Absorption optical sensing principles

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. 

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],... 

Figure 6 illustrates the platform under consideration in this analysis. The principle of sensor operation is as described previously for absorption-based optical sensors employing evanescent wave interrogation of the sensing layer. 

In principle, a great deal of information concerning intramolecular electron transfer is available from IT absorption band measurements. Optical electron transfer is rapid on the vibrational time-scale and, as illustrated in Figure 7, the optical transition is a vertical process in the Franck— Condon sense. 

In this overview, the basic principle of all optical remote gas-sensing methods is introduced and is used to explain how the accuracy and sensitivity with which a target gas can be measured are defined by the spectroscopic properties of the gas itself. These properties define the region of the spectrum in which it has significant absorption features together with their strength and structure. 

Fig. 15.4 Principle of the optical gas-sensing effect, (a) Schematic iiiustration of goid nanoparticles embedded in the volume and on the surface of a semiconducting metal oxide layer with refractive index n. (b) Shift of the absorption peak of a single gold nanocluster (75 nm in diameter) by a variation of the refractive index of the surrounding medium at exposure to a reducing or oxidizing gas (Reprinted with permission from Schleunitz et ai. 2007, Copyright 2007...

Dyes, and their absorption changes, can be used to sense chirality and differentiate between two enantiomers with the naked eye , i.e. without the use of a polarimeter or any other device that measures optical activity. In principle every diastereomeric interaction between a chiral medium and the two enantiomers of a chiral dye must lead to different spectral characteristics. In practice, however, the effects are... 

---