THEORETICAL ASPECTS OF BULK OPTODE MEMBRANES

The principles of optodes, which are based on a reversible mass transfer of analyte from the sample into the bulk of the sensing layer, have been elucidated in detail. It is demonstrated that the sensor response of many different types of bulk optode membranes described in the literature can be related to the same formalism. A universal description is given that is independent of the type of the optical transduction process and can be based on absorption (transmission or attenuated total reflection mode) or on fluorescence. The theoretical performance of bulk optodes based on plasticized PVC is described in terms of selectivity, response time, dynamic measuring range, sensitivity and lifetime, and whenever possible a comparison with experimental data is given. The geometrical arrangement and optimization of the membrane composition for a set of specific requirements are discussed in detail. By means of the spin-on technique, a simple manufacture of very thin optode membranes is described. With this technique, highly homogeneous membranes of thickness from 0.5 to 10-mu-m can be produced in only a few seconds.