Synthesis, properties, and biosensor applications of cycloruthenated 2-phenylimidazoles

The cycloruthenation of 2-phenylimidazole (phim) by [Ru(η6-C6H6)(μ-Cl)Cl]2 in acetonitrile in the presence of NaOH has been carried out. The unstable intermediate [Ru(phim)(MeCN)4]PF6 formed has been converted into the complexes [Ru(phim)(4,4′-Me2bpy)(MeCN)2]PF6 (2) and [Ru(phim)(LL)2]PF6 (3, LL=phen (a), bpy, 4,4′-Me2bpy), which were characterized by the mass-spectrometry, 1H-NMR spectroscopy, UV–vis spectrophotometry, and cyclic voltammetry. The RuII/III redox potentials of complexes 3 equal 130–250 mV (vs. Ag–AgCl) at pH 7 (0.01 M phosphate). Such potential range is favorable for fast exchange of electrons with the active sites of redox enzymes. In fact, the second-order rate constant for the oxidation of reduced glucose oxidase (GO) from Aspergillus niger by the electrochemically generated RuIII derivative of complex 3a equals (8.1×106 M−1 s−1). The second-order rate constant for the oxidation of 3a by the Compound II of horseradish peroxidase is 9.3×107 M−1 s−1. Complexes 3 were used as mediators for the fabrication of enzyme electrodes by simple co-adsorbing with GO or horseradish peroxidase on graphite electrodes. These electrodes were tested in flow-injection systems and showed linear responses in the range of d-glucose and H2O2 concentrations 0.1–30 mM and 1–200 μM, respectively. The new mediators reported herein seem promising for the construction of amperometric biosensors based on GO, horseradish peroxidase, and similar enzymes.