Chemical force microscopy

This thesis describes principles and applications of Chemical Force Microscopy (CFM)--a novel scanning probe microscopy technique that allows direct probing of intermolecular interactions and imaging with chemical sensitivity. Probe tips of an atomic force microscope were modified with self-assembled monolayers (SAMs) that terminated in a variety of terminal chemical functionalities. Use of these tips allowed to measure and quantify adhesion and friction forces between the functional groups on the tip and on the sample.;Adhesion force studies between SAMs that terminate with hydrogen-bonding and hydrophobic functional groups have shown that CFM can reproducibly distinguish between different types of intermolecular interactions. The trends observed in these experiments follow chemical intuition and can be interpreted based on modern theory of contact mechanics. This theory also allowed to show that the contact area between the sharp (;A new method--force titrations--has been developed to study surface ionization and determine local pK values. The interactions observed between tip and sample surfaces modified with ionizable functional groups in aqueous solutions also agree with predictions of double-layer and modified JKR models. CFM measurements have been used to extract double layer parameters that are essential to understanding interactions in aqueous media.;CFM allowed to observe chemical specificity in friction forces. The magnitude of the friction forces follows the same trend as adhesion forces. The predictable dependence of the friction forces on the tip and sample functionality has been exploited for mapping surface domains of different functional groups with chemical sensitivity. It has been demonstrated that lateral force images can be rationally interpreted in terms of the strength of the interactions between functional groups on the probe and on the surface. It is also shown that CFM approach can be extended to imaging in tapping mode, which opens us possibilities for chemically-sensitive imaging of soft and delicate surfaces of polymers and biological objects.;Chemical force microscopy has been also extended to a realm of complex interactions relevant to biophysics. Development of appropriate attachment chemistry enabled direct measurements of forces necessary to elastically stretch, structurally transform and break apart a single DNA duplex formed from short (14 base pair) synthetic oligonucleotides. Complementary and non-complementary sequences can be reproducibly distinguished on the basis of the differences in binding forces. Force microscopy and DNA synthesis can now be used to study the effect of specific DNA sequences on forces and the binding in the presence and absence of DNA binding proteins and molecules.;These studies show that chemical force microscopy represents a practical and versatile approach to studying various aspects of intermolecular interactions and mapping surface functional groups on a nanometer scale.