Small probes

used FRAP to monitor the diffusion of extremely small probes (1 20 nm) in solutions of guar galactomannan and polyethylene oxide(24). 

The probes were labeled dextrans (20,40,70, and 500 kDa), ovalbumin, and sixth 

and 17.9 nm hydrodynamic radius dextrans, and 55 nm polystyrene spheres in aqueous 300 kDa HPC, as obtained with FRAP by Bu and Russo(22). Lines are simple exponentials. 

Phillies reports on the diffusion of bovine serum albumin through solutions of 100 kDa and 300 kDa polyethylene oxides(27). The Dp value depended measurably on the probe concentration. At elevated polymer c and low protein concentration. Dp was as much as a third faster than expected from the c-dependent solution fluidity. With increasing protein concentration. Dp fell toward values expected from the macroscopic r. This study pushed the technical limits of then-current light-scattering instrumentation. 

Ullmann, et a/.(28) studied with QELSS the diffusion of 52, 322, and 655 nm radius polystyrene spheres in solutions of bovine serum albumin (BSA) in 0.15M 

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Small probed regions down to 1-2 pm are possible using microscope lenses. Lasers can supply as much pump power as needed to compensate for weaker signals, but a limit is reached when sample heating or nonlinear optically induced processes become significant. 

Analysis of individual catalyst particles less than IMm in size requires an analytical tool that focuses electrons to a small probe on the specimen. Analytical electron microscopy is usually performed with either a dedicated scanning transmission electron microscope (STEM) or a conventional transmission electron microscope (TEM) with a STEM attachment. These instruments produce 1 to 50nm diameter electron probes that can be scanned across a thin specimen to form an image or stopped on an image feature to perform an analysis. In most cases, an electron beam current of about 1 nanoampere is required to produce an analytical signal in a reasonable time. 

This kind of analysis is most effectively conducted in a dedicated FEGSTEM instrument, which has an increased beam current while maintaining a small probe size. Such instruments also permit the detection of light elements such as B at boundaries. It has become possible to map the distribution of grain-boundary... 

Small probes based on conformational or structural changes... 

It is generally accepted that localization and coordination of monovalent Cu ions in different zeolites have significant influence on the catalytic activity. The localization and coordination of Cu ions was studied by means of adsorption of small probe molecules, in particular, carbon monoxide was used often due to its ability to form a stable mono-carbonyl complex with the Cu+ ion. The formation of this complex was investigated by the FTIR and by the microcalorimetry [1-3]. 

L. Bokobza, C. Pham-Van-Cang, C. Giordano, L. Monnerie, J. Vandendriessche and F. C. De Schryver, Relation between excimer formation in small probes and free-volume theory in polymer melts, Polymer 28, 1876 (1987). 

CBED is formed by focusing electrons to form a small probe at the specimen (fig. 1). Compared to selected area electron diffraction, CBED has two main advantages for studying perfect crystals and the local structure ... 

For convergent electron beams with a sufficiently small probe, the diffraction geometry can be approximated by a parallel crystal slab whose surface normal direction is n. To satisfy the boundary condition, letting... 

Measure the intensity of light by "looking through the wall" or measure conductivity with a small probe, etc. 

Figure 3.9. Schematic representation of a ZFC relaxation experiment. The sample is cooled to the measurement temperature under zero field, after a wait time a small probing field h is applied and the ZFC magnetization is recorded as a function of time.

HREM methods are powerful in the study of nanometre-sized metal particles dispersed on ceramic oxides or any other suitable substrate. In many catalytic processes employing supported metallic catalysts, it has been established that the catalytic properties of some structure-sensitive catalysts are enhanced with a decrease in particle size. For example, the rate of CO decomposition on Pd/mica is shown to increase five-fold when the Pd particle sizes are reduced from 5 to 2 nm. A similar size dependence has been observed for Ni/mica. It is, therefore, necessary to observe the particles at very high resolution, coupled with a small-probe high-precision micro- or nanocomposition analysis and micro- or nanodiffraction where possible. Advanced FE-(S)TEM instruments are particularly effective for composition analysis and diffraction on the nanoscale. ED patterns from particles of diameter of 1 nm or less are now possible. 

Fig. 1.4 Diagram showing the principle of operation of a time-of-flight atom-probe. The tip is mounted on either an internal or an external gimbal system. The tip orientation is adjusted so that atoms of one s choice for chemical analysis will have their images falling into the small probe-hole at the screen assembly. By pulse field evaporating surface atoms, these atoms, in the form of ions, will pass through the probe hole into the flight tube, and be detected by the ion detector. From their times of flight, their mass-to-charge ratios are calculated, and thus their chemical species identified.

Scanning tunneling (STM) was invented a decade ago by Binnig and Rohrer [72], and was first applied to the solid-liquid interface by Sonnenfeld and Hansma in 1986 [73]. Since then, there have been numerous applications of STM to in situ electrochemical experiments [74-76]. Because the STM method is based on tunneling currents between the surface and an extremely small probe tip, the sample must be reasonably conductive. Hence, STM is particularly suited to investigations of redox and conducting polymer-modified electrodes [76,77],... 

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