TECHNOLOGY FOCUS - BIOLOGY - Preferred Enzyme: The blue copper oxidase enzyme is laccase, human ceruloplasmin, plant ascorbate oxidase, Bilirubin oxidase, fumarate reductase and/or their analogues, preferably laccase. The laccase is a high potential laccase.    TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Material: The electrode or its precursor comprises carbon, gold or silicon. The electrode or its precursor is a carbon electrode or its precursor.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Method: The electrode or its precursor is modified with residue (R) by electro reduction reaction with diazonium salt or heating in presence of the diazonium salt. The electrode or its precursor is further contacted with Nafion, its derivative or ion-exchanged form. Preferred Compound: The diazonium salt is obtained by reaction of amino derivative with nitrite. The aromatic rings in the aromatic cation is (un)substituted by 1, 2 or 3 unsubstituted substituents chosen from halogen atoms and hydroxy, 1-4C alkyl, 2-4C alkenyl, 2-4C alkynyl, 1-4C alkoxy, cyano, nitro, -SCN, -COR, -COOR, -SR, -SO3R, -NR'R", -SO2NR'R", phenyl, 3-7C carbocyclyl, 5- or 6-membered heteroaryl and 5- or 6- membered heterocyclyl groups.R=H or 1-4C alkyl, preferably H or 1-2C alkyl;andR',R"=H, 1-4C alkyl or phenyl.The substituents are halogen atoms and hydroxy, nitro, 1-2C alkyl, -COR -COOR, -SH, -SO3R, cyano and -SCN groups. The aromatic cation is cation of formula: RN2+, or at least one cation of formulae (IA,IB,IC).R=non-fused aromatic moiety comprising 2-4 aromatic rings linked directly or by -CR1=CR1- or -C=C- linker;R1=H or 1-2C alkyl, preferably 2 or 3 phenyl or pyridinyl rings linked directly or by -CH=CH- linker;A= bond, or carbon or nitrogen;B-D=carbon, nitrogen, -NR2-, -O- or -S-;R2=H or 1-4C alkyl;E,F=bond, or carbon or nitrogen;G,H=carbon, nitrogen, -NR2-, -O- or -S-;R1,R2=H or substituent, however R1 and R2 bonds with carbon atoms to form phenyl ring or 5- or 6-membered heteroaryl ring, said phenyl ring or 5- or 6-membered heteroaryl ring being unfused or fused to a further phenyl ring or 5- or 6-membered heteroaryl ring, and the carbon atoms in each ring are (un)substituted by substituent chosen from halogen atoms, hydroxy, 1-4C alkyl, 2-4C alkenyl, 2-4C alkynyl, 1-4C alkoxy, cyano, nitro, -SCN, -COR, -COOR, -SR, -SO3R, -NR'R", -SO2NR'R", phenyl, 3-7C carbocyclyl, 5- or 6-membered heteroaryl and 5- or 6-membered heterocyclyl groups;R,R',R"=same as defined above, however optionally A-H=(=C(=O)), =S(=O)2 or =C(=NOR);I,J=bond, or carbon or nitrogen;m=0, 1, 2 or 3;n=0,1 or 2;p=0,1,2, 3 or 4;Ra=halogen atom, hydroxy, 1-4C alkyl, 2-4C alkenyl, 2-4C alkynyl, 1-4C alkoxy, cyano, nitro, -SCN, -COR, -COOR, -SR, -SO3R or -SO2NR'R";Rb=halogen atom, hydroxy, 1-4C alkyl, 2-4C alkenyl, 2-4C alkynyl, 1-4C alkoxy, cyano, nitro, -SCN, -COR, -COOR, -SR, -SO3R or -SO2NR'R";andRc=halogen atom, hydroxy, 1-4C alkyl, 2-4C alkenyl, 2-4C alkynyl, 1-4C alkoxy, cyano, nitro, -SCN, -COR, -COOR, -SR, -SO3R, -NR'R", -SO2NR'R", phenyl or 5- or 6-membered heteroaryl group.The aromatic ring is phenyl, pyrrolyl, furanyl, thienyl, pyridinyl and/or benzoquinone. The aromatic cation comprises an anthracene or acridinyl residue having -N2+ substituent, preferably anthracene-2- diazonium.