THICK-FILM BIOSENSORS FOR UREA

Two types of thick film biosensors with different substrate materials were tested for the preparation of conductivity sensors for the determination of urea. Urea is hydrolyzed by urease to ammonia and carbon dioxide resulting in an increase in conductivity. Urease was immobilized by cross-linking with glutaraldehyde. The response times for both sensor fabrications were approximately 10 min. The first type of urea sensors was fabricated by printing two electrodes on a conventional thick film Al2O3 substrate using a platinum paste. The analytical range was from 0.1 to 50 mmol/l urea with a linear range from 6.1 to 4 mmol/l urea. The second type of thick film urea sensors was fabricated using 'green tape' technology which resulted in multi-layer strips with four electrodes printed with a silver palladium paste. This sensor could be used over a concentration range from 10-mu-mol/l to 5 mmol/l urea. The linear range was limited to 350-mu-mol/l urea. The conductivity devices were characterized by admittance measurements. The admittance and the phase angle were monitored varying the amplitude of the a.c. voltage from 100 mV to 2 V peak-to-peak and the frequency from 0.5 to 50 kHz. The influence of electrolyte concentration was also investigated in the range from 1 mmol/l to 1 mol/l KCl.