Direct Imaging of Water/Lipid Interface by Frequency Modulation Atomic
Force Microscopy at Sub-Angstrom Resolution
Takeshi Fukuma, Michael J. Higgins, Suzanne P. Jarvis
Centre for Research on Adaptive Nanostructures and Nanodevices, Trinity College Dublin,
Lincoln Place Gate, Dublin 2, IrelandE-mail: takeshi.fukuma@tcd.ie
Frequency modulation atomic force microscopy (FM-AFM) has been used in ultra highvacuum (UHV) environments for investigating subnanometer-scale structures and functionsof various surfaces. Until recently, however, its operating environment had been limited only
to UHV, which has prevented its practical applications in air and liquids. Recently, Fukuma etal. presented a way to overcome this limitation using an ultra low noise cantilever deflection
detection system and thereby operating FM-AFM with extremely small cantilever oscillation
amplitude [1]. This has made it possible to obtain true molecular [2] and atomic [3] resolution
with FM-AFM in liquid. One of the most interesting applications of this new technique ishigh-resolution imaging of biological systems under physiological conditions. However, true
subnanometer resolution with FM-AFM in liquid has not been demonstrated on biologicalsystems.
Biological membranes are amongst the most fundamental elements in biological systems.
They form the walls of cells and boundaries between the organelles therein with a selectivelypermeable structure. The structure and function of the membranes are determined by thechemical interactions between the constituent molecules mediated through water and ions inphysiological solution. Thus, understanding of the interactions between lipid molecules (mainconstituents of biological membranes) and water or ions are of great importance. To date,various spectroscopic methods have been utilized for investigating water/lipid interface.
However, these methods provide only global information averaged over micrometer-scale
area and hence molecular-scale details of water-lipid and ion-lipid interactions have mostlyremained unknown.
Here we investigate a dipalmitoylphosphatidylcholine (DPPC) lipid bilayer in phosphatebuffer solution as a model biological membrane under physiological conditions by FM-AFM[4]. The force vs. distance curves measured between the bilayer and the AFM tip show
oscillatory force profiles with a peak spacing of 0.28 nm, indicative of the existence of up to
two hydration layers next to the membrane surface. FM-AFM imaging at the water/lipid
interface visualizes individual hydration layers in three-dimensions with molecular-scalecorrugations corresponding to the lipid headgroups. Furthermore, we visualize extensive
lipid-ion interaction networks and their transient formation between headgroups in the bilayer.
The spatial distribution of ion occupancy, visualized in real-space with the unprecedented
lateral resolution of 90 pm, reveals the existence of two equivalent binding sites associatedwith the phosphate groups and the network formation between them.
[1] T. Fukuma, M. Kimura, K. Kobayashi, K. Matsushige, H. Yamada, Rev. Sci. Instrum. 76,053704 (2005).
[2] T. Fukuma, K. Kobayashi, K. Matsushige, H. Yamada, Appl. Phys. Lett. 86, 193108(2005).
[3] T. Fukuma, K. Kobayashi, K. Matsushige, H. Yamada, Appl. Phys. Lett. 87, 034101(2005).
[4] T. Fukuma, S. P. Jarvis, Rev. Sci. Instrum. 77, 043701 (2006).