Plasma-treated nanoparticles, particularly those made of gold (Au) and copper (Cu), have emerged as promising tools for pathogen inactivation through reactive species generation. These nanoparticles leverage non-thermal plasma treatments to modify their surfaces, enhancing their ability to generate reactive oxygen and nitrogen species (RONS) that exhibit potent antimicrobial effects. The process involves careful control of plasma parameters, surface activation mechanisms, and considerations of residual toxicity to ensure efficacy and safety in biomedical and environmental applications.

The effectiveness of plasma-treated nanoparticles in pathogen inactivation depends on several key plasma parameters. These include the type of plasma gas used, discharge power, treatment duration, and working pressure. Common plasma gases include argon, oxygen, and nitrogen, each contributing distinct reactive species. For instance, oxygen plasma generates high concentrations of oxygen radicals, while nitrogen plasma enhances nitrogen-based reactive species. Discharge power typically ranges between 10 and 100 W, with higher power leading to increased surface activation but also potential nanoparticle aggregation. Treatment durations vary from 1 to 30 minutes, with longer exposures generally increasing surface functionalization but risking structural damage to the nanoparticles. Working pressure is maintained in the low-to-medium vacuum range (0.1 to 10 Torr) to sustain stable plasma conditions.

Surface activation of nanoparticles via plasma treatment introduces functional groups that enhance reactive species generation. Gold nanoparticles, when exposed to oxygen plasma, develop surface oxides and hydroxyl groups that facilitate the production of superoxide radicals (O2•−) and hydrogen peroxide (H2O2). Copper nanoparticles, on the other hand, form copper oxides (CuO and Cu2O) under similar conditions, which are known for their catalytic activity in generating hydroxyl radicals (•OH). These reactive species disrupt microbial cell membranes, oxidize proteins, and damage nucleic acids, leading to pathogen inactivation. The surface charge of plasma-treated nanoparticles also plays a critical role, as a more negative zeta potential enhances colloidal stability and interaction with microbial surfaces.

The antimicrobial efficacy of plasma-treated nanoparticles has been demonstrated against a broad spectrum of pathogens, including bacteria, viruses, and fungi. For example, plasma-treated gold nanoparticles have shown a reduction of over 99% in Escherichia coli viability within 30 minutes of exposure, attributed to the sustained release of reactive oxygen species. Copper nanoparticles exhibit even stronger antimicrobial effects due to the combined action of reactive species and the intrinsic toxicity of copper ions. Studies report complete inactivation of Staphylococcus aureus within 15 minutes when treated with plasma-functionalized copper nanoparticles. The synergistic effect of plasma-induced surface modifications and the inherent properties of the nanoparticles enhances their pathogen-killing efficiency.

Residual toxicity is a critical consideration when deploying plasma-treated nanoparticles for pathogen inactivation. While the reactive species generated are highly effective against microbes, their persistence in the environment or biological systems must be carefully evaluated. Gold nanoparticles, being chemically inert, pose minimal toxicity risks once the reactive species are depleted. However, copper nanoparticles may leach copper ions, which can accumulate and cause cytotoxicity in mammalian cells. Studies indicate that residual copper concentrations above 10 ppm can impair cell viability, necessitating precise control over nanoparticle dosage and application conditions. Post-treatment washing or coating with biocompatible materials like polyethylene glycol (PEG) can mitigate these effects.

The stability of plasma-treated nanoparticles under operational conditions also influences their long-term performance. Exposure to ambient air and moisture can lead to the gradual loss of surface functional groups, reducing their reactive species generation capacity. Encapsulation or embedding these nanoparticles in polymer matrices has been explored to prolong their activity. For instance, incorporating plasma-treated gold nanoparticles into chitosan films maintains their antimicrobial properties for extended periods while preventing aggregation.

Comparative studies between plasma-treated and untreated nanoparticles highlight the advantages of plasma functionalization. Untreated gold nanoparticles exhibit minimal antimicrobial activity, whereas plasma-treated versions achieve significant pathogen inactivation. Similarly, untreated copper nanoparticles rely solely on ion release, which is slower and less effective compared to the rapid action of plasma-induced reactive species. The combination of plasma treatment with nanoparticle size optimization further enhances performance, with smaller particles (below 50 nm) showing higher surface area and reactivity.

Applications of plasma-treated nanoparticles extend beyond direct pathogen inactivation. They can be integrated into filtration systems, wound dressings, and surface coatings to provide continuous antimicrobial protection. For example, embedding these nanoparticles in air filters reduces microbial load by over 90%, demonstrating their potential for improving indoor air quality. In medical settings, plasma-treated nanoparticle coatings on catheters and implants prevent biofilm formation, reducing infection risks.

Future research directions include optimizing plasma parameters for specific nanoparticle compositions and scaling up production for industrial applications. The development of real-time monitoring techniques to assess reactive species generation and pathogen inactivation efficiency is also crucial. Additionally, exploring the environmental impact of these nanoparticles, particularly their degradation pathways and long-term ecotoxicity, will ensure sustainable deployment.

In summary, plasma-treated gold and copper nanoparticles represent a versatile and effective approach for pathogen inactivation through reactive species generation. By carefully controlling plasma parameters, surface activation, and residual toxicity, these nanoparticles can be tailored for diverse applications in healthcare, environmental remediation, and public health. Their ability to combine the unique properties of nanomaterials with plasma-induced reactivity opens new avenues for combating microbial threats.