Plain fiber sensors

Miscellaneous Plain Fiber Sensors for Chemical and Biochemical Species 942... 

Plain fiber sensors are simpler in design and manufacture than are indicator-phase sensors (Section 18.2.2). However, they are mostly less selective and suffer from interferences by any substance that adsorbs at the same analytical wavelength, even small changes in the optical parameters such as refractive index at the sensing tip, and ambient light. They therefore are usually operated at at least two wavelengths and/or with background subtraction. 

Indicator-mediated optrodes frequently have an optical isolation at the fiber tip to prevent ambient light and sample from interfering in the optical system, and a fairly constant tip chemistry with its almost invariable refractive index (see Figure 18-11). However, as with plain fiber sensors, additional discriminations such as pulsed excitation plus electronic background subtraction or sequential excitation are useful techniques for improving selectivity. 

Some analyte chemical species are themselves capable of exhibiting optical changes related to their concentrations, which can be measured photometrically using optical fibers. Such sensors, in which separate chemical transduction systems will not be required, are commonly referred to as plain-fiber sensors. Since many analytes do not themselves possess suitable optical characteristics, they may be sensed indirectly through their interaction with an appropriate chemical transduction system. The sensor response function in such chemical transducer-based optical sensors depends on the manner in which the analyte interacts with the reagent phase. For example, in a simple system where a reagent R reacts with an analyte species A forming a product AR can be represented by the reaction... 

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. 

One may differentiate between two types of sensors. In one type, a plain fiber is used to monitor the intrinsic properties of an analyte such as hemoglobin with its intense red color, or reduced nicotine adenine dinucleotide (NADH) with its strong blue fluorescence under UV excitation. In the other type, the indicator-mediated sensors for pH, PO2, PCO2, electrolytes, and other uncolored low molecular weight species discussed in previous sections may also be used in vivo. In fact, many of those have been designed especially for biomedical applications. Some of them have found their first commercialization in blood analyzers. 

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