Atomic Force Microscopy of Nanoparticles and Biological Cells

Nanoparticles are of great interest in science and industrial application. The high surface to volume ratio offer very distinct physical properties compared to their corresponding bulk material. One of the most powerful tools to investigate nanoparticles and generally the nanoworld is the Atomic Force Microscopy (AFM). This instrument offers unique applications for the analysis of nanoparticles, from imaging to manipulation up to assignment of their intrinsic physical properties. It also enables to perform experiments in various environments from liquids to ultra-high-vacuum (UHV) and temperatures down to the low Kelvin regime. The following presented thesis is structured into three parts. In the first part, the retention properties of calcium fluoride (CaF2) nanoparticles on mica and human tooth substrate in liquid and at room temperature were discussed. These nanoparticles are promising candidates as additives in dental care products, which could serve as possible fluoride-container to prevent carries. The adhesion strength of as-synthesized calcium fluoride nanoparticles adsorbed on mica and on tooth enamel in liquid with Amplitude Modulation AFM (AM-AFM), depending on the substrate roughness and the chemical interplay between substrate and nanoparticles were investigated. By comparing the frequency distribution of the dissipated power of the tip to induce manipulation of the particles, showed that up to 10-times higher retention was observed for particles adsorbed on tooth enamel compared to mica. Although the enamel had an increased surface roughness compared to mica resulting in a decreased contact area of the particle with the substrate, more power was needed to dislocate the particles. We related this to the strong chemical interaction of the CaF2 nanoparticles with the tooth enamel. Further, we observed that particles with an ordered, smooth and plane surface structure show higher retention than rough and spherical ones. Thus, the nano-morphology of the particles was shown to strongly influence the mobility. The evidence that the interplay of calcium fluoride nanoparticles with the tooth enamel is so strong, makes calcium fluoride nanoparticles in respect to their adhesive strength on tooth enamel a promising candidate to be used in dental care products preventing teeth demineralization. In the second part, pathogen Escherichia coli (E. coli) bacteria were investigated under ambient conditions with AM-AFM. Treatment of these bacteria with the human antibody immunoglobulin A (IgA) was found to inhibit the pathogenicity of these bacteria. The aim was to explore how the IgA affects the morphology of native bacteria and to show where and how this biomolecule can be found on the cell. Images of native and incubated bacteria revealed a much higher stability of the incubated bacteria against the drying procedure as part of the sample preparation. Less lesions of the bacteria body and a much more homogeneous body structure after treatment with the antibody were observed. The incubated bacteria also showed a higher degree in surface roughness after incubation, a clear indication that the antibody covers the bacteria body. The statistically evaluated values for the width and the height of the entire bacteria and the flagellum revealed a slightly lower thickness of the flagella. An indication where the antibody IgA exactly binds to the bacterium was obtained from AM-AFM images, where a high tendency of the amplitude to show bistability was observed. Each bistability event was interpreted as a result of the presence of the antibody. A high tendency of the amplitude to show bistable behaviour was seen on the bacteria body and on the whole flagella for IgA incubated bacteria. We showed that the IgA binds to the bacteria body and the flagellum, where the modification of the flagellum strongly could influence the mobility and hence the possibility of pathogenic E. coli bacteria to enter into blood circulated tissues to cause infections. The last part of the thesis was a combination between both previous questions. It combines nanoparticles and bacteria. The magnetic properties of one single magnetotactic bacteria (MTB) were investigated with AFM under UHV conditions at cryogenic temperatures. Magnetotactic bacteria have magnetosomes incorporated in their body. These magnetosomes consist of nanometer-sized iron oxide (magnetite) particles used for the bacteria to sense the earth magnetic field to find optimum living conditions. Biogenic produced iron oxide nanoparticles are interesting for various fields in science. We were able to position one single bacterium at the free end of sensitive tip-less cantilever with the help of a micro mechanical manipulator and performed dynamic cantilever magnetometry measurements. The data showed excellent agreement with the theoretical model. The magnetic moment of a single magnetosome chain fitted with the derived equation of a ferromagnetic rod, gave a value, which was in very good agreement with the values found in the literature. Compared to other studies, the cantilever magnetometry method also provided information about anisotropy constant (Keff) or the coercive field (Hc) of a single magentosome chain. This anisotropy constant was found to be higher than the first order magnetocrystalline constant for bulk magnetite. The anisotropy constant was found to be higher than the first order magnetocrystalline constant for bulk magnetite indicating that for an ensemble of nano-sized magnets the interparticle interaction is stronger than the intrinsic magnetic anisotropy behaviour of single domain particles. The coercive field value was found to be in very good agreement with the literature values for biogenic produced magnetite crystals. From the value of the anisotropy constant Keff and the coercive field Hc we concluded that the magnetic interaction between the single magnetite crystals dictate the magnetic properties of the magnetosome chain. Discrete steps in the f-H experiment were attributed to a change of the volume magnetization due to switching of magnetic single domains.