S. Chiruvolu

Applications of atomic force microscopy to structural characterization of organic thin films

Abstract The atomic force microscope (AFM) has created exciting new possibilities for imaging thin organic films under ambient conditions at length scales ranging from tens of microns to the sub-molecular scale. We present images of thin organic films prepared by the Langmuir-Blodgett (LB) and self-assembly (SA) techniques that demonstrate the possibilities and limitations of the AFM. Atomic force microscope images of LB films show that manganese arachidate (MnA2) monolayers are short-range ordered and lead stearate (PbSt2) monolayers are long-range ordered on crystalline mica substrates, but disordered on amorphous oxidized silicon substrates. The lattice structures of PbSt2 and MnA2 monolayers on mica were previously unknown and have larger lattice parameters and molecular areas than do multilayer films of the same materials, indicating the strong interactions with the larger mica lattice. Multilayer films of PbSt2, cadmium arachidate (CdA2), and MnA2, have centered rectangular “herringbone” lattices on both silicon and mica substrates. After sufficient layers, the effect of the mica substrate is eliminated and the lattice parameters and area per molecule of films deposited on mica relax to those of multilayer films on amorphous oxidized silicon. This limiting area per molecule correlates well with the degree of ionic versus covalent bonding as estimated by the Pauling electronegativity, with barium arachidate (BaA2) > MnA2 > CdA2 > PbSt2. For BaA2 and MnA2 the increased molecular area is sufficient to induce a tilt in the molecular packing. The lattice parameters, symmetry, and area per molecule are independent of the length of the alkane chain of the fatty acid for all cations and substrates examined. AFM images also show that self-assembled monolayers of octadecyltrichlorosilane (OTS) form on mica by nucleating isolated, self-similar domains. With increasing coverage, the fractal dimension of the growing domains evolves from 1.6 to 1.8. At higher coverage, continued growth is limited by adsorption from solution.

Direct Measurements of Specific Ligand-Receptor Interactions Between Model Membrane Surfaces

The Surface Forces Apparatus technique has made it possible to directly measure the long-range interaction forces and adhesion between two model membrane surfaces containing receptor and ligand molecules. Both long-range electrostatic and hydrophobic forces and short-range adhesion or specific binding forces can be directly measured at the angstrom resolution level, and the rearrangements of the proteins and lipids during these interactions can also be studied. Results are presented of the measured forces between two surfaces, the one supporting a lipid-protein membrane exposing Streptavidin receptors, the other exposing Biotin ligands. At surface separations greater than 4A three types of forces are operating: repulsive or attractive electrostatic forces and attractive van der Waals and hydrophobic forces. Closer in, a highly specific “lock and key” binding force suddenly switches on as soon as the surfaces approach within about 4A of each other. The final binding is very strong, but the results show that the number of bonds formed, which determines the final adhesion strength, also depends on the fluidity of the supporting membranes and on the rates at which the ligands approach the surfaces. Our results also show which forces are responsible for different aspects of receptor-ligand interactions; for example, while the longer-ranged electrostatic and hydrophobic forces are found to have little effect on the final adhesion energy, they do affect the rates of association and thereby play the dominant role in modulating the on rates of association in solution. Preliminary results are also presented on the experiments using biotin analogues having different binding constants to Streptavidin, and on parallel studies on specifically adhering vesicles in solution employing rapid freezing electron microscopy imaging techniques. These studies show that biospecific interactions such as are involved in immunological recognition and cell-cell contacts may be studied at the molecular level and in real time by the Surface Forces Apparatus (SFA) and Electron Cryo-Microscopy techniques.