Sensors absorbance, fiber-optic-based

Coleman J.T., Eastham J.F., Sepaniak M.J., Fiber optic based sensor for bioanalytical absorbance measurements. Anal. Chem. 1984 56 2246. 

A concept of the fiber-optic-based absorbance sensor is shown in Figure 18-8. Again, the fiber is threaded in a standard catheter, thus allowing its insertion into tissue or body fluids. A piece of aluminium foil is attached to the end of the inner needle (which contains the optical fiber). Fluids can be drawn into the sample irradiation cavity by aspiration, the volume between foils and Hber being Hlled through the hole shown. Typical pathlengths (twice the distance from the Hber tip to the foil) are O.S-4.3 mm. 

Optical transduction modes applied in combination with enzyme based fiber-optic sensors include absorbance, reflectance, fluorescence,... 

Absorbance- and reflectance-based measurements are widespread, as there are many enzymatic reaction products or intermediates that are colored or if not, can react with the appropriate indicator. Sensors using acetylcholinesterase for carbamate pesticides detection are an example of indirect optical fiber biosensors. This enzyme catalyses the hydrolysis of acetylcholine with concomitant decrease in pH41 ... 

From a general point of view, a chemical sensor is a device capable of continuously monitoring the concentration of an analyte. The two main classes are electrochemical sensors and optical chemical sensors. The latter are based on the measurement of changes in an optical quantity refractive index, light scattering, reflectance, absorbance, fluorescence, chemiluminescence, etc. For remote sensing, an optical fiber is used, and the optical sensor is then called an optode because of... 

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. 

Fiber optic sensors based on polymer swelling offer several potential advantages. They can be designed so that the optical measurement is separated from the polymer by a diaphragm so that the measurement can not be affected by the optical properties of the sample. Unlike fiber optic sensors based on indicator absorbance or luminescence, photodegradation is not a potential source of sensor instability. Measurements can be made in the near infrared region of the spectrum and take advantage of inexpensive components available for fiber optic communications. 

Hales B., Burgess L., and Emerson S. (1997) An absorbance-based fiber-optic sensor for C02(aq) measurements in pore waters of seafloor sediments. Mar. Chem. 59, 51-62. 

These criteria limit the applicahility of fluorescent dye-based sensors to only a handful of analytes. If a fluorescence-based method does not exist or is Incompatible with an optical sensor, an absorption sensor is the only alternative. There exist a large number of different absorbing dyes that are sensitive, selective and absorb in convenient regions of the spectrum. Several absorbance-based fiber-optic sensors have been described (8,25.26). Unfortunately, a major drawback with absorption spectroscopy is its inherent insensitivity. We decided to explore the possibility of using other optical techniques that can be used with indirect methods. A successful example has been presented which combines the sensitivity of the fluorescence measurement with the specificity of an absorbing dye and is based on an energy transfer mechanism (15). 

Systems based on diffuse reflectance from solid surfaces rather than absorbance in solution are also commercially available, particularly for clinical analysis, and fiber optic sensors based on spectro-photometric principles have been developed for... 

In most fiber-optic chemical sensor systems, the light transmitted from the sensing element (output light) is measured by using photon detection devices, which absorb photons and convert them into electrical signals. Several photon detectors are available. The simplest and cheapest detector is the silicon photodiode. In this solid-state device, photon detection is based on p-n semiconductor... 

The fiber-optic, absorbance-based pH sensor monitors the absorbance change of the immobilized dye as a function of the pH change of the analytical medium. The Beer-Lambert law can be applied to an absorbance based pH sensor. The concentration of the dye [Dy] is related to the absorbance according to the following equation ... 

A fiber-optic pH sensor based on fluorescence energy transfer can be construeted by coimmobilizing a pH-sensitive fluorophore and apH-sensitive absorber. For example, eosin (donor) and phenol red (acceptor) were coimmobilized in a polymer on the distal end of a silanized single-core optical fiber. Eosin s emission spectrum overlaps with the absorption of the basic form of phenol red. The concentration of the basic form of phenol red increases with an increase in pH. As a result, energy transfer from eosin to phenol red increases and the fluorescence intensity of eosin decreases. Thus, the pH-dependent absorption change of phenol red can be detected as changes in the fluorescence signal of eosin. 

Sol-gel optical sensors were developed for many different applications the determination of pH [160], measuring the absorbance of common pH indicators immobilized in thin-film silica matrices [161,162] or as a coating in fiber-optics, where the evanescent wave absorption is measured [163,164], and sensors based on fluorescent pH indicators in hybrid matrices [165,166] were also developed, showing excellent reproducibility and reversibility. Zirconia in hybrid films doped with methyl-red showed reversible response to acid and base vapors [167]. Silica-titania thin films with entrapped sensing molecules have been developed as pH sensors [168]. 

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]. 

Absorbance evanescent-based sensors are based on the absorption or dispersion of light outside the core. They rely on light attenuation in the evanescent field following the Beer-Lambert law (ATR sensors), but owing to the low intensity of the field, they offer poor sensitivity. This can be improved because the effective optical path length can be increased, especially when using optical fibers, capillary [62] or planar waveguides [114]. 

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