Surface potential mapping of dispersed proteins

We describe a method for detecting proteins after transfer to PVDF membranes, based on the surface potential attributed to each protein. Proteins separated by classical two-dimensional polyacrylamide gel electrophoresis could be detected by scanning the membrane surface with a vibrating capacitor (also called a Kelvin probe) on the basis of differences between their surface potential and that of the membrane. Coupled to colloidal gold staining, the technique enables detection of proteins previously undetectable by classical staining methods. Plotting variations of the surface potential in two dimensions visualizes proteins which migrate close together. Finally, we demonstrate that the Kelvin probe detects proteins over a concentration range from micro to sub-nanogram with increased sensitivity at lower concentrations, and unlike other methods, appears to be similar for all proteins tested so far. The method described is fast, reliable, and it can be automated for high throughput.