Chapter 18
Optical Sensors – Fundamentals
Analytical chemistry uses light on a large scale to investigate the chemical composition of various kinds of sample. Methods based on light interaction with the sample are known as optical methods of analysis [1]. In many instances, the compound of interest is able to interact with light without any prior chemical modification. However, quite often the analyte cannot be detected directly by an optical method. In such cases, the analyte is converted into a detectable compound by a chemical reaction involving a specific reagent. This procedure involves chemical reactions in the solution phase in which both the analyte and the reagent are free to diffuse.
In order to obtain an optical sensor, the specific reagent is included in a sensing layer. The analyte–reagent reaction is monitored by a light beam that is conveyed by an optical fiber (or other type of waveguide) integrated with the sensing layer. Therefore, waveguiding is a crucial process in optical sensors.
Optical transduction can be achieved by measuring the light power absorbed or emitted by a component of the sensing layer at a specific wavelength. As the dependence of light power on the wavelength represents an optical spectrum, such methods are classified as spectrochemical methods. Application of spectrochemical methods needs a component of the sensing layer to be able to absorb or emit light. If this condition cannot be fulfilled, one must resort to optical signaling labels. ...
