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A conductometric enzyme biosensor using proteinase K was developed and then tested to relate its electrical signal to the number of proteinase K hydrolysis sites in bovine serum albumin (BSA) and angiotensin, a ten amino acid peptide, with one cleaving site.The conductometric sensor presents a large linear range of response for BSA and angiotensin ranging from 0.5 to 8 mg/l and from 4 to 8 mg/l, respectively. For a same tested concentration (mg/l), the response for native BSA is 12 times higher than for angiotensin. Aspartam was used as negative test and no response was obtained with the proteinase K biosensor. The conductometric sensor permitted also to detect difference on enzyme activity on native and non-native BSA, a response three times higher was obtained for non-native BSA. Keywords: Conductometric biosensor, Proteinase K, Bovine serum albumin, Angiotensin, Aspartam

We report a new approach for immunoassays based on magnetite nanoparticles for Escherichia coli (E. coli) detection using conductometric measurements. Biotinylated antibodies, anti-E. coli, were immobilized on streptavidin modified magnetite nanoparticles by biotin–streptavidin interaction. A layer of functionalized nanoparticles were directly immobilized on the conductometric electrode using glutaraldehyde cross-linking.The specific test with E. coli cells and the non specific test using Staphylococcus epidermidis (S. epidermidis) were investigated by conductometric measurements. Results show a good response as a function of antigen additions. The detection of 1 CFU/ml of E. coli induces a conductivity variation of 35 μS. The negative test shows good selectivity using the conductometric immunosensor. Conductometric measurements allow to detect 500 CFU/l. Keywords: E. coli, Antigen–antibody interaction, Conductometric immunobiosensor, Magnetite nanoparticles, Atomic force microscopy