| Summary: | Atomic force microscopy (AFM) is an important measurement methodology for the study of interactions at the micro and nanoscale. The study of colloidal interactions at microbial cell or membrane surfaces can be significantly extended by the application of AFM imaging and force measurement capabilities to provide unique insights into the surface properties and their relationships. The zeta-potential of membrane and cell surfaces can be mathematically described with Boris Derjaguin, Lev Landau, Evert Verwey and Theodoor Overbeek (DLVO) theories and linked to surface interactions. The research of this thesis analyses AFM force-distance measurements and has developed a FORTRAN program to calculate surface properties from AFM force spectroscopy. In the first instance the developed AFM measurement platform allowed the determination of zeta-potential at the nanoscale across a membrane surface (DK). The mapping of the zeta-potential across a surface within a process relevant environment alongside the measurement of other surface properties is unique to AFM and the presented thesis. The distribution of zeta-potential across the membrane or microbial surfaces was found to be as large as ±20mV with the average zeta potential ranging from 10mV to a maximum as 35mV depending on the surface and aqueous environment. The results were compared to other zeta potential measuring methods; zeta-sizer for cells and streaming potential for DK membrane surface. Zeta potential mapping across the surfaces could also be achieved with the AFM method. The surface adhesion is a prominent feature of force curves and the research of the thesis extended the FORTRAN program for numerical analysis of the force curve. Maximum adhesion could be measured as more than 10000pN, while minimum could be less than 100pN. Hydrophobicity of cells was also measured to aid interpretation of AFM data. With a combination of reaction equilibrium and Gaussian distribution, the research demonstrates that the method can identify the type of functional groups on the sample surface. To illustrate the application of the developed AFM analysis the influence of chemical additives on the surface interactions was also investigated. The effect of Sodium tripolyphosphate (STP) on zeta potential at bacterial and yeast cell surfaces was studied. The effect of STP was to narrow the distribution of zeta potential from 10 – 20mV to 10 – 15mV for both yeasts and bacteria. The influence of the antibiotic amoxicillin was also examined and there was a significant adhesion detected with the non-amoxicillin treated cells; maximum of about 3000pN for NCYC-1324 and maximum of around 30000pN for NCYC-1681. The adhesion was reduced to a few hundred pN within 15mins in low amoxicillin (0.1mg/l). A longer time of exposure or higher concentration caused damage to the cell and reduced the validity of the cell adhesion measurement. In conclusion, the work of the thesis has developed an AFM analysis platform that allows the novel interrogation of AFM force-distance curves measured across surfaces. This provides unique insight into the interactions found at the surface which govern the behaviour of colloids and bio-colloids and impacts within medicine, bioprocess engineering and the natural environment.
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