The American physicist Irving Langmuir identified plasma as a fourth state of matter in 1922 [1]. Plasmas are gases that have been totally or partially ionized. In order to remove electrons from atoms, enough energy is given to the gas, resulting in a combination of free electrons, free radicals, neutrals, and positively charged species ^[2][3][4][2,3,4]. Nonthermal plasma (NTP) or cold atmospheric plasma differs from thermal plasma because of the properties of electrons, ions, and neutrality. The charge species, reactive oxygen species (ROS), reactive nitrogen species (RNS), electromagnetic field, electric field, ability to influence pH, visible light, charged particles, neutral species, ozone, and ultraviolet (UV) radiations were all found in NTP as shown in Figure 1, making it a feasible tool for a variety of tasks ^{[5][6][7][8][9]}[5,6,7,8,9]. The temperature of plasma electrons might reach tens of thousands of kelvin, significantly exceeding the neutral gas’ temperature of roughly room temperature. In bulk, most plasmas are electrically neutral, which adds to plasma’s unique properties and categorization as the fourth state of matter. Plasma parameters can vary dramatically depending on the energy source and amount delivered. Furthermore, NTP devices such as jet plasma, dielectric barrier discharge (DBD) plasma, and spark plasma depicted in Figure 2a often operate in a room environment, making them ideal for life science research ^{[9][10][11][12]}[9,10,11,12] and a wide range of biomedical applications ^{[13][14][15][16][17][18][19][20][21][22]}[13,14,15,16,17,18,19,20,21,22]. Sterilization, skin disinfection, oral/dental disease treatment, blood coagulation, wound healing, cancer therapy, and immunotherapy have all seen new possibilities in medicine by using NTP [23]. Gliding arc discharge (Figure 2d) and corona discharge (Figure 2e) devices are commonly used for environmental applications such as the removal of gaseous pollutants.

As plasma makes up 99% of the universe, it is the most prevalent kind of matter. The Sun and other stars, galaxies, solar winds, lightning, and the aurora borealis are all examples of the plasma state. Plasma televisions, neon and fluorescent lights, and plasma displays are well-known applications for man-made plasma. From Figure 1 and Figure 2c, it can be seen that plasma is made up of neutral, ionized, and/or excited charged particles, ions, and molecules, and ozone with the ability to influence the pH of solutions, as well as the existence of different ROS and RNS. A significant source of UV and vacuum UV radiation is plasma made up of several gases [24]. Plasma is a unique material treatment technology since it can employ a single component or a mix of components.

All of the particles in a thermal plasma are at about the same temperature and completely ionized. Hot, completely ionized, and equilibrium plasma are other names for thermal plasma. Depending on the demands, equilibrium plasma is used in a variety of applications [25]. Plasma is a (partially) ionized gas made up of neutral species (molecules, radicals, excited species), ions, photons, and electrons. As the electron temperature is much higher than the temperature of heavy species (ions and neutrals) in nonthermal plasma (NTP) or non-equilibrium plasma, radicals and excited species are formed at temperatures that are closer to room temperature. This nonthermal energy distribution provides a potential route to getting around both the kinetic and thermodynamic constraints on the chemical conversion of reactants into desired products. Cold, partially ionized, atmospheric pressure, low temperature, and non-equilibrium plasma are other names for NTP. The NTP technology is suitable for treating a range of biological materials, including solids, liquids, and aerosols because it is at a low temperature when applied. There are several uses for two different types of NTP, low pressure and atmospheric pressure ^{[26][27][28][29]}[26,27,28,29].

2. Generation of Reactive Species in NTP Discharge

The NTP is well recognized for generating exceptionally high concentrations of reactive species. NTP produces a variety of reactive oxygen species (ROS), including OH radicals, O₂, H₂O₂, O, O₃, and ¹O₂, as well as reactive nitrogen species (RNS), such as NO, NO₂, N₂O, N₂O₅, and atomic N [5]. Some of them, such as OH radicals, O, NO, N, and ¹O₂, have a short lifespan [19]. Helium plasma discharge was also applied by a number of studies ^{[30][31][32][33]}[31,32,33,34]. The mechanisms by which ROS are produced in helium plasma jets have been well reported ^[34][35]. A number of species, including N₂⁺, atomic (He, O) radicals, and molecular OH, were discovered in helium discharge ^[31][33][32,34]. The RONS, also referred to as primary reactive species, are generated by the energy released during collisions between accelerating electrons and neutrals. In the gas phase right after the collision, electrons (e⁻), ionized neutrals and gas (M⁺), excited neutrals and gas (M*), N, O, atomic H, NO, and O₂*^- are produced [13]. They are categorized as primary reactive species ^[35][36], and the intensities of these species are very high in the plasma region. The lifetimes of primary reactive species are relatively short; for example, the lifespan of OH radicals, NO, and O₂*⁻, is 2.7 μs, 1.2 μs, 1.4 μs, and 1.3 μs, respectively ^[36][37]. Some of these reactive species immediately experience radiative decay, while others combine with neutrals, water molecules, and other reactive species. The main reactive species change into secondary reactive species such as H₂O₂, NO₂, NO₃, and O₃ ^[37][38] in an ambient environment_, as shown in Figure 3. The liquid phase (or another target) is where the RONS produced in the gas phase dissolve and form tertiary reactive species. The tertiary reactive species are formed when the RONS produced in the gas phase dissolve in the liquid phase ^[35][36]. Long-living reactive species include O₃, H₂O₄, NO₃, and NO₂ because of their lifespans of a few milliseconds over several days ^[38][39]. Water can dissolve H₂O₂, NO₂, and NO₃; NO₂ and NO₃ are immediately converted into NO₂⁻ and NO₃⁻, respectively ^[13][39][13,40]. Depending on the plasma source, working gas, power source, treatment time, and sample volume, these reactive species can reduce the pH of the target liquid by up to 2 ^[39][40]. However, whether a target is dry or aqueous affects the chemistry of ROS and RNS formation in a target ^[39][40]. The target, gas region, plasma/target interface, and discharge region of RONS formation are schematically depicted in Figure 3. DBD plasma and jet plasma have been shown to produce high levels of ROS ^[26][40][41][26,41,42]. Indirect plasmas are produced between two electrodes of certain devices and transferred to the application region through a gas flow. ROS are often produced at the border of jets with the surrounding air by a variety of causes. Several authors claim that ROS generated by plasma can induce morphological alterations, membrane depolarization, lipid peroxidation, and DNA damage in cells ^{[42][43][44][45]}[43,44,45,46]. The transport of reactive oxygen/nitrogen species (RONS) is the major mechanism of NTP anti-neoplastic action ^[46][47]. The quantity of reactive species created in plasma-treated liquids is extremely important in plasma therapy. Several lines of study now focus on utilizing plasma to treat cancer using the ROS generation feature ^[47][48][49][48,49,50]. In human cells, plasma treatment induces the mitochondrial membrane potential to depolarize, resulting in the generation of ROS ^[50][51]. The therapeutic effects of air plasma have been linked to the generation of RONS such as H₂O₂, Ox, OH, •O₂, and NO_x as a result of mitochondrial membrane potential depolarization and ROS accumulation ^[48][49]. Previous reports provide a more thorough explanation of plasma–liquid interactions and the plasma roadmap ^[23][51][52][23,52,53].

3. NTP Application for Cancer Treatment

The NTP technology has advanced dramatically in the previous two decades, from theoretical and experimental study to real-world implementations. The generation of plasma at atmospheric pressure and room temperatures to deal with biological systems has given rise to a new multidisciplinary field known as “plasma medicine” [2]. NTP has a variety of potentials in biomedical engineering nowadays ^[53][54]. Figure 4 depicts the interaction of NTP with the biological system, indicating the main molecular mechanisms involved in the use of LTP in cancer treatment. NTP technology has the potential to provide a less intrusive surgical procedure for removing particular cells without causing injury to the surrounding tissue. Traditional laser surgery relies on heat contact, which can result in unintentional cell death (necrosis) and lasting tissue damage. NTP contact with tissue, on the other hand, may allow selective cell elimination without necrosis ^[54][55]. Cell detachment without influencing cell viability, regulated cell death, and other interactions are examples of these interactions. It can also be employed for cosmetic approaches to dermis reticular architecture regeneration. The goal of plasma contact with tissue is to operate below the temperature damage threshold and cause chemically specific reactions or alterations, rather than to denature the tissue. The presence of plasma, in particular, can enhance chemical processes that have the desired effect. Pressure, gas composition, and power may all be adjusted to enhance chemical reactions. As a result, finding plasma conditions that generate a positive influence on tissue without having heat treatment is crucial. NTP produces a variety of reactive species that can be employed to increase cancer cells’ oxidative stress and ultimately kill them ^[40][55][56][41,57,58]. Different ROS can be produced in plasma, and some of them can cause oxidative stress in cells ^[46][47]. As a result, it can alter any pathway that is regulated by or connected to ROS, either directly or indirectly ^[40][57][41,59]. Recent research has found that cancer cells generate more ROS ^[58][60] and are thus prone to oxidative stress higher than normal cells, making them more appropriate for targeting by ROS in combination with plasma technology ^[59][60][61,62]. Because NTP-killing activity is greater in cancer cells than in normal cells, the outcome of an NTP cancer treatment is more optimistic ^[60][62]. NTP medicinal applications have achieved this incredible breakthrough from initial discovery through fundamental scientific research to clinical applications ^[61][63]. Previous studies dealt with the optimal duration of treatment to maximize plasma’s anticancer effects ^{[32][62][63][64]}[33,64,65,66]. The improvement of plasma on-time during treatment is a critical issue that needs to be addressed. The biomedical effects are directly impacted by RONS levels, which are directly impacted by plasma on-time. Recently, research on the effects of plasma on-times was conducted while keeping the duty ratio and treatment time fixed (Figure 5) ^[65][67]. The selected plasma on-times are 25, 50, 75, and 100 ms for 10% and 36% duty ratios, respectively. Figure 5a–c depict, respectively, the experimental setups, a photo taken during the preparation of PTM using a soft plasma jet, and the application of PTM to cells. Figure 5d,e show that the NO_x concentration in PTM corresponds to the plasma on-time, in 10% and 36% duty ratios. Similarly, Figure 5f,g show the concentrations of H₂O₂. Figure 5h,i indicate the viability of the U87-MG cell line when PTM was applied. It is interesting to note that by increasing the plasma on-time, the levels of ROS/RNS dramatically increased in PTM and significantly impacted the viability and ATP levels of the U87-MG cell line. The findings of this study offer a significant indication of progress by introducing the optimization of plasma on-time to enhance the effectiveness of the soft plasma jet for biomedical applications ^[65][67]. It is interesting to observe from this research that the ROS/RNS levels can be altered to suit needs by adjusting the plasma on-time within the fixed duty ratio and treatment time. A new cancer treatment modality is being developed by the multidisciplinary field of plasma medicine, which combines plasma physics, chemistry, biology, and clinical medicine. In order to specifically target malignant cells for the prevention of cell proliferation and tumor progression, it primarily relies on the use of low-temperature plasmas in atmospheric pressure. A comprehensive discussion is given of intracellular mechanisms of action, important pathways, and the selectivity of NTP against cancer cells as shown in Figure 4. The in vivo, in vitro, and in ovo experiments are all studied using the NTP technology. Figure 6 depicts two recent in vivo and in ovo experiments using NTP for anticancer purposes. According to reports and widespread knowledge, the NTP raises the rate of apoptosis in cancer cells when compared to the control group (Figure 6a) ^[66][68]. There is still a lack of understanding of the molecular mechanisms underlying NTP’s therapeutic effect on thyroid cancer. Understanding the anticancer effects of NTP-activated medium (NTPAM) on thyroid cancer cells and clarifying the signaling mechanisms causing NTPAM-induced thyroid cancer cell death was explored in a recent study ^[66][68]. The in vivo analysis demonstrates that, when compared to control groups, NTPAM-treated groups had significantly lower tumor weights (Figure 6b). The findings of the reported study showed that NTPAM inhibited THCA cell growth more potently than the control did and that ROS controlled EGR1/GADD45α to mediate NTPAM-induced apoptotic cell death (Figure 6c). Recent research offers a fresh understanding of the basic chemical mechanisms underlying NTP–cancer cell interactions as well as a previously unrecognized benefit of current NTP cancer therapy: lowering immunosuppressive signals on the surface of cancer cells (Figure 6g–k) ^[67][69]. Researchers can better explain many disagreements by having a thorough understanding of the underlying mechanisms. This includes choosing the best plasma parameters to regulate the combination and concentration of reactive species, delivering plasma to deep-lying tumors, and figuring out the best plasma dose to achieve particular clinical translational outcomes. Designing low-temperature plasma sources that satisfy medical device technical standards is a unique approach for cancer therapy in clinical trials, but it still has to be safer and more effective.

4. Role of Plasma Technology in Food Decontamination and Storage

4.1. Microbial Inactivation

Plasma technology’s source of ROS and RNS is mainly concerned with increasing the yields of cultivars by stimulating seedling growth and inactivating microorganisms to increase the shelf life of food storage and fulfill the scarcity of food from all over the world ^[68][69][90,91]. Several treatment methods increased the concentration of intrabacterial reactive species such as H₂O₂, NO₃, NO₂, and O₃. These reactive species are caused by oxidative stress, which prevented the ability of biofilms to regenerate ^[70][92]. The inactivation of E. coli bacteria was reported due to the UV radiation produced during the PAW ^[71][93]. The PAW has bactericidal properties due to the RONS. Conidium is an asexually generated fungal spore, a notorious plant disease inhibited by the reactive species in the PAW ^[72][94]. Most crops are destroyed by serious diseases by wall-less bacteria called phytoplasmas. Yellow grapevines can easily become infected with phytoplasma ^[73][95]. Phytoplasma caused disease in yellow grapevines controlled by the plasma-treated liquids which stimulate the defensive enzymes stilbene synthase and phenylalanine ammonia. The presence of RONS in PAW solution results in oxidative stress, which activates the stilbene metabolic pathway and increases the antioxidant properties. Resultantly, the research shows that PAW treatment can be used to enhance plants’ resistance to diseases ^[74][96].

4.2. Effect of NTP on Biofilms

Biofilms are multicellular communities of microbial cells, connected to a surface indefinitely and enveloped by a matrix predominantly consisting of polysaccharides, extracellular nucleic acid, and protein. In nature, more than 95% of the bacteria exist as a biofilm which favors their growth on solid surfaces ^[75][97]. Bacterial biofilm formation involves many steps. Quorum sensing is a special type of signaling required which occurs between microbial cells, and the transcription of a different set of genes is also a basic requirement for biofilm formation. Bacterial inactivation or bio-decontamination, as well as the sterilization of surfaces by NTP, have gained a lot of interest in recent years ^[76][98]. It is well-known that NTP has antibacterial effectivity and can inactivate planktonic bacteria, yeast, and spores. The effects of plasma application on biofilms seem to be a promising new direction in biofilm removal technology ^[77][99]. Since NTP, particularly atmospheric-pressure plasma jets (APPJs), are frequently operated at temperatures close to room temperature, they can be used directly on heat-sensitive materials such as human tissues. Due to the APPJs’ transitory character and accompanying thermal non-equilibrium, the plasma chemistry is improved, and charged species are transported to the targets quickly and efficiently ^[78][100]. The reactive species formed by plasmas are expected to comprise a combination of charged particles and chemically active species producing ultraviolet (UV) light (e.g., O₃, O₂, NO, H₂O₂, and OH) which contribute to the anti-microbial effects by inflicting damage on DNA, lipids, proteins, etc., leading to cellular structure degradation ^[79][101]. BAs we know, bacterial biofilms consist of DNA, proteins, and polysaccharides which are affected or damaged by plasma-generated reactive species which are made up of charged particles and chemically active species and the generation of UV radiation.

4.3. Sustaining Food Freshness and Storage

NTP technology has been suggested as an advanced method to increase food freshness and food storage for longer use. The freshness of freshly cut lettuce was reported to be maintained for a long time after washing with PAW. The lettuce tissue organelles were sustained as fresh properties such as color, texture, and taste after washing with PAW treatment ^[80][81][102,103]. PAW treatment has the ability to maintain the freshness of fresh-cut kiwifruit. Kiwifruits were sprayed with PAW solution or deionized water and submerged in S. aureus suspension. The reactive species in PAW generated oxidative stress, which in turn damaged the membrane of bacterial cells and ultimately lead to cell death. Furthermore, PAW treatment had no detrimental effects on the kiwifruits’ quality. They were less affected by reactive species oxidative species damage due to the antioxidant enzymes in PAW ^[82][104]. Plasma therapy effectively suppressed B. cinerea spore germination and mycelial development in vitro, as well as gray mold decomposition in blueberries infected with B. cinerea during postharvest storage ^[83][105]. Figure 7 shows the action of NTP on bacterial cell structures, resulting in functional loss and sterilization which helps in food preservation. The microbial inactivation in a liquid substrate is mostly caused by oxidative stressors brought on by the reactive species, and NTP is regarded as an improved oxidation approach. Typically, seed treatment is conducted to guard against infections that could harm seeds during the early stages of seedling development. If this phase is skipped, pathogens, insects, and plant diseases will only attack, disrupting the germination process and plant growth. However, some seed treatments that use chemicals can be extremely detrimental since they expose people to chemicals when treating large amounts of seeds, which can lead to inadvertent poisoning, damage to the seeds, and risk for farmers or workers. The presence of microorganisms (bacteria and fungus) that will impact crops is the industry’s top concern right now. Recently, the agriculture sector has implemented a number of methods to limit the growth of bacteria and fungi on crops. The creation of bacterially resistant strains, the use of chemical fumigants, the drainage of the soil, and particular plowing techniques are a few of the methods used to manage bacteria in the agriculture sector. The denaturation of proteins, lipids, and other micronutrients during some of these reactions, however, could result in unfavorable alterations to the nutritional quality of food. Furthermore, when applying this technology for food applications, it is important to thoroughly research the genotoxicity of reactive species. The application of cold plasma for microbial eradication can be used across a variety of food substrates, including cheese, fruits, and meat products. In addition, it is utilized to modify the rate at which seeds germinate. It is an environmentally friendly method that is utilized as an alternative to conventional methods for food preservation and other possible uses.