Simultaneous height and adhesion imaging of antibody-antigen interactions by atomic force microscopy

Specific molecular recognition events, detected by atomic force microscopy (AFM), so far lack the detailed topographical information that is usually observed in AFM. We have modified our AFM such that, in combination with a recently developed method to measure antibody-antigen recognition on the single molecular level (Hinterdorfer, P., W. Baumgartner, H. J. Gruber, K. Schilcher, and H. Schindler, Proc. Natl. Acad. Sci. USA 93:3477-3481 (1996)), it allows imaging of a submonolayer of intercellular adhesion molecule-1 (ICAM-1) in adhesion mode. We demonstrate that for the first time the resolution of the topographical image in adhesion mode is only limited by tip convolution and thus comparable to tapping mode images. This is demonstrated by imaging of individual ICAM-1 antigens in both the tapping mode and the adhesion mode. The contrast in the adhesion image that was measured simultaneously with the topography is caused by recognition between individual antibody-antigen pairs. By comparing the high-resolution height image with the adhesion image, it is possible to show that specific molecular recognition is highly correlated with topography. The stability of the improved microscope enabled imaging with forces as low as 100 pN and ultrafast scan speed of 22 force curves per second. The analysis of force curves showed that reproducible unbinding events on subsequent scan lines could be measured.