Optical urea biosensor based on ammonium ion selective membrane

An optical urea biosensor was developed by immobilizing an urease enzyme layer on a thin ammonium-selective polymer membrane. The ammonium optical membrane utilized dichlorofluorescein octadecyl ester (DCFOE) as anionic chromophore and nonactin as neutral ionophore. The urease layer was coated on the top of the ammonium layer by gelatin entrapment combined with glutaradehyde cross-linking. Hydrolysis of urea catalyzed by urease produced ammonium ion, which was extracted into the polymer film to form complexes with nonactin. A proton was released which resulted in a color change of the optical membrane due to charge neutrality principle. The biosensorshowed response to urea in the range of 10(-5) to 0.1 M in 0.1 M Tris buffer at pH 7.4. The response time is 3-8 min. under non-stir conditions and it is completely reversible. The biosensor was found to be reliable in diluted serum samples for urea detection from 0.1 to 10 mM and good correlation was obtained with a potentiometric urea sensor.