Academic literature on the topic 'Atmospheric plasma applications'

Plasma is an electrically conducting medium that responds to electric and magnetic fields. It consists of large quantities of highly reactive species, such as ions, energetic electrons, exited atoms and molecules, ultraviolet photons, and metastable and active radicals. Non-thermal or cold plasmas are partially ionized gases whose electron temperatures usually exceed several tens of thousand degrees K, while the ions and neutrals have much lower temperatures. Due to the presence of reactive species at low temperature, the biological effects of non-thermal plasmas have been studied for application in the medical area with promising results. This review outlines the application of cold atmospheric pressure plasma (CAPP) in dentistry for the control of several pathogenic microorganisms, induction of anti-inflammatory, tissue repair effects and apoptosis of cancer cells, with low toxicity to healthy cells. Therefore, CAPP has potential to be applied in many areas of dentistry such as cariology, periodontology, endodontics and oral oncology.

The use of plasmas for biomedical applications in encountering a growing interest, especially in the framework of so-called “plasma medicine”, which aims at exploiting the action of low-power, atmospheric pressure plasmas for therapeutic purposes [...]

Plasma-based treatment of chronic wounds or skin diseases as well as tissue engineering or tumor treatment is an extremely promising field. First practical studies are promising, and plasma medicine as an independent medical field is emerging worldwide. While during the last years the basics of sterilizing effects of plasmas were well studied, concepts of tailor-made plasma sources which meet the technical requirements of medical instrumentation are still less developed. Indeed, studies on the verification of selective antiseptic effects of plasmas are required, but the development of advanced plasma sources for biomedical applications and a profound knowledge of their physics, chemistry, and parameters must be contributed by physical research. Considering atmospheric-pressure plasma sources, the determination of discharge development and plasma parameters is a great challenge, due to the high complexity and limited diagnostic approaches. This contribution gives an overview on plasma sources for therapeutic applications in plasma medicine. Selected specific plasma sources that are used for the investigation of various biological effects are presented and discussed. Furthermore, the needs, prospects, and approaches for its characterization from the fundamental plasma physical point of view will be discussed.

The ability to produce cold plasma at atmospheric pressure conditions was the basis for the rapid growth of plasma-related application areas in biomedicine. Plasma comprises a multitude of active components such as charged particles, electric current, UV radiation, and reactive gas species which can act synergistically. Anti-itch, antimicrobial, anti-inflammatory, tissue-stimulating, blood flow-enhancing, and proapoptotic effects were demonstrated in in vivo and in vitro experiments, and until now, no resistance of pathogens against plasma treatment was observed. The combination of the different active agents and their broad range of positive effects on various diseases, especially easily accessible skin diseases, renders plasma quite attractive for applications in medicine. For medical applications, two different types of cold plasma appear suitable: indirect (plasma jet) and direct (dielectric barrier discharge—DBD) plasma sources. The DBD device PlasmaDerm® VU-2010 (CINOGY Technologies GmbH), the atmospheric pressure plasma jet (APPJ) kINPen® MED (INP Greifswald/neoplas tools GmbH), and the SteriPlas (Adtec Ltd., London, United Kingdom) are CE-certified as a medical product to treat chronic wounds in humans and showed efficacy and a good tolerability. Recently, the use of plasma in cancer research and oncology is of particular interest. Plasma has been shown to induce proapoptotic effects more efficiently in tumor cells compared with the benign counterparts, leads to cellular senescence, and—as shown in vivo—reduces skin tumors. To this end, a world-wide first Leibniz professorship for plasmabiotechnology in dermatology has been introduced to establish a scientific network for the investigation of the efficacy and safety of cold atmospheric plasma in dermatooncology. Hence, plasma medicine especially in dermatology holds great promise.

Plasma chemistry is a rapidly growing field which covers applications ranging from technological processing of materials, including biological tissues, to environmental remediation and energy production. The so called atmospheric plasma, produced by electric corona or dielectric barrier discharges in a gas at atmospheric pressure, is particularly attractive for the low costs and ease of operation and maintenance involved. The high concentrations of energetic and chemically active species (e.g. electrons, ions, atoms and radicals, excited states, photons) present in such plasmas can promote chemical reactions which are otherwise hardly possible under such mild temperature conditions. This thesis deals with the use of atmospheric plasma to activate two different processes: water purification from organic pollutants and carbon dioxide reforming of methane to produce syngas. Both address major environmental issues, specifically the ever growing demand for drinking water and the need to control carbon emissions in the atmosphere. Due to the very different nature of the two investigated processes, different plasma sources, types of discharge, reactors, experimental conditions and analytical procedures had to be developed and adopted. Despite such differences, however, both lines of research stem from a common background and share a common goal: to understand and exploit the great chemical potential of atmospheric plasma. Thus, a common research approach was used, based on extensive investigation of the discharge and plasma features, notably of its reactive species, and of the process efficiency, products and intermediates. The mechanistic investigations involved quantitative product and kinetic studies, spectroscopic determinations and some modeling. An already available prototype reactor was used for water treatment, in which dielectric barrier discharges (DBD) are generated in the air above the liquid. The strong oxidants formed in humid air plasma (OH radicals, atomic oxygen, ozone) interact with the aqueous solution and induce the oxidation of even the most resistant organic pollutants. Phenol was used as a model organic pollutant and found to be decomposed quite efficiently, especially in dilute solutions, the rate of reaction increasing linearly with the reciprocal of phenol initial concentration. Despite its high reactivity air plasma displays some selectivity. The rate of oxidation of monosubstituted phenols (m-(CH3)2N-, m-Cl-, p-NO2- and m-NO2-) depends linearly on the Hammett substituents constant yielding a rho value of -0.48 which is characteristic of electrophilic attack by the OH radical. The main products and intermediates of phenol decomposition were determined quantitatively. The behavior of two such intermediates, maleic acid and fumaric acid, was investigated in detail since they are very common water secondary pollutants formed in the oxidative degradation of most aromatic compounds. The reaction mechanisms and the role of the major oxidizing species – hydroxyl radical and ozone – were investigated in experiments in which the two acids were treated separately and also in mixture under different pH conditions. Most interesting and useful was also the comparison with the results obtained in experiments of ozonation conducted under the same experimental conditions except for the fact that ozone was produced ex situ. These experiments show that under any conditions plasma treatment is more efficient due to the contribution of short lived highly reactive species. As for the oxidation mechanism of the two acids in the plasma system, it is concluded that due to their high reactivity with ozone, the decomposition process of maleic and fumaric acids is mainly due to this species. Depending on the pH of the solution, ozone reacts directly with the organic molecules or is converted to OH radicals. However, additional OH radicals produced directly by the electrical discharge also contribute to the oxidation of maleic and fumaric acids in the air-liquid plasma system, independently on the pH used. Thus, the direct formation of •OH by the discharge in situ constitutes a big advantage of plasma treatment over reaction with ozone produced ex-situ, in particular at acidic pH values for compounds which do not react with ozone itself. In fact, contrary to ozone, OH radicals react efficiently with any organic compound and when directly produced by the discharge their concentration is independent on pH. The obtained results are also very useful to show the importance of ozone mass transfer from the gas phase to the solution. Both in plasma treatment and in ozonation ozone is not accumulated into the solution but reacts as it is transferred in water or directly on its surface. However, comparing the behavior of maleic and fumaric acids in plasma treatment and in ozonation, it was demonstrated that the ion wind present in the DBD reactor, due to the charged species formed by the discharge, plays an important role in mixing the solution. In fact, when ozone produced ex-situ is used magnetic stirring of the solution is required to allow the reaction to take place also in the bulk and not only on the surface of water, while in the case of plasma treatment magnetic stirring increases the rate of the reaction but does not change significantly the shape of the oxidation curves. The reactor and experimental apparatus for performing plasma driven carbon dioxide reforming of methane and product analysis had to be designed and developed from scratch since this line of research started with this Thesis. To allow emission spectroscopy measurements and in view of future investigations on the combination of plasma with heterogeneous catalysis, the reactor was made of quartz: two flanges are welded on the extremities of a tube of 570 mm of length and 37 mm of diameter, while a ring is welded in the middle of the tube to support a stainless steel tip which constitutes the high voltage electrode. The grounded counter electrode has the shape of a funnel and is covered by a stainless steel mesh. Most of the quartz tube is filled with ceramic cylinders, while the plasma zone occupies a volume of about 40 cm3 in the middle of the tube for allowing its heating in a vertical furnace for future investigations with heterogeneous catalysts. The setting-up of the experimental apparatus was a major task which was followed by preliminary tests with different types of discharge for determining the most efficient regime for transformation of methane and carbon dioxide to the mixture of hydrogen and CO. The best results in terms of efficiency and product selectivity were obtained with a spark discharge, self-triggered by a simple and efficient power supplying. The average electron density of the plasma, 5.7 x 1014 cm-3, was measured by emission spectroscopy techniques and the temperature of the bulk gas, approaching 100°C, by a thermocouple. However, the main characteristic of spark is the development of discharge filaments, in which the electron density and the temperature of the species, such as electrons, radicals, ions, but also atoms and molecules, are significantly higher than those of the bulk. In the present reactor these filaments fill completely the plasma zone. Thus, it is assumed that the elementary processes of the reaction between methane and carbon dioxide take place inside the discharge filaments. The major products, hydrogen and carbon monoxide were determined quantitatively by GC/FID/TCD. A few byproducts were also detected in low percentages and identified by means of GC/MS analysis. These include ethane, ethylene and acetylene. Based on quantitative product data and on precise measurements of the input and output flows, the reagents conversion, the products yield and selectivity and the energy efficiency of the process were calculated. The quite high conversion of CH4 (74%) and CO2 (69%), the high selectivity for the desired products (78% H2 and 86% CO) and the good energy efficiency (2.4 mmol/kJ) obtained make this system competitive with other reactors/processes described in the literature. Moreover, no carbon deposition was observed and CO2/CH4 ratios between 0.5 and 1.5 could be used without significant changes in the characteristics of the process. Easy power control and self-triggering of the system eliminate the need for expensive high-voltage switches, making this setup attractive for scaling up and further development.
La chimica dei plasmi è un settore in rapida espansione che conta un gran numero di applicazioni, dal trattamento di materiali, inclusi materiali biologici, alla decomposizione di inquinanti e produzione di energia. Il cosiddetto plasma atmosferico, prodotto da scariche elettriche corona o a barriera di dielettrico in un gas a pressione atmosferica, è particolarmente attraente grazie ai costi contenuti e alla facilità di impiego e manutenzione. L’elevata concentrazione di specie ad alta energia chimicamente attive (ad esempio elettroni, ioni, atomi, radicali, specie eccitate, fotoni) presenti in questi plasmi può promuovere reazioni chimiche che in condizioni più blande sarebbero difficilmente realizzabili. La Tesi riguarda l’impiego del plasma atmosferico per attivare due diversi processi: la purificazione dell’acqua da inquinanti organici e il reforming di metano con anidride carbonica per produrre gas di sintesi. Entrambi i processi mirano a dare un contributo nella risoluzione di un problema ambientale, la crescente domanda di acqua potabile in un caso, la necessità di limitare le emissioni di carbonio nell’atmosfera nell’altro. A causa della natura molto diversa dei due processi indagati, essi richiedono lo sviluppo e l’impiego di sorgenti di plasma, tipi di scarica, reattori, condizioni e procedure sperimentali diversi. Tuttavia, nonostante queste differenze, entrambe le linee di ricerca derivano da conoscenze comuni e condividono lo stesso obiettivo: comprendere e sfruttare l’enorme potenziale chimico dei plasmi atmosferici. Anche nella ricerca è stato quindi applicato un approccio comune, basato su uno studio approfondito delle caratteristiche della scarica elettrica e del plasma, in particolare per quanto riguarda le specie reattive, dell’efficienza del processo e dei prodotti e degli intermedi che si formano nel processo. Gli studi meccanicistici sono basati sull’analisi quantitativa dei prodotti, sulla cinetica del processo, su misure spettroscopiche e su simulazioni. Il reattore impiegato per il trattamento delle acque è un prototipo realizzato in precedenza, in cui vengono generate scariche a barriera di dielettrico (DBD) nell’aria sovrastante la soluzione. I potenti ossidanti formati nel plasma in aria umida (radicale OH, ossigeno atomico, ozono) interagiscono con la soluzione acquosa e inducono l’ossidazione anche dei più resistenti inquinanti organici. Il fenolo, usato come inquinante organico modello, viene decomposto efficacemente, soprattutto in soluzioni diluite. La sua velocità di scomparsa aumenta linearmente con il reciproco della sua concentrazione iniziale. Nonostante l’elevata reattività, il plasma in aria mostra una certa selettività. La velocità di ossidazione di fenoli monosostituiti m-((CH3)2N-, m-Cl-, p-NO2- and m-NO2-) dipende linearmente dalle costanti di Hammett. Il valore di rho ottenuto, pari a -0.48, è caratteristico dell’attacco elettrofilo da parte del radicale OH. I principali prodotti ed intermedi della decomposizione del fenolo sono stati determinati quantitativamente. Il comportamento di due di questi intermedi, l’acido maleico e l’acido fumarico, è stato analizzato in dettaglio poiché si tratta di comuni inquinanti secondari delle acque derivanti dalla degradazione ossidativa della maggior parte dei composti aromatici. Esperimenti in cui i due acidi sono stati trattati separatamente e in miscela a diversi pH hanno permesso di indagare i meccanismi di reazione e il ruolo delle principali specie ossidanti – radicale ossidrile e ozono - nella decomposizione dei due acidi. Molto interessante ed utile è stato anche il confronto con i risultati ottenuti in esperimenti di ozonizzazione realizzati nelle stesse condizioni sperimentali ma in cui l’ozono veniva prodotto ex situ. Questi esperimenti dimostrano che in tutte le condizioni sperimentali il trattamento al plasma è più efficiente del trattamento con solo ozono grazie al contributo aggiuntivo da parte di specie a vita breve altamente reattive. Per quanto riguarda il meccanismo di ossidazione dei due acidi nel plasma, è stato concluso che a causa dell’elevata reattività con ozono, il processo di decomposizione degli acidi maleico e fumarico è dovuto principalmente a questa specie. A seconda del pH della soluzione, l’ozono reagisce con le molecole organiche come tale oppure viene convertito in radicali OH. Nel sistema al plasma, radicali OH vengono prodotti anche direttamente dalla scarica elettrica e contribuiscono anch’essi all’ossidazione degli acidi maleico e fumarico, indipendentemente dal pH della soluzione. E’ quindi evidente che la formazione diretta di •OH in situ da parte della scarica costituisce un enorme vantaggio del trattamento al plasma rispetto al caso in cui l’ozono venga prodotto ex-situ, in particolare nel caso di composti che a pH acidi non siano in grado di reagire direttamente con l’ozono. Infatti, contrariamente all’ozono, il radicale OH reagisce in modo efficiente con qualsiasi composto organico, inoltre, quando viene prodotto direttamente dalla scarica la sua concentrazione è indipendente dal pH. I risultati ottenuti si sono rivelati molto utili anche per dimostrare l’importanza del trasferimento di massa dell’ozono dalla fase gas alla soluzione. Sia nel trattamento al plasma che nell’ozonizzazione l’ozono non si accumula nella soluzione ma reagisce non appena viene trasferito in acqua o direttamente sulla superficie dell’acqua. Comunque, confrontando il comportamento degli acidi maleico e fumarico nel trattamento al plasma e nell’ozonizzazione, è stato dimostrato che il vento ionico attivo nel reattore DBD e dovuto al trasferimento di specie cariche generate dalla scarica, svolge un ruolo importante nel mescolamento della soluzione. Infatti, quando l’ozono viene prodotto ex-situ è necessario agitare la soluzione con un’ancoretta magnetica perché la reazione abbia luogo nell’intera massa di acqua e non solo sulla sua superficie; al contrario, nel caso del trattamento al plasma il mescolamento magnetico aumenta la velocità della reazione ma non cambia significativamente la forma dell’andamento dell’ossidazione in funzione del tempo. Nel caso del processo di reforming di metano con CO2 attivato da plasma è stato necessario progettare e sviluppare il reattore e l’intero sistema sperimentale da zero poiché questa linea di ricerca è stata iniziata con questa Tesi. Per poter realizzare misure di spettroscopia di emissione e in vista di studi futuri sulla combinazione del plasma con la catalisi eterogenea, il reattore è stato realizzato in quarzo: due flange sono saldate alle estremità di un tubo lungo 570 mm e largo 37 mm (diametro interno), mentre un anello è saldato nel mezzo del tubo per supportare una punta di acciaio inossidabile che costituisce l’elettrodo ad alto voltaggio. Il controelettrodo, posto al potenziale di terra, ha la forma di un imbuto ed è ricoperto da una retina di acciaio. Il tubo è in buona parte riempito con cilindri di ceramica forati, mentre la zona del plasma occupa un volume di circa 40 cm3 nel mezzo del tubo, soluzione che ne permetterebbe il riscaldamento in una fornace verticale in eventuali studi futuri con catalizzatori eterogenei. La realizzazione dell’apparato sperimentale ha richiesto un grosso impegno. Il passo successivo è stato l’esecuzione di esperimenti di prova con diversi tipi di scarica per determinare il regime più efficiente per realizzare la trasformazione di metano e anidride carbonica in una miscela di idrogeno e monossido di carbonio. I risultati migliori in termini di efficienza e selettività dei prodotti sono stati ottenuti con una scarica di tipo spark, auto-innescante grazie ad un sistema di alimentazione elettrica semplice ed efficiente. La densità elettronica media del plasma, pari a 5.7 x 1014 cm-3, è stata misurata tramite tecniche di spettroscopia di emissione e la temperatura del gas, poco inferiore a 100°C, tramite una termocoppia. La caratteristica principale della scarica di tipo spark è lo sviluppo di canali filamentari di scarica, in cui la densità degli elettroni e la temperatura delle specie, vale a dire elettroni, radicali, ioni, ma anche atomi e molecole, sono significativamente maggiori di quelle della massa del gas. Nel reattore in questione questi canali filamentari di scarica occupano interamente la regione in cui si sviluppa il plasma. Di conseguenza, si può assumere che i processi elementari della reazione tra metano e anidride carbonica si verifichino all’interno di tali canali. I prodotti principali della reazione, idrogeno e monossido di carbonio, sono stati determinati quantitativamente tramite GC/FID/TCD. Alcuni sottoprodotti sono stati rivelati in basse percentuali e identificati tramite analisi GC/MS: si tratta di etano, etilene ed acetilene. Sulla base dei dati quantitativi relativi alla formazione dei prodotti e delle misure precise dei flussi di entrata ed uscita del gas nel e dal reattore, sono state calcolate le percentuali di conversione dei reagenti e di resa e selettività dei prodotti. I risultati di conversione di CH4 (74%) e CO2 (69%), di selettività per i prodotti desiderati (78% H2 and 86% CO) e di efficienza energetica sono risultati molto buoni e rendono il sistema competitivo con altri reattori e processi descritti nella letteratura. Non viene inoltre osservata deposizione di carbone e il rapporto CO2/CH4 può essere variato tra 0.5 e 1.5 senza variazioni significative delle caratteristiche del processo. La facilità di controllo della potenza e la caratteristica di auto-innesco del sistema fanno sì che non siano necessari costosi sistemi di controllo che lavorano ad alto voltaggio e rendono promettente il ridimensionamento dell’apparato sperimentale e interessante il suo impiego in ricerche future.

Les plasmas froids produits par les décharges électriques sont des gaz faiblement ionisés, ce qui maintient la température du gaz à une température proche de la température ambiante, contrairement à la température de l'électron qui peut atteindre plusieurs électron-volts. Les applications des plasmas froids en médecine et en agriculture sont des nouveaux domaines de recherche multidisciplinaires basés sur les interactions de ces plasmas avec des organismes vivants. Le champ électrique ainsi que les espèces réactives de l’oxygène et de l'azote peuvent inactiver les bactéries, stimuler la régénération de la peau (dermatologie), la réduction tumorale (oncologie) et la germination des graines (agriculture). Ces nouveaux domaines de recherche, basé sur la chimie produite par l’interaction plasma-liquide est très prometteur et se développe rapidement. L’objectif de ce travail est d’étudier les interactions entre les plasmas froids et l’eau pour les applications biologiques, d’une part la promotion de la germination des graines au moyen d’une décharge à barrière diélectrique (DBD) et, d’autre part, l’effet ex vivo d’un traitement par jet de plasma froid sur la peau.Ce manuscrit est divisé en cinq chapitres: i) On présente tout d'abord une revue de la littérature présentant l'état de l’art concernant l'interaction plasma-liquide et les principales avancées en matière d'applications des plasmas froids à la germination des semences. Ii) Deuxièmement, les dispisitifs expérimentaux sont décrits, en particulier la fabrication de réacteurs à plasma utilisant l’impression 3D. Iii) Ensuite, la production d'espèces réactives gazeuses et aqueuses formées par des plasmas de type DBD a été mesurée quantitativement et l'interaction plasma-liquide a été analysée. Iv) Puis, plusieurs variétés de graines ont été sélectionnées pour évaluer l’effet un traitement par plasma DBD ; l'étude des mécanismes de promotion de la germination du plasma a été spécifiquement étudiée en traitant les graines de soja vert dans différentes conditions de décharge, dans différents milieux, avec un champ électrique seul et dans différentes conditions de cultures ou de niveau d'hydratation des graines.v) Enfin, l'imagerie paramétrique de Muller (MPI) a été appliquée pour la modification de la peau de souris ex vivo traitées par un plasma à jet d'hélium
Non-Thermal-Plasmas (NTP) produced by electric discharges are weakly ionized gases, which keeps the gas temperature at near room temperature contrary to the electron temperature which can reach several electron-Volts. Applications of NTP to medicine and agriculture are new multidisciplinary research fields based on interactions of the Non-Thermal-Plasmas with living organisms. Electric field as well as Reactive Oxygen and Nitrogen Species produced by NTP may inactivate bacteria, stimulate skin regeneration (dermatology), tumor reduction (oncology) and seeds germination (agriculture). These new fields of research are based on the plasma-liquid chemistry. The objective of this work is to study the NTP interacting with water for biological applications including on one hand, the promotion of the germination of seeds using a Dielectric Barrier Discharge (DBD) and on the other hand, the effect of a plasma jet treatment ex vivo on skinThis manuscript is divided in five chapters: i) First a literature review is presented showing the state of the art of the plasma-liquid interaction, and the main advances of the application of non thermal plasmas to seed germination. Ii) Second, experimental set ups are described, in particular the manufacturing of plasma reactors using 3D printing. Iii) then , the production of gaseous and aqueous reactive species formed by DBD plasmas was measured quantitatively and plasma-liquid interaction was analyzed. Iv) Next, different varieties of seeds were selected to evaluate the effect of a DBD plasma treatment and the study of the mechanisms of plasma germination promotion was specifically investigated by treating mung bean seeds in different discharge conditions, in different mediums, in electric field alone and in different hydration levels of seeds.v) Finally, Muller parametric imaging (MPI) was applied to study the modification of ex vivo mice skin treated by a helium jet plasma

In this study, an in-house built atmospheric. pressure non-thermal plasma jet has been investigated for its potential utilisation as a new alternative antimicrobial tool for a variety of medical applications. Anti - biofilm activity of this plasma jet has been evaluated against biofilms of a selected panel of bacterial species, grown on different abiotic surfaces, where complete eradication of all tested bacterial biofilms was achieved after relatively short plasma exposures of up to 10 minutes. Multiple approaches of cell viability evaluation were adopted to show the nature, extent and distribution of the remarkable anti-biofilm activity of the plasma jet including colony counting, XTT metabolic assay, scanning electron microscopy examination and differential Live/Dead fluorescent staining followed by confocal laser scanning microscopy examination. Antibacterial efficacy of the plasma jet has also been evaluated against similar bacterial species in their planktonic mode of growth where plasma exposures even shorter than those required for biofilm eradication were sufficient to cause complete inactivation of these planktonic bacteria. Such excellent bactericidal activity resulted from the ability of plasma exposure to mediate an oxidative damage to multiple cellular targets including cellular membrane, DNA and proteins of bacterial cells. However, damage of cellular membrane and the resultant disruption of its integrity and permeability were shown to be the primary rate-determining step in the plasma mediated bacterial cell death. Furthermore, in depth investigation of the plasma- mediated bacterial destruction mechanism has been carried out to identify the plasma-produced reactive species that were responsible for mediating its bactericidal activity. Based on the findings of this study, a hypothesis was formulated to describe the mechanism of bacterial cell destruction after plasma exposure. This hypothesis assumed a two-part mechanism; one part was a rapid H20 2-dependent mechanism associated with Fenton's or Fenton's-like reaction that was catalysed by metal ions released from the bacterial cells initially damaged by another proposed H20 2 - independent mechanism.

My Ph.D. activity developed along four lines of research dealing with non-thermal plasma (NTP) induced chemical processes for water remediation and biomedical applications. Specifically, I studied the effectiveness of atmospheric air plasma treatment in decomposing emerging organic contaminants (EOCs). The experimental setup used was a dielectric barrier discharge (DBD) reactor, a prototype developed in collaboration with the Department of Industrial Engineering of the University of Padova. Among EOCs, I chose six different contaminants, notably sulfamethoxazole, a veterinary antibiotic, triclosan, an antibacterial agent, perfluorooctanoic acid (PFOA), a perfluorinated organic contaminant, and the herbicides irgarol, metolachlor and mesotrione. Kinetics of their removal by plasma, intermediates of oxidation, possible degradation pathways and conversion to CO2 were evaluated. The achievement of more than 93% of conversion was observed for all the contaminants used at the initial concentration of 5 μM, except for PFOA (42%). An important advancement in my research involved the assessment of residual toxicity of plasma treated water samples. For this purpose, in collaboration with Prof. Giovanni Libralato (University of Naples), we tested the efficiency of plasma treatment in producing water free from ecotoxicological effects due to potentially toxic by-product residues. We tested one of the pollutants mentioned above, sulfamethoxazole (SMZ), an antibiotic listed among the most important emerging organic contaminants. A battery of acute and chronic toxicological test were employed: Daphnia magna, Raphidocaeilis Subcapitata and Vibrio Fischeri. It was found that toxicity of SMZ 5×10-4 M is minimized (V.fischeri) or reduced to zero (D. magna, R. Subcapitata) after 4 h of plasma treatment. To improve the efficiency of our DBD reactor, we tested the effect of addition of a photocatalyst, TiO2. We compared the kinetics of degradation of Irgarol in photocatalytic plasma process with those obtained when TiO2 was not included. The results obtained suggest that the effect of photoactivation by titanium dioxide in our reactor was negligible under the conditions employed. Possible reciprocal effects of different organic pollutants dissolved in water subjected to plasma induced advanced oxidation in our dielectric barrier discharge (DBD) reactor were then evaluated. As case study for this investigation, I chose the herbicides S-metolachlor and mesotrione, which are commonly applied in mixture. Results revealed that metolachlor does not affect mesotrione kinetics and viceversa when they are in solution, in 1:1 ratio. A new reactor was developed in our lab, in collaboration with Dr. Bosi from the Department of Industrial Engineering (University of Padova) with improved design and features with respect to the existing DBD reactor. The new reactor, operating in streamer discharge regime, was exhaustively characterized in collaboration with Dr. Gabriele Neretti (University of Bologna) and Dr. Barbara Zaniol (Consorzio RFX), and tested on phenol and metolachlor. Finally, during a four-month stage at the University of Bochum (Germany) I had the opportunity to work on a project dealing with plasma applications in the biomedical field under the supervision of Profs. Julia Bandow and Jan Benedikt. In particular, the effects of two plasma sources were tested in vitro on glyceraldehyde 3-phosphate dehydrogenase and E. coli. The results obtained for the enzyme suggest the importance of oxidation of the thiol group of the active site in plasma mode of action. The same approach was applied to assess the effect of ionic components of plasma by a new source developed by Prof. Benedikt (University of Bochum). The study of inactivation of the enzyme via plasma, with and without ions, showed a synergic effect between radicals and ions.
La Tesi riporta e discute i risultati ottenuti nell’applicazione di plasmi non termici per il trattamento ossidativo di inquinanti modello e ulteriori risultati relativi all’utilizzo del plasma in campo biomedico. L’apparato sperimentale impiegato è stato progettato e realizzato in collaborazione con il Dipartimento di Ingegneria Elettrica e produce una scarica a barriera di dielettrico (reattore DBD). Il sistema era già in uso nel periodo antecedente l’inizio della mia attività di dottorato. Le specie reattive che si generano a causa della scarica elettrica nell’aria umida sovrastante la fase liquida entrano in contatto con essa e possono reagire con l’inquinante organico in soluzione. Le specie reattive possono essere distinte in primarie, cioè generate direttamente dalla scarica per reazione del gas con gli elettroni energetici formando radicali, ioni e specie eccitate altamente reattive ed instabili, e secondarie prodotte per reazione delle stesse specie con le molecole del gas oppure con l’umidità presente. Il primo passo è stato quello di applicare tali scariche elettriche per il trattamento di diverse categorie di inquinanti emergenti allo scopo di valutare le potenziali applicazioni di questa tecnologia in relazione alle proprietà chimico fisiche degli inquinanti trattati. Sono stati selezionati i seguenti contaminanti organici persistenti: il sulfametossazolo, un antibiotico veterinario, il triclosan, un antibatterico, l’acido perfluoroacetico e tre erbicidi, l’irgarol, il metolachlor ed il mesotrione. Per tutti i composti in esame ho ottenuto profili esponenziali di degradazione in funzione del tempo di trattamento, da cui sono state ricavate le costanti cinetiche di pseudo-primo ordine. L’analisi HPLC-MS ha consentito l’identificazione degli intermedi e prodotti di degradazione, compatibili con possibili reazioni dovute all’azione dell’ozono e dei radicali ∙OH. Sono stati proposti inoltre i meccanismi di degradazione dei composti organici trattati. Lo scopo finale nell’uso di processi di degradazione avanzata è la completa conversione della componente organica a CO2. In seguito al trattamento al plasma, sono state riscontrate percentuali di mineralizzazione pari o maggiori al 93% per tutti gli inquinanti considerati, usati in concentrazione pari a 5 μM, fatta eccezione per l’acido perfluoroottanoico per cui la percentuale di mineralizzazione è stata considerevolmente più bassa (42%). Lo studio dei processi di degradazione al plasma è inoltre servito in alcuni casi da punto di partenza per ulteriori approfondimenti. È questo il caso dell’irgarol, in cui si è cercato di implementare l’effetto del plasma aggiungendo un fotocatalizzatore ampiamente utilizzato, TiO2. Non sono stati riscontrati tuttavia miglioramenti nell’effetto della scarica su tale inquinante indicando un trascurabile effetto fotocatalitico nelle condizioni sperimentali adottate. Un ulteriore avanzamento nelle ricerche in questo ambito è consistito nell’applicazione della scarica DBD su una miscela di inquinanti, il metolachlor e il mesotrione, solitamente utilizzati in combinazione in diverse formulazioni agricole. Gli studi cinetici effettuati hanno evidenziato che i due composti non si influenzano reciprocamente quando subiscono il trattamento al plasma in soluzioni miste in cui sono presenti in rapporto molare 1:1. Un importante parametro nella valutazione di una tecnica di depurazione consiste nell’analisi ecotossicologica del campione acquoso dopo il trattamento. A tale scopo, in collaborazione con il Prof. Giovanni Libralato del Dipartimento di Biologia dell’Università di Napoli, sono stati effettuati test tossicologici su campioni contenenti sulfametossazolo (SMZ), prima e dopo il trattamento nel reattore DBD. Allo scopo è stata utilizzata una batteria di test acuti e cronici per Vibrio Fischeri, Daphnia magna e Raphidocaelis subcapitata. I dati ottenuti a partire da una soluzione di SMZ 5·10-4 M hanno mostrato un elevato livello di tossicità della soluzione iniziale e la riduzione (V.fischeri) o l’azzeramento di tali effetti (D.magna e R.subcapitata) a seguito del trattamento nel reattore al plasma. Un nuovo reattore è stato inoltre ideato e realizzato in collaborazione con il Dr. Franco Bosi, del Dipartimento di Ingegneria Industriale dell’Università di Padova. La sorgente di plasma utilizza una scarica di tipo streamer ed è stata realizzata allo scopo di favorire un migliore trasporto delle specie reattive prodotte dalla scarica e ottimizzare la loro interazione con la soluzione da trattare. Il reattore è stato quindi caratterizzato in collaborazione con il Dr. Gabriele Neretti (Università di Bologna) e la Dr.ssa Barbara Zaniol (Consorzio RFX, Padova) e collaudato nel trattamento di due inquinanti organici, il fenolo ed il metolachlor. Infine nel corso di un periodo di quattro mesi di attività di ricerca presso il laboratorio della Prof.ssa Bandow dell’Università di Bochum (Germania) ho avuto modo di approfondire alcuni aspetti legati alle applicazioni del plasma atmosferico in campo biomedico. In particolare ho partecipato a studi sugli effetti di due diverse sorgenti al plasma su un enzima, gliceraldeide-3-fosfato deidrogenasi, in vitro e sul batterio E. coli. Il sito di attacco principale è risultato essere il sito attivo cisteina con conseguente ossidazione del gruppo -SH. Lo stesso approccio è stato applicato, in collaborazione con il Prof. Benedikt per lo studio degli effetti del plasma, in assenza e in presenza delle specie ioniche. I risultati ottenuti hanno evidenziato un effetto sinergico dovuto alla copresenza di specie neutre e ioniche.

The use of plasmas in aerodynamics has become a recent topic of interest. In particular, over the last ten years, plasma actuation has received much attention as a promising active method for airflow control. Flow control consists of manipulating the properties of a generic moving fluid with the aim of achieving a desired change, but flow dynamics in proximity of a solid object is usually considered, being a consistent and significant issue in many engineering applications, such as engine, automobile or airplane design. Plasma control of airflows along surfaces has been the subject of several experimental studies whose aim was to reduce turbulence, to decrease drag, to enhance airfoil lift or to prevent flow detachment. The fast temporal response and the absence of moving parts are the most promising features from which plasma actuators could benefit. Different types of plasma sources are currently studied as good candidates for plasma actuation, but Dielectric Barrier Discharges (DBDs) are usually preferred, being characterized by the presence of an insulating barrier between the electrodes. This allows the generation of a non-thermal plasma at atmospheric pressure and prevents the discharge from collapsing into an arc. Surface Dielectric Barrier Discharges (SDBDs) are particularly suitable for these kinds of applications, since plasma is created by ionizing a thin portion of air nearby the surface of the dielectric barrier and this can effectively influence the local properties of the boundary layer associated to an external flow. This thesis deals with SDBDs in an asymmetric configuration where one electrode is glued into an insulating material and to other one is exposed to air, so that plasma is created in correspondence of just one side of the dielectric barrier. The buried electrode is connected to the ground, whereas a sinusoidal high-voltage is applied to the exposed one. It has been noticed that, when these discharges are operated in quiescent air, an airflow of several metres per second is observed above the dielectric sheet and near the plasma region. This is usually called ionic wind because the main mechanism responsible for its generation is believed to be momentum transfer from the ions drifting in the discharge electric field to the surrounding fluid, by particle-particle collisions. When the electric field imposed by the voltage difference between the electrodes is sufficiently high, plasma is created and electrical charges are transported through the gap and accumulated on the insulating surfaces. This charge accumulation generates an electric field that locally weakens the external one. When the total electric field falls below the threshold necessary for plasma ignition, the discharge extinguishes. If the voltage imposed to the fed electrode is increased, the discharge can be locally initiated again, and that is the reason why a sinusoidal high-voltage supply is adopted instead of a continuous one. Consequently, the presence of the insulating barrier usually leads to a regime where charge is mainly transported in sub-millimetre regions consisting of current filaments with temporal duration limited to a few tens of nanoseconds. These plasma microdischarges are concentrated into two phase intervals of the sinusoidal voltage supply, when the modulus of the applied voltage difference is high enough and is increasing in time. These two phases of plasma activity are often called Backward Stroke (BD) and Forward Stroke (FD), depending if the high-voltage signal is rising from its minimum to its maximum or decreasing from its maximum to its minimum. This thesis is motivated by the fact new studies focusing on plasma properties and dynamics are required in order to get better and better aerodynamic results, to understand which parameters mainly affect the actuator performances and to validate numerical models trying to forecast the aerodynamic effects induced by the discharge. This has brought to a scientific collaboration between the Centre of Excellence PlasmaPrometeo of University of Milano-Bicocca and the Aerodynamics and Wind Tunnel Department of the aerospace company Alenia Aermacchi. During these years I have studied the properties of these discharges by means of electrical and optical diagnostics (mainly Rogowski coils, capacitive probes, a photomultiplier tube and a thermal camera). With some of them a temporal resolution high enough for studying several characteristics of plasma microdischarges has been achieved. This is important because these strokes manifest as series of current and light pulses, lasting tens of nanoseconds and a few nanoseconds respectively. I have first of all carried out a detailed investigation of the properties of these events and of their evolution in space and time in the course of the FD and BD. It has been pointed out that there are several analogies between the BD and FD, but that not all plasma properties are identical for the two semi-cycles, because of the asymmetrical configuration adopted. These investigations let think that light and current signals give insights about different microdischarge properties. Light is presumably ascribable to electrons that excite nitrogen immediately after the passage of the ionizing wave that initiates the microdischarge. In contrast, the current signal is due to the movement of charges into the plasma channel and thus reflects the microdischarge temporal evolution, rather than its formation. In the following experiments I have thus focused mainly on the electrical properties of plasma microdicharges, with the aim of better understanding which plasma characteristics are responsible for the ionic wind generation and properties. Several SDBDs with different geometrical configurations and operating parameters have been considered. It has been found that both the discharge and ionic wind characteristics are mainly affected by the dielectric thickness, whereas other properties of the SDBD are less decisive. These studies are of practical interest because optimizations of SDBD characteristics are still needed for adopting these discharges as plasma actuators for active flow control. In particular, it has been found that at first the speed of the induced wind increases quite linearly with the voltage amplitude, but then this velocity and thus the aerodynamic effects induced by the discharge tend to saturate. This is particularly evident when thin panels are adopted as dielectric barriers. I thus focused on this topic and I found that an asymmetry in the total charge transported by plasma microdischarges during the backward and forward strokes is favourable for obtaining a ionic wind with a greater velocity, and that the velocity saturation at the highest voltages is associated to a change in discharge regime, which is visible first of all because a pattern of plasma filaments appears superimposed to the more homogeneous plasma. I have thus characterized how this regime transition affects the dynamics of the backward and forward strokes. Three groups of microdischarges have been identified, depending on their temporal duration, and results let think that they don't contribute equally to the electric wind generation. These studies pave the way to a better understanding of the discharge peculiarities and ionic wind formation, with the aim of understanding if an intrinsic limit exists in plasma actuator potentialities or if new optimization strategies are possible. Eventually, I proposed to implement the Background Oriented Schlieren (BOS) technique for the visualization and characterization of the airflow induced by the discharge. The potentialities of this technique have been evaluated in relation to the specifics of the available scientific equipment. The technique has then been proved to be able to visualize density changes induced by plasma. A spatial characterization of the air near the discharge was made in stationary wall jet conditions as well as in the transient period following the discharge ignition when a starting vortex is generated.

This PhD thesis is focused on cold atmospheric plasma treatments (GP) for microbial inactivation in food applications. In fact GP represents a promising emerging technology alternative to the traditional methods for the decontamination of foods. The objectives of this work were to evaluate: - the effects of GP treatments on microbial inactivation in model systems and in real foods; - the stress response in L. monocytogenes following exposure to different GP treatments. As far as the first aspect, inactivation curves were obtained for some target pathogens, i.e. Listeria monocytogenes and Escherichia coli, by exposing microbial cells to GP generated with two different DBD equipments and processing conditions (exposure time, material of the electrodes). Concerning food applications, the effects of different GP treatments on the inactivation of natural microflora and Listeria monocytogenes, Salmonella Enteritidis and Escherichia coli on the surface of Fuji apples, soya sprouts and black pepper were evaluated. In particular the efficacy of the exposure to gas plasma was assessed immediately after treatments and during storage. Moreover, also possible changes in quality parameters such as colour, pH, Aw, moisture content, oxidation, polyphenol-oxidase activity, antioxidant activity were investigated. Since the lack of knowledge of cell targets of GP may limit its application, the possible mechanism of action of GP was studied against 2 strains of Listeria monocytogenes by evaluating modifications in the fatty acids of the cytoplasmic membrane (through GC/MS analysis) and metabolites detected by SPME-GC/MS and 1H-NMR analyses. Moreover, changes induced by different treatments on the expression of selected genes related to general stress response, virulence or to the metabolism were detected with Reverse Transcription-qPCR. In collaboration with the Scripps Research Institute (La Jolla, CA, USA) also proteomic profiles following gas plasma exposure were analysed through Multidimensional Protein Identification Technology (MudPIT) to evaluate possible changes in metabolic processes.

This PhD thesis is focused on cold atmospheric plasma treatments (GP) for microbial inactivation in food applications. In fact GP represents a promising emerging technology alternative to the traditional methods for the decontamination of foods. The objectives of this work were to evaluate: - the effects of GP treatments on microbial inactivation in model systems and in real foods; - the stress response in L. monocytogenes following exposure to different GP treatments. As far as the first aspect, inactivation curves were obtained for some target pathogens, i.e. Listeria monocytogenes and Escherichia coli, by exposing microbial cells to GP generated with two different DBD equipments and processing conditions (exposure time, material of the electrodes). Concerning food applications, the effects of different GP treatments on the inactivation of natural microflora and Listeria monocytogenes, Salmonella Enteritidis and Escherichia coli on the surface of Fuji apples, soya sprouts and black pepper were evaluated. In particular the efficacy of the exposure to gas plasma was assessed immediately after treatments and during storage. Moreover, also possible changes in quality parameters such as colour, pH, Aw, moisture content, oxidation, polyphenol-oxidase activity, antioxidant activity were investigated. Since the lack of knowledge of cell targets of GP may limit its application, the possible mechanism of action of GP was studied against 2 strains of Listeria monocytogenes by evaluating modifications in the fatty acids of the cytoplasmic membrane (through GC/MS analysis) and metabolites detected by SPME-GC/MS and 1H-NMR analyses. Moreover, changes induced by different treatments on the expression of selected genes related to general stress response, virulence or to the metabolism were detected with Reverse Transcription-qPCR. In collaboration with the Scripps Research Institute (La Jolla, CA, USA) also proteomic profiles following gas plasma exposure were analysed through Multidimensional Protein Identification Technology (MudPIT) to evaluate possible changes in metabolic processes.

In this dissertation are reported the most relevant results obtained during my three years Ph.D. project. An open-air plasma source has been developed to treat plastic and metallic films typically used in food packaging manufacturing. Among others, the DBD configuration was chosen due to its many advantages such as high intensity and uniformity of the treatment, possibility of operating in ambient air as well as ease of scale up. Biological experiments were performed to assess the microbial reduction induced by the plasma treatment. Different operative conditions have been tested in order to identify the most efficient configuration and two distinct behaviours have been observed: low-power density treatment allowed to achieve microbial inactivation values below log 2 independently on treatment time; high-power density treatment where the microbial reduction grew with increasing treatment time. Subsequently, the plasma discharge has been characterized by means of three investigation methods: thermal, electrical and optical absorption spectroscopy (OAS) analysis. The thermal and electrical analyses were employed to identify the best dielectric materials for food packaging manufacturing purposes. Once defined the optimal DBD configuration, OAS was used to measure the absolute concentration of ozone and nitrogen dioxide. Results showed that at low-power density the chemistry is governed by ozone; while at high-power density ozone is consumed by the poisoning effect and only nitrogen dioxide is detectable. Lastly, a numerical simulation has been used to deeper investigate the chemistry governing the plasma discharge; by means of PLASIMO a global model and a fluid model were implemented.

Abstract Parametric drifts and fluctuations occur during plasma spraying. These drifts and fluctuations originate primarily from electrode wear and intrinsic plasma jet instabilities. One challenge is to control the manufacturing process by identifying the parameter interdependencies, correlations and individual effects on the in-flight particle characteristics. Such control is needed through methods that (i) consider the interdependencies that influence process variability and that also (ii) quantify the processing parameter-process response relationships. Artificial intelligence is proposed for thermal spray applications. The specific case of predicting plasma power parameters to manufacture grey alumina (Al2O3-TiO2, 13% by wt.) coatings was considered and the influence of the plasma spray process on the in-flight particle characteristics was investigated.