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Magnetic Fe 3 O 4 nanoparticles (Fe 3 O 4 NPs) were synthesized by co-precipitation (of Fe 2+ and Fe 3+ ) and surface-carboxylated by poly(styrene-co-acrylic acid) (PSA). Then they were thoroughly characterized by Scanning Electron Microscope and Transmission Electron Microscope analyses. Using glutaraldehyde cross-linking immobilization strategy, cholesterol oxidase (ChOx) was immobilized onto Fe 3 O 4 /PSA microspheres. The Fe 3 O 4 /PSA-ChOx has been successfully synthesized on the planar platinum polyaniline (PANi) modified electrode. PANi/Fe 3 O 4 /PSA-ChOx sensing properties were studied using cyclic voltammetric and chronoamperometric techniques. Under the optimized experimental conditionsthe biosensor shows excellent features, such as wide linear range, 0.2 to 1.8 mM (R 2 ¼ 0.9901), low detection limit (0.02 mM), short response time (5-10 s), and high sensitivity (8796 mA mM À1 cm À2 ). The interference test with selected organic compounds, such as glucose, ascorbic acid, uric acid, and acetaminophen indicated high selectivity of the above sensor. It can be proven that the proposed PANi/ Fe 3 O 4 /PSA-ChOx based sensing concept could be successfully used for other biodetections.

. Electroactive poly(1,8-diaminonaphthalene) is known to have a high affinity for metal ions thanks to amine and imine groups in the polymer chain. However, electrochemical sensors based on pristine P(1,8-DAN) have a major drawback concerning its poor electrical conductivity. To solve this problem, recently P(1,8-DAN) has been modified with some advanced nanomaterials such as carbonaceous materials or different metallic elements. In this research, we reported the synthesis and electrochemical characterization of a poly(1,8-diaminonaphthalene)/graphene composite film capable of adsorbing Cu2+ ions towards the application of nitrite sensing. P(1,8-DAN) was directly electropolymerized on graphene-coated glassy carbon electrode by a potential cycling between –0.15 and +0.95 V (vs. SCE) at a scan rate of 0.05 V/s, in aqueous solution  containing 1.0 M HClO4 and 1.0 mM monomer 1,8-DAN,. The adsorption of Cu2+ ions onto the P(1,8-DAN) thin film was caried out in 0.1 M Cu(NO3)2 solution at 80 oC, followed by electrochemically redution to metal Cu0 by applying -0.4 V. The obtained copper decorating poly(1,8-diaminonaphthalene)/graphene (Gr/P(1,8-DAN)-Cu) electrodes acted as a catalyst in the enhancement of electrochemical signal for the determination of nitrite. The linear voltammetric response to the nitrite concentration was observed by a square wave voltammetric technique in the range of 0.69 to 1.12 mM with a detection limit of 0.13 mM. The results open up the path for designing other nitrite sensing based on our novel approach.