams of the applied voltage, breakdown, and plasma generation stages in the SPP with the background of the current and voltage waveforms obtained by low-pass filter circuits [45].

Besides, the plasma discharges can be altered in different fluids to induce ionization, accelerate ions, and multiply the initial seed of electrons into the electron avalanche. The breakdown depends on the dipole moment of the fluids, including the dielectric behavior, ionization potential, band gap, and dipole moment [4,44]. For example, in water, the electrical discharge requires a large electric field (67–70 MV/m), while benzene requires a much higher energy field for the breakdown due to its higher dielectric strength (163 Besides, the plasma discharges can be altered in different fluids to induce ionization, accelerate ions, and multiply the initial seed of electrons into the electron avalanche. The breakdown depends on the dipole moment of the fluids, including the dielectric behavior, ionization potential, band gap, and dipole moment [4,44]. For example, in water, the electrical discharge requires a large electric field (67–70 MV/m), while benzene requires a much higher energy field for the breakdown due to its higher dielectric strength (163 MV/m).

#### *2.2. Formation of Reactive Species*

MV/m).

*2.2. Formation of Reactive Species* Considering how the SPP can be used to modify chitin and chitosan, the plasmas in aqueous solutions or water plays a key role because almost all modifying processes for Considering how the SPP can be used to modify chitin and chitosan, the plasmas in aqueous solutions or water plays a key role because almost all modifying processes for chitin and chitosan have been reported in aqueous solutions [46–49]. Several reactive species are generated when the electrical discharge is carried in water through molecules collision, mass transfer, vaporization, sputtering, and ultra-violet (UV) [44]. Many previous studies demonstrated that both short-lived reactive species (e.g., hydroxyl radical (•OH) and hydrogen radical (•H), superoxide (O<sup>2</sup> – )) and long-lived reactive species (e.g., hydro-

gen peroxide (H2O2), ozone (O3)) are continuously formed and further reacted during the activation of water by the electrical discharge without the addition of a catalyst or chemical agents [50,51]. Figure 4 reveals the optical emission spectrum (OES) of reactive species generated in water during the SPP, which was measured in our research group. The short-lived reactive species, like •OH and •H, are firstly generated from the main reaction via electron impact dissociation and continuous collision of reactive species to surrounding molecules, as shown below in Equation (1) [52]. The activity of •OH and •H, is found to initiate and prolong modification reactions of chitin and chitosan, such as deacetylation, degradation, and altering the crystal structure, in not only plasma treatment but also in other methods, such as oxidative degradation [53]. Subsequently, hydrogen peroxide (H2O2), the most common long-lived reactive species in the plasma-activated water, is formed via the recombination of •OH, as shown in Equation (12). In addition, in the water, there are dissolved oxygen molecules that can also be excited or ionized to reactive oxygen species (ROS), such as exited atomic oxygen (O(1D) or O<sup>I</sup> ) and triplet ground-state atomic oxygen (O(3P)), singlet oxygen (1O2), and triplet oxygen (3O2). H2O<sup>2</sup> and these ROS also evidently contribute to the formation of •OH [51,52]. activation of water by the electrical discharge without the addition of a catalyst or chemical agents [50,51]. Figure 4 reveals the optical emission spectrum (OES) of reactive species generated in water during the SPP, which was measured in our research group. The shortlived reactive species, like •OH and •H, are firstly generated from the main reaction via electron impact dissociation and continuous collision of reactive species to surrounding molecules, as shown below in Equation (1) [52]. The activity of •OH and •H, is found to initiate and prolong modification reactions of chitin and chitosan, such as deacetylation, degradation, and altering the crystal structure, in not only plasma treatment but also in other methods, such as oxidative degradation [53]. Subsequently, hydrogen peroxide (H2O2), the most common long-lived reactive species in the plasma-activated water, is formed via the recombination of •OH, as shown in Equation (12). In addition, in the water, there are dissolved oxygen molecules that can also be excited or ionized to reactive oxygen species (ROS), such as exited atomic oxygen (O(1D) or OI) and triplet ground-state atomic oxygen (O(3P)), singlet oxygen (1O2), and triplet oxygen (3O2). H2O2 and these ROS also evidently contribute to the formation of •OH [51,52].

chitin and chitosan have been reported in aqueous solutions [46–49]. Several reactive species are generated when the electrical discharge is carried in water through molecules collision, mass transfer, vaporization, sputtering, and ultra-violet (UV) [44]. Many previous studies demonstrated that both short-lived reactive species (e.g., hydroxyl radical (•OH) and hydrogen radical (•H), superoxide (O2–)) and long-lived reactive species (e.g., hydrogen peroxide (H2O2), ozone (O3)) are continuously formed and further reacted during the

*Int. J. Mol. Sci.* **2021**, *22*, x FOR PEER REVIEW 5 of 23

**Figure 4.** Optical emission spectrum (OES) of reactive species generated in water; its conductivity adjusted by NaCl at a concentration of 0.02 M during the SPP. **Figure 4.** Optical emission spectrum (OES) of reactive species generated in water; its conductivity adjusted by NaCl at a concentration of 0.02 M during the SPP.

The reactions revealing the possible formation of •OH, •H, H2O2, and excited O are shown below [51,52]: The reactions revealing the possible formation of •OH, •H, H2O2, and excited O are shown below [51,52]:

e− + H2O → •OH + •H + e− (Ionization of water molecule by plasma discharge) (1) e <sup>−</sup> + H2O → •OH + •H + e<sup>−</sup> (Ionization of water molecule by plasma discharge) (1)

e− + H2O → H2O\* + e− (Excitation of water molecule by plasma discharge) (2) e <sup>−</sup> + H2O → H2O\* + e<sup>−</sup> (Excitation of water molecule by plasma discharge) (2)

e− + M → M\*+ e− (Sputtering of metal atom to excited metal atom) (3) e <sup>−</sup> + M → M\*+ e<sup>−</sup> (Sputtering of metal atom to excited metal atom) (3)

$$\stackrel{\cdot}{\cdot} \stackrel{\cdot}{\cdot} + \stackrel{\cdot}{\cdot} \stackrel{\cdot}{\bullet} \stackrel{\cdot}{\bullet} \to \stackrel{\cdot}{\bullet} \stackrel{\cdot}{\bullet} \stackrel{\cdot}{H} + \stackrel{\bullet}{\cdot} \stackrel{\cdot}{H} + \stackrel{\cdot}{M} \tag{4}$$

$$\text{Fe}^- + \text{O}\_2 \rightarrow \text{O}(^3\text{P}) + \text{O}(^1\text{D}) + \text{e}^- \text{ (Ionization of O}\_2 \text{ molecule by plasma discharge)} \tag{5}$$

$$\text{O}(^{1}\text{D}) + \text{H}\_{2}\text{O} \rightarrow \text{"OH} + \text{"OH} \tag{6}$$

$$\bullet \text{H} + \text{O}\_2 \rightarrow \bullet \text{OH} + \text{O} \tag{7}$$

$$\text{O} + \text{O} \rightarrow \text{O}\_2 \tag{8}$$

$$\text{UV} + \text{H}\_2\text{O} \rightarrow \text{H}\_2\text{O}^\* \text{ (Exidation of O}\_2 \text{ molecule by UV)}\tag{9}$$

$$\text{UV} + \text{H}\_2\text{O}^\* \rightarrow \text{OH}^\cdot + \text{H}^+ \tag{10}$$

$$\text{e}^- + \text{OH}^- \rightarrow \text{"OH} + \text{e}^- \tag{11}$$
