In the framework of this dissertation, a microwave-assisted synthesis strategy for the production of zinc oxide nanoparticles, at the lower end of the nanoscale, is presented. The radius of these spherical particles can be adjusted by choosing a reaction temperature between 125 °C and 200 °C in the range of 2.6 nm to 3.8 nm. In this range, semiconductor particles such as those made of zinc oxide, exhibit size-dependent optical and structural properties, and tailor-made particles can aid their detailed investigation. This is demonstrated by the changes in optical band gap, photoluminescence and crystal lattice stress.
Due to their functionalisation with oleate, these zinc oxide nanoparticles are hydrophobic and only dispersible in organic solvents such as cyclohexane or tetrahydrofuran. However, some applications -- for example as luminescent probes in biological systems -- may require hydrophilic particles in aqueous dispersion. For this, a phase transfer procedure is presented in which the particles are transferred into an aqueous medium employing the surfactant polysorbate 80. This process is associated with further particle growth, and the final particle size can be adjusted to up to 5.7 nm via the duration of heating this dispersion to 90 °C.
In this context, the particle size distributions are determined via small-angle X-ray scattering, which, in contrast to other methods such as electron microscopy, can also be reliably applied in complex media. This is demonstrated with studies on the artificial digestion of zinc oxide nanoparticles. Zinc oxide, as well as other ionic zinc compounds, are added to food and cosmetic products due to their antibacterial effect or as food supplements and are ingested by humans. For this reason, it is essential to examine in detail any changes in the chemical and structural composition of these additives that end up being digested.
When zinc oxide and zinc chloride come into contact with artificial saliva, new, small nanoparticles with radii around 3 nm are formed, whereby the size, concentration or functionalisation of the ingested substances has no significant influence on particle formation. After the dissolution of these nanostructures in gastric juice, particles form again in the intestinal juice, but their size distribution could not be determined unambiguously. It is assumed that the newly formed particles consist of zinc phosphate, regardless of which zinc compound is initially ingested. Ion release measurements confirm the findings on the formation and dissolution behaviour of the particles obtained from small-angle X-ray scattering.
In order to investigate the particle formation from a zinc chloride solution in artificial saliva more precisely, X-ray scattering and X-ray absorption spectroscopy were employed. Within five minutes of the addition of zinc chloride to artificial saliva, nanoparticles with a radius of 1 nm are formed, and this radius remains constant for at least two weeks. The particles are found as aggregates with a radius of ca. 15 nm, in which a protein layer presumably binds the particles together. It is assumed that the particles initially consist of amorphous zinc phosphate, which begins to crystallise within seven hours. At the same time, the aggregates densify, i.e. the distance between the particles becomes smaller. In the subsequent two weeks, the radius of the aggregate increases to ca. 22 nm as the number of bound particles also increases.
In summary, the first part of this dissertation focuses on the synthesis of reference candidate nanoparticles from zinc oxide at the lower end of the nanoscale. These particles, with adjustable size and in turn inherent optical and structural properties, are provided in both polar and apolar solvents. The second half of this dissertation, focuses on the application of small-angle X-ray scattering, and the suitability of the method for the characterisation of nanoparticles in complex media is demonstrated at an artificial digestion process. Thus, a contribution is made to the understanding of biological processes induced by the application of particulate and dissolved zinc compounds in food and cosmetic products.
