Fluorescence evanescent-wave sensors

The pH measurement can by realized using sol-gel films and evanescent-wave sensors method74. To incorporate a near-infrared pH sensitive fluorescent dye, a thin-film coating on the core of a multimode fiber was used. By evanescent wave excitation an absorption or fluorescent based sensor can be realized for use in high pH regions. 

The fiber optic evanescent wave sensor (FO-EWS) belongs to a sensor in which the fiber core interacts with the analyte. This interaction occurs through the attenuated total reflection (ATR) and the evanescent wave excitation in a dielectric medium of smaller refractive index in the vicinity of fiber core. If the surrounding medium is fluorescent, then the fluorescence signal in the reaction region of evanescent wave field is excited and detected. This is illustrated in Figure 8.2. 

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. 

A novel fiber optic sensor concept using antibody-antigen reactions at a glass-liquid interface was reported by Daehne146. The reaction of antibodies immobilized onto the surface of fused silica fiber optic or planar waveguides with antigens in solution was detected by interaction with the evanescent wave. By detecting in-line fluorescence, the measurement of human IgG is described. 

MacCraith B.D., Ruddy V., Potter C., O Kelly B., McGilp J.F., Optical waveguide sensor using evanescent wave excitation of fluorescent dye in sol-gel glass. Electron. Lett. 1991 27 1247. 

Evanescent Wave Effects and Fluorescence-Based Optical Sensors... 

While planar optical sensors exist in various forms, the focus of this chapter has been on planar waveguide-based platforms that employ evanescent wave effects as the basis for sensing. The advantages of evanescent wave interrogation of thin film optical sensors have been discussed for both optical absorption and fluorescence-based sensors. These include the ability to increase device sensitivity without adversely affecting response time in the case of absorption-based platforms and the surface-specific excitation of fluorescence for optical biosensors, the latter being made possible by the tuneable nature of the evanescent field penetration depth. 

The background problem can be further overcome when using a surface-confined fluorescence excitation and detection scheme at a certain angle of incident light, total internal reflection (TIR) occurs at the interface of a dense (e.g. quartz) and less dense (e.g. water) medium. An evanescent wave is generated which penetrates into the less dense medium and decays exponentially. Optical detection of the binding event is restricted to the penetration depth of the evanescent field and thus to the surface-bound molecules. Fluorescence from unbound molecules in the bulk solution is not detected. In contrast to standard fluorescence scanners, which detect the fluorescence after hybridization, evanescent wave technology allows the measurement of real-time kinetics (www.zeptosens.com, www.affinity-sensors.com). 

OF optical fibers, IOS integrated optical sensors, A absorbance, R reflectance, F fluorescence, ev evanescent wave, ISP isopropyl alcohol, DOS bis(2-ethylhexyl)sebacate, o - NPOE ortho-nitrophenyl octyl ether, TOP tris(2-ethylhexyl)phosphate... 

Luminescent evanescent wave-based sensors use optical fibers and planar waveguides [105,106] as fight-guiding structures, and they are more complex than the absorbance ones. However, such optodes have been satisfactorily applied to measure fluorescence of indicators or labels for the measurement of gas molecules, proteins or labeled antigen-antibody interactions as well as directly in solution [24,107] when immobilized in matrices [23,109]. 

The utihzation of fluorescence dyes for analytical measurements enhances the sensitivity for the detection of the molecules of interest. First, Cronick and Little made use of evanescent wave excitation for a fluorescence immunoassay, in 1975. By using totally internally reflected light, they excited the fluorescence of a fluorescein-labeled antibody which has become bound to a hapten-protein conjugate adsorbed on a quartz-plate in an antibody solution [41]. Contrary to the label-free high-refractive-index sensors where the mass of the molecule of interest is... 

The products of hybridization are detected through the use of fluorescent labeling. These molecular complexes can either be homogeneously distributed in the liquid core or be bound to the interior surface of the capillary through covalent bonding. In both cases, labeled molecules can be excited either by direct illumination with the leaky modes of the liquid filled core, or by the evanescent waves arising from the guided modes of the capillary wall. Direct excitation is less wasteful of incident photon flux and is the method of choice in conventional fluorometers. Evanescent wave excitation becomes a necessity when direct excitation is either not feasible or results in undesirable sensor performance. Both methods of illumination are possible for the CWBP. 

Aptamer-based protein sensors have been developed since 1998. Thus, 0.7 amol of thrombin in 140-pL sample (0.5 pM concentration) was detected in a single-aptamer sensor through binding to a fluorescently labeled DNA aptamer by evanescent-wave-induced fluorescence anisotropy in less than 10 minutes [45]. Performance of DNA... 

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