Τμήμα Χημικών Μηχανικών (ΔΔ)

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Standardization of human genome-scale metabolic models for fluxomics and systems biology applications
(2026-10-21) Παντζίρη, Μαριλένα; Pantziri, Marilena
Human genome-scale metabolic models (GEMs) provide systematic representations of human metabolism and support flux analysis and multi-omics integration. Yet human GEMs have evolved through successive generations of genome annotation, biochemical databases, identifier systems and reconstruction practices. Because many reconstructions originated when these resources and their interconnections were less mature, individually valid annotations may coexist within the same model reaction without describing mutually consistent biological relationships. Existing standards and quality-control frameworks improve structure and annotation coverage, but do not systematically test whether enzyme, protein, reaction and metabolite annotations converge on the same biochemical transformation. The main objective of this thesis was therefore to make cross-layer annotation consistency testable against a common representation of the human metabolic reactome. Four specific aims were pursued: (1) to construct an enzyme-centric reference connecting enzyme activities, reviewed human proteins, biochemical transformations, metabolites and stoichiometry, while retaining directionality and subcellular localization as biological context; (2) to characterize the reconstructed reactome as a quantitative knowledge landscape; (3) to evaluate cross-layer consistency using the Human2 model (https://github.com/SysBioChalmers/Human-GEM) as a proof of concept and (4) to derive general annotation requirements and curation guidelines for the reproducible comparison of human GEMs. A reference human metabolic reactome was reconstructed by integrating curated resources, principally Rhea, UniProt, ExplorEnz, ChEBI, KEGG and MetaCyc, with Reactome and other resources retained as supporting cross-reference and pathway evidence. Biochemical transformations were defined by chemical participants and stoichiometric relationships rather than identifiers alone; directionality and localization were represented separately as biological-context properties, and source provenance was preserved. The reference comprises several thousand retained EC classifications, associated reviewed human proteins, a source-derived reaction set, participating metabolites and a separately generated transport layer. Its characterization showed frequent many-to-many relationships between enzyme activities and reviewed human proteins, and an uneven distribution of evidence across the reactome. A cross-layer-complete core supported by all evidence layers coexists with regions in which the underlying knowledge remains incomplete or conflicting. Human2 was used as a highly curated proof of concept. The analysis did not assess biological correctness or predictive performance; it tested whether annotations attached to the same reaction were mutually consistent with the reference. Cross-layer consistency was evaluated at two levels: the biochemical transformation, at which the enzyme classification, the protein, the reaction and the metabolite participants and their stoichiometry were examined, and the biological context, at which directionality and subcellular localization were examined against the reference. Identifier correspondence was not treated as sufficient evidence of equivalence. Candidate mappings were evaluated through independent biological and chemical evidence layers, and the outcomes were retained separately, so that support, incompleteness, ambiguity, conflict and cases outside the scope of the reference remained distinguishable. The application showed that recognized identifiers and successful model-level quality control do not necessarily guarantee cross-layer consistency. Of the transformations that remained unresolved, 73.7% still carried at least one principal annotation type. Therefore, the unresolved set is not an unannotated region of the model, and the distinction between insufficient or non-convergent evidence and missing annotation cannot be made by counting identifiers alone. The resulting guidelines extend identifier- and model-level standardization by requiring consistency among chemical identity, stoichiometry, reaction-enzyme relationships and enzyme-protein support, while treating directionality and localization as separate biological-context layers and preserving missing, ambiguous or conflicting evidence as distinct states. The principal contributions are therefore a reference, provenance-preserving human metabolic reactome and a general framework for testing annotation consistency across biological layers. Future work should apply the framework to additional human GEM lineages, maintain the reactome as a versioned resource and formalize the guidelines into explicit annotation consistency criteria.
Open Access
Atmospheric aging of biomass burning emissions : formation of secondary aerosols and changes in toxicity
(2026-09-24) Γεωργοπούλου, Μαρία; Georgopoulou, Maria
Biomass burning (BB) is a major global source of particulate matter (PM) and volatile organic compounds (VOCs). After emission, BB pollutants undergo atmospheric aging driven by sunlight and oxidants such as hydroxyl (OH) and nitrate (NO3) radicals. These processes alter aerosol mass, chemical composition, oxidation state, optical properties, and oxidative potential (OP), thereby modifying their effects on air quality, human health, and climate. However, the mechanisms governing secondary organic aerosol (SOA) formation and toxicity across realistic day–night cycles remain incompletely understood. This doctoral thesis investigates how these processes control SOA formation, chemical evolution, and water-soluble oxidative potential (WS-OP), an acellular indicator of the ability of aerosol components to induce oxidative stress. The work combines atmospheric simulation-chamber experiments, high-resolution gas- and particle-phase measurements, and size-resolved ambient observations. The aerosol WS-OP was quantified using a semi-automated dithiothreitol (DTT) assay. A novel experimental platform was developed by coupling, for the first time, the FORTH mobile atmospheric simulation chamber with the CESAM reactor at the LISA laboratory (Laboratoire Interuniversitaire des Systèmes Atmosphériques, CNRS) in Créteil, France. The system enabled the continuous production of stable and atmospherically representative fresh and aged aerosols from pellet combustion for up to 72 h under controlled conditions, supporting realistic simulations of urban pollution and long-term exposure studies in preclinical models. Photochemical aging increased the aerosol oxygen-to-carbon (Ο:C) ratio by approximately 35% and enhanced oxidative potential by up to a factor of two, demonstrating that atmospheric processing can increase the intrinsic oxidative activity of BB aerosol. Subsequently, experiments with olive-wood and olive–pine emissions showed that oxidation history and relative humidity determine the trajectory of aerosol mass, composition, and oxidative activity. Under initially dry conditions (RH 12–24%), organic aerosol mass increased by 35–90% and O:C by 56% on average. Despite sequence-dependent differences during intermediate stages, the final day–night and night–day mass spectra converged (spectral angle θ=3°). In addition, atmospheric aging increased the WS-OP by approximately 60%, with the final mean values of both experimental sequences converging to 68 ± 18 pmol min⁻¹ μg⁻¹. In the next phase of the work, the SOA formation during the nighttime and daytime oxidation of some of the major volatile organic compounds emitted during biomass buring: phenol, m-cresol, and catechol was investigated. During nighttime oxidation, more than 95% of the products of phenol- and m-cresol-derived remained in the gas phase, forming reservoirs of oxygenated and nitrogen-containing secondary vapours. Subsequent OH exposure of these products increased m-cresol SOA 9.4-fold even though approximately 96% of the precursor had already reacted at night, demonstrating that nighttime products can prime aerosol formation after sunrise. Catechol followed a different pathway: 24% of the nighttime products were already in the particle phase, and subsequent OH exposure mainly increased aerosol oxidation state rather than mass. Finally, the water-soluble oxidative potential of fine (PM2.5) and ultrafine (PM0.1) ambient particles was compared, for the first time in Greece, between a regional-background site in Poros and an urban site (National Observatory of Athens; NOA) in Thissio, Athens. At Poros, the mass-normalized DTT activity (DTTm) of PM0.1 averaged 50.9 ± 20.8 pmol min-1 μg-1 and was approximately four times that of PM2.5. This OP was associated mainly with organic aerosol and nitrate, whereas PM2.5 activity was more closely related to chloride, sulfate, and more-oxidized oxygenated organic aerosol (MO-OOA), indicating marine, shipping, and transported aged influences. In Athens the size dependence of the WS-OP had the opposite behaviour. The PM2.5 activity was on average 33.3 ± 6.8 pmol min-1 μg-1, twice that of PM0.1. Covariance between biomass-burning organic aerosol (ΒΒΟΑ) factor and potassium (K+) confirmed fresh biomass-burning influence on PM0.1 composition. The reversal between the two environments demonstrates that intrinsic particle oxidative activity is controlled by source mixture, chemical composition, and atmospheric processing rather than by particle mass or size alone. Overall, the thesis links oxidation history and molecular product formation with aerosol phase transfer, size distribution, and potential toxicity. The results demonstrate that PM mass and bulk oxidation state are insufficient descriptors of health-relevant aerosol properties and support the inclusion of precursor-specific chemistry, sequential day–night processing, and size-resolved composition in assessments of biomass-burning pollution and its potential health effects.
Open Access
Mass concentration, composition and sources of atmospheric nanoparticles
(2026-09-23) Αργυροπούλου, Γεωργία; Argyropoulou, Georgia
Ultrafine particles (UFPs or PM0.1), are atmospheric particles with diameters smaller than 100 nm. Although these particles dominate atmospheric particle number concentration, they contribute relatively little to particulate matter (PM), rendering their mass and composition measurement challenging. They have physicochemical properties that distinguish them from larger particles. Their small size and large surface area per unit mass facilitate deep penetration into the respiratory system and enable translocation to other organs through the bloodstream, while their large surface area and high surface reactivity enable them to adsorb, for a given mass of PM, greater quantities of hazardous metals. As a result, ultrafine particles are receiving increasing attention as a potentially important component of ambient particulate matter associated with adverse health effects. Nevertheless, the understanding of their atmospheric behavior, chemical composition, and sources remains limited, partly because their typically low mass concentrations make accurate time-resolved measurements particularly challenging. In the first part of this thesis, the relationship between PM0.1 and particle number concentration above different lower size cut-offs was investigated using particle size distribution measurements from several European locations, together with predictions from the three-dimensional chemical transport model PMCAMx-UF. The analysis showed that PM0.1 and the concentration of particles larger than 50 nm (N50) were strongly correlated. Hourly observations from six European urban locations showed that PM0.1 had strong spatial and temporal correlation with N50 (R2 > 90%). The line passing through zero had an average slope of 0.16 ± 0.01 x 10-9 μg, for an assumed unit particle density, indicating that PM0.1 can be measured indirectly but quite accurately and continuously by measuring N50. Based on the above finding, the second part of this thesis focused on the development of a novel approach for the continuous PM0.1 mass measurements using a condensation particle counter (CPC). A water-based CPC was modified to measure N50 (CPC50), and its performance was evaluated by comparing its ambient measurements with measurements from a Scanning Mobility Particle Sizer (SMPS) in Rio, Greece. The PM0.1 concentration was estimated from the CPC50’s N50 measurements and was in good agreement with PM0.1 measured by the SMPS, with a fractional bias of less than 5% and a fractional error of 21% for hourly measurements. These results demonstrated that accurate N50 measurements can be used to estimate PM0.1 continuously and that a CPC-based approach has the potential to provide a substantially simpler and cheaper alternative to an SMPS. Such an approach could facilitate the deployment of PM0.1 measurements over larger spatial scales and provide high-temporal-resolution mass concentration datasets that are currently scarce. Measuring the PM0.1 mass concentration alone, however, does not provide information about its sources or chemical composition. The next part of the thesis therefore focused on the development of a system for continuous chemical characterization and source apportionment of ultrafine PM. An Aerodynamic Aerosol Classifier (AAC) was modified to separate PM0.1 from larger particles and was combined with a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS), a single-particle soot photometer (SP2-XR), and an Xact625i elemental analyzer. This approach enabled the high-temporal-resolution measurements of organic aerosol (OA), sulfate, nitrate, ammonium, chloride, black carbon (BC), and elemental composition, as well as the source-apportionment analysis of the ultrafine OA. The method was first evaluated during a summertime campaign in Rio, Greece. PM0.1 averaged 0.7 μg m-3 and OA was its most abundant component, contributing approximately 45% of the total mass. Sulfate, calcium, ammonium, BC, and the remaining detected elements contributed approximately 14%, 18%, 7%, 4%, and 7%, respectively, while nitrate and chloride contributed less than 4% each. Source apportionment showed that approximately 80-90% of the ultrafine OA was oxygenated, while primary OA accounted for approximately 10–15%. These measurements demonstrated the feasibility of obtaining detailed chemical and source-related information for PM0.1 continuously, rather than relying on the daily or longer sampling periods required by conventional filter-based approaches. The developed measurement system was subsequently deployed for one-month at a Mediterranean background site on the island of Poros, Greece, during late summer, to investigate ultrafine PM under conditions of strong atmospheric processing, secondary formation, and long-range transport. PM0.1 averaged 0.5 μg m-3 and consisted predominantly of OA and sulfate (40% and 30%, respectively), followed by ammonium and nitrate at approximately 10% each. Frequent new particle formation events were observed near the site, associated with high sulfur dioxide concentrations in the area, which led to sharp increases in PM0.1, sulfate, and particle number around midday. Elemental measurements detected relatively high concentrations of calcium and iron in the ultrafine fraction, potentially associated with the nearby shipping emissions. Particularly notable were periods influenced by volcanic emissions from Mount Etna, approximately 750 km from the measurement site. During these events, sulfur dioxide concentrations increased by approximately a factor of ten and were accompanied by elevated concentrations of zinc, lead, and arsenic in PM0.1, demonstrating the persistence of these heavy metals in the ultrafine fraction even after long-range transport over hundreds of kilometers and substantial atmospheric aging. PM0.1 and PM1 showed only a weak overall correlation (R2 = 0.25), despite both being dominated by OA and sulfate, emphasizing that fine and ultrafine particles can respond differently to regional and atmospheric processes. Finally, the developed chemical characterization system was deployed for a month-long wintertime campaign at an urban background site in central Athens, Greece, to investigate the composition and sources of PM0.1. The average PM0.1 concentration was 1.1 μg m-3, with OA accounting for most of the mass (75%), followed by BC (11%). Total PM0.1 and OA exhibited their highest concentrations during the nighttime, whereas ultrafine BC showed a pronounced morning maximum associated with traffic emissions. Source apportionment analysis identified four PM0.1 OA factors: hydrocarbon-like OA (HOA), cooking OA (COA), biomass-burning OA (BBOA), and oxygenated OA (OOA), contributing 19%, 19%, 13%, and 49% of the ultrafine OA, respectively. Thus, primary OA accounted for approximately half of the PM0.1 OA, with cooking and biomass burning emissions making a particularly important contribution to the ultrafine fraction despite their commonly assumed association with larger accumulation-mode particles. Although PM0.1 and PM1 mass were strongly correlated (R2 = 0.78), aspects of their detailed composition and temporal behavior diverged. OA represented a larger fraction of PM0.1 than of PM₁ (75% compared with 55%), and primary OA factors generally exhibited sharper and earlier peaks in the ultrafine fraction, suggesting their initial emission into smaller particles followed by growth toward larger sizes. In contrast, cooking OA showed little temporal shift between the two size fractions, suggesting a particularly direct contribution to ultrafine PM. Secondary inorganic species also exhibited pronounced differences in their size-dependent behavior, demonstrating that a strong correlation between total PM0.1 and PM1 mass does not necessarily mean identical chemical composition or source dynamics.
Open Access
Numerical modelling and simulation of multiphase flows : applications to granular materials and heat transfer
(2026-09-25) Μπαλαχτσής, Αθανάσιος; Balachtsis, Athanasios
Rotary kilns are critical high-temperature reactors used in a wide range of industrial processes, including cement production, mineral calcination, waste incineration, biomass pyrolysis, and solid-state reactions. Their ability to process heterogeneous granular materials under extreme conditions makes them essential in process industries. However, the physics governing their operation remains only partially understood due to strongly coupled, multiscale transport phenomena involving granular flow, heat transfer, phase change, and chemical reactions. Consequently, kiln operation is still largely guided by empirical knowledge and reduced-order models that do not fully resolve the underlying multiphysics behaviour. This thesis develops a unified continuum computational framework for rotary kiln processes, enabling a rigorous description of dense granular flow, thermal transport, phase change, viscoplastic melt rheology, and the melting of granular beds and their transition into cohesive aggregates within a multiphase finite element formulation. A central objective is to enhance predictive capability by resolving the transition from granular to partially or fully molten regimes, with particular emphasis on melting dynamics as previously underrepresented aspects of kiln modelling. The proposed framework provides a structured foundation for future extensions toward fully coupled multiphysics simulations of industrial pyroprocessing systems.The first part of the thesis addresses granular flow using a pressure-dependent () rheology. Benchmark simulations of granular column collapse reveal mesh-dependent instabilities in the computed fields, associated with the ill-posed character of the constitutive model under certain flow regimes. These numerical difficulties are addressed through partial rheological regularization combined with shock-capturing stabilization terms in the momentum equation. The stabilized formulation accurately reproduces collapse dynamics across all stages (free fall, spreading, and arrest), while distinguishing between yielded regions, where the material flows, and unyielded regions, where it exhibits solid-like behaviour. Building on this foundation, the framework is extended to simulate dense granular flow in the transverse cross-section of a partially filled rotating cylinder. The model reproduces experimentally observed flow structures and captures the formation of characteristic regions, including an active layer (with intense shear deformation) and a passive layer undergoing near-solid-body rotation. Extensive parametric studies quantify the influence of angular velocity, filling degree, friction coefficient, and particle size on flow topology, active layer thickness, and dynamic angle of repose, as well as the transition from rolling to cascading regimes. The second stage of the thesis extends beyond isothermal mechanical processes by incorporating thermal transport and phase change. A single-domain enthalpy formulation is employed in the energy equation, combined with a temperature-dependent viscosity model, allowing the solid–liquid transition to be captured implicitly. This approach avoids mesh deformation and explicit interface tracking, as the solid–liquid boundary is represented through the temperature field. Initially, the melt is approximated as a Newtonian fluid. The simulations show that melting kinetics are governed by the balance between buoyancy-driven natural convection and rotation-induced forced convection. Subsequently, to represent more accurately partially molten clinker agglomerates, the melt is modelled as a viscoplastic Bingham fluid using Papanastasiou regularization. The introduction of yield stress modifies the flow structure by promoting yielded and unyielded regions, thereby altering mixing behaviour within the melt. Higher yield stress leads to increased adherence of the fluid to the rotating wall, which enhances local heat transfer and can reduce the overall melting time compared to the Newtonian case. While this viscoplastic fluid model captures the non-Newtonian dynamics of the liquid phase, a complete representation of the physical process requires a more rigorous description of the solid state, which up to this point had been simplified as an extremely viscous Newtonian fluid limit. To address this limitation and provide a dedicated rheological description of the granular medium under thermal transition, the final stage of the thesis unifies dense granular mechanics and phase-change thermodynamics through a cohesive granular analogy. Utilizing an empirical melt-fraction-dependent yield stress function integrated into the original ()-model, the framework captures the progressive transformation of the dry powder into cohesive, partially molten aggregates. Crucially, the liquid phase acts as a rheological binder rather than a heat-transfer medium, creating a thermal bottleneck that dampens convective recirculation. This framework provides a useful starting point for developing future predictive tools that aim to integrate coupled chemical reactions and phase-change dynamics in high-temperature industrial processing.
Open Access
Sources of anthropogenic secondary organic aerosol in the atmosphere
(2026-10-07) Παυλίδης, Δαμιανός; Pavlidis, Damianos
Organic aerosol is a major component of fine particulate matter and a large fraction of it is secondary in origin, formed through the atmospheric oxidation of gas-phase precursors. Anthropogenic activities, and in particular motor vehicles, are an important source of these precursors, yet their contribution to secondary organic aerosol (SOA) formation remains poorly understood. This thesis addresses these knowledge gaps through a combination of laboratory studies using a smog chamber and an oxidation flow reactor, together with real-world field measurements. The findings contribute to improving the representation of vehicle emissions in emission inventories and SOA formation and chemical transport models. This work first addresses a methodological challenge that affects the interpretation of chamber SOA formation experiments. The high-resolution time-of-flight aerosol mass spectrometer (AMS) has become one of the most important instruments in the field of atmospheric sciences, providing real-time measurements of the mass concentration, size distribution, and chemical composition of submicrometer non-refractory aerosol, along with the scanning mobility particle sizer, which measures the particle number size distribution. However, AMS measurements require correction for the instrument’s collection efficiency, which is commonly assumed to be independent of particle size. This assumption often leads to significant discrepancies between the measurements of the two instruments in SOA formation experiments with new particle formation. To address this, a new methodology is proposed for the estimation of a size- and composition-dependent collection efficiency. Applied to two sets of chamber experiments, nonylcyclohexane oxidation and α-humulene ozonolysis, the new approach reduced the average deviation between the two instruments to below 10% for both total aerosol and SOA mass. SOA formation from the photooxidation of large anthropogenic volatile and intermediate-volatility organic compounds with hydroxyl radicals under high-NOx conditions, often encountered in urban areas, was investigated next. The precursors studied included cyclic alkanes (amylcyclohexane, hexylcyclohexane, nonylcyclohexane, and decylcyclohexane) and aromatic compounds of increasing size (1,3,5-trimethylbenzene, 1,3,5-triethylbenzene and 1,3,5-tritert-butylbenzene). A combination of measurements was performed for each precursor to develop SOA parameterizations for use in chemical transport models. Specifically, the volatility and effective vaporization enthalpy of the oxidation products were estimated using an algorithm that combined the observed SOA yields with results from two evaporation techniques, thermodenuding and isothermal dilution. Significant SOA formation was observed for all investigated compounds, with cycloalkanes exhibiting higher yields than aromatic compounds, while aromatic precursors produced more oxidized SOA. Additionally, the oxidation products of cycloalkanes were dominated by semivolatile organic compounds, whereas those of aromatic precursors were mainly distributed in the intermediate-volatility range. To enhance our understanding of cold start emissions from passenger cars, which dominate urban traffic, measurements of approximately 21,000 cold starts were carried out inside an underground parking garage from November 29 to December 20, 2024. The underground facility, located beneath a shopping mall in Patras, Greece and consisting of 246 parking spaces, provided a controlled environment representative of real-world cold start conditions. Measurements of traffic and pollutant concentrations were combined to derive average emissions of gas- and particulate-phase pollutants. The average parking time of vehicles was 1.5 h and vehicle emissions were driven almost entirely by cold starts. The emitted submicron particulate matter consisted primarily of organic aerosol (61%) and black carbon (39%), while volatile organic compounds were dominated by C6–C9 aromatics (65%). The estimated cold start emission factors were then compared with existing emission inventory estimates to assess the representation of cold start emissions in European emission inventories. In the final part of the thesis, individual on- and off-road vehicles were tested on a dynamometer, with their exhaust introduced into an atmospheric simulation chamber and an oxidation flow reactor to investigate SOA formation across a range of photochemical ages. In total, tailpipe emissions from 14 vehicles, including 4 gasoline cars, 4 diesel cars, 4 scooters, and 2 tractors, covering a range of engine types and emission control technologies, were examined. The SOA formation potential (SOAFP, SOA formed per kilogram of fuel) estimated using the two reactors showed similar trends across vehicle categories and was strongly correlated (R2=0.90). However, the oxidation flow reactor consistently produced more oxidized SOA than the smog chamber at comparable OH exposures, with oxygen-to-carbon ratios on average 47% higher. Scooters (pre-Euro–Euro 5) and the off-road vehicles had the highest SOAFP values, exceeding those of all on-road gasoline and diesel vehicles. For these vehicles, SOA also accounted for more than 80% of the total aged organic aerosol. The volatility characterization further showed that the aged emissions (with a saturation concentration below 106 μg m-3) of most vehicles were dominated by intermediate-volatility organic compounds (58 ± 19%). Overall, these findings improve the characterization of vehicle-related SOA, highlighting scooters and off-road vehicles as potentially important SOA sources, and support the oxidation flow reactor as a practical tool for SOAFP evaluation in the development of future vehicle emission control policies.
Open Access
Sources and concentrations of atmospheric organic pollutants in gaseous and particulate phase
(2026-07-24) Ματραλή, Αγγελική; Matrali, Angeliki
Air pollution is one of the leading factors driving premature mortality currently on the globe, with an estimated contribution in the order of 10 million deaths annually attributed to inhalation of airborne fine particles. A major fraction of submicron aerosol is organic (OA) and can be either primary (directly emitted in the particle phase) or secondary (formed from gas to particle conversion processes). The secondary OA (SOA) has been found to be more hazardous and harder to regulate, with chemical transport models (CTMs) often underestimating its concentration. A central reason for this discrepancy is the incomplete understanding of the multigenerational oxidation pathways that lead to SOA formation. This thesis investigates the formation, chemical evolution, and atmospheric relevance of secondary organic aerosols from both anthropogenic and biogenic precursors. It combines controlled smog chamber and oxidation flow reactor experiments with field measurements in urban and remote environments. Emphasis is placed on the role of intermediate volatility organic compounds, the interaction between anthropogenic and biogenic emissions, and the ability of measured gas phase precursors to explain observed SOA formation. The first part of this work investigates the mass balance gap between observed and predicted SOA produced during the oxidation of road transport emissions. Parallel analysis of the levels of organic vapors and their photooxidation was conducted in two environments heavily influenced by vehicular emissions. The precursors collected on sorption tubes were analyzed offline with thermal desorption gas chromatography mass spectrometry (TD-GC-MS). The potential SOA that could be formed in these environments was quantified using an oxidation flow reactor. For both environments the estimated SOA from the measured precursors could account for practically all the observed SOA. However, the importance of intermediate-volatility organic compounds (IVOCs) varied strongly between the two environments. In the Fréjus tunnel, where diesel-fueled vehicles had a stronger influence, IVOCs dominated the predicted SOA formation, contributing approximately 70%. In contrast, in the gasoline-influenced environment of an underground parking, IVOCs accounted for only about one quarter of the total SOA. These results highlight the importance of intermediate volatility precursors in the diesel-dominated vehicular European fleet. The second part of the thesis focuses on biogenic SOA formation from sesquiterpenes. Although sesquiterpene emissions are substantially lower than monoterpene emissions, their ozonolysis can produce semi-volatile and intermediate-volatility products that continue to react in the atmosphere. This work shows that this sesquiterpene SOA source has been seriously underestimated, with the aging of ozonolysis products reacting with the OH radical producing SOA at mass yields exceeding 100%. This later generation SOA is quite oxidized (O:C above 0.5) and its composition is consistent with field observations. The corresponding multigenerational chemistry was parameterized and implemented in a chemical transport model. The simulations suggest that, during summer in European forested regions, inclusion of this chemistry can increase sesquiterpene-derived SOA concentrations by up to 1.5 μg m-3, corresponding to a factor of 3–5 enhancement compared with previous estimates. The third part of the thesis examines organic vapors and particulate organics in an urban environment. A month-long measurement campaign was conducted at an urban background station in Athens during January and February 2025. Individual organic compounds in both the gas phase and the submicron particulate phase were measured using a PTR-ToF-MS coupled with a CHARON inlet. Source apportionment of the VOCs indicated contributions from various primary and oxidized factors. The concentrations of major compounds did not appear to have changed significantly over the last decade, despite the factor of 2.6 reduction in estimated anthropogenic emissions assumed in the most recent European emissions inventory for this area. Comparison with a collocated PTR-Quad-MS and low-cost total VOC electrochemical sensors was conducted assessing the sensitivity of these instruments to specific VOCs. In the particle phase, OA components showed significant fragmentation in the PTR-ToF-MS, and the 215 identified species or fragments represented approximately 10% of the total OA. Biomass-burning tracers, including dehydroabietic acid, abietic acid, nitrophenol, and eugenol, were detected, while oleic, palmitic, and stearic acids were quantified as cooking-related OA tracers, corresponding to approximately 0.5–1% of the cooking OA factor. The final part of the thesis investigates summertime aerosol composition and sources at a remote forested site in the Eastern Mediterranean. The SPRUCE-22 field campaign was conducted in July 2022 at a remote mountainous site in Greece. Particle chemical composition, size distributions, and concentrations of organic and inorganic gases were measured at high time resolution, complemented by offline analysis of collected samples. Despite the remote location, average PM₁ concentrations were unexpectedly high, reaching 13 μg m-3 dominated by organics, in a previously sulfate rich area pertaining to reductions in SO₂ emissions from coal-burning power plants in Greece. The observed concentrations were uniformly distributed across Greece with PM2.5 concentrations at the site occasionally higher than, those observed in Athens, in the absence of influences from local sources. This background aerosol was highly oxidized with an average O:C ratio of 0.83. Mean isoprene and monoterpene mixing ratios were 0.6 ppb and 0.5 ppb, respectively. Source apportionment identified highly and moderately oxidized OA as well as biogenic SOA. Radiocarbon analysis was used in conjunction with the source apportionment factors to further elucidate the origin of the observed PM. The biogenic SOA was also relatively aged, with an O:C ratio of 0.63, and was primarily transported to the site. Nearby new particle formation events are related to the concentrations of precursor species and origin of the air masses. The oxidative potential of the particulate matter per unit mass was high, emphasizing the potential health relevance of regionally transported aerosol even at remote locations. The results of this work highlight the importance of specific gaseous precursors in explaining organic aerosol by accounting for the in situ observed SOA of traffic gaseous precursors to highlighting the importance of multigeneration reactions of biogenic IVOCs like β-caryophyllene. Field measurements indicate the dominance of long-range transport on a regional scale during a photochemically active period. Conversely, the urban wintertime pollution of the Greek capital is assessed (with respect to organic vapors) and related to equivalent measurements of more than a decade ago. New insights are gained with respect to SOA precursors, while on-line measurements of major PM constituents on the molecular level are first conducted in Athens using high resolution mass spectrometry.
Open Access
Molecular dynamics simulations of organic aerosol components
(2025-12-19) Σιαχούλη, Παναγιώτα; Siachouli, Panagiota
Atmospheric aerosols influence climate, air quality, and human health through their effects on our planet’s energy balance, cloud formation, and heterogeneous chemistry. Organic compounds constitute a major fraction of aerosols and play a central role in determining their physicochemical properties. Most of these compounds have a wide range of functional groups and complex molecular architectures, forming a chemically rich organic mixture that remains only partially understood. Investigating their physicochemical properties is essential for understanding their role in atmospheric processes. Glass transition temperature (Tg) and self-diffusion coefficient (D) influence the phase state of organic compounds, thereby affecting key processes such as gas-particle partitioning, heterogeneous chemistry, and long-range transport. In this work, Molecular Dynamics (MD) simulations were employed to investigate Tg and D for a diverse set of atmospherically relevant organic compounds, including ketones, alcohols, carboxylic acids, and multifunctional ones. Tg was determined from changes in density and non-bonded energy during cooling simulations, with results compared against available experimental data and semi-empirical equations. For a few compounds, the method of segmental relaxation was also utilized. Self-diffusion coefficients at 298 K were either directly obtained from simulations or, for compounds with melting points above this temperature, evaluated through temperature-dependent simulations followed by extrapolation. For both properties, the dataset encompassed a broad range of organic compounds with varied molecular weights, functional groups identities, and molecular architectures, enabling the identification of systematic trends. In the first part of the thesis, the framework of predicting Tg via MD simulations is established. The scope was to examine the influence of functional groups, carbon chain length, and molecular architecture on Tg for a set of atmospherically relevant organic compounds. The MD simulations agreed within 20% with the available experimental measurements, while consistently overpredicting Tg. Tg was found to be most sensitive to functional group type, following the hierarchy -COOH > -OH > -C=O. Increasing the number of carbon atoms increased Tg. No clear correlation between oxygen to carbon (O:C) ratio and Tg was observed, whereas molecular weight was positively correlated with the increase of Tg. Molecular architecture was also found to influence Tg, with non-aromatic cyclic compounds showing higher Tg than linear or branched corresponding counterparts. In the second part of the thesis, the Tg framework was extended to a broader and more diverse set of organic compounds, and the MD results were used to develop a Tg parameterization based on molecular characteristics. Both the MD predictions and the parameterization linked Tg to functional group identity and number, molecular architecture, and elemental composition. The simulations confirmed the functional group hierarchy -COOH > -OH > -C=O, a trend maintained even when multiple functional groups coexisted within a molecule. Cyclic structures consistently exhibited higher Tg values than their linear counterparts, and Tg increased with the number of carbon atoms. The parameterization reproduced the MD simulation results within 10% and, after applying a scaling factor of 0.9, achieved agreement within 10% with the available experimental measurements. The robustness of the parameterization was evaluated through a leave-one-out approach, which identified the functional groups and molecular features that most strongly influence Tg and highlighted compounds whose limited representation leads to greater variability in predicted contributions. In the third part, MD simulations were carried out to quantify the self-diffusion coefficient D of atmospherically relevant organic compounds at 298 K. The study began with simple monofunctional compounds—ketones, alcohols and carboxylic acids—before extending to multifunctional ones that had previously been examined for their Tg. Predicted D values agreed with available measurements within a factor of 1.9 and with prior MD studies within a factor of 1.7. Diffusivity decreased with increasing carbon chain length, molecular weight, and functional group multiplicity. The hierarchy of sensitivity was found to be –COOH > –OH > –C=O, consistent with their Tg behavior and reflecting the importance of hydrogen-bond network formation in controlling molecular mobility. Multifunctional compounds exhibited the same trend, with compounds containing carboxyl-hydroxyl functional group combinations showing the lowest D values. The spatial arrangement of functional groups also influenced D, with closely positioned groups producing the strongest mobility constraints and the lower predicted values. Molecular weight showed a general inverse relationship with D in monofunctional systems, but its predictive relevance diminished in multifunctional compounds, where intermolecular association and local packing effects dominated; elemental descriptors such as oxygen to carbon ratio were likewise informative only within closely related chemical families. Overall, these results provide a molecular-level understanding of the molecular features influencing Tg and D in organic aerosol components, offering predictive capability for compounds lacking measurements and supporting improved representation of aerosol phase state and molecular mobility in atmospheric models.
Open Access
Improving the simulation of intermediate volatility organic compounds and their contribution to secondary organic aerosol formation in the atmosphere
(2025-12-16) Μανάβη, Στέλλα Ευτυχία; Manavi, Stella Eftychia
In the atmosphere, secondary organic aerosol (SOA) is an important constituent of fine particulate matter (PM2.5). SOA is formed through the condensation of organic vapors produced from the oxidation of volatile precursors. Intermediate volatility organic compounds (IVOCs) are organic vapors with an effective saturation concentration (C*) at 298K between 103 and 106 μg m-3, and they can be an important source of SOA formation in the atmosphere. In the first part of this thesis, we develop a new approach for simulating IVOCs, and SOA formed from IVOCs (SOA-iv) in chemical transport models (CTMs). The main idea is to treat IVOCs as lumped species that retain their chemical characteristics, allowing for a more explicit representation of their SOA formation potential and chemistry than previous highly parameterized schemes. Moreover, this new lumped species approach allows the quantification of both the overall role of IVOCs in SOA formation and the role of individual compounds in the IVOC range. Overall, eleven lumped species are introduced in a CTM, representing intermediate length alkanes, small polycyclic aromatic hydrocarbons, intermediate length aromatics, phenolic compounds, benzaldehydes and ketones in the IVOC range. The first application of the lumped species approach focused on IVOC emitted from on-road diesel and gasoline vehicles over Europe. The simulated SOA-iv is the highest in major European cities, such as Paris, Athens, and Madrid, where on-road SOA-iv can account for up to 25% of the predicted total SOA. Among the simulated IVOCs, unspeciated cyclic alkanes in the IVOC range contribute up to 72% of the predicted SOA-iv mass, with C15-C20 compounds being the most prominent precursors. Sensitivity tests show that multigenerational aging has the strongest effect on SOA-iv levels, increasing concentrations by about 67%. The second application of the lumped species approach aimed at biomass burning IVOCs over Europe. During winter, reactions with the nitrate radical account for roughly 40% of the formed biomass burning SOA-iv, while the resulting aerosol can account for up to 20% of the total SOA over Europe. In summer, near wildfires, the hourly predicted biomass burning SOA-iv concentrations can reach 240 μg m-3, contributing 60% of the SOA in the wildfire plumes. The IVOCs identified as the most important precursors differ between the simulation periods, with alkylphenols being more prominent during winter and ten-carbon-atom methylbenzenes during summer. Across both seasons, 1-propylbenzene is identified as a key individual contributor to biomass burning SOA-iv formation. In addition to on-road and biomass burning IVOCs, we also developed a new parametrization to describe SOA formed from the ozonolysis of sesquiterpenes and the sequential aging of the ozonolysis products based on smog chamber experiments conducted with β-caryophyllene. The implementation of the new approach in a CTM substantially enhanced the role of sesquiterpenes in summertime SOA formation over Europe. Monthly average concentrations of biogenic SOA formed can reach up to 1.2 μg m-3, with sequential aging reactions accounting for approximately 90% of the formed aerosol. Overall, compared to the previous model, the updated parameterization increases the predicted sesquiterpene SOA by a factor of 7, and hourly biogenic SOA concentrations increase by a factor of 4. As a result, biogenic SOA becomes a major contributor to total SOA, even exceeding the contribution of anthropogenic SOA in some urban areas. Although both the updated models still underpredict OA and SOA levels, the new IVOC and sesquiterpene schemes significantly improve the predicted concentrations and provide a more accurate representation of SOA sources and chemistry.
Open Access
Μη καταστροφική μέτρηση καμπτικών συχνοτήτων ταλάντωσης για την ανίχνευση ρωγμών σε μεταλλικές, πλαστικές και τσιμεντένιες δοκούς με τη χρήση μαγνητοελαστικών αισθητήρων σε μορφή μικροσύρματος και κορδέλας
(2025-12-10) Ταπεινός, Χρήστος; Tapeinos, Christos
Σε αυτή την εργασία έγινε χρήση των μαγνητοελαστικών αισθητήρων (σε μορφήκορδέλας και μικροσύρματος) για την ανίχνευση των ρωγμών σε τσιμεντένιεςδοκούς, καθώς και των καμπτικών συχνοτήτων ταλάντωσης σε τσιμεντένιες καιμεταλλικές δοκούς. Όλη η διατριβή χωρίζεται σε τρία μέρη.Στο πρώτο μέρος, έγινε η προσπάθεια να μετρηθούν οι συχνότητεςκαμπτικής ταλάντωσης μιας αλουμινένιας δοκού προβόλου, έμμεσα με τη χρήσηπλαστικών μικρότερων δοκών εκτυπωμένων με τρισδιάστατο εκτυπωτή. Ημεταλλική δοκός πακτώθηκε με τη βοήθεια μιας πρέσας και συνδέθηκε με έναμηχανικό ταλαντωτή ο οποίος συνδεόταν με μια γεννήτρια συχνοτήτων. Ηπλαστική δοκός κολλήθηκε με ταινία στο ελεύθερο άκρο της μεταλλικής. Οαισθητήρας κολλήθηκε εξωτερικά της πλαστικής δοκού αλλά και στο εσωτερικόσε διάφορα βάθη. Στη διάταξη προστέθηκε ένα πηνίο ανίχνευσης και έναπολύμετρο. Έτσι, όταν λειτούργησε ο ταλαντωτής διέγειρε (εξανάγκασε) τη δοκόσε συγκεκριμένες συχνότητες και όταν συνέβαινε συντονισμός, ο αισθητήραςπαρήγαγε ΗΕΔ, η οποία ανιχνευόταν από το πολύμετρο. Έτσι καταγράφηκαν όλεςοι φυσικές συχνότητες της εταλλικής δοκού. Να σημειωθεί πως οι συχνότητεςπου μετρήθηκαν διέφεραν από τις συχνότητες απλής μεταλλικής δοκού λόγω τηςπροσθήκης της πλαστικού δοκού. Τέλος, πρέπει να αναφερθεί πως το ίδιο πείραμαέγινε και άλλες φορές με τον αισθητήρα να βρίσκεται εξωτερικά αλλά και σεδιάφορα βάθη μέσα στην πλαστική δοκό, για να εξακριβωθεί αν το βάθος επηρεάζειτην ανίχνευση.Στο δεύτερο κομμάτι έγινε η προσπάθεια να ανιχνευτούν οι ρωγμές σετσιμεντένιες δοκούς με ελεύθερα άκρα με τη χρήση αισθητήρα σε μορφή κορδέλαςμέσω της εύρεσης των φυσικών συχνοτήτων ταλάντωσης της δοκού. Οι δοκοίδιαστάσεων 4x4x16cm3κατασκευάστηκαν σε ειδικά καλούπια από εμπορικότσιμέντο σύμφωνα με τα πρότυπα EN 76 196-1 σε ειδικό εργαστήριο. Οιαισθητήρες κολλήθηκαν στην επιφάνεια των δοκών με ειδική κολλητική ταινία δύοόψεων. Για τη στήριξη της δοκού χρησιμοποιήθηκε μια μεταλλική και μια ξύλινηβάση για σύγκριση και η ανίχνευση του σήματος έγινε με τη χρήση πηνίου. Τοπηνίο συνδέθηκε στη θύρα μικροφώνου ενός υπολογιστή και το πρόγραμμαspectrum analysis έδινε μετά από μετασχηματισμό Fourier τις συχνότητεςταλάντωσης των τσιμεντένιων δοκών. Λαμβάνοντας τις συχνότητες της δοκού μεκαι χωρίς ρωγμή, από τις διαφορές στο σήμα ανιχνευόταν η ρωγμή.Στο τρίτο μέρος μελετήθηκε η συμπεριφορά των δύο αισθητήρων κορδέλαςκαι σύρματος συγκριτικά. Μετρήθηκαν οι βρόχοι υστέρησης και η αντίδραση τωναισθητήρων στην εφαρμογή εξωτερικού εναλλασσόμενου μαγνητικού πεδίου 1-300KHz. Στη συνέχεια, πάλι συγκριτικά, ανιχνεύθηκαν οι συχνότητες καμπτικήςταλάντωσης και η ρωγμή σε τσιμεντένιες δοκούς σε μια διάταξη όμοια με τουδευτέρου μέρους. Τέλος ανιχνεύθηκαν οι συχνότητες τσιμεντένιας δοκού με τουςαισθητήρες ενσωματωμένους στο εσωτερικό της δοκού.
Open Access
Activation of persulfate for sustainable water purification : heterogeneous, electrochemical, and hybrid strategies
(2025-09-10) Liu, Zhongda; Liu, Zhongda
The global water crisis is one of the most pressing challenges facing humanity today. Uncontrolled water pollution has intensified the water scarcity caused by rapid population growth, urbanization, and industrialization. According to the United Nations, 44% of global household wastewater (113 × 10⁹ m³) was discharged without safe treatment in 2022. In the same year, more than 2.2 billion people still lacked access to safely managed drinking water. Industrial and household pollutants such as azo dyes, endocrine-disrupting chemicals, and pharmaceuticals have long-term environmental persistence and can cause chronic toxic effects on aquatic ecosystems. Through bioaccumulation in the food chain, such pollutants can ultimately pose risks to human health, causing problems like hormonal disruption, neurological disorders, and other chronic diseases. Considering the significant gap between current global wastewater treatment capabilities and demand, there is an urgent need to develop innovative, efficient, and sustainable wastewater treatment technologies.Persulfate-based advanced oxidation processes (AOPs) are promising approaches to deal with such problems. Two types of persulfates, i.e., peroxydisulfate (PDS, S₂O₈²⁻) and peroxymonosulfate (PMS, HSO₅⁻), can be activated by certain methods to generate various reactive species for the degradation of organic pollutants, involving hydroxyl radicals (•OH), sulfate radicals (SO₄•⁻), superoxide radicals (O₂•⁻), singlet oxygen (¹O₂), high-valent metal (HVM)-oxo species, and mediate direct electron transfer (DET) pathways. Heterogeneous activation, electrochemical activation, and hybrid activation are of particular interest due to their multiple advantages. Delicate design strategies enable the simultaneous attainment of high activity and stability in heterogeneous catalysts. Besides, electrochemical processes exhibit high sustainability due to high treatment efficiencies and less reliance on chemicals. Hybrid strategies bring together the merits of faster degradation kinetics, deeper mineralization rate, improved catalyst lifetime, reduced chemical dosage, and reduced electricity energy consumption. This project aims to develop innovative and sustainable water purification technologies by employing persulfate activation via heterogeneous, electrochemical, and hybrid strategies. In total, four processes were developed. The first process is a heterogeneous system for PMS activation and acid orange 7 (AO7) degradation, in which the carbon-iron-cobalt co-doped graphitic carbon nitride (g-C₃N₄/C-FeCo) was prepared with a co-doping strategy. The co-doping boosted the PMS activation due to improved charge transfer mobility, and non-radical pathways dominated the AO7 degradation. Remarkably, the required catalyst dosages (5–50 mg/L) were one order of magnitude lower than commonly used levels, but ultrahigh degradation rate constants (up to 2.985 min⁻¹) were achieved with complete removal (>99.8%) finished within one minute. A stabilized performance was also demonstrated during a 36-h test in a flow reactor. Mechanistic study revealed that the PMS activation proceeded via the non-radical pathway, giving rise to O₂•⁻, ¹O₂, high-valent metal-oxo species, and mediated DET mechanism.In the second process, another heterogeneous catalyst was developed for PMS activation and bisphenol S (BPS) degradation, in which the carbon-encapsulated cobalt ferrite (CoFe₂O₄/CoFe@C) was designed with a unique core-shell structure. The structure and activity were highly dependent on the preparation temperature. The hydrothermal process resulted in a core shell product with metal carbonate cores and amorphous carbon shells. A sufficiently high temperature (>300°C) is crucial for the decomposition of the metal carbonate cores to form the metal oxide cores. With increasing temperature up to 900°C, the amorphous carbon shell gradually graphitized and reduced the metal oxide to zero-valent CoFe interlayer. The optimal temperature of 500°C balanced the formation of CoFe and over-graphitization of the carbon shell and thus achieved the optimal activation performance. The BPS removal reached 97.0% in 15 min with a rate constant of 0.199 min⁻¹ and low metal leaching (0.66 mg/L of Co and 0.04 mg/L of Fe). Both radical and non-radical pathways contributed to the activation, with SO₄•⁻, ¹O₂, and high-valent metal-oxo species as the dominant reactive species.The third process is an electrochemical system for AO7 degradation assisted by both PMS and PDS. Reticulated vitreous carbon (RVC) cathode demonstrated excellent performance in generating H₂O₂ and activating persulfate, especially PMS. Abundant H₂O₂ (up to 117 mg/L) was produced from oxygen reduction with current efficiencies between 78–24% in 1 hour. Moreover, the addition of 0.5 mM of PMS boosted the degradation rate by 9.6 times reaching 0.447 min⁻¹ and the energy consumption was reduced by 90.1% to 0.0243 kWh/m³/order. Also, the RVC cathode was very robust with little deterioration observed in 10 cyclic tests. The H-cell test, scavenging experiments and electrochemical tests demonstrated that more than 90% of degradation took place on the cathode, with the occurrence of O₂•⁻ and DET pathway.In the fourth process, a hybrid system was developed for PDS activation and ciprofloxacin (CIP) degradation by combining heterogeneous and electrochemical activation. Considering the low activity of CoFe₂O₄/CoFe@C catalyst for PDS activation, the catalyst was applied as the particle electrode assisted by the RVC cathode. PDS enhanced the generation of H₂O₂ from the RVC cathode, and both PDS and H₂O₂ were efficiently activated by the particle electrode in the electric field. As a result, a remarkable synergy was observed in the hybrid process, removing 95.5% of CIP in 10 min with a rate constant of 0.309 min⁻¹. The process showed a high sustainability with reduced electric energy consumption (0.31 kWh/m3/order) by making full use of cathodic electrons. Scavenging, probing, and electrochemical tests revealed that •OH, O₂•⁻, and ¹O₂ were the major contributing reactive species.For all the four processes, the validity of the catalyst or process design was first demonstrated, followed by the systematic investigation of key affecting process parameters, environmental applicability, role of reactive species, degradation pathways, and in silico toxicity assessment of the identified degradation products. A comprehensive conclusion of obtained results, limitations, and prospects of the developed processes was provided. In summary, the innovated heterogeneous, electrochemical, and hybrid strategies for persulfate activation in this project pave the way for the development of greener sustainable technologies for water purification.
Open Access
Motion, deformation and interaction of immersed bodies in complex fluids
(2025-10-06) Esposito, Giancarlo; Esposito, Giancarlo
The dynamics of multiphase systems consisting of rigid or deformable particles suspended in complex fluids is central to both fundamental research and a wide range of applications, including biomedical, food, and consumer product industries. Multiphase systems such as air bubbles in sanitizer gels, oil droplets in food pastes, and cells traveling through blood exemplify this class of materials. Among the various categories of complex fluids, this thesis focuses mainly on yield stress materials. Their intriguing characteristic, differentiating them from other complex fluids, is that they behave as solids under low stress but flow as liquids after yielding, i.e., when a critical stress threshold is exceeded. Understanding the microscopic origin of this dual behavior remains a significant challenge and an active research topic, as it varies with the microstructure of the systems, whether it be colloidal suspensions, non-Brownian systems, or colloidal gels. However, macroscopic constitutive models can describe their rheology despite their microscopic differences. Motivated by recent experimental findings highlighting elastic phenomena in yield stress materials, either before or after yielding, this thesis adopts an elastoviscoplastic (EVP) constitutive model to describe the rheological behavior of such materials. This framework captures the combined viscous, elastic, and plastic characteristics of yield stress materials. With the EVP model providing a robust description of the continuous phase, the thesis proceeds to examine its implications for the dynamics of single and multiple inclusions under different flow conditions. In the first part of the thesis, we investigate single-object dynamics in three different settings. Initially, we examine the rise of a viscous drop in a polymeric fluid without yield stress, emphasizing the interplay between fluid elasticity and interfacial forces and the formation of trailing filaments observed in experiments. Secondly, we analyze the sedimentation of a drop in EVP materials, focusing on the conditions that lead to the entrapment of the object and how material parameters influence settling behavior. Thirdly, we explore the rising and migration of an air bubble through EVP fluids near walls employing fully 3D simulations, revealing how confinement, material properties, and initial shapes of the bubble affect its migration patterns. In the second part of the thesis, we address the interaction of multiple particles. We study the sedimentation of two coaxial viscous drops in EVP materials, capturing numerically the experimental trends and describing the mechanism of approach or separation of the pair. A parametric analysis leads to the discovery of master curves that link the relative velocity of the drops to their current distance. Furthermore, inspired by novel microfluidics techniques, we investigate the behavior of rigid and deformable particle pairs in pressure-driven microflows of viscoelastic and EVP materials. We identify a critical initial separation that indicates whether particles attract or repel, and we quantify the dependence of this threshold on the rheological parameters. The gained insights offer valuable guidance for the design of microfluidic technologies aimed at non-invasive particle manipulation and the controlled assembly of structures in soft-matter systems. Finally, moving beyond the constraints of axial symmetry, we present a 3D numerical analysis of two viscous drops sedimenting in elastoviscoplastic materials. This novel approach allows us to explore non-symmetric scenarios and shed light on the stability of the previously assumed axisymmetric configuration.
Open Access
Modeling secondary organic aerosol formation : from atmospheric simulation chambers to the field
(2025-09-12) Ουρούτσης, Πέτρος; Uruci, Petro
Particulate matter (PM) refers to the suspended solid or liquid particles in the atmosphere. These particles can pose significant human health risks and influence the Earth's energy balance, thereby contributing to climate change. A major component of PM is organic aerosol (OA), which includes both primary organic aerosol (POA) emitted directly into the atmosphere and secondary organic aerosol (SOA) formed through gas-phase oxidation of organic compounds. This work explores SOA formation through a combination of laboratory and field observations, supported by modeling studies. This thesis begins by investigating the gas-phase chemistry of toluene, a common anthropogenic volatile organic compound (VOC), with a focus on its photooxidation. Our analysis draws on an extensive series of photooxidation experiments conducted under varying conditions in nine atmospheric simulation chambers across Europe, all part of the EUROCHAMP-2020 (EUROpean simulation CHambers for investigating AtMospheric Processes) infrastructure. To synthesize these results, we used a model incorporating the Master Chemical Mechanism (MCM), a near-explicit mechanism for VOC gas-phase oxidation, along with an SOA formation module. The combined data from these multi-chamber toluene experiments support our hypothesis that combining different facilities and experimental conditions provides a more comprehensive understanding of the system's behavior, as each chamber probes different regions of the chemical space. However, the synthesis also reveals significant gaps in our current understanding, especially during the later stages of toluene oxidation. The MCM-based model struggled to accurately capture the concentrations of both first-generation and oxygenated products, and it consistently underpredicted SOA mass across the experiments. These findings highlight critical gaps in the representation of SOA-forming pathways in the current version of the MCM, indicating that the uncertainties are greater when simulating SOA formation. The oxidation of VOCs produces numerous organic compounds through complex reaction networks, making detailed analysis of atmospheric simulation experiments difficult and the incorporation of the corresponding results into chemical transport models (CTMs) challenging. To manage this complexity, the one-dimensional volatility basis set (1D-VBS) framework has been developed to simulate SOA formation in CTMs. Traditionally, SOA parameterizations in the 1D-VBS have relied on fitting SOA yield data from atmospheric simulation chamber experiments. To reduce the uncertainties of this approach, we developed an algorithm that estimates the volatility distribution and effective vaporization enthalpy of SOA products. This algorithm combines SOA yield data with results from two types of evaporation techniques, thermal and isothermal dilution. We evaluated the algorithm using pseudo-experimental data from SOA systems with known properties and found it accurately reproduced SOA yields under atmospherically relevant conditions. The new algorithm developed in this work was used to parameterize the oxidation of various compounds, including the ozonolysis of α-humulene (a sesquiterpene) and the photooxidation of four cyclohexanes and three aromatic compounds. The resulting yields and parameterizations revealed that the current scheme in the CTM PMCAMx-SR underestimates the SOA formed during the ozonolysis of sesquiterpenes by a factor of two. Additionally, the results show that larger cyclohexanes produce higher SOA yields than aromatics, while aromatic compounds generate more highly oxidized SOA. The derived VBS parameterizations are recommended for future implementation in CTMs. In the final part of this research, we use two CTMs, PMCAMx-SR and PMCAMx-Trj, based on the 1-D and 2-D VBS, respectively, to simulate OA levels and its degree of oxidation during a field campaign conducted in a forested background area in Greece in July 2022. The 2D-VBS extends the 1D-VBS by incorporating the oxygen-to-carbon (O:C) ratio, a key metric for assessing OA oxidation degree. Both models predicted a high influence of wildfires across Europe during this period, with PMCAMx-SR attributing nearly 40% of OA to biomass burning SOA (bbSOA) in the campaign site. However, PMCAMx-SR consistently underestimated total OA, especially biogenic SOA (bSOA). PMCAMx-Trj accurately predicted O:C for total OA (0.84 vs. 0.82 observed) and biogenic OA (0.59 vs. 0.63 observed), but it underpredicted OA concentrations, indicating probably the same model limitations as in PMCAMx-SR. Despite this agreement between the predicted and observed O:C for biogenic OA, the O:C distribution of first-generation SOA products used in PMCAMx-Trj remains uncertain. To better constrain these 2D-VBS parameterizations, further smog chamber experiments are needed. Sensitivity analyses indicated that the underestimation of OA levels by the PMCAMx-SR and PMCAMx-Trj models results from several factors. Doubling bVOC emissions improved model performance (FBIAS by 90%, FERROR by 30%), though emissions alone did not fully explain temporal discrepancies. While PMCAMx-SR predicted that 90% of biogenic SOA was transported, this likely reflects incomplete aging chemistry treatment. Incorporating aging for sesquiterpene-derived SOA significantly reduced model biases (up to 100% in FBIAS and 40% in FERROR), highlighting the importance of multi-generational oxidation processes. These findings underscore the need for better representation of SOA yields and aging rates, potentially through smog chamber constraints.
Open Access
Βιομηχανικά υλικά υψηλής απόδοσης απο νανοκυτταρίνη : μοριακός χαρακτηρισμός, μελέτη ανισοτροπίας και μορφολογίας φιλμ/ινών
(2025-09-25) Παπαπέτρος, Κωνσταντίνος; Papapetros, Konstantinos
Η έρευνα στον τομέα των βιοαποικοδομήσιμων υλικών συσκευασίας ή και των πλαστικών φυσικής προέλευσης παίζει ήδη καθοριστικό ρόλο στη βιώσιμη ανάπτυξη. Η συνεχής βελτιστοποίηση των ιδιοτήτων τους, όπως η μηχανική αντοχή, η ικανότητα φραγής και η βιοστατική δράση, είναι απαραίτητη προκειμένου να ανταποκριθούν στις απαιτήσεις εφαρμογών συσκευασίας τροφίμων. Παράλληλα, η ανάπτυξη καινοτόμων τεχνικών που θα μειώνουν το κόστος παραγωγής αποτελεί κρίσιμο βήμα ώστε τα βιοπολυμερή και τα βιοαποικοδομήσιμα πολυμερή να καταστούν ανταγωνιστικά σε σχέση με τα καθιερωμένα συνθετικά υλικά συσκευασίας, όπως το πολυπροπυλένιο (PP) και το πολυαιθυλένιο (PE). Η κατεύθυνση αυτή είναι καθοριστική για τη μετάβαση σε ένα πιο βιώσιμο και κυκλικό μοντέλο βιομηχανικής παραγωγής. Η παρούσα διδακτορική διατριβή εκπονήθηκε στο πλαίσιο του έργου HIPERION (Βιομηχανικά Υλικά Υψηλής Απόδοσης από Νανοκυτταρίνη, Τ2ΕΔΚ-01394) και εστιάζει στην ανάπτυξη και αξιολόγηση βιοαποικοδομήσιμων πολυμερικών συστημάτων, ενισχυμένων με νανοκυτταρίνες, για εφαρμογές στη συσκευασία τροφίμων. Κεντρικός στόχος υπήρξε η μελέτη και βελτιστοποίηση των ιδιοτήτων του πολύ(βινυλικού οξέως) (PVA) και της χιτοζάνης, καθώς και των νανοσυνθέτων τους με νανοκυτταρίνη, ώστε να αναδειχθούν ως υποσχόμενα υλικά φιλικά προς το περιβάλλον, με μειωμένο αποτύπωμα και υψηλή λειτουργικότητα. Στο πρώτο μέρος εξετάστηκε λεπτομερώς η επίδραση της θερμικής κατεργασίας (ανόπτηση) στη δομή και στις ιδιότητες μεμβρανών PVA. Αναδείχθηκε η σχέση μεταξύ της κατανομής δεσμευμένου νερού, του βαθμού κρυσταλλικότητας και της πυκνότητας του πολυμερικού δικτύου, και συσχετίστηκαν οι παράμετροι αυτές για μελλοντική έρευνα σε μηχανικές ιδιότητες και ιδιότητες φραγής των μεμβρανών, σύμφωνα με τις απαιτήσεις της ευρωπαϊκής νομοθεσίας για τις συσκευασίες τροφίμων. Το δεύτερο μέρος επικεντρώθηκε στην ενίσχυση της PVA μέσω της ενσωμάτωσης νανοκυτταρίνης διαφορετικών μορφολογιών (νανοκρυσταλλική κυτταρίνη CNC και νανολιγνοκυτταρίνη NLC). Τα αποτελέσματα έδειξαν ότι η ομοιόμορφη κατανομή των νανοκυτταρινών βελτιώνει τον μοριακό προσανατολισμό, με άμεσο αντίκτυπο στη μηχανική αντοχή, στη μείωση της διαπερατότητας σε κρίσιμα αέρια (O₂, CO₂, N₂) και στον περιορισμό της υδροφιλικότητας, καθιστώντας τα σύνθετα υλικά κατάλληλα για συσκευασίες τροποποιημένης ή ελεγχόμενης ατμόσφαιρας. Στο τρίτο μέρος μελετήθηκαν συστήματα χιτοζάνης, καθώς και τριαδικά μίγματα χιτοζάνης/PVA με νανοκυτταρίνες. Διερευνήθηκε η αναμιξιμότητα και οι φυσικοχημικές αλληλεπιδράσεις των συστατικών, ενώ καταγράφηκαν οι βελτιωμένες μηχανικές ιδιότητες και ιδιότητες φραγής των συνθέτων υλικών. Ιδιαίτερη έμφαση δόθηκε στη βιοστατική δράση της χιτοζάνης και στη δυνατότητα ανάπτυξης βιοδιασπώμενων μεμβρανών κατάλληλων για εφαρμογές ελαχιστοποιημένου περιβαλλοντικού αποτυπώματος. Συνολικά, η διατριβή παρέχει ένα ολοκληρωμένο πλαίσιο γνώσης και τεχνολογικής προσέγγισης για την ανάπτυξη προηγμένων βιοπολυμερικών και βιοαποικοδομήσιμων συστημάτων συσκευασίας τροφίμων. Τα αποτελέσματα αναδεικνύουν τον κρίσιμο ρόλο των νανοκυτταρινών στη βελτιστοποίηση λειτουργικών ιδιοτήτων και προσφέρουν εφικτές λύσεις για την κλιμάκωση παραγωγής και τη βιομηχανική αξιοποίηση υλικών μειωμένου περιβαλλοντικού αποτυπώματος.
Open Access
Σύνθεση καινοτόμων υλικών και ανάπτυξη νέων μεθόδων για τη συντήρηση ιστορικών μνημείων
(2025-07-09) Παυλάκου, Ευσταθία; Pavlakou, Efstathia
Η πολιτιστική κληρονομιά χαρακτηρίζει την ιδιαιτερότητα μιας κοινωνίας. Αποτελεί, δε, το κληροδότημα άυλων και υλικών χαρακτηριστικών από προηγούμενες γενιές, τα οποία διατηρούνται στο παρόν και παραχωρούνται στις μελλοντικές γενιές. Είναι μοναδική και αναντικατάστατη για την εκάστοτε κοινωνία, γεγονός το οποίο καθιστά τον σύγχρονο πολιτισμό υπεύθυνο για την διαφύλαξη της. Τα περισσότερα ιστορικά μνημεία αποτελούνται κυρίως από μάρμαρο και ασβεστόλιθο. Με την πάροδο του χρόνου, την επίδραση των περιβαλλοντικών συνθηκών, (όξινη βροχή, υγρασία, άνεμος και μεταβολές της θερμοκρασίας), σε συνδυασμό με τη δομή του υλικού, καθώς και με τις ανθρώπινες επεμβάσεις, υφίστανται εκτεταμένη φθορά. Τα άλατα τα οποία σχηματίζονται στο εσωτερικό των πόρων των δομικών υλικών, καθώς και η ανάπτυξη μικροοργανισμών στις επιφάνειές τους, αποτελούν επιβαρυντικούς παράγοντες για την ακεραιότητα των δομικών στοιχείων των μνημείων, το κύριο συστατικό των οποίων είναι ο ασβεστόλιθος. Κρίνεται, λοιπόν, απαραίτητη η ανάπτυξη μεθόδων με σκοπό την προστασία και την αποκατάσταση της φθοράς των δομικών υλικών των ιστορικών μνημείων. Στην παρούσα εργασία μελετήθηκαν τρόποι προστασίας της επιφάνειας ασβεστολιθικών δομικών υλικών με την ανάπτυξη αιωρημάτων ναοσωματιδίων ανθρακικού ασβεστίου και αμόρφου διοξιδίου του πυριτίου τα οποία έχουν δομική συνάφεια σε μοριακό επίπεδο με τους κρυσταλλίτες του ανθρακικού ασβεστίου, της βασικής χημικής δομικής μονάδας των ασβεστολιθικών υλικών. Το πρώτο μέρος της εργασίας περιλαμβάνει τη σύνθεση αιωρημάτων ανθρακικού ασβεστίου (CaCO3) και τη μελέτη της σταθερότητάς τους συναρτήσει του χρόνου. Για το σκοπό αυτό έλαβε χώρα καταβύθιση ανθρακικού ασβεστίου, υπό συγκεκριμένες πειραματικές συνθήκες και συγκεντρώσεις υδατικών διαλυμάτων χλωριούχου ασβεστίου (CaCl2) και ανθρακικού διμεθυλίου (dimethyl carbonate, DMC). Τα στερεά που προέκυψαν χαρακτηρίστηκαν με τις παρακάτω φυσικοχημικές μεθόδους: Περίθλαση ακτίνων-Χ (XRD), μέτρηση της ειδικής επιφάνειας με φυσική ρόφηση αζώτου (BET), θερμοσταθμική ανάλυση (TGA), φασματοσκοπία απορρόφησης υπέρυθρης ακτινοβολίας (IR) και φασματοσκοπία Raman (microRaman Spectroscopy). Ο μορφολογικός χαρακτηρισμός, έγινε με τη βοήθεια ηλεκτρονιακής μικροσκοπίας σάρωσης (SEM). Επετεύχθη σύνθεση κρυσταλλικών σωματιδίων (βατερίτη, ασβεστίτη και αραγωνίτη) με μέση διάμετρο μερικών μικρομέτρων αλλά και αμόρφου ανθρακικού ασβεστίου, οι κρυσταλλίτες του οποίου είχαν μέση διάμετρο 750nm. Η έκθεση των αιωρημάτων των παρασκευασμάτων σε υπερήχους (37kHz, 2 λεπτά) ευνόησε την αποσσυσωμάτωση των σωματιδίων, για χρονικό διάστημα τουλάχιστον 100 ωρών. Μελετήθηκε, επιπλέον, η επίδραση της παρουσίας δύο τασιενεργών ουσιών, ενός ανιοντικού (sodium sodecyl sulfate, SDS) και ενός κατιοντικού (cetrimonium bromide, CTAB) σε διάφορες συγκεντρώσεις στα αιωρήματα του ανθρακικού ασβεστίου τα οποία παρασκευάσθηκαν. Μελετήθηκε η κατανομή μεγεθών των σωματιδίων και οι τιμές του ζ δυναμικού με στόχο τη διερεύνηση της σταθερότητας των αιωρημάτων συναρτήσει του χρόνου. Παρουσία του τασιενεργού CTAB παρατηρήθηκε μείωση της σταθερότητας των αιωρημάτων αυξονομένης της συγκέντρωσής του. Αυξανομένης, δε, της συγκέντρωσης του τασιενεργού SDS, μετά τις 100 ώρες, παρατηρήθηκε αύξηση της συσσωμάτωσης. Το δεύτερο μέρος της εργασίας περιλαμβάνει την σύνθεση αιωρημάτων νανοσωματιδίων οξειδίου του πυριτίου (σίλικας / SiO2) τα οποία έχουν πολύ καλή πλεγματική συμβατότητα22 με τους κρυσταλλίτες του ασβεστίτη κατά κύριο λόγο. Η σύνθεση νανοσωματιδίων σίλικας πραγματοποιήθηκε με την υδρόλυση ορθοπυριτικού τετρααιθυλίου (TEOS) σε θερμοκρασία 25οC, σε συγκέντρωση TEOS 0,29Μ υπό συνεχή ανάδευση. Μελετήθηκε η σταθερότητα των αιωρημάτων σε διαφορετικές συνθήκες σύνθεσης. Μετά το διαχωρισμό τους από την υγρή φάση τα στερεά χαρακτηρίσθηκαν με τις μεθόδους οι οποίες προαναφέρθηκαν. Μελετήθηκε η επίδραση της παρουσίας των τασιενεργών ουσιών sodium dodecyl sulfate (SDS) και cetrimonium bromide (CTAB) διαφορετικών συγκεντρώσεων στα αιωρήματα της σίλικας, τα οποία παρασκευάσθηκαν. Τέλος, μελετήθηκε η κατανομή μεγεθών των σωματιδίων καθώς και των τιμών του ζ δυναμικού, προκειμένου να διερευνηθεί η σταθερότητα των αιωρημάτων συναρτήσει του χρόνου. Τα νανοσωματίδια άμορφης σίλικας τα οποία παρασκευάσθηκαν είχαν μέση διάμετρο της τάξης των 200nm, ενώ η χρήση των υπερήχων δεν οδήγησε σε μικρότερο μέσο μέγεθος των σωματιδίων των αιωρημάτων. Επιπλέον, η παρουσία του τασιενεργού SDS στα αιωρήματα στην περιοχή συγκεντρώσεων, είχε ως αποτέλεσμα τη σταθεροποίηση των αντιστοίχων αιωρημάτων. Τόσο το μέγεθος των σωματιδίων, όσο και το ζ δυναμικό των αιωρημάτων παρέμενε σταθερό σε βάθος χρόνου ενός μήνα. Το τρίτο μέρος της εργασίας εριλαμβάνει την παρασκευή αιωρημάτων ανθρακικού ασβεστίου, σίλικας καθώς και δυο σύνθετων υλικών, διαφορετικής σύστασης ως προς ανθρακικό ασβέστιο και τη σίλικα. Τα υλικά είναι: σύνθετο υλικό 1 (ΣY1): 69% CaCO3 / 31% SiO2 και το σύνθετο υλικό (ΣY2): 97% CaCO3 / 3% SiO2. Ακολούθησε η εναπόθεση άμορφου ανθρακικού ασβεστίου, άμορφης σίλικας, ΣY1 και ΣY2 σε υποστρώματα μαρμάρου Διονύσου και τριών κονιαμάτων προέλευσης από ιστορικό μνημείο των Ελληνιστικών χρόνων. Τα τρία κονιάματα , τα οποία μελετήθηκαν διέφεραν ως προς την σύσταση σε ανθρακικό ασβέστιο και σε σίλικα, καθώς και στην ύπαρξη ή μη αργιλοπυριτικών ενώσεων και ήταν τα εξής: 1: 39% SiO2 (και αργιλοπυριτικές ενώσεις) / 61% CaCO3, 2: 50% SiO2 / 50% CaCO3 και 3: 57% SiO2 / 43% CaCO3. Το μάρμαρο Διονύσου είχε κονιοποιηθεί σε σφαιρόμυλο αχάτη, ενώ τα τρία κονιάματα είχαν υποστεί ελαφριά κονιοποίηση. Τα στερεά που προέκυψαν χαρακτηρίστηκαν μέσω μέτρησης της ειδικής επιφάνειας και του όγκου των πόρων τους με φυσική ρόφηση αζώτου (BET), ενώ ο μορφολογικός χαρακτηρισμός έλαβε χώρα με ηλεκτρονιακή μικροσκοπία σάρωσης (SEM). Στη συνέχεια, μελετήθηκε η διάλυση του μαρμάρου και των τριών κονιαμάτων καθώς και όλων των στερεών υλικών που παρασκευάστηκαν, σε ακορεστότητα 0,89 ως προς ασβεστίτη, με σκοπό την μελέτη των ρυθμών διάλυσης των υλικών. Μέσω της σύγκρισης των ρυθμών διάλυσης του μαρμάρου και των κονιαμάτων με τους ρυθμούς διάλυσης του μαρμάρου και των κονιαμάτων με τις εναποθέσεις των υλικών που έλαβαν χώρα, προκύπτουν συμπεράσματα ως προς τον μηχανισμό ο οποίος διέπει τη διεργασία της διάλυσης, αλλά και της αποτελεσματικότητας της επεξεργασίες με τα νανοσωματίδια τα οποία παρασκευάσθηκαν. Τέλος, ως σύστημα αναφοράς για την σύγκριση των αποτελεσμάτων μελετήθηκε ο μηχανισμός διάλυσης του μαρμάρου Διονύσου, μέσω πραγματοποίησης πειραμάτων διάλυσης σε διαφορετικές ακορεστότητες.
Open Access
Valorization of rice for the biotechnological production of natural phenolic antioxidants using microalgae
(2025-06-24) Παπαβασιλείου, Πολυτίμη; Papavasileiou, Polytimi
The object of this work is to propose an alternative way of utilizing rice hull, an underexploited type of lignocellulosic biomass, aiming to produce valuable microalgae biomass rich in natural antioxidant phenolic compounds. This study offers a unique and fresh look into biomass valorization with microal-gae towards the production of value-added products, that can help meet the ever-growing demand from cosmetics and food industry for products of natural origin. On the one hand, rice hull is rich in cellulose, hemicellulose and thus is an excellent candidate for the production of a nutrients-rich hy-drolysate that can serve as an economic and sustainable microalgae feed. On the other hand, microal-gae biomass is considered as one of the most valuable types of biomass, able to biosynthesize a wide variety of biochemicals with endless applications. To this end, microalgae derived natural phenolic extracts with potent antioxidant activity were the target product of this study. The basic steps of the studied fields in this work can be categorized in the following: firstly, study of rice hull hydrolysis, secondly, cultivation of microalgae in rice hull hydrolysate, thirdly, study of the production of phenolic compounds from microalgae and fourthly, study of biomass phenolic extraction techniques. In addition, valuable biosilica recovery from the spent rice hulls was also covered. Finally, a simple technoeconomic analysis took place in the end of the study, to help better understand the feasibility of such a project in real life at a greater scale. The first step of this study consists of the thermochemical hydrolysis of rice hull, in which various types of pretreatments were tested, in particular acid and alkaline pretreatments. Also, the effect of various hydrolytic parameters was tested, such as hydrolysis time, solids loading, acid dose, tempera-ture, whereas the importance of mechanical pretreatment (particle size) was also highlighted. The outcome of this study was the obtainment of the optimum hydrolytic conditions which lead to the production of a sugars- and nitrogen-rich liquid hydrolysate suitable for use as microalgae growth me-dium. Next, a microalga strain suitable for phenolics production was selected among several ones, after their cultivation autotrophically under the same conditions. The selected microalga, Botryococcus braunii, was then grown in rice hull hydrolysate aiming to combine high biomass production with opti-mum phenolics accumulation intracellularly. Various series of cultures in heterotrophic and mixo-trophic conditions were carried out to in order to understand the mechanism which favors the produc-tion of phenolics of this microalga. What is more, a two-stage cultivation scheme was also tested in which B. braunii was grown first mixotrophically or heterotrophically in rice hull hydrolysate and sec-ondly autotrophically, as a means to further improve its phenolics yields. On top of this, further studies on the autotrophic stage of the two-stage culture were done, with the goal to find adequate culture parameters that can enhance the total phenolic content of the produced biomass. From the above study, the optimum cultivation conditions leading to high phenolics accumulation were selected. Furthermore, the extraction of phenolics from microalgae biomass was also greatly investigated during this study. In particular, several solvents and extraction techniques were tested and compared towards their efficiency to effectively extract phenolic compounds, whereas both qualitative and quantitative methods were used for the evaluation of the obtained extracts. Finally, the technoeconomic assessment of the proposed process included the design of a complete flowchart for the entire process, the estimation of the total capital costs for each equipment type, the estimation of the total operational costs on a year-round basis, and an investment plan to calculate the revenues and return of investment in a hypothetic scenario that was selected for analysis. The conclusion of this study is that a multitude of important results were obtained in all the re-search fields studied, while innovative elements were added in the global effort to implement tech-nologies that adhere to the principles of circular economy. The assessment of this specific study is that the combination of lignocellulosic biomass with microalgae cultivation is not only feasible, but a new technology that opens new avenues for the creation of complex biorefineries of lignocellulosic biomass with microalgae, which will in the future be able to combine the production of energy and value-added products with the utilization of these unexploited but valuable natural resources.
Open Access
Development of smart injectable and biodegradable sodium alginate hydrogels for biomedical applications
(2025-06-06) Σαραβάνου, Σοφία Φαλιά; Saravanou, Sofia Falia
Biomedicine has made significant progress driven by continuous innovation and research in healthcare. During the last few decades, functional improved natural polysaccharides have gained great attention for hydrogel development for biological purposes. These materials derived from natural sources are attractive choices for biomedical applications due to their similarities with the extracellular matrix, biocompatibility and negligible toxicity. Therefore, chemical modifications could maximize their potential. As so, modification of natural polysaccharides into “smart” systems by chemical and physical alterations to respond and to adapt to external stimuli, such as temperature and pH, enables their use in a wide range of biomedical applications. These hydrogel systems are continuously being designed and developed and present great potential in bioapplications such as drug delivery and tissue regeneration. Therefore, the aim of this work is to design and investigate the utilization of multi-responsive hydrogels using the natural polysaccharide sodium alginate as the main backbone material in potential bioapplications such as injectable drug delivery systems or as 3D printable bioink cell carrier. First, Chapter 2 highlights the development of a dual crosslinked network based on sodium alginate-based graft copolymers by combining the hydrophobic association of the thermosensitive P(NIPAM-co-NtBAM) grafted chains with a second ionic association through the addition of Ca2+ cations. Oscillatory rheology revealed a two-step synergistic gelation of 5 wt.% NaALG-g-P(NIPAMco-NtBAM)/Ca2+ hydrogels: 1. at room temperature a soft 3D network is formed via the “egg-box model” i.e., ionic interaction between the calcium ions and the carboxyl groups on sodium alginate monomer unit groups and 2. the heating process triggers the hydrophobic association of the thermoresponsive P(NIPAMco-NtBAM) side chains, expanding the network. Surprisingly, the dual crosslinking mechanism resulted in a five-to-eight-fold increase in the storage modulus of the developed hydrogel due to reinforced hydrophobic crosslinkers above the critical gelation temperature, which was further boosted by the ionic bonding of the alginate backbone. Furthermore, the co-existence of the two bonding interactions is crucial for the morphology, injectability and 3D printability of the gels, as they exhibit a shear thinning effect under applied shear stress. Therefore, the proposed copolymer can form arbitrary domains under mild 3D printing conditions indicating its further use as a bioprinting ink. The 3D printed scaffold can promote human periosteum derived cells (hPDCs) growth and their capacity to develop 3D spheroids. Furthermore, as the bioink can behave as a sol-to-gel and gel-to-sol during heating or cooling, the thermoresponsive crosslinking mechanism of the polymer network can be further utilized for the facile recovery of the cell spheroids. As a result, the proposed system presents selfhealing and shear thinning properties under mild conditions and it can host and promote cell spheroid formation. The aforementioned properties enable the utilization of biomaterials in various biomedical applications such as cell spheroid encapsulants for drug screening, injectable therapeutics or 3D in vitro modeling. Moreover, a sodium alginate-based copolymer grafted by pure PNIPAM chains was used as a second gelator combined with methylcellulose as a gelation thickener (NaALG-co-PNIPAM/MC). The gelation properties of the networks were further enhanced through the addition of calcium cations. Once more, the crosslinking mechanism of the proposed network is regulated by the secondary ionic interactions by the addition of calcium ions and by hydrophobic associations due to the thermoresponsive PNIPAM chains. Interestingly, methylcellulose was found to increase the dynamic moduli of the developed hydrogels, rendering them suitable for 3D printing in biomedical applications. The fidelity of the 3D printed scaffolds remained as high as 94% after one week. Furthermore, pre osteoblastic cells adhered to the proposed 3D printed scaffolds showed 80% viability compared to tissue culture polystyrene controls and the sodium alginatebased materials were found to promote significant osteogenic potential, as indicated by increased alkaline phosphatase activity, calcium and collagen production. The proposed materials were found to demonstrate great potential in bone tissue engineering. In addition, in Chapter 3 emphasis is given to a single chain polymer gelator comprising an alginate dually grafted backbone with thermoresponsive P(NIPAM-co-NtBAM) polymer grafting chains and diols/pH sensitive 3aminophenylboronic acid moieties. The resulting material can form robust polymer networks by three cooperative crosslinking mechanisms: 1. the hydrophobic association of the thermo-sensitive polymer grafts above gelation temperature at 24 oC, 2. the boronate esters formation between the boronic acid and the diols of the sodium alginate at physiological pH and 3. the ionic interactions of the alginate carboxylate moieties with calcium ions. The developed polymer presents excellent tunability of the mechanical properties guided by combinations of temperature, pH and glucose stimuli. Also, the graft polymer is found to be non-toxic and could form simple 3D printing structures that can undergo multi-stimuli responsive erosion profiles based on the variation of temperature, pH or glucose and it could be utilized as a drug delivery system for Type I diabetes. Finally, in Chapter 4 the design of a semi-interpenetrated polymer network is presented. The development of the network is based on the intramolecular combination of alginate-based backbones grafted either with the diols/pHsensitive 3-aminophenylboronic acid moieties, i.e., NaALG-g-BA or with the thermoresponsive P(NIPAM-co-NtBAM) polymer, i.e., NaALG-g-P(NIPAM-coNtBAM). The gelation behavior of the resulting networks is determined by the blending ratio of the two alginate-based materials. In this chapter the crosslinking mechanisms are the hydrophobic association due to the thermoresponsive PNIPAM-based chains above 24 oC and the formation of dynamic covalent boronate esters at physiological pH. At a blending ratio 70/30 wt.% of the thermo/diols-responsive copolymers, the hybrid network enhanced the arrangement of cell spheroids with average diameter approximately 60 μm after one day as the boronic acids can spontaneously form boronate esters with the cellular glycocalyx. The network can be dissociated by the enrichment of glucose solution, an antagonistic molecule to the ester blocks. The blending matrix is found to be non-toxic, injectable and 3D printable by forming 3D geometries rendering it promising for use in drug screening and in microtissue formation engineering.
Open Access
Μελέτη διεπιφανειών οργανικών ημιαγωγών με φυλλόμορφα υλικά με τη χρήση επιφανειακά ευαίσθητων τεχνικών για εφαρμογές σε οργανικά ηλεκτρονικά
(2025-06-11) Δρίβας, Χαράλαμπος; Drivas, Charalampos
Η παρούσα διατριβή επικεντρώνεται στη μελέτη των διεπιφανειών μεταξύ οργανικών ημιαγωγών και των διχαλκογενιδίων μετάλλων μετάπτωσης (TMDs), με στόχο την κατανόηση των ηλεκτρονιακών τους ιδιοτήτων για εφαρμογές στην οργανική ηλεκτρονική. Η έρευνα βασίστηκε στις επιφανειακά ευαίσθητες φασματοσκοπικές τεχνικές XPS (Φασματοσκοπία Φωτοηλεκτρονίων από Ακτίνες-Χ) και UPS (Φασματοσκοπία Φωτοηλεκτρονίων από Υπεριώδη Ακτινοβολία), με σκοπό τη μελέτη της ευθυγράμμισης ενεργειακών επιπέδων και της μεταφοράς φορτίου στις διεπιφάνειες. Η διατριβή ξεκινά με τη θεωρητική εισαγωγή στις ιδιότητες των TMDs, τη δομή τους και ορισμένα από τα ηλεκτρονιακά χαρακτηριστικά τους. Παράλληλα, εξετάζονται οι ιδιότητες των οργανικών ημιαγωγών και η σημασία της μοριακής δομής τους για την ευθυγράμμιση ενεργειακών επιπέδων και την ευκολία μεταφοράς φορτίου στις διεπιφάνειες που σχηματίζουν με τα TMDs. Το πειραματικό μέρος της διατριβής περιλαμβάνει την in situ εναπόθεση των οργανικών μορίων P3HT, H2Pc, MnPc και CuPc σε υποστρώματα 2Η-MoS2(0001), 2Η-TaS2(0001), και 2Η-MoS2 παρασκευασμένο με CVD. Στη διεπιφάνεια P3HT/MoS2, εκτός από τις ηλεκτρονιακές ιδιότητες της διεπιφάνειας γίνεται και μελέτη για την επίδραση της κατεργασίας του υποστρώματος στις ιδιότητες αυτές. Φάνηκε πως η θερμική κατεργασία του μονοκρυστάλλου MoS2 έχει επίδραση στο επίπεδο Fermi του υλικού, το οποίο με τη σειρά του επηρεάζει τα φράγματα δυναμικού με το P3HT. Όσον αφορά τις φθαλοκυανίνες μετάλλων μετάπτωσης, ένα σημαντικό κομμάτι της έρευνας επικεντρώνεται στην επίδραση του μεταλλικού κέντρου στη μεταφορά φορτίου από και προς το υπόστρωμα. Από τα πειραματικά αποτελέσματα φαίνεται πως το άτομο μετάλλου στο κέντρο του μορίου δρα ως δίαυλος για τη μεταφορά φορτίου, με το φαινόμενο αυτό να είναι εντονότερο στον χαλκό, σε βαθμό που επηρεάζεται ακόμη και ο δακτύλιος του μορίου. Διαφορές παρατηρήθηκαν ανάμεσα στον μονοκρύσταλλο και στο CVD-MoS2, σχετικά με τη μεταφορά φορτίου προς το υπόστρωμα. Παρά το γεγονός ότι τα φαινόμενα μεταφοράς ηλεκτρονίων ήταν έντονα στον μονοκρύσταλλο, η παρουσία ατελειών και ακμών πλούσιων σε S, οδηγεί σε μείωση των φαινομένων αυτών. Τα αποτελέσματα της εναπόθεσης των φθαλοκυανινών στο υπόστρωμα 2H-TaS2 έδειξαν παρόμοια συμπεριφορά με την εναπόθεση στο 2H-MoS2 παρά τις σημαντικές διαφορές στις ηλεκτρονιακές ιδιότητες των δύο υλικών. Επιπλέον, δεν παρατηρήθηκε δημιουργία χημικού δεσμού ή αλλοίωση της δομής της φθαλοκυανίνης, όπως έχει παρατηρηθεί σε συμβατικά μέταλλα, κάτι που ενθαρρύνει τη χρήση μεταλλικών TMDs ως ηλεκτρόδια σε επαφές μετάλλου – οργανικού. Τέλος, για όλες τις διεπιφάνειες προσδιορίστηκαν τα ενεργειακά τους διαγράμματα που είναι πολύ σημαντικά για εφαρμογές τους σε μικροηλεκτρονικές διατάξεις. Από τα ενεργειακά διαγράμματα γίνεται ο υπολογισμός των φραγμάτων ηλεκτρονίων και οπών, τα οποία καθορίζουν την απόδοση μιας διάταξης. Συνολικά, η διατριβή παρέχει σαφείς ενδείξεις ότι είναι εφικτός ο στοχευμένος σχεδιασμός ετεροδομών οργανικών–TMDs με επιθυμητά ηλεκτρονικά χαρακτηριστικά, χωρίς την ανάγκη χημικής τροποποίησης των TMDs. Οι διαπιστώσεις αυτές είναι ιδιαίτερα σημαντικές για την ανάπτυξη νέων ηλεκτρονικών και οπτοηλεκτρονικών διατάξεων χαμηλού κόστους, υψηλής ευελιξίας και λειτουργικότητας, όπως οργανικά τρανζίστορ πεδίου, φωτοανιχνευτές και διατάξεις μνήμης.
Open Access
Ultrasonically activated nanomedicines for targeted therapy of pancreatic cancer
(2025-05-14) Τούμπα, Δήμητρα; Toumpa, Dimitra
Pancreatic cancer is one of the leading causes of cancer-related deaths in the Western world due to the lack of potent therapeutics and biomarkers for early diagnosis. A primary reason is the asymptomatic nature of the disease, as patients with stage III locally advanced pancreatic cancer (LAPC) exhibit severe symptoms only later, when the disease intersects with the celiac trunk or the superior mesenteric artery. Additionally, gemcitabine (GEM), one of the most commonly used monotherapies, is only marginally effective. In recent years, combination therapies have improved survival rates in patients with various cancer malignancies. These therapies leverage the synergistic effects of multiple drug molecules targeting different pathways. Two combination therapies currently employed in clinical practice for LAPC are FOLFIRINOX (co-administration of folinic acid, fluorouracil, irinotecan, and oxaliplatin) and the co-administration of GEM with nab-paclitaxel (albumin-bound paclitaxel, nab-PTX). Despite the noticeable clinical benefit of the FOLFIRINOX regimen in improving overall survival, only a small number of patients can benefit from this therapy due to its high risk of lethal toxicity. To address these limitations, alternative strategies such as stimulus-responsive drug delivery systems are being explored to enhance efficacy while reducing systemic toxicity. Among these, ultrasound-mediated therapies have gained attention due to their non-invasive nature and ability to spatially control drug release. Ultrasound, a mechanical sound wave, can efficiently induce hyperthermia and generate cytotoxic reactive oxygen species (ROS). The combination of ultrasound waves with sonosensitizers (i.e., porphyrin molecules) to produce ROS is known as sonodynamic therapy (SDT). Although the mechanistic mode of action of SDT is not yet fully understood, it has shown promise in oncology for delivering small anticancer drugs and biologics (i.e., proteins, nucleic acids). Polymer-drug conjugates (PDCs) have recently garnered attention from the pharmaceutical industry, as they enable controlled and targeted drug release through the use of suitable linkers. Therefore, the aim of my thesis is to synthesize and biologically evaluate multidrug PDCs that release anticancer drugs via ultrasound stimulation. Initially, commercially available monomer synthons were functionalized to attach drug molecules through covalent coupling. The choice of linker was a key consideration, as it plays a critical role in drug release. Accordingly, two types of linkers were synthesized: one containing disulfide bonds that can be easily hydrolyzed in cells via enzymes and acidic pH (the fast-reducible linker), and another with no sensitivity to changes in the cellular environment (the slow-reducible linker). Two anticancer drugs, GEM and camptothecin (CPT), were used, resulting in four hydrophobic monomer prodrugs that were ready for polymerization with various drug and linker combinations. To investigate the release rates of the synthesized linkers and the efficacy of the drugs, these hydrophobic monomers were polymerized via reversible addition-fragmentation chain transfer (RAFT) polymerization with hydrophilic polyethylene glycol (PEG). Amphiphilic polymers were formed, and nanoparticles were generated through precipitation. Release studies demonstrated that drug release was enhanced under cancer cell culture conditions when the drug was attached to the disulfide-containing linker. Upon ultrasound irradiation, accelerated drug release was observed in these PDCs, which was further enhanced by the presence of a sonosensitizer due to ROS generation. Systematic in vitro testing across different treatment modalities revealed that some formulations were capable of outperforming the IC50 values of the parent drugs by up to five orders of magnitude. The drug-monomers were further combined with thermoresponsive polymers in a hyperthermia setting. Mild hyperthermia at 41–42°C has been shown to induce multifaceted effects on tumor physiology at both the tissue and cellular levels, including increased cell membrane fluidity, fragmentation of the endoplasmic reticulum, reduced ATP production, heat-shock protein overexpression, and activation of apoptotic caspase signaling pathways. Thus, thermoresponsive PDCs were developed to respond to mild hyperthermia conditions in combination with ultrasound activation. Starting from the polymerizable monomer-drug precursors, RAFT polymerization was performed with oligo(ethylene glycol) methyl ether methacrylate (OEGMA500) and di(ethylene glycol) methyl ether methacrylate (MeO2MA)—which have a lower critical solution temperature (LCST) of approximately 41–42°C, aligned with the desired hyperthermia range—yielding a library of thermoresponsive PDCs. Their release rates were evaluated above and below the LCST, with and without ultrasound, and in acidic or neutral pH. In vitro studies revealed that synergism was observed among the treatment modalities—PDCs, ultrasound, and temperaure—with IC50 values significantly lower than those of the parent drugs. Finally, polyelectrolyte complexes (PECs) were developed to address critical challenges in drug delivery systems, such as improved biocompatibility and enhanced cellular uptake due to better interactions with cell membranes. Polyionic PDCs were synthesized using free radical polymerization (FRP), incorporating 2-(dimethylamino)ethyl methacrylate (DMAEMA) and HPMA-Suc to achieve positive and negative charges, respectively. These polyions were used to form PECs via nanoprecipitation through electrostatic interactions, and release experiments were conducted at two different pH levels. In vitro experiments demonstrated that enhanced cytotoxicity was achieved for PECs, particularly those containing CPT, along with low hemolysis, indicating their potential as novel nanomedicines that maximize therapeutic efficacy while ensuring safety.
Open Access
Production of antioxidants from marine microalgae
(2025-04-09) Ανδριόπουλος, Βασίλης; Andriopoulos, Vasilis
Microalgae have been extensively studied by biologists for at least 80 years, while during the last decades they attracted the interest of biotechnologists due to their high growth rate and attractive biochemical composition, which, depending on the species and cultivation conditions, can be suitable for the production of fuel, food or feed, or medicinal and cosmetic products. While initially biofuel was the main focus of microalgal research, recently interest in that sector has been diminished, with food, feed and pharmaceutical or cosmetic applications taking its place. One category of molecules that was only reported during the last 20 years in microalgae is phenolic compounds. Measured both with quantitative and qualitative techniques, this class of molecules has the potential to make microalgal biorefinery more feasible, introducing one additional high-value product next to pigments and omega-3 fatty acids (FAs). The goal of this thesis was to study the production of phenolics and other antioxidants from selected marine microalgae, while at the same time maintaining a high protein and omega-3 content, suitable for food or feed purposes. The selected marine species were Chlorella minutissima, Dunaliella salina, Nannochloropsis oculata, Isochrysis galbana and Tisochrysis lutea, which had an attractive phenolic content as well as nutritious biomass composition according to the literature. The first step was to screen the proximate biomass composition and phenolic content of those species. To that end, cultivation under saturating light intensity and controlled temperature (25°C) was performed, while biomass was collected at two stages, the early and late stationary phases. C. minutissima and N. oculata displayed the highest growth rates, while all microalgae were rich in protein. Only N. oculata contained C20:5n3 (EPA) in significant amount, while only I. galbana, and T. lutea contained C22:6n3 (DHA). Biomass of all species was extracted sequentially with water (H2O) and methanol (MeOH) to assess its antioxidant content. While the methanolic extract of C. minutissima was the most potent in terms of the Folin-Ciocalteau (FC) assay, a quantitative method for the measurement of phenolics in plant samples, chromatographic analysis showed a complete absence of phenolics from all samples, with the exception of minor peaks in T. lutea extracts. The conclusion was that pigments such as chlorophyll might be significant interfering compounds in the FC assay, while the biomass composition of N. oculata and T. lutea held promise for the production of nutritious biomass and extracts of pigments and omega-3 FAs. To further explore the potential of antioxidant production from N. oculata and T. lutea different stressors were studied, specifically salinity, light intensity and the lack of aeration. A fractional factorial design with two levels was used, applying salinities of 38 and 60ppt (parts per thousand), and aeration of 2.4 or 0 volumes per volume per minute (vvm), while the levels of light intensity were different for the two species due to their different tolerances. Significant effects of lack of aeration were uncovered for N. oculata, with an increase of protein content and decrease of saturated FA content, while the apparent phenolic content according to the FC assay also increased without a correlation to pigments. On the contrary, T. lutea did not display positive effects under stress conditions, while its apparent phenolic content varied with pigment content. Conclusively, the lack of aeration might be a useful tool in the optimization of N. oculata cultivation, while further research is required on the compounds contributing to its apparent phenolic content. The next step was to optimize critical cultivation parameters, specifically temperature, pH, light period and illuminated surface to volume ratio, for the production of nutritious biomass and antioxidant extract from N. oculata. At the same time an extraction protocol was developed in collaboration with the Laboratory of Pharmacognosy and Chemistry of Natural Products (LPCNP), Department of Pharmacy, University of Patras. Like before, a fractional factorial approach was adopted, while two sets of experiments were performed, one in photobioreactors with online pH control via acid or base addition, and one in shake-flasks with daily adjustment of pH to a setpoint. Regression analysis to the combined data from the two experiments showed effects of temperature and light intensity that agree with the literature, for example the negative effects on these two parameters on EPA fraction in total lipids, while a more appealing biomass composition was attained in photobioreactors, which had a lower illuminated surface to volume ratio, at the expense of growth rate, which was higher in shake-flasks. Among the extraction methods that were assessed from LPCNP, ultrasound-assisted ethanolic extraction was chosen, due to the high protein content of the residual biomass, and the high pigment and EPA content of the extracts. The powdery texture and the practically zero moisture content of the residual biomass after freeze-drying were also very appealing for industrial use like incorporation to food products. Using the selected extraction method on the biomass from the optimization experiments showed an interesting effect of temperature, with a higher pigment content in extracts of biomass cultivated under high temperature despite the lower pigment biomass content. That could relate to a weakened cell wall or to a modified packing of pigments in the chloroplast and requires further examination. Co-optimizing the cultivation conditions for high protein and EPA biomass content as well as for high pigment extract content, showed that cultivation under 20-27.5°C, 16-18 hours of light and pH ~ 7.7 in photobioreactors would yield biomass with protein content of 40-60 % ash-free dry weight (AFDW) and EPA content of 3.2-4.5 % AFDW, while producing extract with at least 386 mg chlorophyll a (Chla) and 86 mg carotenoids L-1. Further investigation of the effects of cultivation conditions on the fatty acid content and profile on N. oculata was performed, using novel data from various cultivation systems and the available literature. A new regression method, the high dimensional selection with interactions algorithm, was equipped to assess a very large number of predictor parameters and their interactions, something that would not be possible with least squares regression. The findings confirmed well known effects such as those of temperature, while less studied effects such as that of potassium on palmitic acid were also confirmed, despite the absence of potassium-related studies in the training data. The most significant novel effect was that of aeration on EPA fraction in total lipids. Absence of aeration can modify the fatty acid content, decreasing saturated FAs while maintaining rather constant EPA content in biomass. More research is required to untangle the effects of aeration from that of mixing and clarify whether increase of oxygen under non-aerated conditions enhances fatty acid desaturation. Validation with external data showed a good performance of the models for total lipid content and EPA fraction in total lipids under stable conditions, while under outdoor conditions the EPA content was underestimated, with the overall trend being correctly predicted however. Adding more data to the training dataset should be a target for future improvement. Optimization showed great potential of N. oculata for EPA production, with up to ~ 11 % AFDW EPA in biomass and 0.12 g L-1 EPA achieved under optimal conditions, while the lack of aeration might be equipped to decrease the saturated FAs and achieve a more appealing dietary profile of the biomass. As a final step, the phenolic content of microalgal was revisited. Evaluating the literature on the phenolic content of microalgae according to both the Folin-Ciocalteau method and chromatographic analysis showed several issues with the reported findings and the methodology used. On the one hand, the interference of pigments, free-amino acids and nucleotides on the FC assay has been almost universally neglected, while antioxidant mechanisms of the cells such as the glutathione system have been studied to some extent but not in their effect to the FC assay. On the other hand, chromatographic evidence for phenolics in microalgae might be compromised by contamination from external sources via adsorption to the cell wall or absorption and biotransformation from the cells. The lack of two critical enzymes that initiate the phenylpropanoid pathway (PP) from most microalgae also makes the presence of cinnamic acids, anthocyanins and other products of PP enigmatic. Exclusion of phenolics from all growth media and accurately reported methods should be used in future research, while the removal of phenolics from waste streams by microalgae is very promising. In conclusion, N. oculata holds a great promise for the development of a biorefinery based on an ethanolic extract rich in pigments, EPA and possibly other antioxidants and a residual biomass with an excellent protein content and residual EPA, while the lack of aeration can be further explored as a finishing step for the improvement of the fatty acid profile and antioxidant content. Meanwhile, the phenolic content of microalgae might not be of great practical interest, considering the abundance of plant waste streams rich in phenolics, while the removal of phenolics by microalgae is promising, and the boosting effects of phenolics in microalgal metabolism is also an interesting research subject.
Open Access
Development and chemical functionalization of two-dimensional materials for humidity and gas sensing applications
(2025-03-06) Μάτσαλης, Στέφανος; Matsalis, Stefanos
One of the major challenges of the 21st century is achieving a cleaner and healthier environment. Industrial evolution has led to the release of various gases and pollutants as byproducts of the primary manufacturing processes, contributing to environmental degradation. The excess concentration of these gases is posing daily health risks and are responsible for millions of deaths each year. This has created a growing demand for humidity and gas sensors capable of detecting different vapors with high accuracy, excellent sensitivity, and fast response and recovery times. The primary objective of this thesis is to develop and chemically modify various two-dimensional materials to enhance their humidity and gas sensing capabilities. Following the isolation of graphene in 2004, a new class of materials emerged, the two-dimensional materials, whose unique nature allows them to have a thickness of only a few nanometers. This structure offers a wide range of exceptional electrochemical and mechanical properties, making them perfect candidates for sensing applications, such as high surface area to volume ratio and the ability to be modified. The latter can be achieved by introducing new molecules, removing the already existing or even creating structural defects to their lattice. Initially, novel 3D printed electrodes were produced utilizing the Fused Deposition Modeling (FDM) technique. For this reason, electrically conductive nanocomposites filaments were produced based on the polymer Polylactic Acid (PLA) mixed with the nanofiller multiwall carbon nanotubes (MWCNTs). A melt premixing step was introduced followed by extrusion through a single screw extruder to produce the electrically conductive filament. The nanocomposite filament was then inserted into the FDM 3D printer and interdigitated electrodes were printed over neat PLA substrate. The electrodes were extensively characterized in order to identify their physical and electrical properties along with other critical parameters including flexibility and their mechanical strength. Subsequently, various materials were synthesized and chemically modified in order to investigate their humidity and gas sensing capabilities. Initially, graphene oxide (GO) was produced following a top-down technique, a modification of Hummer’s method. According to this, flakes of graphite were implemented into a two-step reaction which exfoliates the graphite and introduces oxygen functional groups to its lattice. Then, the GO was extensively physicochemically characterized to understand its structure and properties. After coating the 3D printed electrodes, it exhibited excellent humidity sensitivity, and especially pairing with the polymer Polyvinyl Alcohol (PVA), which acted as an intermediate layer between the electrodes and the sensing layer, its stability was significantly enhanced. Furthermore, the effect of temperature on the humidity sensing measurements of the GO/ PVA sensor was investigated, and a mathematical model was derived, capable of providing real-time measurements by converting the capacitance and temperature to relative humidity. Subsequently, the GO/ PVA sensor was integrated into a helicopter’s wing to evaluate its ability to detect humidity under extreme weather conditions simulating flight environments, including elevated humidity levels, freezing temperatures and the presence of ice. The GO/ PVA sensor demonstrated excellent stability, accurately measuring the humidity without the performance being hindered by the harsh environmental conditions. Following this, the effect of mechanical strain on the flexible sensor was examined, revealing that it is capable of detecting the strain, without affecting the humidity measurements, thus providing multifunctional abilities. Finally, the GO was employed as gas sensor detecting various gases including ammonia and formaldehyde. Furthermore, Layered Double Hydroxides (LDHs) were synthesized and tested for their gas and humidity sensing properties. Specifically, different LDHs based on the metals Mg and Al were synthesized using a coprecipitation method followed by an ageing step with intercalated carbonate ions and GO. The final materials were characterized and found that they possess an increased concentration of oxygen functional groups which yielded high sensitivity towards humidity. At the same time, the effect of coating thickness on humidity sensing performance was evaluated, and impedance spectroscopy was used to examine the mechanism of physisorption and absorption of water molecules. Regarding the gas sensitivity, the LDH with intercalated carbonate is able to detect both formaldehyde and ammonia with great accuracy. Finally, another nanomaterial, the hexagonal Boron Nitride (hBN) was employed and underwent chemical modification. After using a single step strong oxidation procedure, the hBN became highly oxidized and were introduced various oxygen functional groups in its lattice, and obtained excellent humidity sensing capabilities and sensitivity towards ethanol, hydrochloric acid and ammonia.