https://www.um.edu.mt/library/oar/handle/123456789/70981 2026-04-11T14:18:34Z 2026-04-11T14:18:34Z https://www.um.edu.mt/library/oar/handle/123456789/136794 2025-07-23T12:56:04Z 2018-01-01T00:00:00Z

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)

2018-01-01T00:00:00Z https://www.um.edu.mt/library/oar/handle/123456789/73713 2021-04-15T07:06:53Z 2018-01-01T00:00:00Z

Title: Genetic and chemometric properties of Maltese olive oils : developing an analytical profile for protected determination of origin certification Abstract: The determination of the singularity of the Maltese olive oil provides an opportunity for local producers to purpose a PDO certification. This objective of this study was to identify chemical and genetic parameters which enable the discrimination of Maltese olive oils. Prior to the identification of markers, a preliminary survey of quality was conducted whereby it was shown that monocultivar olive oils grown locally were of sufficiently high quality to be classified as extra virgin olive oils. The minor constituents present in EVOOs were subjected to a more detailed study as chemical markers. Phenolic compounds were extracted and quantified using microtiter assays. The antioxidant activity of these extracts was determined using different redox-based assays including FRAP, ACC and CUPRAC assays whilst the radical scavenging activity towards different radicals, including DPPH, ABTS and NO, was determined. It was shown that EVOOs derived from the indigenous cultivars had a significantly lower TPC and TFC whilst no significant difference was observed in the TdPC when compared to EVOOs from other Mediterranean countries. The low TPC and TFC reflected the significantly lower antioxidant and radical scavenging activity of the indigenous cultivars. Whilst microtiter plate assays showed that the indigenous cultivars had a lower phenolic content, application of HPLC for phenolic profiling revealed that both the indigenous and locally grown foreign cultivars had a significantly higher content of six phenolic compounds, namely p-coumaric acid, tyrosol acetate, 3,4 DHPEA-EDA, p-HPEA-EDA and two unidentified compounds. Elemental characterisation via the application of semi-quantitative XRF analysis enabled discrimination not only between the geographical origin of the EVOOs but also between EVOOs and other refined seed oils. Application of classical statistics and chemometrics on the phenolic profiling showed that geographical discrimination between the different samples was possible. The application of chemometric techniques namely PCA, PLS-DA, PNN, SLC-DA on data derived from different chemical techniques namely fluorescence (SEEF and EEM), FTIR, NMR and direct infusion mass spectrometry not only enabled the complete discrimination of the different EVOOs but enabled the identification of markers which had the highest discriminatory power. In the case of SEEF spectroscopy, compounds having emissions in the 380-480 nm and 650-700 nm ranges were found to have the greatest discriminating power, as confirmed through the use of 3-way chemometric analysis (PARAFAC and N-PLS) on the EEM offering comparable results to those obtained using SEEF. For NMR, 1H chemical shifts in the 3.5-4.5, 6.7-7.0 and 9.1-9.5 ppm had the most discriminate power and were attributed to the presence of glycerides, terpenic, phenolic and carbonyl containing compounds. In the case of DI-MS under positive ESI, it was found that the data obtained was highly redundant, nonetheless minor phenolic compounds identified through their m/z values were found to be the most discriminate. FTIR spectroscopy was found to be one of the most effective chemical fingerprinting methodologies owing to its simplicity, with compounds having functional groups which vibrated at 500-1500 cm-1 being the most discriminate. Application of genetic analysis on the three indigenous cultivars using SSR markers revealed that cultivar discrimination was possible via the use of two most informative markers namely DCA-3 and GAPU 101. Analysis of these markers on genomic DNA showed that the ‘Bidni’ cultivar consisted of a homozygous population whereby the individual trees are clones of each other showing a very similar allelic pattern to olive cultivars cultivated in northern Tunisia. In the case of the ‘Malti’ cultivar, the population was highly heterozygous, showing very similar allelic patterns to Spanish, Greek and Southern Italian cultivars, suggesting a multi-cultivar population coined under the same nomenclature. In the case of ‘Bajda’ at the two loci studied this could not be distinguished from another Leucocarpic cultivar grown in southern Italy baring the name of Morachia, Cannellina, Bianca, and Chiarita. From the results obtained it was concluded that the authenticity of the Maltese EVOO can be defined using both genetics analysis and chemical fingerprinting methods. Description: PH.D.CHEMISTRY

2018-01-01T00:00:00Z https://www.um.edu.mt/library/oar/handle/123456789/72863 2021-04-05T12:38:17Z 2018-01-01T00:00:00Z

Title: Fluorescent molecular logic gates for pE and pH Abstract: Three novel molecules were designed for sensing pH and redox potential. The fluorophore for two of the molecules was based on an asymmetrical (PDI) motif. Receptors for these molecular logic gates were attached to the N-imide positions of a perylene-3,4,9,10-tetracarboxylic acid diimide, the main precursor. Through asymmetrical attachment of pE and pH receptors these perylene based moieties were coined as ‘Pourbaix’ sensors. An ‘electron-donor-spacer1-fluorophore-spacer2-receptor’ format was used in their design. A third molecule, 1 was built in ‘electron-donor-spacer1-fluorophore-spacer2- receptor1-spacer3-receptor2’ format incorporating a naphthalimide fluorophore. A ferrocene with a methylene spacer was attached on the N-position through an imide bond to the naphthalimide fluorophore. As proton receptor, a piperazine moiety was attached at the 4 position on the naphthalimide. By incorporating a 2-methoxybenzo-1-aza-15- crown-5, 1 could also detect Na+. The presence of a methoxy group on the benzene was devised as a way of increasing specificity to Na+. Photophysical and logic properties of the molecule 1 were studied in 1:1 (v/v) methanol/water and 2 and 3 were studied in THF. The study also investigated the physicochemical behaviour of molecule 19 in polyacrylamide hydrogel and both molecule 3 and 19 in polyurethane polymer. Logic characteristics were determined in aqueous solution with UV-vis absorption and fluorescence spectroscopy. Two mechanisms, photon electron transfer (PET) and internal charge transfer mechanisms (ICT) drive 1 and PET drives 2 and 3. With molecules 2 and 3 having respective fluorescence quantum yields of Ф = 0.20 and Ф = 0.22 respectively. Molecule 2 showed a 3-fold enhancement while 3 showed a 2-fold enhancement. Molecule 1 had a fluorescence quantum yield of Ф = 0.22 and a 9-fold enhancement between the ‘off’ and ‘on’ state. At high H+ concentrations and Na+ presence and high pE. Confocal microscopy was used to determine the progress of corrosion on cold rolled steel coupons coated with a polyurethane coating impregnated with Pourbaix sensor 19 from previous work and 3, one of the novel PDI molecules synthesised in this work. The impregnation of the polyurethane polymer was enough to indicate early signs of corrosion. Description: M.SC.CHEMISTRY

2018-01-01T00:00:00Z