An air-gap enzyme electrode: development and applications

A novel air-gap enzyme electrode was developed by amalgamating the principles controlling ion-selective electrodes, enzyme kinetics, and diffusion of molecules across membranes. Commercially available ammonia and carbon dioxide gas-sensing electrodes were modified to measure the respective gases over a head-space. A plexiglass cell was designed and fabricated to house the modified electrodes. The air-gap electrodes exhibited superior sensitivity and response characteristics compared to the original membrane electrodes. The plexiglass cell was divided into two compartments by virtue of a selectively permeable membrane. Enzymatic reactions were conducted in the cell to determine quantitatively the concentration of the substrate. L-Phenylalanine ammonia lyase was used to determine L-phenylalanine with the ammonia electrode and L-glutamate decarboxylase was utilized for the determination of L-glutamic acid by the carbon dioxide electrode. Near-Nernstian slopes were obtained for the response of the enzyme electrodes. In addition to the good reproducibility, the method provided unique ability to reuse the same enzyme solution for several determinations of various concentrations of the analyte. The performance of the two compartment cell and air-gap enzyme electrode was found to be superior in comparison to conventional enzyme electrodes.