Studying the surface of soft materials (live cells) at high resolution by scanning probe microscopy: challenges faced

Abstract

The formation of transmembrane protein channels in the cells of multicellular organisms is not well understood. These transmembrane channels play important roles in cell signal transduction and, in the case of gap junctions, (J.-P. Revel, J.H. Hoh, S.A. John, D.W. Laird, K. Puranam, S.B. Yancey, Cell Biol. 3 (1992) 21) intercellular communication. Obtaining high resolution images (less than 1 nm) of the surface of living cells (the plasma membrane) would contribute valuable information towards understanding the formation of these channels. At present, atomic or scanning force microscopy (AFM or SFM) shows the greatest potential for reaching this goal. AFM is able to obtain high resolution on the surfaces of hard materials immersed in a solution. Thus, it is possible to image cells with AFM in an aqueous solution which mimics physiological conditions, so they can remain alive. We have taken up this challenge and have found that interpreting data from imaging a cell surface at the nanoscale is difficult due to its soft material properties (when compared to the cantilever and probe materials). Therefore, a better understanding of the resolution achievable by AFM on soft materials is needed. The probe apex structure directly affects the resolution of AFM, so it is important to characterize it accurately (J.A. DeRose, J.-P. Revel, Microsc. Microanal. 3 (1997) 203). In addition, one needs to better understand the probe-sample interaction for AFM to image highly deformable materials, such as cells, with reliable high resolution. We have studied AFM probe structure before and after use for imaging and the results have elucidated some information on the probe-sample interaction.