Interactions of Organic and Biomolecules with Solid Surfaces Probed by Atomic Force Microscopy

Force spectroscopy has become a valuable tool to measure physical and chemical interactions at the molecular level through a variety of techniques. This dissertation focuses on applications of friction force microscopy and single molecule force spectroscopy to measure surface interactions of thin films and single molecules in a quantitative manner. Since the force microscope is capable of distinguishing very small forces (piconewton level), a precise and accurate calibration procedure is required. We present a rapid calibration procedure using the thermal noise spectrum of the cantilever to determine the normal and lateral force sensitivity without contacting any surfaces. Calibration without contacting a surface is advantageous because many experiments may require the force probe to be functionalized with molecules that may be damaged or removed during experiments.;The interactions between DNA and carbon nanotubes provide many potential applications in nanotube sorting and purification and therapeutic treatment of diseases. Fundamental knowledge of interactions between DNA and the surface of carbon nanotubes through simulations and experiments is essential in guiding the development of biomolecule complexes with nanomaterials. In order to model the interaction of DNA with a carbon nanotube, single molecule force spectroscopy was used to remove DNA from graphite. The removal of single-stranded DNA from a graphite surface resulted in steady-state peeling forces for each DNA homopolymer oligomer. The peeling forces for homopolymer oligomers on graphite produced the ranking T ≥ A > G ≥ C. However, it is fundamentally more interesting to directly measure the interaction through force experiments between DNA and individual carbon nanotubes. Horizontally suspended carbon nanotubes were prepared through a simple, self-assembly method for use in DNA peeling experiments. The peeling forces of the DNA homopolymer oligomers on suspended carbon nanotubes decreased compared to graphitic substrates and produced the ranking A ≥ T ≥ G > C. For oligomers where tube wrapping and 3-dimensional structures are important for formation of stable complexes, force curves on suspended CNTs displayed a higher peeling force than force curves measured on flat surfaces. Oligomers having a "special sequence" motif capable of structural identification of CNTs based on size and chirality displayed periodic stretching features in peeling curves indicating the presence of intrastrand interactions.;Additionally, lateral force spectroscopy was used to detect differences in the yield strength of Langmuir-Blodgett bilayer films supported on solid substrates. We were able to damage Langmuir-Blodgett bilayer films controllably by a slow increase in the normal load, resulting in complete film removal. Film damage was detectable by abrupt changes in the friction forces of the films. This procedure enabled us to demonstrate the dramatic increase in the yield strength of Langmuir-Blodgett films due to addition of a poly-ionic interlayer that acted like a glue within the films.