Nanogold and nanosilver hybrid polymer materials
Significant opportunities exist in both the scientific and industrial sectors for the development of new generation hybrid materials. These multifunctional hybrid materials favourably combine the often disparate characteristics of both precursor components in one material. As such, this field can be very innovative due to the many possible combinations of components providing the opportunity to create a wide variety of new generation materials with a range of known and as yet unknown properties. In this manner the research carried out in this PhD research programme combines particular polymer substrates with gold, silver or silver halide nanoparticles, generating multifunctional hybrid materials which exhibit novel and useful optical, antimicrobial and antifouling properties. As such, these hybrid materials are well suited for applications in the healthcare and biomedical devices, food and packaging, surface coatings and the personal hygiene industries. The novel approach developed and used for the production of these nanogold, nanosilver and nanosilver halide hybrid polymer materials did not use conventional external reducing or stabilising agents. Instead, for the nanogold and nanosilver hybrid polymer materials, the Au3+ or Ag+ ions were first absorbed into the polymer substrates (polyurethane, nylon 6,6, polyurethane K5000 latex paint base and amine coated polyethylene terephthalate) and then upon heating the nitrogen-containing functional groups in the polymer matrices reduced the metal ions to their respective metal nanoparticles Au0 and Ag0. Simultaneously a chemical interaction between the metal nanoparticles and the polymer matrix was facilitated. Hence the reduction reaction was effected by the coupled to the oxidation reaction of the nitrogen-containing functional groups. The polymer matrix also afforded control over the nanoparticle size. Silica based BULK ISOLUTE® SORBENTS were used to help elucidate this particular chemistry taking place in the formation of the hybrid polymer materials. The synthesis of the nanosilver halide hybrid polymer materials involved the initial absorption of halide ions into the polymer matrix followed by treatment with silver ions to effect precipitation of nanosize silver halide particles within the polymer matrix, wherein the particle size was similarly controlled by the polymer matrix and precipitation conditions. All formed nanoparticles were therefore stabilised by the polymer matrix. The colour of the resultant hybrid polymer materials is due to the surface plasmon resonance effect of gold and silver nanoparticles. The colour is dependent on the particle size and shape of the nanoparticles and on the refractive index of the surrounding medium. Nanogold hybrid polymers are pink/purple in colour whereas nanosilver hybrid polymers reflect yellow/brown colour. Nanosilver halide hybrid polymers absorb light in the UV range of light and are therefore white in colour. However, due to their photosensitive properties, once exposed to light, silver halides undergo a self-photosensitisation process resulting in formation of silver nanodomains (smaller nanoparticles) on the surface of the silver halide nanoparticles. This gives rise to their absorption in the visible range of light making the hybrid polymer materials appear purple/brown in colour. Nanosilver iodide hybrid polymer materials do not show this effect to any extent and remain as their typical yellow colour. The reflected colours of the hybrid polymer materials and therefore the particle sizes and shapes of metal nanoparticles were investigated by the UV-Vis spectroscopy. The electron microscopy (SEM and TEM) studies showed the morphology of the hybrid polymer materials and that the nanoparticles were not only deposited on the surface but distributed within the polymer matrix. The metal nanoparticles varied in sizes and shapes, particle agglomerates were observed. The confirmation of gold, silver or silver halide species was undertaken using energy dispersive spectroscopy (EDS), scanning transmission spectroscopy (STEM) and X-ray diffraction (XRD). Furthermore, X-ray photoelectron spectroscopy (XPS) was carried out in order to study the nature of the interaction between the formed metal nanoparticles and the polymer matrix. It was demonstrated that the gold and silver nanoparticles are bound to the polymer matrices via Au-N and Ag-N bonds respectively, through the nitrogen-containing functional groups of the polymer matrices. The presence of the oxidised nitrogen species (NOx) confirmed that the electrons required for the reduction of Au3+ and Ag+ to the respective nanoparticles were provided by the coupled oxidation reaction of the nitrogen-containing groups in the polymer matrices. The XPS studies showed there is an interaction between the silver on the surface of the AgX nanoparticles and the nitrogen and oxygen groups present in the polymer matrix. The observation that only very small amounts of Au3+ and Ag+ ions could be leached from the nanogold and nanosilver hybrid materials confirmed the integrity of this chemical bonding between the gold or silver nanoparticles and the polymer matrix. The nanogold, nanosilver and nanosilver halide polymer materials showed effective antimicrobial properties. They were successfully tested against gram negative bacteria Escherichia coli. Additionally, the new generation nanogold and nanosilver hybrid polymer materials have been shown to exhibit antifouling properties.