Optical evanescent field techniques

Interferometric sensors frequently have also been applied to biosensor measurements. Thereby, the evanescent field technique (Mach-Zehnder interferometer) has been compared with other optical detection principles regarding information on layer structure and in case of biosensing30. The... 

A chemical sensor is a device that transforms chemical information into an analytically useful signal. Chemical sensors contain two basic functional units a receptor part and a transducer part. The receptor part is usually a sensitive layer, therefore a well founded knowledge about the mechanism of interaction of the analytes of interest and the selected sensitive layer has to be achieved. Various optical methods have been exploited in chemical sensors to transform the spectral information into useful signals which can be interpreted as chemical information about the analytes [1]. These are either reflectometric or refractometric methods. Optical sensors based on reflectometry are reflectometric interference spectroscopy (RIfS) [2] and ellipsometry [3,4], Evanescent field techniques, which are sensitive to changes in the refractive index, open a wide variety of optical detection principles [5] such as surface plasmon resonance spectroscopy (SPR) [6—8], Mach-Zehnder interferometer [9], Young interferometer [10], grating coupler [11] or resonant mirror [12] devices. All these optical... 

Further investigations using this method will be of considerable interest, not only because of the practical advantages mentioned above, but also because it now becomes possible to study surface reactions, e.g., catalysis, adsorption phenomena and enzymatic reactions. This is especially so with respect to the modern evaporation techniques and the possibilities of chemical conditioning of the surfaces of almost any material. Extremely thin films and monomolecular layers are sufficiently transparent to maintain an evanescent field in the optically rare medium. 

Near-field optics is another promising way to achieve high-density optical memory. Since near-field techniques overcome the diffraction limit of light by the contribution of evanescent wave, it is possible to store a bit datum in a nanometric region. The nanometric resolution is achieved by the contribution of evanescent fields. 

Despite the differences in generation of the evanescent field, the basic binding experiment is basically the same for all the optical biosensors (see Fig. 5.3). One of the interacting partners, the receptor, is attached to the sensor surface while the analyte binds to the receptor from free solution. As the sensor monitors refractive index changes occturing in real time, the amount of receptor, analyte and the rate of binding can be determined. Indeed, the estimation of the interaction kinetics is one of the key advantage of this technique. 

Regarding applications, long-chain phosphoric (and also phosphonic) acid esters are expected to have potential for applications in areas where the specific surface functionalization of oxides by extremely thin films is essential to the quality of a product. One area where application of octadecyl phosphate on tantalum oxide has already proven to be successftd is optical biosensor technology, where the use of well-controlled ODP self-assembled monolayers on optical waveguide chips has been demonstrated to increase both the specificity and selectivity when sensing extremely low quantities of biomolecules using evanescent field and fluorescence techniques. The application potential will be further increased if a second functionality can be introduced in the terminal, phosphoric acid esters. 

Due to the length scales involved, most interesting phenomena probed by the evanescent wave require microscope optics for imaging. In the past, several TIRM studies have been conducted using only scattering from objects in the evanescent field for imaging. More recently, fluorescence microscopy systems have become the standard imaging technique for evanescent wave microscopy... 

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