Probing species

In addition to the many applications of SERS, Raman spectroscopy is, in general, a usefiil analytical tool having many applications in surface science. One interesting example is that of carbon surfaces which do not support SERS. Raman spectroscopy of carbon surfaces provides insight into two important aspects. First, Raman spectral features correlate with the electrochemical reactivity of carbon surfaces this allows one to study surface oxidation [155]. Second, Raman spectroscopy can probe species at carbon surfaces which may account for the highly variable behaviour of carbon materials [155]. Another application to surfaces is the use... 

The refractive index of a medium is the ratio of the speed of light in a vacuum to its speed in the medium, and is the square root of the relative permittivity of the medium at that frequency. When measured with visible light, the refractive index is related to the electronic polarizability of the medium. Solvents with high refractive indexes, such as aromatic solvents, should be capable of strong dispersion interactions. Unlike the other measures described here, the refractive index is a property of the pure liquid without the perturbation generated by the addition of a probe species. 

The OMRC does not account for the orientation or structure of the probe species, nor does it account for the interconnectedness (i.e., the possibility of percolation in the gel whereby certain regions of the gel may be inaccessible to a particular species) of the matrix [361]. The OMRC also assumes a uniform electric field and does not consider the effects of the gel on the electric field. 

Comments There are several suggested controls for this assay, including use of yeast total RNA as a negative control (check for probe species specificity) and a no RNAse control to determine probe stability. In Fig. 6.3A, the positive control marker lane was produced by addition of R-luc-4 sites or F-luc mRNA only to the assay. Also, optimal times for RNAse digestion will vary from probe to probe. In addition, for maximum sensitivity a probe with high specific activity is preferable (yet still in molar excess to the mRNA). 

By choosing a standard probe species i (e.g., a helium atom) of known van der Waals radius, one can probe the steric surface of any doubly occupied NBO j by determining the locus of points for which achieves a specified energy... 

Many probes are now known that display changes in fluorescence lifetime on complexation of the analyte, photophysical properties some of them are summarized in Table 10.2. While we have listed the lifetimes of the free and the bound forms of the probes, there is no straightforward equation to calculate the analyte concentration using the mean lifetime as was in the case of the absorbance and intensity (Eqs. (10.14) and (10.15)). The mean lifetime depends not only on relative concentration of the probe species (free and complexed) but also on their decay times, quantum yields, and to some extent on the measurement (method or conditions). While the mean lifetime is independent of total probe concentration, this value generally depends not only on analyte concentration but also on excitation and observation wavelengths.03 ... 

Nonlinear optical spectroscopies such as second harmonic generation (SHG) and sum frequency generation (SFG) are finding increasing use in probing species at interfaces (e.g., Eisenthal, 1996). For example, SHG was used by Donaldson et al. (1995) to detect a surface-bound S02 species, and SFG has been applied to elucidate the structure of dimethyl sulfoxide at liquid interfaces (Allen et al., 1999). These techiques are... 

The probe species is often radioactively labeled, or it may carry a fluorescent tag, or some other chemical or enzymatic moiety to generate a positional signal. For radioactive labeling, a common choice of radioisotope is phosphorus-32 (or 32P), because it can be incorporated as phosphate into DNA or RNA relatively easily, and it emits energetic beta particles that are easy to detect. The radioactivity on the membrane can be used to expose an adjacent x-ray film in a pattern corresponding to the radioactive spots on the membrane. After a suitable exposure time, one develops the film and studies the location and intensity of the images of the radioactive spots to deduce the position and degree of probe hybridization on the membrane. 

Previous studies on paraffins, rhodamine dyes, and l,3-bis(N-carbozoyl) propane excimers have concluded that there is a relationship between km and polymer viscosity and free volume [103-105], Indeed, this dependence has been investigated in the context of decreasing free volume during methyl methacrylate polymerization [83,84], It has been shown that the nonradiative decay processes follow an exponential relationship with polymer free volume (vf), in which kra reduces as free volume is decreased [see Eq. (5)]. Here, k. represents the intrinsic rate of molecular nonradiative relaxation, v0 is the van der Waals volume of the probe molecule, and b is a constant that is particular to the probe species. Clearly, the experimentally observed changes in both emission intensity and lifetime for/ac-ClRe(CO)3(4,7-Ph2-phen) in the TMPTA/PMMA thin film are entirely consistent with this rationale. 

Recent advances in the development of non-invasive, in situ spectroscopic scanned-probe and microscopy techniques have been applied successfully to study mineral particles in aqueous suspension (Hawthorne, 1988 Hochella and White, 1990). In situ spectroscopic methods often utilise molecular probes that have diagnostic properties sensitive to changes in short-range molecular environments. At the particle-solution interface, the molecular environment around a probe species is perturbed, and the diagnostic properties of the probe, which can be either optical or magnetic, then report back on surface molecular structure. Examples of in situ probe approaches that have been used fruitfully include electron spin resonance (ESR) and nuclear magnetic resonance (NMR) spin-probe studies perturbed vibrational probe (Raman and Fourier-transform IR) studies and X-ray absorption (Hawthorne, 1988 Hochella and White, 1990 Charletand Manceau, 1993 Johnston et al., 1993). 

Electron spin resonance (ESR) studies of radical probe species also suggest complexity. Evans et al. [250] study the temperature dependence of IL viscosity and the diffusion of probe molecules in a series of dissimilar IL solvents. The results indicate that, at least over the temperature range studied, the activation energy for viscous flow of the liquid correlates well with the activation energies for both translational and rotational diffusion, indicative of Stoke-Einstein and Debye-Stokes-Einstein diffusion, respectively. Where exceptions to these trends are noted, they appear to be associated with structural inhomogeneity in the solvent. However, Strehmel and co-workers [251] take a different approach, and use ESR to study the behavior of spin probes in a homologous series of ILs. In these studies, comparisons of viscosity and probe dynamics across different (but structurally similar) ILs do not lead to a Stokes-Einstein correlation between viscosity and solute diffusion. Since the capacities for specific interactions are... 

There is a problem in the investigation of catalysts. The chemisorption experiments carried out in laboratories are performed under high vacuum, whereas the real catalysts work under high pressure to catalyze chemical reactions this may well result in different behavior. Recently, high-pressure adsorption cells and instruments using photons to probe species have been developed to examine catalysts in their original conditions. 

In heterogeneous electron transfer experiments, the gold electrode with a monolayer film is placed in contact with the solution containing a redox couple [such as Fe(CN)63-/Fe(CN)64- etc.]60-63. The shape of the cyclic voltammogram depends on how effectively the monolayer blocks access for the redox probe to the electrode surface. This method is therefore invaluable for permeability studies. Absence of redox waves close to the formal potential of the probe indicates that the monolayer is completely impermeable for redox probe species. On the other hand, the presence of a redox wave shows either that the monolayer is loosely packed and easily penetrated by external molecules, or the presence of numerous defects and pin-holes in the monolayer. 

In the above analysis, the pump frequency (cOp) and the Stokes frequency (coj have been assumed to be ideally monochromatic, which is applicable when each vibration-rotation line is scanned. In a broadband CARS system as described here the Stokes (dye) laser has a broad spectral profile so that the multiplexed CARS spectral profile of the probed species, is generated with each laser pulse. 

The application of PFG NMR to chemical reactions in zeolite catalysts is not necessarily confined to the observation of the reactant and product molecules. By monitoring the diffusivity of an inert molecule it is also possible directly to trace any changes in the transport properties of the catalyst during the reaction. As an example of this procedure, in [236] tetrafluoro-methane is used to follow the transport inhibition within ZSM-5 during ethene conversion. In these studies, an H-ZSM-5 zeolite was loaded with 4 CF4 and 12 ethene molecules per unit cell and kept at a temperature of 343 K. The representation of the results in Fig. 22 shows that the mobility of the probe species drops by a factor of 6 during the first 3 hours of reaction and then remains relatively constant. Obviously, ethene conversion leads to the formation of larger compounds, which more effectively reduce the mobility of the probe molecules than is the case with the ethene molecules. 

With attenuated total reflection spectroscopy, the light absorption by the electrolyte solution and the cell window is no obstacle. The probe beam enters a crystal transparent for infrared light. It is directed to the outer surface of the crystal, which is coated with a thin layer of the electrode material under investigation. The beam is reflected, but a small part (the evanescent wave) penetrates the surface and thus can probe species located immediately on the electrode surface. The returning beam contains exactly this information. As discussed below (p. 91) in detail, this approach shows also serious limitations. 

The kinetic order of the desorption process provides useful information as to the manner in which probe species interact and are removed from silanols on the surface. Provided in Figure 32.8 and Figure 32.9 are respective first and second order plots for the desorption data of the 1445 cm peak area of pyridine as it was removed from the surface of the Davisil sample pretreated... 

When is a probe species dilute Dp is the probe diffusion coefficient at near-zero probe concentration. If Dp is substantially independent of Cp, no extrapolation 0 is needed. 

---