Total internal reflection technique, optical

The thicknesses of free soap films and liquid films adsorbed on surfaces (Figs. 1.26d and 1.26e), which can be measured using optical techniques such as reflected intensity, total internal reflection spectroscopy, or ellipsometry as functions of salt concentration or vapor pressure, can provide information on the long-range repulsive forces stabilizing thick wetting films. We see an example of this in Chapter 11. 

Optical devices have also been used as transducers. Laser fiber-optics allows high intensity light to travel a long distance using fibrous size carrier. The stable and intense light beam not only provides calibration stability but also makes all the detecting techniques faster and more sensitive. In addition to the UV-VIS absorbance and fluorescence intensity, measurements of multiple reflections, surface plasmon resonance, and total internal reflection fluorescence had recently been used (12, 13, 14). 

Apart from optical microscopy, there are some other optical techniques which are truly surface sensitive and have found widespread use. Examples are ellipsometry (see Section 9.4.1), total internal reflection fluorescence (TIRF) [316], and surface plasmon resonance techniques [348],... 

Other methods have appeared more recently for measuring forces between macroscopic surfaces or between a (large) colloidal particle and a surface, immersed in a liquid. These include the total internal reflection microscope in 1990 (TIRM) and the atomic force microscope in 1991(AFM). With TIRM, incredibly weak forces can be measured (-10 N), whilst with AFM forces - 10 N can be determined. With the development of optical tweezers, we now have the ability to measure forces directly between two colloidal particles. Using these latest techniques, not only may interaction forces between surfaces be measured but, by performing dynamic measurements, the hydrodynamic forces can also be examined. We are now at a stage surely undreamt of by Theo Overbeek 50 or so years ago when he made his own measurements. It is surely fitting that he has lived to witness all this, and that he has reached an age almost commensurate with that of the Faraday Society/Division itself ... 

In general, a vast number of optical transduction techniques can be used for biosensor development. These may employ linear optical phenomenon (e.g. adsorption, fluorescence, phosphorescence, and polarization) or nonlinear phenomena (e.g. second harmonic generation). The choice of a particular optical method depends on the analyte and the sensitivity needed. Total internal reflection fluorescence (TIRF) has been used with planar and fibre-optic wave-guides as signal transducers in a number of biosensors. 

As discussed later, in Section 3.4, this technique is particularly well adapted to coupling to optical fibres, so a probe that can be immersed in a polymer reaction stream may be fabricated to enable spectra to be collected in real time. The principles of total internal reflection and its application in fibre-optics are discussed in more detail in Section 3.4.1. 

As a consequence, researchers from different disciplines of the life sciences ask for efficient and sensitive techniques to characterize protein binding to and release from natural and artificial membranes. Native biological membranes are often substituted by artificial lipid bilayers bearing only a limifed number of components and rendering the experiment more simple, which permits the extraction of real quantitative information from binding experiments. Adsorption and desorption are characterized by rate constants that reflect the interaction potential between the protein and the membrane interface. Rate constants of adsorption and desorption can be quantified by means of sensitive optical techniques such as surface plasmon resonance spectroscopy (SPR), ellipsometry (ELL), reflection interference spectroscopy (RIfS), and total internal reflection fluorescence microscopy (TIRE), as well as acoustic/mechanical devices such as the quartz crystal microbalance (QCM)... 

Sensing of evanescent waves with an optical tip has been proposed for use as an optical device to sense AFM forces by means of an optical microlever which is illuminated by a laser under conditions of total internal reflection and which is connected to an atomic force tip [77], Thus tunneling photons from the microlever to the optical tip at the evanescent light coupling may be used for the feedback loop. This instrument combines noncontact AFM and PSTM techniques. 

Because infrared spectroscopy is an optical technique, the sample must be transparent in the spectral region of interest. Infrared spectroscopy can be used for heterogeneous samples and adsorbates on catalyst surfaces [11, 17]. If the sample cannot be made thin enough to be transparent, special probing techniques such as attenuated total internal reflectance can be use to monitor the concentration. 

During the last few years optical visualization techniques have also been introduced. Among them the total internal reflection fluorescence excitation (TIFR) microscopy [4] and optical interference-enhanced reflection microscopy [5] appear to be the most promising nonintrusive techniques. Their resolution, however, does not even approach the resolution of atomic force microscope and optical techniques may thus serve as an image survey of nanobubbles at 300 nm level (diameter) which is so far their resolution limit. 

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