Phase imaging by atomic force microscopy: Analysis of living homoiothermic vertebrate cells

Atomic force microscope-based phase imaging in air is capable of elucidating variations in material properties such as adhesion, friction, and viscoelasticity. However, the interpretation of phase images of specimens in a fluid environment requires clarification. In this report, we systematically analyzed atomic force microscope-derived phase images of mica, glass, and collagen under the same conditions as used for living cells at various tapping forces; the resulting data provide critical information for the interpretation of phase images of living cells. The peripheral regions of COS-1 cells consistently show a more negative phase shift than the glass substrate in phase images at set-point amplitude: free amplitude (A(sp)/A(o)) = 0.6-0.8. In addition, at all A(sp)/A(o) values suitable for phase imaging, tapping frequency appears to be high enough to ensure that phase shifts are governed primarily by stiffness. Consequently, phase imaging is capable of high resolution studies of the cellular surface by detecting localized variations in stiffness. We demonstrate that phase imaging of a bifurcating fiber in COS-1 cell cytoplasm is readily capable of a lateral resolution of similar to 30 nm.