Design of Optical Sensors

Let us now focus on the selective layer and how it is incorporated in optical sensors. Unless there is a specific reason for it, we use the term optical fiber in the general sense, meaning both optical fibers and flat optical waveguides. The selective layer can be placed on the fiber in several different ways depending on the type of interaction involved (Kuswandi et al., 2001). The general arrangement is shown in Fig. 9.22. 

The reflecting surface can be a mirror or a membrane with a light-scattering surface. In any case, the sensor has the appearance of a monolithic probe (i.e., a dip-stick probe). Optical sensors based on absorption, fluorescence, phosphorescence, and luminescence can employ such a configuration. Various highly optimized fiberoptic probes for UV-Vis, NIR, and IR ranges are now commercially available, and their designs are shown in Fig. 9.23. 

The second option is to make use of the continuous evanescent field and to locate the selective layer within this field. To that end, the protective coating and 

1 For the definition of attenuation of intensity, the multiplier in front of the log is 20 rather than 10. 

Finally, there is another possibility of arranging interaction between light and the analyte. It is called open optical path sensing. Here, no physical waveguide is used. The beam of light is emitted from a coherent, narrow bandwidth source and 

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Energy transfer [3,12] between molecules has also been used in the design of optical sensors. Here, an excited molecule (donor) can transfer its electronic energy to another species (acceptor). This process occurs without the appearance of a photon and results from dipole-dipole interaction between the donor-acceptor molecules. The rate depends on the fluorescence quantum yield of the donor, the overlap of the emission spectra of the donor with the absorption of the spectrum of the acceptor, and their relative orientation and distance. It is the overlap of... 

Morf W.E., Seiler K., Lehmann B., Behringer C., Hartman K., Simon W., Carriers for chemical sensors design features of optical sensors (optodes) based on selective chromoionophores, Pure Appl. Chem. 1989 61 1613. 

This chapter provides an overview of the basic principles and designs of such sensors. A chemical sensor to detect trace explosives and a broadband fiber optic electric-field sensor are presented as practical examples. The polymers used for the trace explosive sensor are unpoled and have chromophores randomly orientated in the polymer hosts. The electric field sensor uses a poled polymer with chromophores preferentially aligned through electrical poling, and the microring resonator is directly coupled to the core of optical fiber. 

Fe(III), Cu(II), Ni(II), and even NH4. The second type of optical sensor can be designed in the same way as the proton sensors if a fluorogenic or chromogenic compound is found to bind with the cation selectively and reversibly, e.g. 

Fiber optical sensors are popular devices for the design of optical chemosen-sors. They are based on the change of optical properties (such as adsorption or luminescence) of particular chemical indicators. For example, fiber optical oxygen sensors are produced by the immobilization of oxygen sensitive dyes on the tip of an optical fiber and in an appropriate matrix. 

In this chapter, an overview is presented of the main optical biosensors, the operating principle of the different devices, the design of the sensors, the technology of fabrication, the resolution, the dynamic range and detection limit of each device, the most important applications and the commercial devices on the market. Finally, an outlook of futtu"e prospects for this technology is given. 

Earlier, the electrochemical detections were mostly employed in chemical sensors and biosensors, and until now they are most commonly used, especially in commercially available sensors, mainly for clinical and environmental analyses. An intensive development of optical sensors (optodes) in recent 30 years has resulted in numerous designs and commercial products, which are increasingly competitive to electrochemical sensors. A more limited importance, especially as mass production is concerned and applications in routine analyses, have thermal and mass-sensitive sensors and biosensors. 

This study also explored the scope of the utility of direct dyes as Immobilized Indicators at hydrolyzed cellulose acetate films for the construction of optical sensors which exhibit a rapid response and are easily fabricated. In addition, and perhaps more importantly, the "proof of concept" design of a sensor and preliminary evaluation of characterization with a response over a broad pH range was demonstrated. Illustrating opportunities provided by this approach for the fabrication of sensors with new and/or Improved performance characteristics. Studies to explore the fundamental Interactions that govern the formation, structure, reactivity, and stability of such sensors are currently underway. 

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