CHAPTER 6 O PTICAL AND F IBER O PTIC C HEMICAL S ENSORS G. Korotcenkov B. K. Cho R. Narayanaswamy F. Sevilla III 1. INTRODUCTION An optical sensor can be described as a device which can be employed for the detection and determina- tion of physical and chemical parameters through measurements of optical properties. Optical methods have played dominant roles and continue to do so in various fields of analytical science. Chemical and biochemical analytes have been qualitatively detected by spot tests using spectroscopic techniques including colorimetry and photometry. Such techniques, in conjunction with the use of dry reagent chemistries, have revolutionized the field of optical chemical sensing, particularly in the field of clinical analysis. A very familiar example of an optical sensor is the pH indicator strip, which utilizes immobi- lized pH-sensitive reagents on cellulose for qualitative measurements of pH over a wide range. Optical fibers allow transmission of light over great distances and for chemical sensing, typical monitoring dis- tances required range from 1 to 100 m. In addition, availability of low-cost optoelectronic components such as light-emitting diodes (LEDs), laser diodes, photodetectors, etc., and of rapid data acquisition and data processing techniques, including chemometrics, has made considerable impact in the growth and interest in optical chemical sensors over the past 30 years. In general, optical sensors incorporate optical fibers as light guides. 311