Fiber-optic absorbance sensors

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

For both type of microwave reactors, if the reactor is not supplied with a temperature sensor or more likely accurate temperature measurment is prerequisited during an experiment, the fiber-optic temperature sensor is directly applied to the reaction mixture. In order to secure the sensor from harsh chemicals, the sensor is inserted into a capillary that in turn is inserted into the reaction mixture. In such a case, it is strongly advocated to use capillaries that are made of quartz glass and are transparent to microwave irradiation. Any capillary that is made of glass or even borosilicate glass can always slightly absorb microwave energy, in particular, while the reaction mixture does not absorb microwaves efficiently, and in turn lead to failures of fiber-optic thermometer performance. 

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. 

C. Ion sensing. Several different schemes can be applied to fiber-optic chemical sensors for detecting ions other than hydrogen. One approach is to design a system, similar to pH fiber-optic chemical sensors, in which a dye that selectively binds a metal ion of interest is immobilized in an ion-permeable polymer such as cellulose or a hydrogel at the tip of an optical fiber. The reaction between the dye and the ion changes the absorbance or fluorescence of such dyes. Absorbance or fluorescence intensity changes are measured as a function of ion concentration, but this... 

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

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. 

Spectrophotometry, 42 Absorbance, 42 Infrared, 44 Luminescence, 45 Raman, 48 Fiber Optics, 50 Refractive Index, 52 Piezoelectric Mass Sensors, 53 New Chemistry, 54 Immunochemistry, 54 Polymers and New Materials, 56 Recognition Chemistry, 57 Chromatography and Electrophoresis, 61 Flow Injection Analysis and Continuous Flow Analysis, 63 Robotics, 65 Chemometrics, 68 Communications, 70... 

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. 

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. 

The evanescent wave depends on the angle of incidence and the incident wavelength. This phenomenon has been widely exploited to construct different types of optical sensors for biomedical applications. Because of the short penetration depth and the exponential decay of the intensity, the evanescent wave is absorbed mainly by absorbing compounds very close to the surface. In the case of particularly weak absorbing analytes, sensitivity can be enhanced by combining the evanescent wave principle with multiple internal reflections along the sides of an unclad portion of a fiber optic tip. 

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