Chapter 9: Interaction of organic molecules with layer silicates, oxides and hydroxides and related surface-nanocharacterization techniques
Giovanni Valdré, Daniele Moro and Gianfranco Ulian
Knowledge of the surface properties of layered minerals is of great importance to understand both fundamental and applied technological issues, such as, for example, liquid–surface interactions, microfluidity, friction or tribology and biomolecule self-assembly and adhesion.
Recent developments in Scanning Probe Microscopy (SPM) have widened the spectrum of possible investigations that can be performed at a nanometric level at the surfaces of minerals. They range from physical properties such as surface potential and electric field topological determination to chemical and spectroscopic analysis in air, in liquid or in a gaseous environment.
After a brief introduction to new technological developments in SPM, we present recent achievements in the characterization and application of nanomorphology, surface potential and cleavage patterns of layer silicates, in particular chlorite.
Two general research directions will be presented: interaction of organic molecules with layer silicates and synthetic substrates, and mineral hydrophilicity/phobicity and friction/adhesion issues. SPM is used to assess the force-curve, force-volume, adhesion and surface potential characteristics of layer silicates by working in Electric Force Microscopy (static and dynamic EFM) and in Kelvin probe modes of operation. For instance, EFM allows us to measure the thickness of silicate layers and, from frequency, amplitude, phase modulation and Kelvin analysis, to derive the electrostatic force experienced by the probe. We can relate these measurements directly to the electrostatic force gradient at the mineral surface.
Transverse dynamic force microscopy, also known as shear force microscopy is introduced here and examples of the investigation of attractive, adhesive and shear forces of water on layer silicates will be presented. The study of water in confined geometries is very important because it can provide simple models for fluid/mineral interactions.
The ability to control the binding of biological and organic molecules to a crystal surface is fundamental, especially for biotechnology, catalysis, molecular microarrays, biosensors and environmental sciences. For instance, recent studies have shown that DNA molecules have different binding affinities and assume different conformations when adsorbed to different layer silicate surfaces. On certain crystals the electrostatic surface potential anisotropy is able to order and stretch the DNA filament and induce a natural change in its conformation. The active stretching of DNA on extensive layer silicates is a clear indication of the basic and technological potential carried by these minerals when used as substrates for biomolecules. Other examples including amino acids, proteins, nucleotides, nucleic acids and cells are discussed here. Finally, a comparison between experimental data and simulation is presented and discussed.
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