Optical sensor FOCS

This section covers early indirect fiber optic chemical sensors (FOCS) for species that cannot be sensed directly but require the use of indicators, probes, labeled biomolecules, or color-forming reactions. 

The signal conversion, occurring in the fibre optic chemical sensors (FOCS), proceeds in several steps, which are shown in Figure 1. 

In the last decade, fiber-optic chemical sensors (FOCS), also known as optrodes, have emerged as alternatives to conventional methods of analysis. FOCS development for a particular analyte depends on the availability of reversible indicating schemes to detect the analyte of interest. Typically, the indicating schemes use commercially available colorimetric or fluorometric indicators (e.g. fluorescein to measure pH (1)). However, the utility of these indicators is limited. Furthermore, indicators may not exist for many analytes. Several reviews discuss the scope of this approach (2,3,4). 

This section attempts to present a broad review of this technique in the light of recent research on fiber-optic chemical sensors (FOCS). A discussion on the advantages and performance of pH optodes will be broached by considering the various applications planned and the resulting pH measurements, including titration and ionic strength. Sensors for low molecular weight electrolytes, particularly optodes for anions and cations in solution, are also considered. 

The use of fiber-optics in the preparation of the optical sensors allows performing spectroscopy measurements at sites that are inaccessible to conventional spectroscopy and also over large distances. Moreover, the possibility of using evanescent wave spectroscopy and spatially resolved lifetime spectroscopy makes FOCS (fiber optics chemical sensors) a very interesting option for building the sensors. Comprehensive reviews [183,184] on fiber-optic chemical sensors are available, where the most interesting applications of sol-gel coatings for optical sensors are described. 

Fiber optic chemical sensors (FOCS) have been reported for several different types of analytes such as ions, metals, pH, gases, enzymes, etc. These optical sensors can often be used as alternatives to electrochemical sensing devices. However, with a few exceptions (e.g., halides, cyanide, etc.), there is a lack of anion-selective FOCS. 

While several optical chemical sensors and biosensors in use that do not rely on fiber optics, the commercialization of FOCS technology started slowly, probably because of the limitations imparted to optical sensing if... 

By incorporating these three material phenomena for chemical measurements with fiber optic structures creates a basis for a fiber optical chemical sensor or FOCS. Two classes of FOCS have been reported in the literature ... 

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