Synthesis and Characterisation of Barium Titanate Nanoparticles for Second Harmonic Generation Applications.

This thesis presents findings of hydrothermally synthesised barium titanate nanoparticles for biomarker applications. Hydrothermal barium titanate (H-BT) and barium strontium titanate (H-BST) nanoparticles were successfully synthesised and were characterised for their second harmonic generation applications. X-Ray powder diffraction (laboratory and synchrotron) highlighted that H-BT and H-BST had a mixed tetragonal and cubic phase fraction present by Rietveld peak fitting analysis. Regardless of the phase fractions present, all nanoparticles emitted SHG (including a commercial cubic BaTiO3 sample that appeared cubic by XRD). The smaller sized H-BST nanoparticles (45 nm) required an increase in incident laser power compared to the H-BT sample (~140 nm).
The phase of the nanoparticles and origin of SHG was investigated by electron diffraction, electron energy loss spectroscopy and high resolution HAADF-STEM imaging. In-situ electron diffraction of barium titanate showed that the tetragonal diffraction pattern transformed to a cubic pattern when heated above the Curie point. The phase transition was also investigated by EELS measurements of the Ti-L3 edge t2g-eg peak separation at room temperature and 400 oC showing the reduction in t2g-eg peak separation when the sample transforms from a tetragonal to cubic phase. The surface of the nanoparticles also showed an atomically rough layer with incomplete unit cells, and the ‘bulk’ of the nanoparticles showed random Ti-atom distortions by HAADF-STEM Ti-atom displacement analysis. This suggests the origin of SHG is likely to be both a cause of surface roughness and local asymmetric distortions in the nanoparticle bulk.
The hydrothermally prepared and PLL-coated nanoparticles were measured to assess the cell viability and DNA damage of cells after a 24-hour exposure. The nanoparticles were measured by dynamic light scattering to understand the behaviour of uncoated and PLL-coated nanoparticles suspended in different media. The uncoated nanoparticles showed little reduction in cell viability and genotoxicity, whereas the PLL coated nanoparticles showed a reduction in cell viability and a failed comet assay at concentrations ≥10 µg/mL. The nanoparticles were confirmed to be taken up into the cells by electron microscopy of critically point dried and resin embedded cell sections. Cryo-TEM of the H-BT-PLL nanoparticles suspended at 100 µg/mL in complete cell culture media showed that some nanoparticles were coated with a calcium phosphate coating and others not. This resulted in, either cells having a direct exposure to PLL and positively charged nanoparticles, or all the calcium was removed from the media that is required for cell signalling pathways which could lead to a reduction in cell viability.