Plasma-Activated Water (PAW)

PAW has undergone plasma treatment, resulting in significant chemical and physical modifications. The production of various reactive species drives these modifications during the interaction between plasma and water. PAW can be classified based on the dominant reactive processes or mechanisms involved in its creation, including plasma-activated ozone, nitrogen-enriched, hydrogen peroxide-enriched, mixed reactive species, and customized water.  

Ozone-Enriched Plasma-Activated Water (Ozone Water): Ozone water is a type of PAW that contains dissolved ozone (O₃). It is created when plasma-generated oxygen species react with water. Ozone water has strong oxidizing, antibacterial, and antifungal properties, making it highly effective for disinfection, sanitation, cleaning, and environmental remediation. 

Nitrogen-Enriched Plasma-Activated Water (Nitrogen-Fixation PAW): When plasma interacts with nitrogen from the atmosphere or nitrogen-containing gases, it generates reactive nitrogen species (RNS) such as nitrates (NO₃⁻), nitrites (NO₂⁻), and peroxynitrites. These RNS are transferred into the water, creating nitrogen-enriched PAW with distinct reactive and biochemical properties, including high nitrogen content and enhanced biochemical activity. This form of PAW offers significant benefits for biological systems, such as improved nutrient availability. Nitrogen-Fixation PAW has valuable applications in agriculture (e.g., as fertilizers), biological medicine, sterilization, and various industrial processes. 

Hydrogen Peroxide-Enriched Plasma-Activated Water: Plasma treatment produces hydrogen peroxide (H₂O₂) in water, either alone or in conjunction with other reactive oxygen species (ROS). This hydrogen peroxide-enriched plasma-activated water (PAW) is highly effective in oxidative and antimicrobial processes and may work better when combined with other ROS. It has proven useful in food safety, sterilization, and oxidative cleaning applications. 

Mixed Reactive Species Plasma-Activated Water: Plasma treatment can produce a diverse combination of reactive oxygen species (ROS) and reactive nitrogen species (RNS) in water. This mixed-species PAW gains its versatile properties from the synergistic effects of these reactive components. Its broad-spectrum reactivity includes oxidation, antimicrobial, and biochemical effects. These types of PAW are widely applicable in agriculture, healthcare, environmental sanitation, and industrial processes. 

Customized Plasma-Activated Water: Advances in plasma technology have enabled the customization of PAW composition by adjusting factors such as gas type, plasma source, and treatment duration. By precisely controlling the levels of reactive species, PAW can be optimized for specific needs. This customized approach offers significant benefits for applications in therapeutic treatments, precision agriculture, and specialized industrial processes. 

These classifications of PAW provide a framework for understanding the diversity of plasma-activated liquids and their potential applications. By altering plasma parameters and treatment conditions, researchers and engineers can develop highly specific types of PAW to address various scientific, medical, and environmental challenges. 

Since PAW is formed by treating water with plasma, its generation involves the same key plasma-based methods as those of other plasma-activated liquids (PALs): liquid-phase plasma (LPP) and gas-phase plasma diffusion (GPPD). However, some differences may arise—such as characteristics and application use—because of water's unique composition and role as a base medium compared to other liquids.  

The generation of Plasma-Activated Water (PAW) relies on the same core techniques used for other plasma-activated liquids: Liquid-Phase Plasma (LPP) and Gas-Phase Plasma Diffusion (GDDP). However, differences in the generation process and outcomes arise due to water's unique chemical properties and the desired applications of PAW. These differences include variations in reactive species composition, production efficiency, and scalability. 

Liquid-Phase Plasma (LPP) 

The LPP method involves generating plasma directly within the liquid itself. This is typically achieved for water using electrical discharges, such as pulsed high voltages or dielectric barrier discharges, that create plasma directly in liquid water. Since the interaction occurs directly in the liquid phase, the close proximity of charged particles, free radicals, and excited species to water molecules results in highly localized and efficient chemical reactions. This proximity can enhance the production of specific reactive oxygen (e.g., hydrogen peroxide, hydroxyl radicals) and nitrogen species (e.g., nitrites and nitrates). Compared to other PALs, generating PAW with the LPP method may require additional optimization due to the unique role of water's ionization and polar properties in plasma generation. 

• Advantages and Disadvantages of LPP: LPP's direct interaction of plasma with water ensures the efficient energy transfer of reactive species, leading to enhanced chemical reactivity and customized PAW properties. However, LPP-based generation often demands more sophisticated equipment and energy input to sustain plasma in the liquid phase, making it potentially more resource-intensive than other methods. 

Gas-Phase Plasma Diffusion (GPPD) 

The GPPD method generates plasma in a gas phase and then transfers the reaction products (reactive species) into the liquid. For PAW generation, plasma is typically created by applying energy (e.g., electrical discharge, microwave energy) to a gaseous medium such as air, oxygen, or nitrogen. The reactive species diffuse into the water, chemically activating it. When applied to PAW, the GPPD method may produce somewhat different ratios or compositions of reactive species in the water compared to LPP. For example, reactive nitrogen species, such as NO₂⁻ and NO₃⁻, are often more prominently generated due to interactions in the gas phase, while reactive oxygen species like OH radicals and H₂O₂ may be less concentrated compared to LPP-treated water.   

• Advantages and Disadvantages of GPPD: The gas-phase method is generally less complex than LPP and can be implemented using simpler equipment. This approach is also easily scalable for large-scale PAW production, making it suitable for industrial or agricultural applications. However, since reactive species must diffuse from the gas phase into water, there may be reduced efficiency in transferring some plasma-generated species to the liquid phase compared to LPP. This may result in differing PAW properties. 

Plasma-activated water (PAW) is generated using the same core techniques as other plasma-activated liquids: Liquid-Phase Plasma (LPP) and Gas-Phase Plasma Diffusion (GDDP). However, differences in the generation process and outcomes arise due to water's unique chemical properties and the desired applications of PAW. These differences include variations in reactive species composition, production efficiency, and scalability.  

PAW is characterized by a unique combination of physical and chemical properties resulting from the interaction between plasma-generated reactive species and water. These properties make PAW an innovative, versatile solution for agriculture, healthcare, and food safety applications. Key characteristics, including chemical composition, antimicrobial properties, benefits to plant growth and agriculture, physical and functional properties, environmental friendliness, safety and toxicity, and versatility, have a significant impact.  

Chemical Composition: PAW is defined by its high content of reactive oxygen species (ROS) and reactive nitrogen species (RNS), which vary depending on the plasma generation method, gas composition, and treatment conditions. 

• Reactive Oxygen Species (ROS): Compounds such as hydrogen peroxide (H₂O₂), ozone (O₃), and hydroxyl radicals (•OH) are common products in PAW. These species contribute strongly to PAW's antimicrobial and oxidative properties, making it effective in disinfection and oxidation-related processes. 
• Reactive Nitrogen Species (RNS): PAW also contains nitrite (NO₂⁻), nitrate (NO₃⁻), nitric oxide (NO), and other RNS. These are important in applications such as agriculture as they enhance nutrient availability and plant disease resistance. 
• pH Changes: Plasma treatment often lowers the pH of water, making it more acidic. This shift enhances the water's reactivity, impacting its ability to serve as a disinfectant and a catalyst for other chemical reactions.  

Antimicrobial Properties: PAW stands out for its powerful, broad-spectrum antimicrobial capabilities, which are largely driven by its ROS and RNS content. 

• Enhanced Disinfection: PAW effectively eradicates various bacteria, viruses, and fungi. Its disinfection efficacy makes it a promising, non-toxic alternative to conventional methods in healthcare, food safety, and environmental sanitation.  
• Activity Against Resistant Pathogens: PAW has demonstrated effectiveness against antibiotic-resistant strains, addressing urgent public health concerns related to antimicrobial resistance. 

Benefits to Plant Growth and Agriculture: PAW has been shown to offer multiple benefits for crop production and sustainable agriculture. 

• Growth Promotion: PAW enhances seed germination, nutrient uptake, and root development in certain crops. These effects may lead to improved plant growth and higher yields.  
• Disease Resistance: Plants treated with PAW exhibit increased resistance to pathogens, reducing the need for chemical pesticides and bolstering their environmental sustainability credentials. 

Physical and Functional Properties: PAW's unique physical properties contribute directly to its functionality and effectiveness in various applications. 

• Oxidation-Reduction Potential (ORP): PAW has a high ORP, which reflects its strong oxidizing capability. This property underpins its antimicrobial activity and enhances its utility for decontamination purposes. 
• Electrical Conductivity: Adding ions and reactive species alters the conductivity of PAW. This can impact its interactions with biological systems and its effectiveness in certain applications.  
• Reactive Species Stability: The stability of reactive species in PAW depends on various factors, including temperature, pH, storage conditions, and the plasma generation method. Short-lived species (e.g., hydroxyl radicals, nitric oxide) act quickly, making PAW highly reactive in the short term. Longer-lived species (e.g., hydrogen peroxide, nitrate) contribute to a residual effect, extending the functional lifespan of PAW for practical use. 

Environmentally Friendly: PAW is recognized for its sustainability and the reduced environmental footprint of its by-products. 

• Biodegradability: PAW by-products typically break down into harmless compounds, such as water, oxygen, and nitrogen, thereby reducing risks to human health and the environment. 
• Reduced Chemical Inputs: PAW minimizes the use of synthetic fertilizers, chemical pesticides, and disinfectants, promoting cleaner agricultural and industrial practices. 

Safety and Toxicity: PAW is widely regarded as a safe and non-toxic solution for various applications. 

• Low Human and Animal Toxicity: PAW, derived from air and water, poses minimal risks compared to traditional chemical sanitizers and pesticides. 
• Sustainable Composition: PAW's fundamental components—air and water—are inherently safe and non-harmful, making it an attractive choice for sensitive applications like wound treatment and food safety. 

Application Versatility: The multifunctional properties of PAW enable its use across a diverse range of fields:  

• Agriculture: Used for irrigation, improving crop yield, enhancing nutrient uptake, and promoting natural plant defenses. 
• Food Industry: Effective in washing produce, extending shelf life, and enhancing food safety by reducing microbial contamination.  
• Healthcare: Used for disinfecting medical instruments, treating wounds, and decontaminating healthcare environments. 

PAW's defining characteristics—its reactive species content, physical properties, safety, and environmental compatibility—make it a game-changing tool in modern applications. Its antimicrobial activity addresses key challenges in healthcare and food safety, while its plant growth-promoting effects offer sustainable solutions for agriculture. Additionally, its environmentally friendly and non-toxic nature aligns with global efforts to reduce reliance on harmful chemical treatments.  

Due to its unique chemical properties, primarily reactive oxygen and nitrogen species (ROS and RNS), PAW has emerged as a promising technology for disinfection, sterilization, and other applications. However, its efficacy depends on numerous factors—such as activation time, storage conditions, and nozzle distance—that highly depend on the environment and influence its chemical composition, stability, and antimicrobial capabilities. A detailed understanding of these factors is critical for optimizing PAW for specific applications and maximizing its effectiveness.  

Understanding the factors influencing PAW efficacy is essential for designing controlled applications and achieving consistent performance. Each factor plays a role in determining the type, concentration, and stability of reactive species that drive PAW's antimicrobial properties. By recognizing and optimizing these variables, researchers and engineers can expand the practical applications of PAW in diverse sectors, including healthcare (as a disinfectant for surgical instruments and wounds), the food industry (for decontaminating fresh produce and surfaces), and environmental remediation (treatment of wastewater or biofilms). Further research into these factors will pave the way for more precise and effective utilization of PAW technologies in both existing and emerging applications. 

Factors 

Several factors play a critical role in determining the effectiveness of PAW: 

Activation Time: Activation time, or the duration of plasma exposure to water, is a fundamental parameter influencing the concentration and types of reactive species generated in PAW, such as hydrogen peroxide (H₂O₂), hydroxyl radicals (•OH), nitric oxide (NO), and nitrate/nitrite ions (NO₃⁻/NO₂⁻). Generally, longer activation times facilitate the accumulation of these reactive species, thereby increasing the antimicrobial efficacy of PAW (Miranda et al., 2023). However, prolonged activation can also lead to secondary reactions affecting stability or functionality. Optimizing activation time is crucial, as insufficient exposure may result in suboptimal reactive species concentrations, while excessive activation could lead to diminishing returns or chemical imbalances. 

Storage Conditions: The stability of PAW over time is strongly influenced by storage conditions. Reactive species in PAW are not indefinitely stable and can degrade due to environmental factors, such as light exposure, temperature fluctuations, and the presence of contaminants. Studies have demonstrated that prolonged storage often leads to a decline in bactericidal activity, as the concentrations of ROS and RNS gradually decrease (Wang et al., 2023). For example, exposure to UV light can accelerate the decomposition of reactive species, whereas refrigeration may extend the stability of PAW. Therefore, proper protocols, such as dark and cool conditions, are necessary for preserving its efficacy, particularly in scenarios requiring extended periods between production and application. 

Distance from Plasma Nozzle: In plasma gas diffusion applications, the physical distance between the plasma generation source (nozzle) and the activated water significantly impacts the properties and antimicrobial capabilities of PAW (Shah et al., 2023). Optimal distances ensure efficient energy transfer from the plasma to the water, enabling maximal production of reactive species without significant energy dissipation. Excessive distance may reduce ROS and RNS production due to dispersion losses, while too little distance may cause uneven activation or suboptimal mixing. Understanding and standardizing this parameter is particularly important in industrial or continuous-flow systems where consistent PAW production is required. 

Liquid Conductivity: The electrical conductivity of the water being activated is another critical factor determining the efficacy of PAW. Conductivity levels, which depend on the dissolved ions in the water, influence the generation and diffusion of ROS and RNS during plasma activation. Higher water conductivity can promote the formation of these reactive species, thereby enhancing antimicrobial activity (Rahman, 2022). However, excessively high conductivity might lead to the rapid neutralization of reactive species, reducing their stability. Adjusting the conductivity of the source water, for example, by controlling dissolved salts or electrolytes, can optimize PAW production for specific applications. 

Microbial Targets: The efficacy of PAW varies depending on the type of microorganisms being targeted. While PAW has demonstrated broad-spectrum antimicrobial activity against bacteria, viruses, and fungi, its effectiveness can differ based on the organism's structure, physiology, and resistance mechanisms (Zhang et al., 2024; Mai-Prochnow et al., 2021; Wang et al., 2023). For example:  

• Gram-negative bacteria are generally more susceptible to PAW than Gram-positive bacteria due to differences in cell wall structure.  
• PAW's ROS may more effectively inactivate lipid-enveloped viruses than non-enveloped viruses.  
• Fungal spores with robust protective structures may require higher concentrations of reactive species or longer exposure times for effective inactivation. 

Recognizing these variations is crucial for customizing PAW treatments to specific microbial threats in healthcare, agriculture, and food safety environments. 

pH Levels: PAW's pH, which typically becomes acidic during plasma activation, contributes significantly to its antimicrobial properties (Mai-Prochnow et al., 2021). The generation of nitric acid from reactive nitrogen species during activation lowers the pH, creating an environment hostile to many microorganisms. Acidic conditions can disrupt bacterial membranes, denature proteins, and inhibit enzymatic processes. However, overly low pH levels may limit the practical use of PAW in certain applications, such as those involving sensitive materials or surfaces. Monitoring and potentially adjusting pH levels can help balance efficacy and compatibility. 

Environmental Factors: The presence of organic matter or other environmental contaminants can negatively affect the antimicrobial efficacy of PAW (Wang et al., 2023). Organic molecules act as scavengers, reacting with ROS and RNS to produce less active or inactive byproducts, thereby reducing the availability of reactive species for microbial inactivation. This means that heavily soiled or contaminated surfaces may require pre-cleaning or higher PAW dosages to achieve effective sterilization. Understanding the interplay between PAW and environmental factors is essential to ensure consistent results in real-world applications. 

Heating: Thermal treatment of PAW has been shown to enhance its antimicrobial activity (Wang et al., 2023). Heating can influence the chemical kinetics of reactive species, increasing their reactivity and bioavailability. Researchers have observed improved performance against resistant microorganisms by heating PAW to moderate temperatures. However, excessive heating may cause the degradation of some reactive species or unintended chemical changes, so temperature optimization needs careful consideration. 

Temperature Stability: One of PAW's advantages is its ability to retain bactericidal activity across a range of storage temperatures, including room temperature, refrigeration, and frozen conditions (Tsoukou et al., 2020). This stability is particularly valuable for applications requiring transport or storage in varied climates. While reactive species remain active across these temperatures, refrigeration and freezing may further slow the degradation of ROS and RNS, extending PAW's shelf life. This makes PAW a practical solution for a wide range of industries, from healthcare to food preservation. 

PAW has become a highly versatile and innovative tool, harnessing its antimicrobial properties and rich composition of reactive species for a wide range of applications. Below are some of the prominent areas where PAW has shown significant potential. 

Disinfection and Sanitation: PAW is highly effective at inactivating a broad spectrum of pathogens, including bacteria, viruses, and fungi, making it a powerful agent for disinfection and sanitation. It has been effectively used to sanitize surfaces in healthcare facilities, food processing plants, and public spaces, contributing to reduced microbial contamination. Unlike conventional methods, PAW offers an effective alternative to harsh chemical disinfectants, reducing the risk of chemical residues while maintaining excellent antimicrobial efficacy. 

Agriculture: PAW has demonstrated the ability to enhance agricultural practices in several ways. It improves seed germination rates, promotes stronger and healthier plant growth, and suppresses soil-borne pathogens responsible for crop diseases. Its application minimizes the reliance on synthetic chemical pesticides, enabling more sustainable and environmentally friendly farming practices. Additionally, it can potentially enhance crop yield and quality, making it a compelling solution in the context of global food security challenges. 

Wound Healing: PAW is being explored as a novel approach in the medical field, particularly for promoting wound healing and tissue regeneration. Its antimicrobial activity prevents wound infections, while its unique physicochemical properties help create an environment conducive to faster and more effective healing. Current research highlights its potential for treating chronic wounds, burn injuries, and other dermatological conditions, positioning PAW as a promising alternative or adjunct in healthcare. 

Food Safety and Decontamination: PAW is important for enhancing food safety by efficiently decontaminating food surfaces and raw products. Its ability to inactivate harmful pathogens, including those responsible for foodborne illnesses, improves the hygiene of food handling and storage processes. Moreover, PAW extends the shelf life of perishable goods by reducing microbial spoilage, providing consumer safety while reducing food waste. Its non-toxic nature and effectiveness across a variety of food types make it an invaluable asset in the food industry. 

Due to its unique chemical properties, primarily reactive oxygen and nitrogen species (ROS and RNS), PAW has emerged as a promising technology for disinfection, sterilization, and other applications. However, its efficacy depends on numerous factors—such as activation time, storage conditions, and nozzle distance—that highly depend on the environment and influence its chemical composition, stability, and antimicrobial capabilities. A detailed understanding of these factors is critical for optimizing PAW for specific applications and maximizing its effectiveness. 

Environmental Remediation: PAW is gaining attention for its potential in environmental cleanup efforts, particularly in wastewater treatment and the degradation of pollutants. PAW's reactive oxygen and nitrogen species (RONS) can effectively degrade hazardous or persistent organic compounds, such as dyes, pesticides, and pharmaceutical residues. This makes it a powerful tool for remediating contaminated water and soil environments. Its eco-friendly approach to pollutant removal aligns with the growing need for sustainable and green technologies in environmental management. 

Advantages  
PAW offers several key advantages, primarily due to its ability to generate reactive species directly within the water. These benefits include: 

• Enhanced Reactivity: The in-situ generation of reactive species yields higher concentrations and a more diverse range of reactive agents, thereby improving the system's overall reactivity. 
• Environmental Sustainability: Compared to traditional chemical methods, PAW is more eco-friendly, producing fewer harmful by-products and reducing the need for toxic chemicals. This makes it a greener alternative for industrial and scientific applications. 
• Improved Efficiency and Productivity: PAW processes are typically energy-efficient, operating at lower energy costs while achieving faster reaction times. These attributes make PAW a viable economic option for various industries. 

Challenges  
Despite its advantages, PAW faces many challenges, including: 

• Scalability: Although highly effective at smaller scales, scaling PAW processes for industrial applications can prove challenging, as maintaining efficiency and uniformity at larger scales requires careful adjustments and optimizations. 
• Operational Complexity: The precise control of parameters such as voltage and liquid composition during plasma generation demands advanced monitoring and regulation systems, which can increase the complexity of operations.