Supercritical carbon dioxide is an attractive replacement for traditional organic solvents due to its reduced environmental impact and simplified work-up procedures. In addition, the enhanced diffusivity observed in supercritical solvents can often accelerate reaction rates. Bonded stationary phases have previously been prepared in supercritical fluids at University College, Cork, with improved surface coverage as the greater mobility of reagents in the reaction medium allows access to normally inaccessible silanols. Chapter 1 acts as a general introduction to the area of supercritical fluids and the application of supercritical carbon dioxide to the preparation of bonded phases for chromatography. Both physical and chromatographic methods for characterisation of stationary phases are discussed. In chapter 2 the preparation of alkyl silica is investigated, using a wide range of potential reagents, to identify octadecyldichloromethoxysilane as best, giving a high surface coverage (4.8-5.4æmol/m2) under mild conditions. Rapid, efficient methods to bind both mono- and poly-functional reagents to silica particles in supercritical carbon dioxide were optimised. Sc-CO2 endcapping protocols were also developed. The resulting phases possessed chromatographic efficiencies of up to 80,000 plates/m. In chapter 3, this study is extended to prepare six stationary phases containing embedded polar groups, which have a complementary selectivity to traditional alkyl phases. Each phase was prepared in supercritical carbon dioxide {u2013} five amide-embedded phases via a versatile 3-mercaptopropyl silica intermediate and the sixth, containing a benzoyl phenol moiety, by direct addition of a commercially available trifunctional ligand. This extends the range of supercritical fluid derivatisation methodology, enabling the use of polar reagents. In chapter 4, two Frechet polyethers were synthesised and bonded to the 3-mercaptopropyl silica intermediate. These structurally complex phases were readily prepared and endcapped using sc-CO2 as the reaction medium, representing a further extension of supercritical bonding methodology towards larger molecules. These novel phases display enhanced recognition for aromatic molecules and have potential in the separation of polynuclear aromatic hydrocarbons. Silicas prepared in chapters 2, 3 and 4 were characterised by microanalysis, TGA, DRIFT, and solid-state NMR spectroscopy, where possible. Several of the more interesting phases were further deactivated by endcapping in sc-CO2. Key phases were packed and their chromatographic performance evaluated using a number of well-known test mixtures (Engelhardt, Neue, SRM869, SRM870). An acid test-mix was also developed in-house to assess the performance of phases with embedded functionality. Chapter 5 reports results of an investigation into the preparation of shaped semi-conductor nanoparticles using quaternary ammonium salts as shape-directing agents. These highly fluorescent species are promising candidates in next generation solar power cells. They also show potential for incorporation into novel chromatographic phases.