Since the creation of atomic force microscopy (AFM), there has been a continual trend to transform what is essentially surface-characterization tool into an instrument for addressing analytical problems. Efforts have focused on the instrumental development of a variety of operating modes as well as the subsequent chemical or biochemical enhancement of these modes. The selectivity of many of the AFM modes has been enhanced by the use of chemically, electrochemically, biomolecularly, and even biologically modified AFM probes. Technical and scientific motivations associated with these modified probes have led to the recent development of an inverted AFM design and its subsequent use for a new enhanced mode of operation called combinatorial atomic force microscopy. This paper presents an overview of the research performed in our laboratory toward the development of combinatorial atomic force microscopy. The current state of the art is described and examined with the aim of determining the plausibility of developing this technique as an analytical tool.

Atomic force microscopy (AFM) has been used to study a wide range of systems. Chemically and biologically modified probes have extended AFM by coupling chemical and biological information with the physical measurements. In an effort to further expand the capabilities of modified AFM probes, previous studies investigated the use of an inverted AFM design (i-AFM), wherein a microfabricated tip array is used to image a cantilever-supported sample. This report details developments in cantilever and tip array fabrication which are aimed at improving the applicability and performance of this i-AFM design. Using an epoxy-based procedure, commercial cantilevers were modified with a series of standard substrates, including template-stripped gold, highly oriented pyrolytic graphite, and mica. The samples on these cantilevers were imaged with i-AFM, and lateral force images are obtained. This paper demonstrates the first use of i-AFM for measuring friction.

Interactions between molecules and at interfaces are vital to many scientific and technological fields. Techniques such as atomic force microscopy (AFM) have been used to measure forces and force gradients associated with interactions between individual molecules as well as interactions between interfaces. A recent alteration of the AFM configuration combined a tipless cantilever with an array of substrate supported tips. Herein, we present a further extension of AFM force measurement capabilities, by chemically patterning both the cantilever and the tip array. In a proof-of-concept experiment a gold-coated cantilever and tip array were patterning with alkylthiolate monolayers, and the interfacial forces were measured for the various combinations. This patterning allows many different interactions to be rapidly measured in situ, under identical conditions, thereby improving reliability and opening the door to combinatorial applications. Future developments are discussed including the means to measure hundreds of different interactions.

Several developing technologies rely upon the activity of surface-bound biomolecules to control the functionality of an interface. The specificity of these biomolecules can be compromised by mechanical stresses present during their preparation or use. Moreover, many new biomechanical devices are also sensitive to mechanical stress. This paper is concerned with the direct assessment of the mechanical limits beyond which biomolecules are rendered inactive. These mechanical limits are evaluated by a combined approach involving an atomic force microscope (AFM), an optical microscope, and standard colorimetric techniques. The AFM is used to systematically stress micrometer-sized domains of biomolecule-laden surfaces; the results of which can be directly observed with an optical microscope following histochemical staining of the surface with biologically specific dyes. Using this method, we examined the limiting conditions of two benchmark systems, i.e., streptavidin and DNA. Because of the breadth of histochemical staining combined with the nature of the study, our approach is equally applicable to a wide range of biomolecular systems, perhaps even including the surfaces of living cells.

Variability in the coverage or usable lifetime of active molecules at the apex of an atomic force microscope (AFM) tip is a key limitation to biomolecular force measurements with AFM. Microfabricated tip arrays make it possible to measure molecular forces between large arrays of biological molecules with AFM. The forces are measured between a probeless microfabricated cantilever and a microfabricated array containing approximately 105 addressable probes with variable radii. We measure intermolecular forces between the model ligand–receptor pair streptavidin–biotin, to demonstrate that these tip arrays can circumvent these coverage and lifetime obstacles. Further development of these arrays promises to provide a means for measuring millions of different intermolecular interactions, paving the way for AFM to be realistically applied to screen combinatorial libraries.

This paper reports on the use of a scanning force microscope (SFM) for the tip-assisted base hydrolysis of an ester-terminated alkanethiolate monolayer on Au(111). We have found that contact imaging accelerates the base hydrolysis of a dithiobis(succinimido undecanoate) monolayer relative to the surrounding unimaged area. It is proposed that (1) the mechanical disruption by the SFM probe tip of the steric barrier imposed by the neighboring adsorbates facilitates access of hydroxide ions to the buried acyl carbons in the adlayer, and (2) the surface area hydrolytically transformed by this disruption can be controlled by the SFM imaging conditions, Findings in support of our conclusions are presented, and potential implications to nanotechnology are briefly discussed.

During the past year, scanning probe microscopy, especially atomic force microscopy (AFM), has taken root in the biological sciences community, as is evident from the large number of publications and from the variety of specialized journals in which these papers appear. Furthermore, there is a strong indication that the technique is evolving from a qualitative imaging tool to a probe of the critical dimensions and properties of biomolecules and living cells. The next stage of the evolution involves the development of microinstruments for process control and sensing applications. Recent advances have been reported in AFM instrumentation and method. For example, the tapping mode of operation is becoming the method of choice to image biological molecules; work to extend tapping-mode operation in liquids has been reported. Biological molecules can also be imaged at low temperature in a cryo-AFM with improved resolution. The measurement of recognition forces between individual molecules continues to attract much attention and has spawned new concepts for ultra-sensitive biosensors. The AFM is being used increasingly for property measurements such as determining the viscoelastic properties of biological molecules. Finally, structural studies using the AFM abound. Some specific highlights include the mapping of DNA using restriction enzymes, imaging during DNA transcription and determining the mode of drug binding to DNA.

Mixed distearoylphosphatidylethanol amine (DSPE) and dioleoylphosphatidylethanolamine (DOPE) monolayers and bilayers have been deposited on mica using the Langmuir-Blodgett (LB) technique, as a model system for biomembranes. Investigation with atomic force microscopy revealed phase-separation for both monolayers in air and bilayers in water in the form of microscopic DSPE domains embedded in a DOPE matrix. For the monolayers in air, the step height measured between the higher DSPE phase and the lower DOPE phase was larger than expected from the molecular lengths, and a significant contrast in adhesion and friction was observed despite identical lipid end groups. This unexpected behavior resulted primarily from a difference in the film mechanical properties, the DOPE phase being inelastically deformed by the probe. For the bilayers in water, similar trends were found in terms of height, adhesion, and friction, but an additional short-range repulsive hydration/steric force over the DSPE phase contributed to the observed differences.

This paper demonstrates the real lime monitoring of the electrochemical transformation of a surface-bound redox species using AFM-based adhesion measurements, The measurements were conducted using a monolayer formed by the chemisorption of 11-mercaptoundecyl ferrocenecarboxylate (FcT) at a Au(111) electrode and a gold-coated probe tip modified with an octadecanethiolate (OT) monolayer. Using this probe tip-sample combination, the one-electron oxidation of the ferrocenyl group of the FcT monolayer results in a decrease in the observed force of adhesion (F-ad) at the microcontact formed by the two different surfaces. Using surface tension arguments, this change is attributed to the decrease in the effective miscibility at the microcontact as a consequence of the oxidation of the ferrocenyl group to the ferrocenium ion. Issues related to the underlying changes in the interfacial structure that give rise to the differences in F-ad are discussed.

This paper demonstrates the ability to map chemically distinct domains at nanometer length scales using frictional force microscopy (FFM). The basis of this characterization is the dependence of the frictional interactions on the identity of the chemical functional groups at the outermost few angstroms of microscopic contacting areas, i.e., the surface free energies. Such a dependence is confirmed by characterizations of the frictional force between a variety of end-group-derivatized alkanethiolate monolayers deposited at both gold-coated sample substrates and gold-coated FFM probe tips. Coupled with this dependence, we show that the composition of chemically distinct domains at partially formed bilayer structures can be mapped at a spatial resolution of similar to 10 nm, with the image contrast governed by the surface free energies of the microscopic contacts. Opportunities presented by these findings are briefly discussed.