Wilson Atomic Force Microscopy in Cell Biology

1. Auflage 2002
ISBN: 978-0-08-054944-6
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, Band Volume 68, 415 Seiten

Reihe: Methods in Cell Biology

This is the first book to cover the history, structure, and application of atomic force microscopy in cell biology. Presented in the clear, well-illustrated style of the Methods in Cell Biology series, it introduces the AFM to its readers and enables them to tap the power and scope of this technology to further their own research. A practical laboratory guide for use of the atomic force and photonic force microscopes, it provides updated technology and methods in force spectroscopy. It is also a comprehensive and easy-to-follow practical laboratory guide for the use of the AFM and PFM in biological research.

Autoren/Hrsg.

Wilson, Leslie

Leseproben

Chapter 1 Local Probe Techniques
J.K. Heinrich Hörber    EMEL Meyerhofsrrasse 1 69117 Heidelberg, Germany I Introduction
About 400 years ago, the invention of telescopes and microscopes not only extended our sense of seeing but also revolutionized our perception of the world. Extending this perception further and further has since been the driving force for major scientific developments. Local probe techniques extend our sense of touching into the micro- and nanoworld and in this way provide complementary new insight into these worlds with microscopic techniques. Furthermore, touching things is an essential prerequisite to manipulating things, and the ability to feel and to manipulate single molecules and atoms certainly marks another of these revolutionizing steps in our relation to the world we live in. Local probes are small objects, e.g., the very end of sharp tips, whose interactions with a sample, or better, the surface of a sample, can be sensed at selected positions. Proximity to or contact with the sample is required for high spatial resolution. This, in principle, is an old idea that appeared in literature from time to time, in context with bringing a source of electromagnetic radiation into close contact with a sample (Synge, 1928; O'Keefe, 1956; Ash and Nicolls, 1972) yet found no resonance and therefore was not pursued until recently. Nanoscale local probes require atomically stable tips and high-precision manipulation devices. The latter, based on mechanical deformations of spring-like structures by given forces—piezoelectric, mechanical, electrostatic, or magnetic—to ensure continuous and reproducible displacements with precision down to the picometer level, also require very good vibration isolation. The resolution that can be achieved with local probes is mainly determined by the effective probe size, its distance from the sample, and the distance dependence on the interaction between the probes and the samples measured. The latter can be considered to create an effective aperture by selecting a small feature of the overall geometry of the probe tip, which then corresponds to the effective probe. The first of these local probe instruments was the scanning tunneling microscope (STM), which emerged during the early 1980s as a response to an issue in semicon-ductor technology (Binnig et al., 1982). Inbomogeneities on the nanometer scale had become increasingly important as miniaturization of electronic devices progressed. The STM is an electronic-mechanical hybrid. The probe positioning is mechanics, whereas the interaction sensed by the tunneling current between probe and sample is of quantum mechanical origin. The physical effect of electron tunneling describes the strongly distant-dependent probability of electrons to cross a gap between two conducting solids before they really form a contact. The STM for the first time showed the atomic structure at the crystalline surface of silicon in real space and demonstrated that it was even possible to manipulate single atoms. The importance of this development was recognized when the Nobel Prize in Physics was awarded to Binnig and Rohrer in 1986. In 1986, Bimrig together with Quate and Gerber demonstrated that the short-range van der Waals interaction can also be used to build a scanning probe microscope (Binnig et al., 1986). This new device was called the atomic force microscope (AFM). With no electron transport involved, even insulators could be studied down to atomic resolution. The essential part of an AFM, as for all scanning probe microscopes, is the tip that determines by its structure the type of interaction with a surface; and by its geometry, the area of interaction. The original idea for the AFM was to measure the van der Waals interaction of an atom at the very end of the tip with atoms at a surface of a solid substrate. To bring a single atom at a tip close to within angstrom distance toward a surface is only possible if the surface is atomically flat (Fig. 1c), such as, for example, the crystalline surface of mica. If the surface is rough on a nanometer scale (Fig. 1b), groups of atoms can interact and determine, according to their size, the possible resolution. With a roughness at the micrometer scale (Fig. 1a) the macroscopic level is reached where instruments like the surface profiler are able to measure surface roughness. A similarly important part of the scanning probe microscope is the mechanism which moves the tip closer to the surface and scans it across with precision fitting to the highest resolution. What enables such precise manipulation is the property of some materials to change size proportional to an applied electric field. These materials can also generate an electric field if a force is applied, an effect first described by Pierre and Jacques Curie in 1880 for quartz. The piezo-tube scanner is widely used to produce movements in all three directions easily and consists of a thin-walled hard piezo-electric ceramic that is radially polarized. Electrodes are attached to the internal and external faces of the tube. The external electrode is split into quarters parallel to the axis as shown in Fig. 2. By applying a voltage between the inner and all the outer electrodes, the tube expands or contracts and in this way either moves a tip closer to a surface or retracts it from a surface, respectively. If the voltage is applied just between the inner and one outer electrode, the tube will bend, i.e., moving the tip along the surface, with a precision determined by both the noise of the voltage source used and the overall mechanical stability. The disadvantage of these piezo-tubes is that the tip is not scanned exactly parallel to the surface but is moved on an arc, leading to an effect known as “eyeballing” when large scans are carried out. Another problem of piezo-materials is the hysteresis, which like the arc motion must be corrected by the electronic equipment controlling the movement by providing the necessary voltage. Fig. 1 Scanning probe tip structures shown at different scales. Fig. 2 Piezo-electtic effect of quartz and the piezo-ceramic tube scanner with inner and segmented outer electrodes used in scanning probe microscopes. In the meantime, many other types of scanning probe microscopes using various types of interactions have been developed and are too numerous to mention in this short introduction. I prefer, therefore, to name only one other: the scanning neat-field optical microscope (SNOM), developed by Pohl et al. (1988) which is, as the name implies, the near-field equivalent to the conventional optical microscope working in the farfield of the radiation. The STM, on the other hand, can be seen as the nearfield equivalent to the electron microscope. The optical microscope, like other types of microscopes using radiation in the fat-held range, is limited in its resolution by the wavelength of the radiation. This limit, reported by Abbe in 1873, restricts the optimal resolution to several hundred nanometers for using visible light. The only way of overcoming this limit is by using nearfield effects observed within a wavelength from a radiation source. In high resolution, the very small tip can be used again. The tip of a SNOM is, at least in many instruments, a specially prepared end of an optical fiber, which acts as a light source. The interaction of the electromagnetic nearfield at the tip with the surface determines how much light is radiated from the source and how much is reflected back into the optical fiber. In this way the aspects of the surface structure correlated to the interaction with electromagnetic fields can be studied. Many types of scanning probe microscopes have been developed and can be used not only for measuring surface topologies but also for measuring various material properties at or close to surfaces. This can be done in vacuum, in gas, or in liquids in a broad temperature range with a resolution down to either the atomic or the molecular level. In this way, it is the only type of microscopy that can complement optical microscopy in biology on a smaller scale. Additionally, these instruments allow manipulations at either the single-atomic or the molecular level, making experiments which no one ever dreamed of 20 years ago possible. Experiments at the nanometer scale provide a complete new insight into processes which, before the development of these instruments, were accessible only by ensemble-average processes, where all of the elements can never be identical, and all of the information concerning the behavior of individuals is lost. With the available information on single components using scanning probe techniques we can now learn how processes, which we were previously unaware of, are determined by the properties of the single elements of such ensembles. II Scanning Tunneling Microscopy
It is of particular interest to understand the images of biological structures obtained by the STM, as this technique allows imaging with a signal-to-noise ratio unequalled by other techniques and under near-physiological conditions (Hörber et al., 1988; Hörber, Schuler, Witzemann, Schröter et al., 1991; Hörber, Schuler, Witzemann, Müler et al., 1991; Heckl et al., 1989; Ruppersberg et al., 1989; Göbel et al., 1992; Maaloum et al., 1994). This is an advantage that can only be exploited by having a deeper knowledge...