Scanning force microscopy has been used successfully to produce images of individual protein molecules. However, one of the problems with this approach has been the high mobility of the proteins caused by the interaction between the sample and the scanning tip. To stabilize the proteins we have modified the adsorption properties of immunoglobulin G on graphite and mica surfaces. We have used two approaches: first, we applied glow discharge treatment to the surface to increase the hydrophilicity, favoring adhesion of hydrophilic protein molecules; second, we used the arginine modifying reagent phenylglyoxal to increase the protein hydrophobicity and thus enhance its adherence to hydrophobic surfaces. We used scanning force microscopy to show that the glow discharge treatment favors a more homogeneous distribution and stronger adherence of the protein molecules to the graphite surface. Chemical modification of the immunoglobulin caused increased aggregation of the proteins on the surface but did not improve the adherence to graphite. On mica, clusters of modified immunoglobulins were also observed and their adsorption was reduced. These results underline the importance of the surface hydrophobicity and charge in controlling the distribution of proteins on the surface.

The T4 bacteriophage has been used to investigate protocols for the preparation of samples for scanning force microscopy in air, in order to obtaining reproducible images. The resolution of images and the distribution of bacteriophages on the substrate depends on the buffer type, its concentration, the surface treatment of substrate, and the method of deposition. The best imaging conditions for the phages require dilution in a volatile buffer at low ionic strength and adsorption onto hydrophilic surfaces. When imaging with the scanning force microscopy the quality of the images is influenced by the vertical and lateral forces applied on the sample and by the tip geometry.

Scanning force microscopy offers the possibility of observing protein molecules under liquid environment. The main difficulty in obtaining reproducible images is given by the low adhesion of the molecules to the substrate. Physisorbed molecules are displaced by the scanning tip or are resuspended in the medium. We have therefore performed a covalent immobilization of immunoglobulin G ͑IgG͒ or its monovalent FabЈ fragment on gold surfaces thanks to thiol groups. For this purpose, multiple thiol groups were chemically introduced into the IgG molecule by treatment with Traut's reagent. As an alternative, for a FabЈ fragment, we prepared molecules with a single thiol group located close to the C terminus of the truncated heavy chain. Both immobilization techniques enable us to observe clearly discernable individual molecules in liquid media. The grafting of FabЈ fragments on gold surface open new opportunities to study protein interactions.