A New Kind of Microplasma for Nitrogen-Fixation Multifunctional Nanoparticle Synthesis Towards Selected Applications

Joint Degree Program between the School of Chemical Engineering, University of Adelaide, and the School of Engineering, University of WarwickN-doped carbon quantum dot (NCQDs) is an emerging material in the carbon family, which possess many advantageous qualities such as low toxicity, good compatibility with living cells, stability in chemical reactions, strong photoluminescence, photocatalysis, and efficient transfer of electrons when exposed to light. These properties make CDs a highly promising material to mitigate the current challenges in pest control, environmental treatment and theranostic medicine. This thesis study carried out a comprehensive literature review on the potential applications of these materials in pest control. Although a tremendous benefit from NCQDs has been revealed, generating these materials in large-scale production in a green and sustainable manner is still challenging. Therefore, a comprehensive assessment of all available large-scale production of NCQDs in terms of sustainability is significant. It has been revealed that non-thermal nitrogen fixation microplasma is a potential large-scale synthesis method in terms of sustainability while lacking mass efficiency. Therefore, it is crucial to overcome the improvement of mass efficiency of the non-thermal nitrogen fixation microplasma method. In addition, most of these CD synthesis strategies are classified as the trial-and-error approach. It is a time-consuming journey with cost- and process inefficiency to create CDs with a suitable structure and properties available for a specific application. For this reason, it is also urgent to develop a rational design strategy for synthesising NCQDs towards selected applications (pest control, environmental treatment and theranostic medicine), which are emerging issues mentioned above. It has been revealed that the photoluminescence of the as-prepared NCQD increases by 18.4% in the presence of metal flakes as catalysts. However, the role of plasma-liquid-catalyst interaction in the production of NCQDs is still unclear. Therefore, it is also significant to have a deep understanding of the plasma-liquid-catalyst mechanism in the reaction to control the large-scale production of NCQDs better. Finally, it has also been important to have an insight into the reaction rate in the production of NCQDs and to determine factors (viscosity and liquid surface area) that affect the reaction rate. The primary aim of this thesis is to develop a new kind of microplasma to address the challenges associated with the large-scale production of N-fixation multifunctional N-doped carbon quantum dots (NCQD) for selected applications. The objectives of this thesis are organised into 8 chapters that will be presented in the form of a collection of published and submitted papers, which are the research outcomes. In addition, a literature review has been provided to establish the background of microplasma-assisted synthesis of NCQDs and highlight the potential applications of this material in agriculture. Overall, this thesis's main contributions to developing a new kind of microplasma-assisted synthesis process are discovering, investigating, understanding, designing, fabricating, and improving the concept of using the non-thermal nitrogen fixation microplasma process to generate high-quality NCQDs for selected applications. The main contributions are summarised in the following chapters: • Chapter 2. Literature review: Perspectives on the plasma-assisted synthesis of N-doped nanoparticles as nanopesticides for crop pest control (Published paper 1). • Chapter 3. Sustainability assessment of large-scale synthesis processes of NCQDs (Published paper 2). • Chapter 4. Process intensification for gram-scale synthesis of NCQDs (Published Paper 3). • Chapter 5. Rationally designed microplasma synthesis of NCQDs for targeted applications (Published paper 4). • Chapter 6. Insight into plasma-catalysis in triphasic microplasma synthesis for NCQDs (Ready manuscript 5). • Chapter 7. Stagnant Liquid Layer as “Microreaction System” in Submerged Plasma Micro-Jet for Formation of Carbon Quantum Dots (Submitted).Thesis (Ph.D.) -- University of Adelaide, School of Chemical Engineering, 202

A new kind of microplasma for nitrogen-fixation multifunctional nanoparticle synthesis towards selected applications

N-doped carbon quantum dot (NCQDs) is an emerging material in the carbon family, which possess many advantageous qualities such as low toxicity, good compatibility with living cells, stability in chemical reactions, strong photoluminescence, photocatalysis, and efficient transfer of electrons when exposed to light. These properties make CDs a highly promising material to mitigate the current challenges in pest control, environmental treatment and theranostic medicine. This thesis study carried out a comprehensive literature review on the potential applications of these materials in pest control. Although a tremendous benefit from NCQDs has been revealed, generating these materials in large-scale production in a green and sustainable manner is still challenging. Therefore, a comprehensive assessment of all available large-scale production of NCQDs in terms of sustainability is significant. It has been revealed that non-thermal nitrogen fixation microplasma is a potential large-scale synthesis method in terms of sustainability while lacking mass efficiency. Therefore, it is crucial to overcome the improvement of mass efficiency of the non-thermal nitrogen fixation microplasma method. In addition, most of these CD synthesis strategies are classified as the trial-and-error approach. It is a time-consuming journey with cost- and process inefficiency to create CDs with a suitable structure and properties available for a specific application. For this reason, it is also urgent to develop a rational design strategy for synthesising NCQDs towards selected applications (pest control, environmental treatment and theranostic medicine), which are emerging issues mentioned above. It has been revealed that the photoluminescence of the as-prepared NCQD increases by 18.4% in the presence of metal flakes as catalysts. However, the role of plasma-liquid-catalyst interaction in the production of NCQDs is still unclear. Therefore, it is also significant to have a deep understanding of the plasma-liquid-catalyst mechanism in the reaction to control the large-scale production of NCQDs better. Finally, it has also been important to have an insight into the reaction rate in the production of NCQDs and to determine factors (viscosity and liquid surface area) that affect the reaction rate. The primary aim of this thesis is to develop a new kind of microplasma to address the challenges associated with the large-scale production of N-fixation multifunctional N-doped carbon quantum dots (NCQD) for selected applications. The objectives of this thesis are organised into 8 chapters that will be presented in the form of a collection of published and submitted papers, which are the research outcomes. In addition, a literature review has been provided to establish the background of microplasma-assisted synthesis of NCQDs and highlight the potential applications of this material in agriculture. Overall, this thesis's main contributions to developing a new kind of microplasma-assisted synthesis process are discovering, investigating, understanding, designing, fabricating, and improving the concept of using the non-thermal nitrogen fixation microplasma process to generate high-quality NCQDs for selected applications. The main contributions are summarised in the following chapters: • • Chapter 2. Literature review: Perspectives on the plasma-assisted synthesis of N-doped nanoparticles as nanopesticides for crop pest control (Published paper 1). • • Chapter 3. Sustainability assessment of large-scale synthesis processes of NCQDs (Published paper 2). • • Chapter 4. Process intensification for gram-scale synthesis of NCQDs (Published Paper 3). • • Chapter 5. Rationally designed microplasma synthesis of NCQDs for targeted applications (Published paper 4). • • Chapter 6. Insight into plasma-catalysis in triphasic microplasma synthesis for NCQDs (Ready manuscript 5). • • Chapter 7. Stagnant Liquid Layer as “Microreaction System” in Submerged Plasma Micro-Jet for Formation of Carbon Quantum Dots (Submitted)

Survey of synthesis processes for N-doped carbon dots assessed by green chemistry and circular and ecoscale metrics

Six of the most promising lab-scale synthesis process methodologies for N-doped carbon dots (NCDs) are selected and compared in terms of green chemistry and circular and EcoScale/Good-Manufacturing-Practice metrics. We compare a new innovative route, the low-temperature plasma-enabled synthesis of carbon dots, e.g., from citric acid and monoethanolamine, to more-established literature processes, such as thermochemical processes, from the same or other materials. Along with this study, the advantages and disadvantages of each method are depicted in manifold sustainability facets. It is shown how recycling/reuse of nonconverted starting materials and solvents can improve the sustainability profile. In addition, safety constraints, cost analysis, and energy consumption are considered. The analysis showed that the thermal process from citric acid and monoethanolamine gives the best performance with regard to the sustainability assessment chosen here. It has a material circularity indicator of 0.971, with an EcoScale factor of 56% and an E-factor of 5.56. In continuation of those results, the paper shows how the low-temperature plasma using the same materials and the same recycling strategy can be improved to come closer to the performance of its thermal counterpart. It has the best energy efficiency, while lacking so far in mass efficiency. From this study, we learned more about which of these methods are most promising for scaling-up and industrial manufacturing of N-doped carbon dots

Impact of cold plasma on the biomolecules and organoleptic properties of foods: A review

Cold plasma is formed by the nonthermal ionisation of gas into free electrons, ions, reactive atomic and molecular species, and UV radiation. This cold plasma can be used to alter the surface of solid and liquid foods, and it offers multiple advantages over traditional thermal treatments, such as no thermal damage and increased output variation (due to the various input parameters gas, power, plasma type, etc.). Cold plasma appears to have limited impact on the sensory and colour properties, at lower power and treatment times, but there has been a statistically significant reduction in pH for most of the cold plasma treatments reviewed (

Perspectives on plasma-assisted synthesis of N-doped nanoparticles as nanopesticides for pest control in crops

To enhance crop efficiency and meet the growing global demands for food, a new agri-tech revolution has recently been triggering. Engineered nanomaterials have the potential of lessening environmental impact and making agriculture more efficient, resilient, and sustainable. Nitrogen-doped nanoparticles (N-doped NPs) are a rising star toward the development of a new generation of nanopesticides for advanced green agriculture, providing improved efficiency, new concepts for pest control and reduced pesticide resistance, which are key limitations of conventional pesticides. The objectives of this paper are (1) to provide perspectives of promising applications of N-doped NPs as emerging nanopesticides and (2) to review the opportunities which plasma-enabled NP technology is offering for the scalable production of N-doped NPs based on a green, eco-friendly and sustainable approach. The main advantages of the N-doped NPs are their multifunctionality enabling them to provide enhanced adhesion to leaves or insect bodies and several different modes of action to kill insects, including physical, biochemical and catalytic, that are expected to considerably reduce the insect population. Apart from insects, these nanomaterials can inactivate phytopathogenic bacteria and fungi through various mechanisms and are therefore used for a broad spectrum of plant protection. In this review, N-doped ZnO and N-doped TiO2 NPs will be introduced and reviewed as the first examples of these nanomaterials that have been successfully proven as nanopesticides. Following the first demonstration and the application of N-doped carbon dots (N-doped CDs), various agricultural applications such as nanopesticides, pesticide nanocarriers, disease detection, and pest targeting will be reviewed showing their enormous potential to be translated into real applications. The plasma technology comes into play when the focus is on the manufacturing process of these nanomaterials, since, in pest control, producing high-quality nanopesticides is indispensable and challenging. Plasma-chemical NP processing is a green, simple, and rapid approach compared to conventional methods and this review presents how this technology can be used to produce N-doped NPs in a high-quality, high-quantity, low-cost, energy-efficient and sustainable way with minimal waste and without the use of toxic chemicals. The implementation of this technology and introduction of N-doped NPs as new powerful pesticidal agents could make a significant impact on improved crop production.</p

Nanofertilizers and nanopesticides for crop growth

In the last decades, advanced applications of nanotechnology in agriculture have gained good momentum with many methods being developed to widespread the production and application of nanofertilizers and nanopesticides for plants. Nanotechnology develops new types of nanopesticides and nanofertilizers to enhance crop productivity while reducing the advert effects on the surrounding environment. Nanopesticides can protect plants against phytopathogens, while nanofertilizers stimulate plant growth and ensure large-scale food production all over the world. In this chapter, popular nanofertilizers and nanopesticides and their applications on plants in practice were comprehensively introduced

Process technology and sustainability assessment of wastewater treatment

Removal of heavy metals in wastewater treatment is crucial to protect the environment, wildlife, and human health. Various techniques have been developed focusing on removal of heavy metal ions, pharmaceuticals, and other contaminants from different wastewater sources. The main methods include adsorption, filtration, ion exchange, electrochemical, reverse osmosis, precipitation, flotation/coagulation/flocculation, and photocatalytic-based treatments. This paper comprehensively assesses the sustainability of those common technologies used for wastewater process treatment. The sustainability profile depends mostly on the exact approach followed for each technology, including its energy consumption, type of radiation (where appropriate), auxiliary materials used (e.g., catalysts, adsorbents), and further specific experimental process settings. Thus, while sustainability inevitably provides a multifaceted answer, the review finally aims for sustainability benchmarking of all technologies, by compressing the manifold outcomes toward a compact information set, such as a table and radar plot

Rational design for the microplasma synthesis from vitamin B9 to N-doped carbon quantum dots towards selected applications

N-doped carbon quantum dots (NCQD) are rationally designed and synthesised, for the first time, from folic acid (Vitamin B9) by a non-thermal microplasma jet. A new conceptual design was developed to synthesise the desirable NCQD for three main applications (nanopesticides, water purification, and theranostic treatment). The structural and analytical characterisation confirmed an average size of 3.1 nm for the synthesised NCQD with the multi-functional groups (-OH, –COOH, –NH2) on their surface. The TEM results indicated that the core of NCQD was a multilayered structure, including single defected graphene sheets of graphitic-nitrogen and pyrrolic-nitrogen. In addition, fluorescence performance and stability of the as-prepared NCQD were determined. The quantum yield of NCQD was 35%, which is relatively high, with a strong blue fluorescence. A basis for predicting colloidal behaviours based on balancing molecular attractive and repulsive forces was elucidated by applying the Derjaguin, Landau, Vervey, and Overbeek (DLVO) theory. Finally, compared with other similar microplasma-assisted synthesis processes, this developed method has proven the ability to provide a tailored and scalable synthesis process of high-quantum-yield NCQD at gram-scale production

Process intensification for gram-scale synthesis of N-doped carbon quantum dots immersing a microplasma jet in a gas-liquid reactor

N-doped carbon quantum dots are synthesised by immersing a microplasma jet into a gas–liquid reactor of the size of a microwell, containing an aqueous solution of folic acid (Vitamin B9). The distance of the tip of the microplasma jet to the water surface is changed in three steps, named distant, contact, and deflection modes. As a further variation, the liquid volume is either stirred or unstirred and may contain glass beads or metal flakes. In this way, the mass transfer, hydrodynamics, and the electrical field are influenced and create the specific gas–liquid interface, possibly including plasma-catalytic effects. A thermofluidic analysis confirms a uniform temperature profile and a positive temperature effect on the mass transfer. In this way, the research achieves process intensification, bringing the synthesis towards 1 g per day and maximising the intended performance, the photoluminescence intensity. Recycling further increases the mass yield via centrifugation. An analysis by optical emission spectroscopy reveals the formation of the plasma species from which a reaction mechanism is proposed

Bactericidal Effect of Lauric Acid-Loaded PCL-PEG-PCL Nano-Sized Micelles on Skin Commensal Propionibacterium acnes

Acne is the over growth of the commensal bacteria Propionibacterium acnes (P. acnes) on human skin. Lauric acid (LA) has been investigated as an effective candidate to suppress the activity of P. acnes. Although LA is nearly insoluble in water, dimethyl sulfoxide (DMSO) has been reported to effectively solubilize LA. However, the toxicity of DMSO can limit the use of LA on the skin. In this study, LA-loaded poly(ɛ-caprolactone)-poly(ethylene glycol)-poly(ɛ-caprolactone) micelles (PCL-PEG-PCL) were developed to improve the bactericidal effect of free LA on P. acnes. The block copolymers mPEG-PCL and PCL-PEG-PCL with different molecular weights were synthesized and characterized using 1H Nuclear Magnetic Resonance spectroscopy (1H NMR), Fourier-transform infrared spectroscopy (FT-IR), Gel Permeation Chromatography (GPC), and Differential Scanning Calorimetry (DSC). In the presence of LA, mPEG-PCL diblock copolymers did not self-assemble into nano-sized micelles. On the contrary, the average particle sizes of the PCL-PEG-PCL micelles ranged from 50–198 nm for blank micelles and 27–89 nm for LA-loaded micelles. The drug loading content increased as the molecular weight of PCL-PEG-PCL polymer increased. Additionally, the minimum inhibitory concentration (MIC) and the minimum bactericidal concentration (MBC) of free LA were 20 and 80 μg/mL, respectively. The MICs and MBCs of the micelles decreased to 10 and 40 μg/mL, respectively. This study demonstrated that the LA-loaded micelles are a potential treatment for acne