continuous methods for optically sensing chemical species rely on monitoring the optical properties of the analyte, preferentially via an optical waveguide, or by immobilizing some sort of indicator chemistry on a solid support or at the end of an optical fiber. the resulting devices are called optodes or optrodes. because the reagent is immobilized, its optical properties can be monitored continuously and can give an indication of the concentration of chemical parameters such as ph, oxygen partial pressure, and various electrolytes. by coupling enzymatic reactions to oxygen-consuming or producing reactions, transducer-based biosensors are obtained. aside from established methods that essentially are based in intensity measurements, new techniques have been introduced in the past few years which include lifetime measurements, et studies, and enzyme-coupled sensing methods. in addition, the use of new polymer materials can provide perm-selectivity, improved biocompatibility, and ease of dye and protein immobilization