Optical sensing principles

Many classical optical sensing principles, which are well known from free space optics can be implemented in integrated optical structures, while integrated optics at its turn can act as source of new principles. Many types of integrated optical sensors have been investigated and demonstrators have been realized, mostly at universities and other research institutes. And also at... 

Optical sensors rely on optical detection of a chemical species. Two basic operation principles are known for optically sensing chemical species intrinsic optical property of the analyte is utilized for its detection indicator lor label) based sensing is used when the analyte has no intrinsic optical property. For example, pH is measured optically by immobilizing a pH indicator on a solid support and observing changes in the absorption or fluorescence of the indicator as the pH of the sample varies with time1 20. 

In addition, typical methods of sensing are total internal reflection fluorescence or monitoring of fluorescence resonance energy transfer6,7. The second class is a direct optical detection principle which relies either on reflectometry or refractometry. The latter is connected to evanescent field... 

Ion-selective electrodes are now well understood in terms of the underlying theory, and this has made it possible for new sensing principles to emerge that make use of the thousands of chemical receptors originally developed for ion-selective electrodes. One is the field of optical sensors, which has not been discussed here because it is outside the focus of this chapter. Such so-called bulk optodes do not require electrical connectivity between the sensing and detection unit and are therefore more easily brought into various shapes and sizes, including particle formats, which suit the need of modem chemical analysis. 

We have shown that antiresonant dielectric layers can be used to design low-loss liquid-core waveguides that are suitable for implementing planar sensor device geometries. The following sections will describe in more detail how the design principles laid out here were implemented in silicon-based LC-ARROW chips and used for optical sensing and detection of a wide variety of substances. 

Many compounds exhibit near-IR and mid-IR absorption. By using IR transparent optical fibers, detection of an absorption band in the IR region is possible for optical sensing. Both direct sensing using the absorption property of the analyte or indicator sensing are widely exploited. Most mid-IR sensing schemes are based on the principles of internal reflection spectroscopy, or the attenuated total reflection (ATR) [3,14-21],... 

Fig. 20a. 10. Schematic of the sensing principle of a urea optical sensor based on an ammonium-sensitive membrane employing anionic dye and neutral carrier.

This sensing principle does not truly belong to the reflectance category. It is included here only because it does not fit better in any other place. Yet, its operation is interesting and important. It is a cross between the optical and electrochemical... 

Explain the connection between the attenuation loss of the optical fiber used for transmission of information and the sensing principles utilizing an evanescent field. 

The polymer materials not only act as supports for the dye and other necessary additives in the sensing phase, providing protective covering for the transduction element polymers also play various roles in chemical sensors. They provide a compatible environment for the indicator molecules, maintaining or improving the appropriate photophysical features (compared to those observed in homogeneous solution) on which the sensing principle is based. In many cases they collect and concentrate the analyte molecules on sensor surfaces. In addition, the polymer can play an important role in the sensitivity and selectivity of an optical sensor, and its interactions with indicator and analyte molecules influence the analytical performance of the device. 

In this section an overview of the numerous methods and principles for the discrimination of enantiomers is given. First, the interaction principles of the polymer-based methods adapted from chromatographic procedures are illustrated. The discrimination of enantiomers was achieved some decades ago by using different types of stationary materials, like cyclodextrins or polymer-bonded amide selectors. These stationary-phase materials have successfully been appointed for label-free optical sensing methods like surface plasmon resonance (SPR) or reflectometric interference spectroscopy (RIfS). Furthermore, various successful applications to optical spectroscopy of the well-established method of fluorescence measurements for the discrimination of enantiomers are described. 

Integrated optical devices combine microelectronic production technology with the inherent advantages of optical sensing. Many of these developments are in an early state of research but a variety of optical biosensors can be realized in principle. Integrated optical device manufacturing is nowadays commercially available (IOT) and nearly all optical elements can be integrated and miniaturized on chip [24]. 

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