Evanescent wave excitation

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...

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. 

Figure 5. A planar platform for evanescent-wave excitation of fluorescence.

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. 

E. H. Lee, R. E. Benner, J. B. Fenn, and R. K. Chang, Angular distribution of fluorescence from liquids and monodispersed spheres by evanescent wave excitation, Appl. Optics 18, 862-870 (1979). 

Zhou, C. H., Pivarnik, P., Auger, S., Rand, A., and Letcher, S. (1997). A compact fiber-optic immunosensor for Salmonella based on evanescent wave excitation. Sens. Actuators B Chem. 42,169-175. 

Figure 5.45 (a) Molecular structures of the thiol-terminated phthalocyanine compounds reported by Russell and co-workers [77]. (b) Schematic of the experimental configuration used for evanescent wave excitation and emission detection at Pc SAMs, including a gas flow-through cell. Reprinted with permission from T. R. E. Simpson, D. J. Revell, M. J. Cook and D. A. Russell, Langmuir, 13, 460 (1997). Copyright (1997) American Chemical Society... 

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 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. 

Thus, surface plasmons (SPs) are excited on the dielectric medium - metal interfoce through evanescent wave excitation by attenuated total reflection (ATR) (23, 33, 36-38). One can excite SPs through the Kretschmann-Raether configuration... 

Studies involved with evanescent wave excited surface plasmon coupled fluorescence... 

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. 

Fig. 23 Concentration calibration for the three opto/fluid connectors Tl, T2 and T3. Each data point is averaged over three runs with two cycles per run. Tl - direct excitation, T2 - combination of direct and evanescent wave, and T3 - evanescent wave excitation only. (Reprinted from Dhadwal et al. [2], with permission of Elsevier)...

Figure 12. Fluroescence emission spectra for evanescent wave excited interfaciaF L-tryptophan (A)y adsorbed bovine y-globulin (B)y and adsorbed BSA (C).

We have developed a direct and noninvasive method to determine Tg of polymers at the interface with solid substrates [54, 55]. The strategy is to use fluorescence lifetime measurements using evanescent wave excitation. Figure 15a shows PS-NBD, i.e., PS containing the dye 6-[A-(7-nitrobenz-2-oxa-l,3-diazol-4-yl)amino]hexanoic acid (NBD) [55]. The NBD fraction of PS was sufficiently low to avoid self-quenching of the dye. The NBD dye was excited with the second-harmonic generation of a mode-locked titanium sapphire laser equipped with a... 

It can be used to detect the variations in optical properties of chemical and biological films placed around the fiber [5]. The laser optical detection method based on evanescent waves is widely used in biosensors. For example, a tapered optical fiber is used to analyze the total internal reflectance fluorescence. As light is propagated down the fiber, an evanescent wave excites fluorescent tracers bound to the fiber surface. Because the evanescent wave decays exponentially with the distance from the fiber surface, the excitation radius rally extends about 100 nm into the buffer medium. A portion of the emission is captured and propagated back through the fiber to the detector. 

Biosensor Probes. For the fiber optic biosensor used here, a portion of protective cladding on the exterior of the optical fiber is removed from the distal 10 cm of the fiber to expose a core of fused silica. This exposed region becomes the probe. Antibodies are covalently attached to the exposed core. When the probe is in contact with a sample containing an analyte, the immobilized antibody specifically binds the analyte from the bulk solution and concentrates it on the surface of the fiber within the evanescent zone. Any fluorophore associated with the analyte is also immobilized within the evanescent wave. Excitation of the fluorophore by light in the evanescent wave leads to fluorescent emission which generates a detectable signal. Two different methods of associating a fluorophore with the analyte are described below. 

Figure 3.14 Illustration of the arrangement for prism-based evanescent wave excitation.A collimated laser beam (black) is totally internally reflected off the interface formed by the prism/substrate and water. A microscope objective is used to collect the fluorescence generated by the evanescent field at the surface of the substrate in wide field mode.

Fang, XH and Tan, WH, Imaging single fluorescent molecules at the interface of an optical fiber probe by evanescent wave excitation. Analytical Chemistry 71 (1999) 3101-3105. 

When light traversing an optically dense medium approaches an interface with a more optically rare medium at an angle exceeding a critical value, Bent = sin (rerare/ dens), total internal reflection occurs and an evanescent wave of exponentially deca5ung intensity penetrates the rarer medium. This phenomenon is at the heart of certain spectroscopic methods used to probe biomolecules at interfaces (199). In total internal reflection fluorescence (TIRF) spectroscopy (200-202), the evanescent wave excites fluorescent probes attached to the biomolecules, and detection of the emission associated with their decay provides information on the density, composition, and conformation of adsorbed molecules. In fourier transform infrared attenuated total reflection (FTIR-ATIR) spectroscopy (203,204), the evanescent wave excites certain molecular vibrational degrees of freedom, and the detected loss in intensity due to these absorbances can provide quantitative data on density, composition, and conformation. 

Su L., Lee T. H., and Elliott S. R., Evanescent-wave excitation of surface-enhanced Raman scattering substrates by an optical-fiber taper. Opt Lett., 34, 2685-2687 (2009). 

We should point out that we will not be concerned with many other issues related to fluorescence spectroscopy that are also relevant in the case of evanescent wave excitation in general and SPFS, in particular. E.g., we will not deal with the details cf the distance dependence of fluorescence emission in a quantitative way, and are not concerned with orientational effects taking into account that surface modes have a particular... 

It was carefully checked that free anthracene molecules show no tendency to absorb on the alumina surface. The most sensitive method is to compare the fluorescence spectra obtained from excitation of the bulk solution with the one obtained from evanescent wave excitation. Both... 

MacCraith, B.D., Mcdonagh, C.M., Okeefe, G., Keyes, T.E., Vos, J.G., Okelly, B., and Megilp, J.F. (1993) Fiber optic sensor based on fluorescence quenching of evanescent-wave excited ruthenium complexes in sol-gel derived porous coatings. Analyst, 118, 385-388. 

The Otto configuration is well adapted for the cases when it is necessary to avoid contact between prism and surface. However, similar conditions can be realized if the sample film (medium 2) is deposited directly onto the prism surface. Such a geometry is known as a Kretschmann corfiguratim (Kretschmann and Raether 1968) (Fig. 3.5b). The film thickness in this case must be less than the penetration depth of the evanescent wave excited at the prism/film interface which is usually of the order of a few 100 A. 

Two-photon fluorescence spectroscopy provides a much better signal-to-noise ratio compared with the one-photon excitation scheme. We have already seen its advantages in the course of measuring the fluorescence of atoms adsorbed near to a metal surface (Section 6.5). In this chapter we shall consider a similar technique based on two-photon evanescent wave excitation of a gas near a surface. Due to the reliability of the signal, this kind of spectroscopy allows one to study the fluorescence lineshapes in detail. 

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