Browsing by Author "Cole, Daniel G"

This research describes the development of a state-of-the-art atomic force microscope (AFM) for improved force spectroscopy. Although the AFM has been used extensively in this field of research, the performance of the instrument has been limited by inefficient operation techniques, incorrect experimental assumptions, and inadequate controller design. This research focuses on overcoming these deficiencies by providing precise control over the instrument for specialized research in a manner that is conducive to the natural science researcher.

To facilitate this research, a custom, multi-axis AFM system was constructed. The instrument was designed primarily for AFM-based force spectroscopy and as a result a substantial amount of research focused on the development of a wide variety of approach/retraction methods for the instrument. Defining research in this area included the development of methods to minimize potentially damaging compressive forces, form polymer bridges at different tip-sample gap widths, produce clean, deconvoluted force-extension curves, and limit single molecule force spectroscopy pulling geometry errors. In an effort to increase the efficiency of the instrument, the programs developed during this research were fully automated, allowing autonomous operation of the instrument for long periods of time. To compliment the data collection programs, both manual and automated analysis programs with force-volume imaging capabilities were also developed.

By studying the AFM from a dynamic systems, measurements, and controls approach, the resulting controllers were tailored to meet the process requirements of the intended applications. In doing so, the sensitivity of the instrument was improved for applications of interest. By incorporating control over the environment, contact force, loading rate, and pulling angle, the research has increased the accuracy of the AFM such that molecules and receptor-ligand binding events can be investigated with greater detail. Furthermore, the incorporation of a graphical user interface and automated data collection and analysis tools has made the AFM a more user-friendly, efficient instrument for the natural science researcher.

This research presents new calibration techniques for the characterization of atomic force microscopy cantilevers. Atomic force microscopy cantilevers are sensors that detect forces on the order of pico- to nanonewtons and displacements on the order of nano- to micrometers. Several calibration techniques exist with a variety of strengths and weaknesses. This research presents techniques that enable the noncontact calibration of the output sensor voltage-to-displacement sensitivity and the cantilever stiffness through the analysis of the unscaled thermal vibration of a cantilever in a liquid environment.

A noncontact stiffness calibration method is presented that identifies cantilever characteristics by fitting a dynamic model of the cantilever reaction to a thermal bath according to the fluctuation-dissipation theorem. The fitting algorithm incorporates an assumption of heavy fluid loading, which is present in liquid environments.

The use of the Lorentzian line function and a variable-slope noise model as an alternate approach to the thermal noise method was found to reduce the difference between calibrations preformed on the same cantilever in air and in water relative to existing techniques. This alternate approach was used in combination with the new stiffness calibration technique to determine the voltage-to-displacement sensitivity without requiring contact loading of the cantilever.

Additionally, computational techniques are presented in the investigation of alternate cantilever geometries, including V-shaped cantilevers and warped cantilevers. These techniques offer opportunities for future research to further reduce the uncertainty of atomic force microscopy calibration.