Life time, enzyme electrodes

The ISE life-time is closely connected with the drift and is at least one year for good electrodes. With some systems, e.g. enzyme electrodes (see chapter 8), the life-time is only a few weeks. It follows from the results of Oesch and Simon [119] that the life-time of electrodes based on ionophores in solventpolymeric membranes depends on the kinetics of dissolution of the ionophore and the plasticiser in the analyte. If both the ionophore and the membrane solvent have distribution coefficients between water and the membrane greater than 10 , then the ISE life-time is at least one year. 

Generally speaking, enzyme electrodes are highly specific, have reasonable response and life times, and hold considerable promise for further development. 

The solid line in Figure 4 is the decay curve for a GDH electrode which was overcoated by polyallylamine (PAL) as described else where (Katakis, L Ye, L. Duine, J. Schmidt, H.-L. Heller, A. in preparation for publication). The stability was futher improved to a half life time of 33 hours b use that the top layer of crosslinked PAL prevented the leaching of the enzyme. It is worthwhile to point out that the PAL overcoating increased die half life time of the GDH electrode by a factor of 4 at the expense of only 22% decrease in current output. This suggests that the main cause for the fast decay of the uncoated electrode was the leaching of the enzyme. 

---