Lanthanide-doped upconversion nanoparticles (UCNPs) for biomedical applications


This thesis examines the need for new antibacterial materials to treat small colony variants (SCVs) of Staphylococcus (S.) aureus bacteria and their parental strains. While ZnO-based nanoparticles (NPs) activated by ultraviolet (UV) and short wavelength visible light have been researched for their antibacterial properties, the potential benefits of incorporating UCNPs to allow activation by near-infrared (NIR) light have been overlooked. This study aims to fill this research gap by comprehensively investigating the synthesis and performance of ZnO-coated lanthanide-doped upconversion nanoparticle (UCNP) composites activated by NIR light against S. aureus SCVs and parental strains. Furthermore, this research addresses the limited understanding of the potential risks associated with UV emission from UCNPs used as fluorescent probes in super-resolution microscopy (SRM). Despite extensive research on the usage of UCNPs as fluorescent probes for SRMs, the potential cytotoxic effects of UV emission from UCNPs have not been thoroughly studied. To advance cellular imaging techniques and ensure cellular viability, a comprehensive investigation of UV emission from UCNPs is necessary. This thesis aims to identify and quantify UV emission by UCNPs used in SRM and develop strategies to minimise UV emission and mitigate potential cytotoxic effects. These two main aims are addressed in three results chapters. The first aim, the focus of chapters 2 and 3, focuses on the synthesis UCNP@ZnO composites that can be activated by NIR light for antimicrobial photodynamic therapy (aPDT) applications against S. aureus SCVs and parental strains. Chapter 2 reports the synthesis and performance of these composites, showing these materials to be effective antibacterial therapies against S. aureus SCVs, while chapter 3 improves upon the performance of these composites by careful tuning of the UCNP core and provides enhancements to the ZnO shell composition to improve reactive oxygen species generation and add a second mode of action in the form of silver nanoparticles. The second aim of this research is covered in chapter 4, which reports an investigation into the UV emission from UCNPs used as fluorescent probes in SRM. The work posits the need to understand the UV emission properties of these UCNPs as knowledge of these and the potential for cytotoxic effects are crucial for optimizing cellular imaging experiments and ensuring accurate and reliable results. Chapter 4 identifies design features and compositions that can limit UV emission, thereby minimizing the risk of phototoxicity and advancing the field of cellular imaging. Overall, the findings from this research have the potential to contribute to the development of safer and more effective targeted antibacterial therapies and enhance the understanding of UV emissions in cellular imaging techniques.Thesis (Ph.D.) -- University of Adelaide, School of Chemical Engineering, 202

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