Screen-printed enzyme electrode for the determination of lactose

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Abstract
An amperometric lactose sensing electrode based on thick-film technology was developed. The biosensor was based on β-galactosidase and glucose oxidase co-immobilized by cross-linking with glutaraldehyde onto the working electrodes. The enzymically generated H2O2 was monitored at 600 mV versus an Ag–AgCl reference electrode. The assay buffer was of great importance for the sensor response. Potassium citrate buffer (pH 7.0) was found to supply the best results. The buffer had to include magnesium ions, because these activate β-galactosidase. Compared with buffers without magnesium, the response increased approximately four times. The sensor showed linearity over the concentration range 2 × 10–6–2.5 × 10–3 mol l–1(correlation coefficient, r= 0.99922). Under optimum conditions, the sensor was stable for approximately 3 months without noticeable loss of activity. The lactose sensor was used in a batch system in order to determine the lactose content in milk and various dairy products. No interferences could be detected during the measurement of real samples and no sample pre-treatment was necessary. Sensor data for the determination of lactose were compared with those obtained by the Boehringer Mannheim test-kit method (ultraviolet method)(r= 0.9827, n= 26). The sensor method had a low relative standard deviation (8.81%, n= 16).