A. L. Weisenhorn

Forces in atomic force microscopy in air and water

A new atomic force microscope, which combines a microfabricated cantilever with an optical lever detection system, now makes it possible to measure the absolute force applied by a tip on a surface. This absolute force has been measured as a function of distance (=position of the surface) in air and water over a range of 600 nm. In the absolute force versus distance curves there are two transitions from touching the surface to a total release in air caused by van der Waals interaction and surface tension. One transition is due to lifting off the surface; the other is due to lifting out of an adsorbed layer on the surface. In water there is just one transition due to lifting off the surface. There is also a transition in air and water when the totally released tip is pulled down to touch the surface as the surface and tip are brought together. Based on the force versus distance curves, we propose a procedure to set the lowest possible imaging force. It can now be as low as 10−9 N or less in water and 10−7 N...

Scan speed limit in atomic force microscopy

The scan speed limit of atomic force microscopes has been calculated. It is determined by the spring constant of the cantilever k, its effective mass m, the damping constant D of the cantilever in the surrounding medium and the stiffness of the sample. Techniques to measure k, k/m and D/m are described. In liquids the damping constant and the effective mass of the cantilever increase. A consequence of this is that the transfer function always depends on the scan speed when imaging in liquids. The practical scan speed limit for atomic resolution in vacuum is 0·1 μm/s while in water it increases to about 2 μm/s due to the additional damping of cantilever movements. Sample stiffness or damping of cantilever movements by the sample increase these limits. For soft biological materials imaged in water at a desired resolution of 1 nm the scan speed should not exceed 2 μm/s.

From atoms to integrated circuit chips, blood cells, and bacteria with the atomic force microscope

The atomic force microscope (AFM) can now bridge the gap from imaging objects that can be seen with an optical microscope to imaging atoms: a range in magnification of 104. High magnification images of germanium show single atoms separated by 0.4 nm while low magnification images of entire cells and portions of an integrated circuit chip provide lateral and vertical information over a range of 25 μm.

Immobilized proteins in buffer imaged at molecular resolution by atomic force microscopy

Samples of supported planar lipid-protein membranes and actin filaments on mica were imaged by atomic force microscopy (AFM). The samples were fully submerged in buffer at room temperature during imaging. Individual proteins bound to the reconstituted membrane were distinguishable; some structural details could be resolved. Also, surface-induced, self-assembling of actin filaments on mica could be observed. Monomeric subunits were imaged on individual actin filaments. The filaments could be manipulated on or removed from the surface by the tip of the AFM. The process of the decoupling of the filamentous network from the surface upon changing the ionic conditions was imaged in real time.

Imaging and Manipulating Molecules on a Zeolite Surface with an Atomic Force Microscope

The adsorption of neutral molecules and ions on the surfaces of zeolites was observed in real time with an atomic force microscope (AFM). Direct imaging of the surface of the zeolite clinoptilolite was possible by using a diluted tert-butyl ammonium chloride solution as a medium. Images of the crystal in different liquids revealed that molecules could be bound to the surface in different ways; neutral molecules of tert-butanol formed an ordered array, whereas tert-butyl ammonium ions formed clusters. These absorbed molecules were not rearranged by the AFM tip when used in an imaging mode. However, when a sufficiently large force was applied, the tip of the AFM could rearrange the tert-butyl ammonium ions on the zeolite surface. This demonstration of molecular manipulation suggests new applications, including biosensors and lithography.

Atomic force microscopy of hydrated phosphatidylethanolamine bilayers

We present images of the polar or headgroup regions of bilayers of dimyristoyl-phosphatidylethanolamine (DMPE), deposited by Langmuir-Blodgett deposition onto mica substrates at high surface pressures and imaged under water at room temperature with the optical lever atomic force microscope. The lattice structure of DMPE is visualized with sufficient resolution that the location of individual headgroups can be determined. The forces are sufficiently small that the same area can be repeatedly imaged with a minimum of damage. The DMPE molecules in the bilayer appear to have relatively good long-range orientational order, but rather short-range and poor positional order. These results are in good agreement with x-ray measurements of unsupported lipid monolayers on the water surface, and with electron diffraction of adsorbed monolayers.

Wet lipid-protein membranes imaged at submolecular resolution by atomic force microscopy

Abstract We have employed an AFM to determine the structural properties of supported planar membranes and membrane-bound proteins in an aqueous environment. Images of an asymmetric Langmuir Blodgett film of a charged phospholipid show long range positional as well as orientational order; individual headgroups are resolved. In order to study biofunctional membranes we have employed a recently introduced technique that allows the controlled formation of planar lipid-protein membranes on solid supports from a vesicle suspension. Combining this technique with the AFM permits the nondestructive imaging of these models of cell membranes at molecular resolution under physiological conditions of ionic strength and temperature.

Determination of tilted superlattice structure by atomic force microscopy

We have analyzed the structure of tilted superlattices on atomically stepped surfaces by using atomic force microscopy to detect ridges of GaAs formed by the selective oxidation and removal of intervening AlAs regions. Oxides were removed in a liquid cell of the atomic force microscope while scanning. We have demonstrated plan views which reveal the superlattice length and width uniformity, but the method is also in principle suited for cross‐sectional samples.

The atomic force microscope: A tool for science and industry

Abstract Images of graphite and RuCl 3 show that the atomic force microscope (AFM) is capable of imaging rigid samples with atomic resolution. Images of photographic film showing the emulsion demonstrate the potential of the microscope for industrial quality control. An image of a stoma on a leaf shows that the microscope is gentle enough not to damage surfaces, even of soft biological samples.

Progress in sequencing deoxyribonucleic acid with an atomic force microscope

Atomic force microscope (AFM) images of single‐stranded deoxyribonucleic acid (DNA) showing nucleotide resolution have been obtained using two sample preparation methods: DNA covalently attached to a polymerized lipid monolayer and then imaged under water and DNA on mica rinsed with a Ba(NO3)2 solution and then imaged under ethanol. Some of the bases were identified in the AFM image of DNA on polymerized lipid. Current problems in sequencing DNA with the AFM include movement of the DNA during imaging and the difficulty of reproducing experiments.