https://www.um.edu.mt/library/oar/handle/123456789/70980 Wed, 15 Apr 2026 09:58:40 GMT 2026-04-15T09:58:40Z https://www.um.edu.mt/library/oar/handle/123456789/144069 Title: Modelling indoor household PM2.5 using positive matrix factorization and machine learning algorithms Abstract: People tend to spend most of their time indoors, yet the concentration and composition of indoor fine particulate matter (PM2.5) remain poorly understood in the Maltese Islands, with existing receptor modelling studies focusing solely on ambient air. The first objective of this study was to carry out long-term indoor air sampling of PM2.5 followed by chemical characterisation, in order to identify and quantify the main natural and anthropogenic sources of indoor PM2.5 at an urban background site in Malta using Positive Matrix Factorisation (PMF). The second objective was to explore the use of Machine Learning (ML) algorithms to model and predict indoor PM2.5 concentrations in several households in Malta and Gozo. PMF was used to identify and quantify the major sources of indoor PM2.5. Quartz and PTFE filters were collected and analysed gravimetrically and chemically using ICP-MS, IC, and an OC-EC aerosol analyser to determine concentrations of PM2.5, 18 elements, 5 ions, organic carbon (OC), and elemental carbon (EC). Eight contributing factors were identified, seven outdoor sources and one indoor source, contributing 68% and 26% to indoor PM2.5, respectively. Cooking and e-cigarette use were the main contributors to the indoor factor. Uniquely for Malta, a fireworks factor was isolated indoors, responsible for most of the measured Sb and Ba, raising concerns due to the toxicity of these elements. An RF-SHAP model was integrated with the indoor PMF model to investigate the influence of key drivers on indoor PM2.5 concentrations. An outdoor PMF analysis was also conducted, and a corresponding RF-SHAP model (CV RMSE: 2.79 µg m−3; R²: 0.80) was used to refine the outdoor source contributions. Transboundary contributions (Saharan dust and ammonium sulfate) were higher outdoors (58%) than indoors (33%) due to reduced infiltration when windows are closed. Local anthropogenic sources (Industrial, Fireworks, Traffic, Shipping) contributed more to outdoor PM2.5 (33%) than indoor (25%), with increased indoor infiltration during warmer months coinciding with peak fireworks activity. For the second objective, continuous PM measurements were taken using aerosol spectrometers at seven non-smoking residences. RF and XGBoost models were developed to predict indoor PM2.5 at six-hourly intervals. At sites with low indoor PM generation, predictions were mainly influenced by outdoor PM1 levels. At a site with high indoor emissions, indoor relative humidity was a key predictor, especially during cooking. The RF model performed best overall (RMSE: 30.65 µg m−3; IOA: 0.66). Description: Ph.D.(Melit.) Mon, 01 Jan 2024 00:00:00 GMT https://www.um.edu.mt/library/oar/handle/123456789/144069 2024-01-01T00:00:00Z https://www.um.edu.mt/library/oar/handle/123456789/141614 Title: Molecular logic gates as fluorescent markers and DNA binding agents Abstract: N/A Description: Ph.D.(Melit.) Sun, 01 Jan 2023 00:00:00 GMT https://www.um.edu.mt/library/oar/handle/123456789/141614 2023-01-01T00:00:00Z https://www.um.edu.mt/library/oar/handle/123456789/136794 Title: On folded graphene and its properties : a preliminary study Abstract: Graphene is one of the newly discovered forms of carbon and exists as a single sheet, made from sp2-hybridised carbon atoms, which is flexible enough to be able to adopt a variety of conformations. This material, or its variants, may exhibit wide-ranging properties, including a negative Poisson’s ratio (auxeticity). This property may be imparted to graphene through a process which involves modification through the use of patterned or randomly-placed defects which guide graphene to adopt particular three-dimensional conformations that are amenable to exhibiting negative Poisson’s ratios. This work re-examines and extends recent work which has shown how graphene with defects inserted along equally spaced lines may exhibit giant negative Poisson’s ratios as a result of the corrugated-sheet-like conformation it adopts, which defect lines act as crease lines. It is shown, through a combination of static force-field based simulations using the polymer consistent force-field (PCFF) and more computationally intensive NPT molecular dynamics simulations using the Adaptive Intermolecular Reactive Empirical Bond Order (AIREBO) force-field that the corrugated forms of graphene which had previously been reported are neither the most stable nor the most auxetic forms that these systems may adopt. In fact, it is shown that unless constrained, the low-density corrugated forms of graphene are likely to fold and adopt much denser forms with graphite-like features that impart added stability, which may or may not be auxetic. It was also shown that other stable variants of the less dense form of the corrugated graphene may exist which may exhibit an even higher extent of auxeticity than what was reported so far. Other anomalous properties, such as negative tangential stiffness or zero Poisson’s ratios for some of these forms are also identified and discussed. Description: B.SC.(HONS) Mon, 01 Jan 2018 00:00:00 GMT https://www.um.edu.mt/library/oar/handle/123456789/136794 2018-01-01T00:00:00Z https://www.um.edu.mt/library/oar/handle/123456789/132575 Title: The synthesis of single-phase materials exhibiting a controlled, isotropic positive or negative thermal expansion Abstract: Thermal expansion is a fundamental physical property of a material and it is considered crucial for some high precision applications. The aim of this project is to synthesise and characterise new single-phase materials which exhibit controlled thermal expansion or zero thermal expansion (ZTE). This idea originated from the fact that in the last decade several materials with negative thermal expansion (NTE) were synthesised. Recent publications showed that this property can be altered through chemical and physical modifications, in such a way that the coefficient of thermal expansion (CTE) is tuned from negative to zero to positive. The projects discussed in this work involved both organic and inorganic frameworks. The metal organic frameworks HKUST-1 and UiO-66 were used, as these are known to contract upon heating. These frameworks were subjected to methods that hinder contracting mechanisms, such as the introduction of guest molecules and functionalized chains and also the introduction of defects. In the case of inorganic frameworks, the family of Prussian blue analogues was chosen. Here a methodology was constructed which involved doping of hexacyanoferrates(III) which exhibit negative thermal expansion with first row transition metals, in an attempt to tune the coefficient of thermal expansion. Powder X-ray diffraction data conducted at room temperature showed structural changes within both metal organic frameworks. This was followed by variable temperature powder X-ray diffraction experiments which concluded that the coefficient of thermal expansion did indeed change. This was observed when toluene and n-pentane were added as guest molecules, when 1,2,4,5-benzene tetracarboxylic was added as a functionalised linker and via introduction of defects using acetic acid, formic acid, oxalic acid and isophthalic acid. In some samples the magnitude of negative thermal expansion was enhanced, in others near-zero thermal expansion was reached, whilst in some samples even positive thermal expansion (PTE) occurred. Similar tuning in the coefficient of thermal expansion was observed in one of the series of doped hexacyanoferrates(III) which involved the Cu/Co metal cations. Description: Ph.D.(Melit.) Wed, 01 Jan 2025 00:00:00 GMT https://www.um.edu.mt/library/oar/handle/123456789/132575 2025-01-01T00:00:00Z