Synthesis and biosensing application of highly water-soluble and cross-linkable poly(p-phenyleneethynylene) containing osmium(II) complex and aldehyde groups

A poly(p-phenyleneethynylene) (PPE) containing pyridine and acetal groups (polymer A) was prepared by a palladium-catalyzed Sonogashira coupling reaction. Osmium complex-grafted PPE (polymer B) was prepared by post-polymerization functionalization of the polymer A via a chelate reaction of the pyridine groups with Os(bpy)2Cl2 (bpy: 2,2′-bipyridine). A cross-linkable PPE functionalized with osmium complex and aldehyde groups (polymer C) was obtained by deprotecting the acetal groups of polymer B in a hydrochloric acid solution. Both polymers B and C are highly soluble not only in common solvents such as THF, ethanol, methanol, acetone, ethyl acetate, DMF and DMSO, but also in water. The water-soluble polymer C was used to cross-link and assemble glucose oxidase (GOx) onto amine-terminated monolayers at a gold electrode via a coupling reaction of its aldehyde groups with amine groups of the enzyme, an added polyamine cross-linking agent (triethylenetetraamine), and monolayers via formation of initial Schiff bases, which were reduced to secondary amine groups by using cyanoborohydride, a mild reducing agent. The polymer provided very efficient wiring for the enzyme, resulting in an efficient electron transfer of GOx to the electrode. Such enhancement of electron transfer is due to the hydrophilic, long and flexible tethers of osmium complex to the polymer backbone and a close contact between the polycationic polymer and polyanionic GOx via electrostatic interactions. The biosensor displayed highly sensitive responses to glucose with the apparent Michaelis constant of 8.2 mM, and good stability without substantial change of current response to glucose for 1-month storage in phosphate buffer solution (pH 7.0).