Abstract: The promising advantages of Prussian Blue (PB) as catalyst and of the thick film screen printing technology have been combined to assemble sensors with improved characteristics for the amperometric determination of H2O2. PB-modified screen printed electrodes were applied to detect H2O2 at an applied potential of -0.05 V versus the internal screen printed Ag pseudoreference electrode, showing a detection limit of 10^-7 mol l-1, a linearity range from 10^-7 to 5 x 10^-5 mol l-1, a sensitivity of 234 µA mmol L-1 cm-2, and a high selectivity. Improved stability at alkaline pH values was also observed, which made possible their use with enzymes having an optimum basic pH. Then, the immobilization of a single enzyme (glucose oxidase (GOD) or choline oxidase (ChOX)) or of two enzymes, acetylcholinesterase (AchE) coimmobilized with ChOX, has been performed on the surface of PB modified screen-printed electrodes (SPEs) using glutaraldehyde and Nafion(R). ChOX has been selected as an example of enzyme working at alkaline pH. The choline biosensors showed a detection limit of 5 x 10^-7 mol l-1, a wide linearity range (5 x 10^-7-10-4 mol l-1), a high selectivity and a remarkable long term stability of 9 months at 4°C, and at least 4 weeks at room temperature. Similar analytical characteristics and stability were observed with the acetylcholine biosensors.

Abstract: A simple and rapid flow system for the determination of lactulose in milli samples was developed. It is based on the hydrolysis of lactulose to galactose and fructose by the enzyme β-galactosidase immobilized in a reactor. The amount of fructose produced was measured with an electrochemical biosensor based on the fructose dehydrogenase enzyme, K-3[Fe(CN)(6)] as mediator and a platinum based electrochemical transducer. Parameters such as the enzyme immobilization in the reactor and under the electrode surface, the lifetime of the β-galactosidase reactor and of the dehydrogenase biosensor and the flow parameters were studied and optimized. Fructose was determined in the range 1 x 10^-6-5 x 10^-3 mol L-1 with an RSD of about 2% and a detection limit of 5 x 10^-7 mol L-1. The use of a microdialysis probe as the sampling system permitted the direct measurement of lactulose in milli samples without pre-treatment in the range 1 x 10^-5-5 x 10^-3 mol L-1. The sensitivity of the procedure allowed pasteurised, UHT and in-container sterilised mill, to be distinguished.