Scattered evanescent waves

The dynamics of polymers at surfaces can be studied via dynamic light scattering (DLS), as described in Section IV-3C. A modification of surface DLS using an evanescent wave to probe the solution in a region near the interface has... 

Figure C 1.5.6. Single Ag nanoparticles imaged with evanescent-wave excitation. (A) Unfiltered photograph showing scattered laser light (514.5 nm) from Ag particles immobilized on a polylysine-coated surface. (B) Bandpass filtered (540-580 nm) photograph taken from a blank Ag colloid sample incubated witli 1 mM NaCl and...

In these sensors, the intrinsic absorption of the analyte is measured directly. No indicator chemistry is involved. Thus, it is more a kind of remote spectroscopy, except that the instrument comes to the sample (rather than the sample to the instrument or cuvette). Numerous geometries have been designed for plain fiber chemical sensors, all kinds of spectroscopies (from IR to mid-IR and visible to the UV from Raman to light scatter, and from fluorescence and phosphorescence intensity to the respective decay times) have been exploited, and more sophisticated methods including evanescent wave spectroscopy and surface plasmon resonance have been applied. 

Another complication in the quantitation of TIRF on cells is the effect of the membrane thickness itself on the profile of the evanescent wave. Reichert and Truskey<105) have calculated that, in theory, the thickness of the membrane should have a negligible effect on the fluorescence and that a simplified theory of three stratified layers (glass/water/cytoplasm) should be adequate. The theory approximates for simplicity that scattering plays a negligible role and that fluorescence intensity versus angle of observation and fluorescence lifetime are not functions of distance to the interface z. Experiments that... 

One can distinguish between methods in which absorption of the evanescent surface wave in different wavelength regions is measured (these are often called attenuated total reflection methods), and methods which use the evanescent wave to excite other, spectroscopic phenomena, like fluorescence and Raman scattering or light scattering. As the methods of conventional fluorescence spectroscopy have been shown to be exceptionally successful in studies of proteins and other biopolymers, their evanescent surface-sensitive counterparts will be reviewed first. 

Fig. 7. Schematic diagram of the evanescent wave dynamic light-scattering instrumentation (Reproduced with permission from Fytas et al., Science 274 2041 Copyright (1996) American Association for the Advancement of Science)...

This article shows how the evanescent wave can be used with advantage for spectroscopic purposes in the field of biomedical engineering. Three types of spectroscopy can be done with the evanescent wave in the UV-VIS range of the spectrum (a) attenuated total reflection (ATR) spectroscopy, which is well known in the infrared (b) the excitation of Raman scattering with the evanescent wave and (c) the excitation of fluorescence with the evanescent wave. The first two types will be discussed in this article the third is discussed for example by Hirschfeld U) and more recently by Watkins and Robertson (2). But before going into details a historical review may be of some interest. 

After that the behaviour of the evanescent field rather than possible applications was of primary interest. In 1902 Hall (5) succeeded in photographing the evanescent wave and soon afterwards fluorescence (6) and scattering (7) excited by such waves were observed. In 1910 Schaefer and Gross (8) measured quantitatively the exponential decay of the amplitude of the field with microwaves. 

As previously mentioned, the evanescent wave could interact with the optically rare medium not only by being absorbed but also by being scattered either elastically (Rayleigh Scattering) or inelastically (Raman Scattering). Because it is not within the scope of this paper to review the complete history and theory of Raman scattering, further information is indicated in Ref. 

Raman scattering with the evanescent wave is of special interest in biochemical laboratories for several reasons (36) ... 

These results, though rather qualitative, have been presented here in order to stimulate further work by other groups, especially in the case of Raman scattering with the evanescent wave. 

See D. C. Prieve, "Measurement of colloidal forces with TIRM," Adv. Colloid Interface Sci., 82, 93-125 (1999), for a clear description of technique as well as references also S. G. Bike, "Measuring colloidal forces using evanescent wave scattering," Curr. Opin. in Colloid Interface Sci., 5, 144-50 (2000). 

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