Evanescent wave-based techniques

A fiber-optic device has been described that can monitor chlorinated hydrocarbons in water (Gobel et al. 1994). The sensor is based on the diffusion of chlorinated hydrocarbons into a polymeric layer surrounding a silver halide optical fiber through which is passed broad-band mid-infrared radiation. The chlorinated compounds concentrated in the polymer absorb some of the radiation that escapes the liber (evanescent wave) this technique is a variant of attenuated total reflection (ATR) spectroscopy. A LOD for chloroform was stated to be 5 mg/L (5 ppm). This sensor does not have a high degree of selectivity for chloroform over other chlorinated aliphatic hydrocarbons, but appears to be useful for continuous monitoring purposes. 

In addition, the Biacore system is fully automatic and incorporates a reliable microflnidic system that facilitates accurate and precise sample delivery and flow-rate manipnlation. Significantly, it also permits multichannel analysis and thns, reference-subtraction [71], which is useful for comparative analyses and is a prereqnisite for kinetic and affinity estimations. A significant advantage of SPR over optical detection techniques is that the incident light energy does not actnally penetrate the bnlk sample and thus, measurements can be made equally on colored or tnrbid solntions and on clear samples [72]. Typically Biacore and SPR/evanescent wave-based technologies have been routinely used for analysis of small molecnles. With specific reference to foodstuffs, these include hormones [9], antibiotic residnes [73], and small molecules that are indicative of microbial contamination snch as microbial toxins [10,46,74]. 

When the distance h between a spherical particle of radius a and a solid boundary becomes sufficiently small (hla 1), hydrodynamic interactions between the particle and wall hinder the Brownian motion of the particle. Such effects are critical to near-wall measurements and the accuracy of velocimetry techniques, which rely on an accurate accounting of particle displacements to infer fluid velocity. By applying the evanescent wave-based 3D PTV techniques to freely suspended fluorescent particles, anisotropic hindered Brownian motion has been quantified for particle gap sizes hla 1 with 200 nm diameter tracers [8] and hla 1 with 3 pm diameter tracers [9]. These results confirm the increase of hydrodynamic drag when a particle approaches a solid boundary, and such correction shall be applied to not only Brownian motion but also other translational motion of particles where the drag force is of concern. 

B.D. Gupta and D.K. Sharma, Evanescent wave absorption based fiber optic pH sensor prepared by dye doped sol-gel immobilization technique, Opt. Commun., 140(1-3) (1997) 32-35. 

To increase the speed of the TIRF-based kinetic techniques, the perturbation can be optical rather than chemical. If the evanescent wave intensity is briefly flashed brightly, then some of the fluorophores associated with the surface will be photobleached. Subsequent exchange with unbleached dissolved fluorophores in equilibrium with the surface will lead to a recovery of fluorescence, excited by a continuous but much attenuated evanescent wave. The time course of this recovery is a measure of the desorption kinetic rate k2. This technique1-115) is called TIR/FRAP (or TIR/FPR) in reference to fluorescence recovery after photobleaching (or fluorescence photobleaching recovery). 

Complementary operators, on the other hand, comprise an efficient boundary procedure that improves the absorption of numerical reflections by using previously developed ABCs [13]. Their competence is based on the implementation of two boundary operators that are complementary in their action. In this manner, new ABCs that produce prespecified reflection coefficients are derived. By solving the problem with each of the two operators and then averaging the two solutions, the technique annihilates the first-order artificial reflections of both obliquely propagating and evanescent waves, irrespective of their wave number. More specifically, absorption of the latter occurs even if the original ABC reflects them totally. The modified higher order development of COM starts from a well-posed and stable ABC that can be expressed by a single differential equation, defined as J-. If nonstandard operator [.] is... 

Another ammonia sensor speciHcally designed for use in bioliquids is based on the evanescent wave technique and can be applied to the vapor-phase determination of ammonia above blood and serum [136]. It utilizes the ninhydrin reaction occurring in the polymer coating of the fiber, and the resulting color change is monitored by total internal reflection. The probe is applicable to clinical determinations normally carried out in the vapor phase, but works irreversibly. A linear relationship exists between absorbance and ammonia concentration in the clinically useful range of 0-4.0 pg mL. Comparison with the reference method showed a correlation coefficient of 0.92. 

A fiber-optic flow-injection analysis in which a fluorescent reporter molecule is utilized to monitor a competition reaction between a labelled and an unlabelled antigen molecule for an immobilized antibody has been reported [100]. It is based on the evanescent wave technique. 

Two major optical techniques are commonly available to sense optical changes at sensor interfaces. These are usually based on evanescent wave and surface plasmon resonance principles. 

In principle, the sensor constructions rely on five basic sensing schemes transmission, reflection, grazing angle reflection, attenuated total reflection (ATR), and a variant of the ATR effect known as the fibre-evanescent wave sensor (FEWS) (Melting Thomson, 2002). The investigations described in chapters 3 and 4 are based on the use of the ATR technique, focusing on its advantageous properties, esp edally in terms of mechanical robustness. 

Biosensors are also classified according to the parameter that is measured by the physicochemical transducer of the biological event. Thus, classically biosensors are grouped into optical, electrochemical, acoustic and thermal ones. Optical transducers of most common enzyme biosensors are based on optical techniques such as absorption, reflectance, luminescence, chemi-luminescence, evanescent wave, surface plas-mon resonance, and interferometry. 

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