OAR@UM Collection:

Modelling of folded graphene and related systems

Fri, 01 Jan 2021 00:00:00 GMT

Title: Modelling of folded graphene and related systems Abstract: Graphene, a quasi-planar monolayer of sp2 -bonded carbon atoms known for its exceptional physical properties, is highly amenable to out-of-plane deformation. Recent studies have revealed that the creation of folded, pleated-like domains imparts novel characteristics to this material whilst permitting some of its existing properties to be effectively controlled through straining action via regulation of the emergent folding parameters. Despite the considerable influence that strain can have on the material properties of folded graphene, the literature pertaining to the nano-mechanical unfolding of folded, graphene-type systems remains scarce. In this work, molecular dynamics simulations were performed on three novel forms of folded graphene using an ad hoc protocol executable within LAMMPS to study their mechanical response to uniaxial tensile deformation. Patterned line defects were shown to constrain multiply folded graphene to a quasi-periodic, highly ordered morphology that gave rise to instances of pronounced negative tangent modulus – coincidentally with each fold opening – upon the application of uniaxial stress. The severe lack of periodicity observed in the corresponding profiles of the pristine folded systems was attributed to the absence of defect lines which permitted folds to be more mobile and at times merge, effectively reducing the frequency of fold openings. These structural differences were explained, for the first time, via a macroscale model based on the mechanics of paper folding. Overall, this study attests to the potential for defect-type fold lines to guide the unfolding process of folded graphene, and provides valuable insight into the different mechanisms involved in the unfolding of specific forms of folded graphene. Description: M.Sc.(Melit.)

DNA barcoding and phylogenetics of selected hymenopteran species from Gozo

Fri, 01 Jan 2021 00:00:00 GMT

Title: DNA barcoding and phylogenetics of selected hymenopteran species from Gozo Abstract: DNA barcoding and phylogenetic and morphological analyses have proven to be essential tools in correct species identification, especially for poorly studied taxa or taxa containing a high level of cryptic morphological diversity, such as those found in the order Hymenoptera. In this study, bees, wasps and sawflies (order Hymenoptera) were sampled during a total of 46 fieldwork sessions taking place in Gozo. A total of 179 specimens were morphologically identified down to the lowest taxon possible. Various ecological data, such as the hymenopterans sampled per habitat type, were also evaluated. DNA was extracted from all the sampled specimens during the genetic analyses, and a partial sequence of 658 bp from the cytochrome oxidase subunit 1 gene (COI) was amplified. PCR amplification was confirmed through gel electrophoresis. PCR products were sequenced, and the sequences generated were compared to publicly available data on BOLD and GenBank to confirm species identity. The 179 specimens were genetically confirmed to represent 59 species from 17 families belonging to 8 superfamilies within Hymenoptera. The genetic dataset compromising all the studied specimens was aligned and used to produce a phylogenetic tree using Bayesian Inference. Specimens from the same species grouped with high posterior probability values, confirming that they genetically belong to the same species. While locally some morphological studies focused on Hymenoptera, genetic work on this order for the Maltese Islands is minimal, whilst this is the first genetic study on hymenopterans from Gozo. Only 12% of the studied species were previously scientifically recorded from Gozo, including species such as Xylocopa violacea. Therefore this work contributes highly to the knowledge gap reduction for hymenopteran species in Gozo. Additionally, out of the 80 haplotypes recorded in this study, 42 novel haplotypes belonging to 35 out of 59 studied species were also discovered, further contributing to the genetic resources available on these species at an international level. This study also paves the way towards a better understanding of the conservation needs of identified bees, wasps, and sawflies. These species are increasingly recognised for their valuable ecological roles as pollinators or predators of pest insect species. Description: B.Sc. (Hons)(Melit.)

A predictive occurrence model for ‘Elatine gussonei’ based on environmental factors

Fri, 01 Jan 2021 00:00:00 GMT

Title: A predictive occurrence model for ‘Elatine gussonei’ based on environmental factors Abstract: The occurrence of the amphibious plant Elatine gussonei in Mediterranean Temporary Ponds (EU priority habitat 3170* and Natura 2000 site) is patchy, with a strong stochastic component. The specific environmental conditions and tolerance ranges that determine its presence in a pool are not known. This represented the knowledge gap that this study aimed to address. The chemical, morphometric and ecological conditions in a sample of rockpools were collected and used to construct a preliminary occurrence model. Throughout this study, ca. 170 pools from 10 pool landscapes were surveyed from the Maltese Islands. Data collected and used in analyses included species lists, water quality (pH, Electrical Conductivity and Oxidation Reduction Potential), and basin morphometry (dimensions, surface area, maximum water and sediment depths of the basin and depths at which E. gussonei was present). Analyses carried out included correlation plots, CCA, RDA, t-tests, linear regressions (lm) and binomial logistic regressions (generalized linear models – glm). Elatine gussonei occurrence in the model was based on dichotomous presenceabsence data of the species. Therefore, binomial glms were carried out for environmental factors. Only zwm and surface area (both negatively correlated with occurrence) were statistically significant (p<0.05) and were used to model the species occurrence in a given pool. Once the presence of the species was confirmed via glm, lm were used to model the specific depths at which it occurs. The dependent depths (zwe and zse) were significantly positively correlated with independent maximum basin depths (zwm and zsm). Constraining the lms to pass through the origin, however, increased model efficiency by increasing R2 (0.54 to 0.72 and 0.44 to 0.84 for water and sediment depths), indicating better model fit. Literature states that its phenotypic plasticity and rapid response to environmental changes make it a good sentinel species on which to model climate change and predict further environmental changes and habitat status. The species and its habitat are both protected, entitling them to monitoring and conservation. Description: M.Sc.(Melit.)

Probing the solid-state behaviour of pharmaceuticals under different environmental conditions

Fri, 01 Jan 2021 00:00:00 GMT

Title: Probing the solid-state behaviour of pharmaceuticals under different environmental conditions Abstract: The aim of this study is to understand the solid-state properties of imatinib, the free base of imatinib mesylate (Gleevec ®), an active pharmaceutical ingredient (API) that is used to treat chronic myelogenous leukaemia under different environmental conditions using crystal engineering techniques. The behaviour of imatinib after grinding with acidic co-crystal formers was studied, where the API’s solubility was measured in water using UV-Vis techniques. The solubility of imatinib in water improved significantly when ground with acidic co-crystal formers and when ground with pyridoxine. Imatinib free base was recrystallised from several solvents and solvent mixtures, from which one major polymorphic form was detected. The unit cell parameters as well as the atomic co-ordinates were determined. The crystal structure of imatinib free base has been determined but further parameters are still required. Rietveld refinement would be required so that any differences between the experimental powder pattern and modelled crystal data are minimised. This process would validate the structure solution which can eventually be published. Description: M.Sc.(Melit.)