Abstract: There is increasing interest in plasma technology for CO2 conversion because it can operate at mild conditions and it can store fluctuating renewable electricity into
value-added compounds and renewable fuels. This perspective paper aims to provide a view on the future for non-specialists who want to understand the role of plasma
technology in the new scenario for sustainable and low-carbon energy and chemistry. Thus, it is prepared to give a personal view on future opportunities and challenges. First, we introduce the current state-of-the-art and the potential of plasma-based CO2 conversion. Subsequently, we discuss the challenges to overcome the current limitations and to apply plasma technology on a large scale. The final section discusses the general context and the potential benefits of plasma-based CO2 conversion for our life and the impact on climate change. It also includes a brief analysis on the future scenario for energy and chemical production, and how plasma technology may realize new paths for CO2 utilization.

Abstract: The requirements for new technologies to serve as anticancer agents go far beyond their toxicity potential. Novel applications also need to be safe on a molecular and patient level. In a broader sense, this also relates to cancer metastasis and inflammation. In a previous study, the toxicity of an atmospheric pressure argon plasma jet in four human pancreatic cancer cell lines was confirmed and plasma treatment did not promote metastasis in vitro and in ovo. Here, these results are extended by additional types of analysis and new models to validate and define on a molecular level the changes related to metastatic processes in pancreatic cancer cells following plasma treatment in vitro and in ovo. In solid tumors that were grown on the chorion-allantois membrane of fertilized chicken eggs (TUM-CAM), plasma treatment induced modest to profound apoptosis in the tissues. This, however, was not associated with a change in the expression levels of adhesion molecules, as shown using immunofluorescence of ultrathin tissue sections. Culturing of the cells detached from these solid tumors for 6d revealed a similar or smaller total growth area and expression of ZEB1, a transcription factor associated with cancer metastasis, in the plasma-treated pancreatic cancer tissues. Analysis of in vitro and in ovo supernatants of 13 different cytokines and chemokines revealed cell line-specific effects of the plasma treatment but a noticeable increase of, e.g., growth-promoting interleukin 10 was not observed. Moreover, markers of epithelial-to-mesenchymal transition (EMT), a metastasis-promoting cellular program, were investigated. Plasma-treated pancreatic cancer cells did not present an EMT-profile. Finally, a realistic 3D tumor spheroid co-culture model with pancreatic stellate cells was employed, and the invasive properties in a gel-like cellular matrix were investigated. Tumor outgrowth and spread was similar or decreased in the plasma conditions. Altogether, these results provide valuable insights into the effect of plasma treatment on metastasis-related properties of cancer cells and did not suggest EMT-promoting effects of this novel cancer therapy.

Abstract: The physics and chemistry of plasma–surface interaction is a broad domain relevant to various applications and several natural processes, including plasma etching for microelectronics fabrication, plasma deposition, surface functionalization, nanomaterial synthesis, fusion reactors, and some astrophysical and meteorological phenomena. Due to their complex nature, each of these processes is generally investigated in separate subdomains, which are considered to have their own theoretical, modeling, and experimental challenges. In this review, however, we want to emphasize the overarching nature of plasma–surface interaction physics and chemistry, by focusing on the general strategy for its computational simulation. In the first half of the review, we provide a menu card with standard and less standardized computational methods to be used for the multiscale modeling of the underlying processes. In the second half, we illustrate the benefits and potential of the multiscale modeling strategy with a case study of Si and SiO2 etching by fluorocarbon plasmas and identify the gaps in knowledge still present on this intensely investigated plasma–material combination, both on a qualitative and quantitative level. Remarkably, the dominant etching mechanisms remain the least understood. The resulting new insights are of general relevance, for all plasmas and materials, including their various applications. We therefore hope to motivate computational and experimental scientists and engineers to collaborate more intensely on filling the existing gaps in knowledge. In this way, we expect that research will overcome a bottleneck stage in the development and optimization of multiscale models, and thus the fundamental understanding of plasma–surface interaction.

Abstract: We performed molecular dynamics simulations to investigate the effect of lipid peroxidation products on the structural and dynamic properties of the cell membrane. Our simulations predict that the lipid order in a phospholipid bilayer, as a model system for the cell membrane, decreases upon addition of lipid peroxidation products. Eventually, when all phospholipids are oxidized, pore formation can occur. This will allow reactive species, such as reactive oxygen and nitrogen species (RONS), to enter the cell and cause oxidative damage to intracellular macromolecules, such as DNA or proteins. On the other hand, upon increasing the cholesterol fraction of lipid bilayers, the cell membrane order increases, eventually reaching a certain threshold, from which cholesterol is able to protect the membrane against pore formation. This finding is crucial for cancer treatment by plasma technology, producing a large number of RONS, as well as for other cancer treatment methods that cause an increase in the concentration of extracellular RONS. Indeed, cancer cells contain less cholesterol than their healthy counterparts. Thus, they will be more vulnerable to the consequences of lipid peroxidation, eventually enabling the penetration of RONS into the interior of the cell, giving rise to oxidative stress, inducing pro-apoptotic factors. This provides, for the first time, molecular level insight why plasma can selectively treat cancer cells, while leaving their healthy counterparts undamaged, as is indeed experimentally demonstrated.

Abstract: We report on multi-level atomistic simulations for the interaction of reactive oxygen species (ROS) with the head groups of the phospholipid bilayer, and the subsequent effect of head group and lipid tail oxidation on the structural and dynamic properties of the cell membrane. Our simulations are validated by experiments using a cold atmospheric plasma as external ROS source. We found that plasma treatment leads to a slight initial rise in membrane rigidity, followed by a strong and persistent increase in fluidity, indicating a drop in lipid order. The latter is also revealed by our simulations. This study is important for cancer treatment by therapies producing (extracellular) ROS, such as plasma treatment. These ROS will interact with the cell membrane, first oxidizing the head groups, followed by the lipid tails. A drop in lipid order might allow them to penetrate into the cell interior (e.g., through pores created due to oxidation of the lipid tails) and cause intracellular oxidative damage, eventually leading to cell death. This work in general elucidates the underlying mechanisms of ROS interaction with the cell membrane at the atomic level.

Abstract: Cold atmospheric pressure plasmas are gaining increased interest in the medical sector and clinical trials to treat skin diseases are underway. Plasmas are capable of producing several reactive oxygen and nitrogen species (RONS). However, there are open questions how plasma-generated RONS interact on a molecular level in a biological environment, e.g. cells or cell components. The redox pair glutathione (GSH) and glutathione disulphide (GSSG) forms the most important redox buffer in organisms responsible for detoxification of intracellular reactive species. We apply Raman spectroscopy, mass spectrometry, and molecular dynamics simulations to identify the time-dependent chemical modifications on GSH and GSSG that are caused by dielectric barrier discharge under ambient conditions. We find GSSG, S-oxidised glutathione species, and S-nitrosoglutathione as oxidation products with the latter two being the final products, while glutathione sulphenic acid, glutathione sulphinic acid, and GSSG are rather reaction intermediates. Experiments using stabilized pH conditions revealed the same main oxidation products as were found in unbuffered solution, indicating that the dominant oxidative or nitrosative reactions are not influenced by acidic pH. For more complex systems these results indicate that too long treatment times can cause difficult-to-handle modifications to the cellular redox buffer which can impair proper cellular function.

Abstract: Controlling and steering the growth of single walled carbon nanotubes is often believed to require controlling of the nucleation stage. Yet, little is known about the microscopic mechanisms governing the nucleation from hydrocarbon molecules. Specifically, we address here the dehydrogenation of hydrocarbon molecules and the formation of all-carbon graphitic islands on metallic nanoclusters from hydrocarbon molecules under conditions typical for carbon nanotube growth. Employing reactive molecular dynamics simulations, we demonstrate for the first time that the formation of a graphitic network occurs through the intermediate formation of vertically oriented, not fully dehydrogenated graphitic islands. Upon dehydrogenation of these vertical graphenes, the islands curve over the surface, thereby forming a carbon network covering the nanoparticle. The results indicate that controlling the extent of dehydrogenation offers an additional parameter to control the nucleation of carbon nanotubes.

Abstract: Using reactive molecular dynamics simulations, the melting behavior of nickelcarbon nanoclusters is examined. The phase diagrams of icosahedral and Wulff polyhedron clusters are determined using both the Lindemann index and the potential energy. Formulae are derived for calculating the equilibrium constants and the solid and liquid fractions during a phase transition, allowing more rational determination of the melting temperature with respect to the arbitrary Lindemann value. These results give more insight into the properties of nickelcarbon nanoclusters in general and can specifically be very useful for a better understanding of the synthesis of carbon nanotubes using the catalytic chemical vapor deposition method.

Abstract: Atomic scale simulations of the nucleation and growth of carbon nanotubes is essential for understanding their growth mechanism. In spite of over twenty years of simulation efforts in this area, limited progress has so far been made on addressing the role of the hydrocarbon growth precursor. Here we report on atomic scale simulations of cap nucleation of single-walled carbon nanotubes from hydrocarbon precursors. The presented mechanism emphasizes the important role of hydrogen in the nucleation process, and is discussed in relation to previously presented mechanisms. In particular, the role of hydrogen in the appearance of unstable carbon structures during in situ experimental observations as well as the initial stage of multi-walled carbon nanotube growth is discussed. The results are in good agreement with available experimental and quantum-mechanical results, and provide a basic understanding of the incubation and nucleation stages of hydrocarbon-based CNT growth at the atomic level.

Abstract: Purified water supply for human use, agriculture and industry is the major global priority nowadays. The advanced oxidation process based on atmospheric pressure non-thermal plasma (NTP) has been used for purification of wastewater, although the underlying mechanisms of degradation of organic pollutants are still unknown. In this study we employ two needle-type atmospheric pressure non-thermal plasma jets, i.e., indirect (ID-APPJ) and direct (D-APPJ) jets operating at Ar feed gas, for the treatment of methylene blue, methyl orange and congo red dyes, for two different times (i.e., 20 min and 30 min). Specifically, we study the decolorization/degradation of all three dyes using the above mentioned plasma sources, by means of UV-Vis spectroscopy, HPLC and a density meter. We also employ mass spectroscopy to verify whether only decolorization or also degradation takes place after treatment of the dyes by the NTP jets. Additionally, we analyze the interaction of OH radicals with all three dyes using reactive molecular dynamics simulations, based on the density functional-tight binding method. This investigation represents the first report on the degradation of these three different dyes by two types of NTP setups, analyzed by various methods, and based on both experimental and computational studies.

Abstract: Plasma, the fourth and most pervasive state of matter in the visible universe, is a fascinating medium that is connected to the beginning of our universe itself. Man-made plasmas are at the core of many technological advances that include the fabrication of semiconductor devices, which enabled the modern computer and communication revolutions. The introduction of low temperature, atmospheric pressure plasmas to the biomedical field has ushered a new revolution in the healthcare arena that promises to introduce plasma-based therapies to combat some thorny and long-standing medical challenges. This article presents an overview of where research is at today and discusses innovative concepts and approaches to overcome present challenges and take the field to the next level. It is written by a team of experts who took an in-depth look at the various applications of plasma in hygiene, decontamination, and medicine, made critical analysis, and proposed ideas and concepts that should help the research community focus their efforts on clear and practical steps necessary to keep the field advancing for decades to come.

Abstract: We investigate the theoretical possibility of single layer graphene formation on a nickel surface at different substrate temperatures under far-from-equilibrium high precursor flux conditions, employing state-of-the-art hybrid reactive molecular dynamics/uniform acceptance force bias Monte Carlo simulations. It is predicted that under these conditions, the formation of a single layer graphene-like film may proceed through a combined depositionsegregation mechanism on a nickel substrate, rather than by pure surface segregation as is typically observed for metals with high carbon solubility. At 900 K and above, nearly continuous graphene layers are obtained. These simulations suggest that single layer graphene deposition is theoretically possible on Ni under high flux conditions.

Abstract: The application of coreshell SiSiO2 nanowires as nanoelectronic devices strongly depends on their structure, which is difficult to tune precisely. In this work, we investigate the formation of the coreshell nanowires at the atomic scale, by reactive molecular dynamics simulations. The occurrence of two temperature-dependent oxidation mechanisms of ultra-small diameter Si-NWs is demonstrated. We found that control over the Si-core radius and the SiOx (x ≤ 2) oxide shell is possible by tuning the growth temperature and the initial Si-NW diameter. Two different structures were obtained, i.e., ultrathin SiO2 silica nanowires at high temperature and Si core|ultrathin SiO2 silica nanowires at low temperature. The transition temperature is found to linearly decrease with the nanowire curvature. Finally, the interfacial stress is found to be responsible for self-limiting oxidation, depending on both the initial Si-NW radius and the oxide growth temperature. These novel insights allow us to gain control over the exact morphology and structure of the wires, as is needed for their application in nanoelectronics.

Abstract: The interaction of plasma with liquids has led to various established industrial implementations as well as promising applications, including high-voltage switching, chemical analysis, nanomaterial synthesis, and plasma medicine. Along with these numerous accomplishments, the physics of plasma in liquid or in contact with a liquid surface has emerged as a bipartite research field, for which we introduce here the term “plasma physics of liquids.” Despite the intensive research
investments during the recent decennia, this field is plagued by some controversies and gaps in knowledge, which might restrict further progress. The main difficulties in understanding revolve around the basic mechanisms of plasma initiation in the liquid phase and the electrical interactions at a plasma-liquid interface, which require an interdisciplinary approach. This review aims to provide the wide applied physics community with a general overview of the field, as well as the opportunities for interdisciplinary research on topics, such as nanobubbles and the floating water bridge, and involving the research domains of amorphous semiconductors, solid state physics, thermodynamics, material science, analytical chemistry, electrochemistry, and molecular dynamics simulations. In addition, we provoke awareness of experts in the field on yet underappreciated question marks. Accordingly, a strategy for future experimental and simulation work is proposed.

Abstract: Plasma-based gas conversion has great potential for enabling carbon-free fertilizer production powered by renewable electricity. Sustaining an energy-efficient plasma process without eroding the containment vessel is currently a significant challenge, limiting scaling to higher powers and throughputs. Isolation of the plasma from contact with any solid surfaces is an advantage, which both limits energy loss to the walls and prevents material erosion that could lead to disastrous soil contamination. This paper presents highly energy-efficient nitrogen fixation from air into NOx by microwave plasma, with the plasma filament isolated at the center of a quartz tube using a vortex gas flow. NOx production is found to scale very efficiently when increasing both gas flow rate and absorbed power. The lowest energy cost recorded of ~2 MJ/mol, for a total NOx production of ~3.8%, is the lowest reported up to now for atmospheric pressure plasmas.