Influence of cell surface and nanomechanical properties on the flocculation ability of industrial <i>Saccharomyces cerevisiae</i> strains

In the past few years, atomic force microscopy (AFM) has provided novel information on the ultrastructural and nanomechanical properties of yeast cell walls that play a major role in determining the flocculation characteristics of the yeasts. In this study, we used AFM to visualize at the nanoscale the cell surface topography and to determine cell wall nanomechanical properties (e.g. elasticity) of different strains of S. cerevisiae employed for brewing, winemaking and fuel alcohol production. Cell surface topography was found to correlate with the flocculation behaviour of these strains during their late stationary phase, with the cell surface of flocculent cells being rougher than that of weakly flocculent cells. The elastic modulus of the yeast cell walls showed that weakly flocculent strains had a more rigid cell wall than highly flocculent strains. This difference in elasticity seemed to have an effect on the adhesive properties of the yeast cell walls, with weakly flocculent yeasts displaying higher adhesion energy than the highly flocculent strains. These findings seem to indicate that yeast cell surface nanomechanical properties play an important role in governing flocculation

Accurate Calibration and Uncertainty Estimation of the Normal Spring Constant of Various AFM Cantilevers

Measurement of force on a micro- or nano-Newton scale is important when exploring the mechanical properties of materials in the biophysics and nanomechanical fields. The atomic force microscope (AFM) is widely used in microforce measurement. The cantilever probe works as an AFM force sensor, and the spring constant of the cantilever is of great significance to the accuracy of the measurement results. This paper presents a normal spring constant calibration method with the combined use of an electromagnetic balance and a homemade AFM head. When the cantilever presses the balance, its deflection is detected through an optical lever integrated in the AFM head. Meanwhile, the corresponding bending force is recorded by the balance. Then the spring constant can be simply calculated using Hooke’s law. During the calibration, a feedback loop is applied to control the deflection of the cantilever. Errors that may affect the stability of the cantilever could be compensated rapidly. Five types of commercial cantilevers with different shapes, stiffness, and operating modes were chosen to evaluate the performance of our system. Based on the uncertainty analysis, the expanded relative standard uncertainties of the normal spring constant of most measured cantilevers are believed to be better than 2%

Accurate Calibration and Uncertainty Estimation of the Normal Spring Constant of Various AFM Cantilevers

Measurement of force on a micro- or nano-Newton scale is important when exploring the mechanical properties of materials in the biophysics and nanomechanical fields. The atomic force microscope (AFM) is widely used in microforce measurement. The cantilever probe works as an AFM force sensor, and the spring constant of the cantilever is of great significance to the accuracy of the measurement results. This paper presents a normal spring constant calibration method with the combined use of an electromagnetic balance and a homemade AFM head. When the cantilever presses the balance, its deflection is detected through an optical lever integrated in the AFM head. Meanwhile, the corresponding bending force is recorded by the balance. Then the spring constant can be simply calculated using Hooke’s law. During the calibration, a feedback loop is applied to control the deflection of the cantilever. Errors that may affect the stability of the cantilever could be compensated rapidly. Five types of commercial cantilevers with different shapes, stiffness, and operating modes were chosen to evaluate the performance of our system. Based on the uncertainty analysis, the expanded relative standard uncertainties of the normal spring constant of most measured cantilevers are believed to be better than 2%

Micro electro-mechanical system design, fabrication and application for atomic force microscopy probe elasticity characterisation

The work in this thesis is focused on characterising elastic behaviour of micro-cantilever probes found at the core of Atomic Force Microscopy (AFM) tools. It is an essential property to AFM force measurements, since interpretation of the cantilever deflection is directly related to the measure of the tip-sample force via its elastic conduct. It was demonstrated that the conventional micro-cantilevers can be well characterised using analytical and FE methods, together with the ready developed tools and experimental techniques, which formed the insights for the design practices used in further and more complex structure development efforts. However, this project’s emphasis was placed on assessment of non-traditional AFM cantilever structures that are not supported by the said methods, and hence necessitated in the conceptualisation and development of a dedicated micro electro-mechanical system (MEMS) solution. The sensitive force and elasticity measurement device, based on metal film resistive strain sensing, was realised using semiconductor and MEMS fabrication approaches in the James Watt Nanofabrication Centre, allowing the desired performance to be achieved with an in-house process optimisation. The functional MEMS tool paired with bespoke instrumentation was then employed to characterise a range of commercial cantilevers as well as non-standard probes consisting of complex composition multi-structures and non-linear elasticity, providing novel insights into their elastic character

Express analysis of actual bluntness of AFM probe tips

The Atomic Force Microscope (AFM) is an invention that has enabled a significant number of studies and discoveries in the field of nanotechnology. It is well-known that the resolution of AFM-based applications is critically dependent on the tip bluntness of the probe utilised. Numerous researchers have proposed different approaches to assess the condition of AFM probe tips. In spite of these efforts, further advances are still needed for the express analysis of the bluntness of such tips. In this context, the overall aim of the research work presented in this Thesis was to investigate a novel in-situ technique for assessing the apex condition of AFM tips. In particular, this technique relies on the analysis of depth-sensing data obtained from the nanoindentation of the probe tip into a soft elastic sample. Nanoindentation is a process that is readily implemented on AFM devices. For this reason, the proposed technique could be a fast an efficient approach for deciding when AFM probes should be replaced. The theoretical argument on which the technique is based is that the current shape of the tip apex in its working position within an AFM device can be approximated as a power-law function and that the exponent of this function can be used as a quantitative measure of the tip bluntness. Based on this approximation and the use of the self-similar (scaling) approach to depth-sensing indentation, it is possible to extract this bluntness parameter, herein also referred to as the degree of tip bluntness, from AFM nanoindentation data. The practical implementation of this technique was realised using a commercial AFM device and commercial probes. The actual geometry of the apex of these probe was also studied in details using additional experimental methods via the use of Scanning Electron Microscopy and also via the so-called “reverse imaging” method to obtain two- and three-dimensional data about the tip apex of these probes. Among the different iv contributions made from the work carried out in this research, the most important conclusion is that a good agreement was found between values of the bluntness parameter evaluated by the proposed technique and the effective bluntness obtained from analysing the actual three dimensional geometry of the AFM tips. Thus, it can be argued that the technique put forward in this work for the express analysis of the bluntness of AFM probe tips using depth-sensing nanoindentation can be considered as a valid method when assessing the condition of AFM probe