The atomic force microscope (AFM) is a unique tool for studying interfacial phenomena. It operates by scanning a very sharp tip across a sample, which 'feels' the contours of the surface in a manner similar to the stylus tracing across the grooves of a record. In this way it can follow the contours of the surface and so create a topographic image, often with sub-nanometer resolution. The AFM, by its very nature, is extremely sensitive to intermolecular forces and has the ability to measure force as a function of distance. In fact measurement of interactions as small as a single hydrogen bond have been reported. However the major limitation of the AFM remains its lack of chemical specificity.
Chemical force microscopy (CFM) is a technique which combines the force sensitivity of the AFM with chemical discrimination. This is achieved by chemically derivatising the probe tip with a specific functional group, so measured forces may be interpreted in terms of the specific interaction between molecules on the tip and those on the surface. In addition it is possible to map the spatial distribution of the ligands patterned on the surface using friction force microscopy or affinity mapping. CFM experiments have been used to probe chemical interactions between self-assembled monolayers in order to study fundamental adhesion and friction forces at the solid-liquid interface. CFM has also been used to probe biological interactions including the interaction between biotin and streptavidin, oligonucleotides and antibody with antigens.
The most fundamental question that surrounds this technique is how much chemistry is added? Put another way is it valid to interpret image and adhesion contrast in terms of difference in surface chemistry? Three different aspects of this problem will be described. Firstly the role of the substrate is discussed. Secondly, a series of experiments concerned with the interaction of self-assembled monolayers terminating in p electron aromatic systems is described. These show that it is not possible to interpret CFM solely in terms of electronic or intermolecular interactions. The third section will review CFM experiments with chiral surfaces. Surfaces patterned with alternate areas of chiral molecules were found to be an ideal system to study interfacial processes since the molecules are made of precisely the same atoms but arranged as mirror images. These experiments show that chiral discrimination is not only possible but that the results are in accordance with parallel experiments using high performance liquid chromatography (HPLC).
Dr Rachel McKendry
Department of Chemistry, University of Cambridge
Lensfield Road, Cambridge, CB2 1EW UK
TEL 01223 336 528; FAX 01223 336 362