Bioelectrochemical Conversion of Urea on Carbon Black Electrode and Application

The amperometric biosensor for urea determination is designed. The decomposition product of urea produced by urease is oxidized in an electrochemical way. The specially developed carbon black (CB) paste electrode is covered by a semipermeable membrane containing immobilized urease. Three types of the urea biosensor action are identified. At low electrode working potentials (0-0.1 V), one electron electrochemical oxidation of carbamic acid can be monitored. Cation-radical of the carbamic acid whereupon undergoes dimerization to hydrazine. At higher electrode potentials (0.2-0.5 V), the electro-oxidation of both carbamic acid and hydrazine are observed. In addition, at more high potential (> 0.6 V), electro-oxidation of ammonia and amination of the electrode surface are observed. The first type of biosensor is less serviceable because of the slow equilibrium process. However, the third type of biosensor, because of the high potential of action and irreproducible response, is also not valuable. The working potential of 0.35 V is selected for optimal urease-CB electrode operation, and the response properties of the electrode are also characterized. The biosensor possesses a linear range of response up to 5 mM of urea, a coefficient of variation equaling 3.7%, a response time of 1.5 min. The biosensor is tested for urea detection in milk.