Reports: DNI748917-DNI7: Reconstituting Enzymes for Direct Electron Transfer through Surface Initiated Polymerization of Conjugated Polymers

Jason Locklin, PhD , University of Georgia

To utilize bioelectronic power sources powered through enzymatic reactions at the electrodes surface, this project is aimed at improving the electrical contact between the protein and electrode surfaces through conducting polymer wires grown through surface initiated polymerization of conjugated polymers. For most redox active proteins, the electroactive cofactors are buried deep within an insulating protein shell, which causes poor direct electron transfer (ET) between proteins and an electrode surface. The chemical nature of the linker molecule, which connects the electrode to the redox site on the protein, is important since electrons must be shuttled along some conductive pathway (or tunnel through some barrier) to the electrode surface. Self-assembled monolayers (SAMs), especially alkane thiols on gold, have been employed as linkers to attach enzymes to electrode surfaces. The electron transfer rate constant in these systems decays exponentially with distance. Enhancements in the ET rate have been observed in molecules containing delocalized bridges (“molecular wires”) based on oligo(phenylene vinylene) and oligo(phenylene ethynylene) with redox active species attached to the end of the conjugated bridge. These systems have also shown distance independent ET rates up to ~ 3 nm. The problem with conjugated tethers is that they are difficult to synthesize, have limited synthetic versatility, and become increasingly insoluble with extended conjugation length. To overcome the electron transfer bottleneck in biofuel cells, the Locklin group has developed the synthetic methodology that allows for the direct growth of conjugated polymers from electrode surfaces using surface-initiated polymerization, where conjugation length (and film thickness) is controlled by reaction time and monomer concentration.

The initial success in the first year of our project involved determining conditions suitable for the formation of surface-initiated Kumada-type catalyst-transfer polycondensation (SI-KCTP).  This has led to densely packed conjugated polymer films that vary in thickness from 5-150 nm.  We were successful in doing this, but there was a considerable amount of work remaining to fully understand the mechanism of polymer growth and thereby optimize the conditions for film formation.  In the second year of the project, we have further explored this surface mediated coupling reactions and developed a more versatile method for fabricating SI-KCTP initiators on a variety of planar substrates incorporating bidentate phosphine ligands has been demonstrated. Indirect evidence of interfacial disproportionation reactions with nickel catalysts were observed electrochemically through the use of an aryl organomagnesium ferrocene probe. With respect to all self-assembled monolayers studied, incorporation of a methyl substituent ortho to the halogen on the thiophene ring helps prevent disproportionation, however an overall lower yield of initiators was observed due to the reduced packing density of SAMs containing the bulky methyl substituent. Initiators formed with phosphonic acids (for binding with transparent electrodes such as indium tin oxide (ITO)) anchor groups did not demonstrate any significant disproportionation reactions among the bidentate phosphine ligands used, since molecules on ITO are not as densely packed as SAMs formed on Au. We also observed that the use of LiCl or high concentration of Grignard reagent was incompatible with alkanethiol monolayers.  Aryl(Ni(II)-Br monolayers could then be used to fabricate poly(3-methylthiophene) films on SiO2 and ITO surfaces. Uniform P3MT films with thicknesses between 40 and 65 nm were characterized using a variety of techniques. Studies using these films as modified interfaces in organic electronics are now possible and currently underway in our laboratory.  We have also started to study the use of other metal catalysts to overcome the problems of disproportionation.  Pd catalysts have shown to be the most effective, and films that are densely packed, with a high grafting density have been observed.  Thicknesses greater than 300 nm are now possible using this catalyst combination.

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