Ion- and substrate-selective optode membranes and optical detection modes

Abstract

A brief overview covering the state-of-the-art of ion-selective bulk optode membranes and their characteristics is given in the first part. Detection principles involving attenuated total reflection (ATR) measurements and measurements of refractive-index changes are reviewed. In contrast to ion-selective optodes where an indicator is coupled to the ion-exchange reaction, chemical host-guest interactions for neutral analytes, where the chemically reactive ligand is chromogenic, are discussed. In the second part, an alcohol-selective optode membrane and its application in continuous ethanol monitoring is presented. The ethanol-sensitive membrane is used to monitor ethanol in the vapour phase of a bioreactor. Data derived from determining ethanol using the optode membrane are compared to ethanol concentrations in the liquid phase of the reactor filling by using a reference method (distillation and density measurements) simultaneously. The data are analysed by first-order linear regression ( R 2 = 0.935). The repeated use of a pair of optode membranes for six different fermentation runs shows, first, the reversible response behaviour of the membranes; secondly, reproducible starting values of the membrane absorbance at 305 nm wavelength; thirdly, a lifetime of the membranes of more than six days when analysing the vapour phase; fourthly, for each new batch of fermentation, a different but typical production rate of ethanol, which mirrored the production rate of the yeast cells, is monitored. This might well be the first demonstration of the feasibility of using a chemical sensor for continuous monitoring in biotechnology. References for preliminary studies on host-guest interactions of diols and oligools, such as catechol, fructose and glucose, with phenylboronic acid derivatives are provided.