Atomic force microscope measurements and manipulation of Langmuir-Blodgett films with modified tips

Analytical approaches to study domain formation in biomimetic membranes

Novel characterization methods open new horizons in the study of membrane mixtures.

S-Layer-Based Nanocomposites for Industrial Applications

This chapter covers the fundamental aspects of bacterial S-layers: what are S-layers, what is known about them, and what are their main features that makes them so interesting for the production of nanostructures. After a detailed introduction of the paracrystalline protein lattices formed by S-layer systems in nature the chapter explores the engineering of S-layer-based materials. How can S-layers be used to produce "industry-ready" nanoscale bio-composite materials, and which kinds of nanomaterials are possible (e.g., nanoparticle synthesis, nanoparticle immobilization, and multifunctional coatings)? What are the advantages and disadvantages of S-layer-based composite materials? Finally, the chapter highlights the potential of these innovative bacterial biomolecules for future technologies in the fields of metal filtration, catalysis, and bio-functionalization.

Visualizing the bacterial cell surface: an overview

The ultrastructure of bacteria is only accessible by electron microscopy. Our insights into the architecture of cells and cellular compartments such as the envelope and appendages is thus dependent on the progress of preparative and imaging techniques in electron microscopy. Here, I give a short overview of the development and characteristics of methods applied for imaging (components of) the bacterial surface and refer to key investigations and exemplary results. In the beginning of electron microscopy, fixation of biological material and staining for contrast enhancement were the standard techniques. The results from freeze-etching, metal shadowing and from ultrathin-sections of plastic-embedded material shaped our view of the cellular organization of bacteria. The introduction of cryo-preparations, keeping samples in their natural environment, and three-dimensional (3D) electron microscopy of isolated protein complexes and intact cells opened the door to a new dimension and has provided insight into the native structure of macromolecules and the in situ organization of cells at molecular resolution. Cryo-electron microscopy of single particles, together with other methods of structure determination, and cellular cryo-electron tomography will provide us with a quasi-atomic model of the bacterial cell surface in the years to come.

Intermittent contact hydration scanning probe microscopy

Hydration scanning probe microscopy is a technique similar to scanning tunneling microscopy, in which the probe current, sustained by the slight surface conduction of a thin hydration layer covering an insulating support surface, is essentially electrochemical in nature instead of electronic tunneling. Such a technique allows the imaging of a great variety of samples, including insulators, provided that they are hydrophilic, as well as the study of molecular samples of biological interest (such as DNA) fixed on a suitable supporting surface. The main problem to obtain stable and reproducible images comes from the very critical determination of the operating conditions under which the probe-hydration layer interaction does not lead to the formation of a relatively large water meniscus. It has been suggested that this issue can be removed by adding a high frequency oscillation to the probe movement, as in tapping atomic force microscopy. Meniscus formation and breakup have been investigated in order to determine the best values for the amplitude and the frequency of the oscillation. Results obtained in this mode are discussed in comparison with the usual continuous contact mode.

Single Polymer Chains as Specific Transducers of Molecular Recognition in Scanning Probe Microscopy

A new approach for the specific detection and mapping of single molecule recognition is presented, based on the nonlinear elastic behavior of a single polymer chain. The process of molecular recognition between a ligand and a receptor is inherently accompanied by a decrease in the translational and rotational degrees of freedom of the two molecules. We show that a polymeric tether linked to the ligand can effectively transduce the configurational constraint imposed by molecular recognition into a measurable force, which is dominated by the entropic elasticity of the polymer. This force is specifically characterized by a strong nonlinearity when the extension of the polymer approaches its contour length. Thus, a polymer chain tethering the ligand to an oscillating cantilevered tip gives rise to a highly anharmonic motion upon ligand-receptor binding. Higher-harmonics atomic force microscopy allows us to detect this phenomenon in real time as a specific signature for the probing and mapping of single-molecule recognition.

Advances in the characterization of supported lipid films with the atomic force microscope

During the past decade, the atomic force microscope (AFM) has become a key technique in biochemistry and biophysics to characterize supported lipid films, as testified by the continuous growth in the number of papers published in the field. The unique capabilities of AFM are: (i) capacity to probe, in real time and in aqueous environment, the surface structure of lipid films; (ii) ability to directly measure physical properties at high spatial resolution; (iii) possibility to modify the film structure and biophysical processes in a controlled way. Such experiments, published up to June 2000, are the focus of the present review. First, we provide a general introduction on the preparation and characterization of supported lipid films as well as on the principles of AFM. The section 'Structural properties' focuses on the various applications of AFM for characterizing the structure of supported lipid films: visualization of molecular structure, formation of structural defects, effect of external agents, formation of supported films, organization of phase-separated films (coexistence region, mixed films) and, finally, the use of supported lipid bilayers for anchoring biomolecules such as DNA, enzymes and crystalline protein arrays. The section 'Physical properties' introduces the principles of force measurements by AFM, interpretation of these measurements and their recent application to supported lipid films and related structures. Finally, we highlight the major achievements brought by the technique and some of the current limitations.

Pushing, pulling, dragging, and vibrating renal epithelia by using atomic force microscopy

Renal physiologists focus on events that take place on and around the surfaces of cells. Various techniques have been developed that follow transport functions at the molecular level, but until recently none of these techniques has been capable of making the behavior of molecular structures visible under physiological conditions. This apparent gap may be filled in the future by the application of atomic force microscopy. This technique produces an image not by optical means, but by "feeling" its way across a surface. Atomic force microscopy can, however, be modified in a number of ways, which means that besides producing a high-resolution image, it is possible to obtain several types of data on the interactions between the ultrastructural components of cell membranes (such as proteins) and other biologically active molecules (such as ATP). In this review we describe the recent use of the atomic force microscope in renal physiology, ranging from experiments in intact cells to those in isolated renal transport protein molecules, include examples of these extended applications of the technique, and point to uses that the microscope has recently found in other areas of biology that should prove fruitful in renal physiology in the near future.

Comprehensive surface analysis of hydrophobically functionalized SFM tips

Tip-sample interactions have been of interest since the early development of the scanning force microscope. Investigations of interfacial interactions at the molecular level are of importance for fundamental studies of bi-molecular interactions and for possible applications in biomedical research and industrial settings. By engineering the surface chemical properties of the SFM probes, specific force interactions may be measured. However, as these modification schemes become more widely applied, detailed chemical analysis of the modified cantilever surfaces becomes crucial. In this paper, we describe two approaches to coat SFM cantilevers with hydrophobic coatings: a silanization protocol and ratio frequency plasma enhanced chemical vapor deposition.

Discrimination of DNA hybridization using chemical force microscopy

Atomic force microscopy (AFM) can be used to probe the mechanics of molecular recognition between surfaces. In the application known as "chemical force" microscopy (CFM), a chemically modified AFM tip probes a surface through chemical recognition. When modified with a biological ligand or receptor, the AFM tip can discriminate between its biological binding partner and other molecules on a heterogeneous substrate. The strength of the interaction between the modified tip and the substrate is governed by the molecular affinity. We have used CFM to probe the interactions between short segments of single-strand DNA (oligonucleotides). First, a latex microparticle was modified with the sequence 3'-CAGTTCTACGATGGCAAGTC and epoxied to a standard AFM cantilever. This DNA-modified probe was then used to scan substrates containing the complementary sequence 5'-GTCAAGATGCTACCGTTCAG. These substrates consisted of micron-scale, patterned arrays of one or more distinct oligonucleotides. A strong friction interaction was measured between the modified tip and both elements of surface-bound DNA. Complementary oligonucleotides exhibited a stronger friction than the noncomplementary sequences within the patterned array. The friction force correlated with the measured strength of adhesion (rupture force) for the tip- and array-bound oligonucleotides. This result is consistent with the formation of a greater number of hydrogen bonds for the complementary sequence, suggesting that the friction arises from a sequence-specific interaction (hybridization) of the tip and surface DNA.

Submicron structure in L-alpha-dipalmitoylphosphatidylcholine monolayers and bilayers probed with confocal, atomic force, and near-field microscopy

Langmuir-Blodgett (LB) monolayers and bilayers of L-alpha-dipalmitoylphosphatidylcholine (DPPC), fluorescently doped with 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (diIC18), are studied by confocal microscopy, atomic force microscopy (AFM), and near-field scanning optical microscopy (NSOM). Beyond the resolution limit of confocal microscopy, both AFM and NSOM measurements of mica-supported lipid monolayers reveal small domains on the submicron scale. In the NSOM studies, simultaneous high-resolution fluorescence and topography measurements of these structures confirm that they arise from coexisting liquid condensed (LC) and liquid expanded (LE) lipid phases, and not defects in the monolayer. AFM studies of bilayers formed by a combination of LB dipping and Langmuir-Schaefer monolayer transfer exhibit complex surface topographies that reflect a convolution of the phase structure present in each of the individual monolayers. NSOM fluorescence measurements, however, are able to resolve the underlying lipid domains from each side of the bilayer and show that they are qualitatively similar to those observed in the monolayers. The observation of the small lipid domains in these bilayers is beyond the spatial resolving power of confocal microscopy and is complicated in the topography measurements taken with AFM, illustrating the utility of NSOM for these types of studies. The data suggest that the small LC and LE lipid domains are formed after lipid transfer to the substrate through a dewetting mechanism. The possible extension of these measurements to probing for lipid phase domains in natural biomembranes is discussed.

Scanning probe microscopy for the characterization of biomaterials and biological interactions

The scanning probe microscopies provide a unique view of biological and biomedical systems at a nanoscale appropriate to appreciate molecular events. The advent of these methods has brought the ability to acquire quantitative information at the molecular level. Given the proliferation of microscopes and associated methods, the probability for important discoveries is high. If tempered with an appreciation for the potential for artifacts, the SPMs may revolutionize our view of biological systems and biomaterials interactions with those systems.