Imaging mixed lipid monolayers by dynamic atomic force microscopy

Phase imaging with tapping mode atomic force microscopy (AFM) and force mod ulation microscopy were used to probe the mechanical properties of phase-se parated lipid monolayers made of a mixture (0.25:0.75) of the surface activ e lipopeptide surfactin and of dipalmitoylphosphatidylcholine (DPPC). The p i -A isotherms and the result of a molecular modeling study revealed a loos e, 2-D liquid-like organization for the surfactin molecules and a closely p acked, 2-D solid-like organization for DPPC molecules. This difference in m olecular organization was responsible for a significant contrast in height, tapping mode phase and force modulation amplitude images. Phase imaging at light tapping, i.e., with a ratio of the set-point tapping amplitude with respect to the free amplitude A(sp)/A(0) approximate to 0.9, showed larger phase shifts on the solid-like DPPC domains attributed to larger Young's mo dulus. However, contrast inversion was observed for A(sp)/A(0) < 0.7, sugge sting that at moderate and hard tapping the image contrast was dominated by the probe-sample contact area. Surprisingly, force modulation amplitude im ages showed larger stiffness for the liquid-like surfactin domains, suggest ing that the contrast was dominated by contact area effects rather than by Young's modulus. These data emphasize the complex nature of the contrast me chanisms of dynamic AFM images recorded on mixed lipid monolayers. (C) 2001 Elsevier Science B.V. All rights reserved.