An apparent direct electron transfer between various electrode materials and peroxidases immobilized on the surface of the electrode has been reported in the last few years. An electrocatalytic reduction of hvdrogen peroxide promoted by the immobilized peroxidase starts at about +600 mV (vs. Ag/AgCl) at neutral pH. The efficiency of the electrocatalytic current increases as the applied potential is made more negative and starts to level off at about -100 mV (vs. Ag/AgCl). Amperometric biosensors for hvdrogen peroxide can be constructed with these kinds of peroxidase modified electrodes. By coimmobilizing a hydrogen peroxide-producing oxidase with the peroxidase, amperometric biosensors can be made responding to the substrate of the oxidase within a potential range essentially free of interfering electrochemical reactions. Sensors for L-and D-amino acids are shown based on coimmobilizing HRP with L-amino acid oxidase (L-MOD) or D-amino acid oxidase (D-MOD). The effects of the immobilization procedure, buffer (flow carrier), pH, taken amounts of enzymes, flow rate, and injection volume on the response were investigated. The addition of polvethylenimine (PEI) into the paste was found to increase the response considerably. All 20 of the most common L-amino acids could be detected.

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