UREA BIOSENSOR BASED ON AMPEROMETRIC AMMONIUM ION ELECTRODE

An amperometric urea biosensor is described: urease is immobilized on the amperometric ammonium ion electrode surface, and the ammonium ion produced by hydrolytic decomposition of urea by urease is determined by the pulse amperometric technique. Theoretical considerations on the steady-state concentration distribution of the substrate (urea) and the product (sum of ammonia and ammonium ion) in the immobilized-enzyme layer show that the concentration of the product at the ion-selective electrode-surface is proportional to the concentration of the substrate at the solution side surface of the immobilized-enzyme layer up to a certain limiting concentration. The limiting concentration depends on the enzyme-loading factor (Thiele modulus) as well as the Michaelis constant of the enzyme reaction. A practical urea sensor was constructed by immobilizing urease on an amperometric ammonia gas sensor. Preliminary experimental results and discussion are given. With the amperometric urea sensor, the correction for the residual ammonium ion in test solution can be easily achieved with an auxiliary amperometric urease-removed urea sensor.