Biology and classical technology are separated by a large gap of mutual optimization. Applications which aim to combine both worlds are likely to fall in this gap... This is also true for applications where proteins come into contact with solid electrodes. Neither are proteins evolutionarily optimized for contact to a solid, nor is most of solid state electronics compatible with the liquid environment of a protein. The concept of a solid protein surface tries to bridge this gap to safely carry applications that try to combine advantages of both worlds.
One main motivation to combine solid electrodes with proteins is the world-record catalytic activity of proteins. For an impression of proteins in action I highly recommend Drew Berry's TED talk
. Proteins are the only realistic prospect for Ångström technology! A sheer unbelievable amount of it is used by nature even in simpler catalysts. Shown is the reaction center of laccase, with the distances between the metal atoms given in Ångström. Such a catalyst could be used in biofuel cells, where "combustion" of a fuel with oxygen can drive an electric current over a resistor. The kinetics of the reactions at the electrode surfaces of such a fuel cell is what determines its efficiency. And this is where the biocatalysts are needed!
To use proteins as catalysts in an energy conversion system, they have to be immobilized on the electrode surface. This immobilization has to be stable in two respects: the protein needs to stay on the surface, but also its three dimensional structure needs to remain stable. Counterintuitively, the latter is the tricky part - remember the optimization gap? The conventional solution to this is the introduction of a chemically variable intermediating layer, which is compatible to the surface chemistry of the protein. The solid protein surface should do the same with a variable surface chemistry of the electrode surface - only that now no additional electron tunneling barrier lies between the reaction center of the protein and electronic states in the electrode, which allows fast electron transfer.
To find the ideal surface chemistry for the solid electrode, a look at the protein-protein binding of natural binding partners is worthwhile. Often, the main driving force for binding is hydrophobic interaction, while selectivity is mediated by electrostatic interaction.
The solid protein surface should mimic this interplay of hydrophobic and hydrophilic binding areas. The surface of diamond electrodes can offer this by adjustment of the surface chemistry between hydrogen and oxygen termination.
As simple as the adjustment of the surface chemistry sounds, as difficult is its realization. The main challenge is to find techniques to create and maintain an atomically clean surface, which do not damage the surface.
With electrochemical hydrogen termination, a suitable technique was found to reproducibly generate an atomically clean hydrogen terminated diamond surface. This surface is hydrophobic, due to the low polarity of the C-H bond.
The inverse process, electrochemical oxygen termination can be used to generate an electrode surface with a precisely defined ratio of oxygen and hydrogen termination and therefore a defined degree of hydrophobicity.
With electrochemical surface termination, electrodes with different oxidation degrees can be produced and the respective protein activity can be tested. Two example proteins show an activity maximum at intermediate oxidation degrees.
An example electron transfer protein shows 100 times faster electron transfer on the optimized diamond electrode than on unoptimized solid electrode surfaces. This surface is even competitive to self assembled monolayers (SAMs).
The optimized activity and electron transfer velocity can be explained by an ideal combination of electrostatic and hydrophobic orientation of the protein and the surface, while denaturation is prevented.
The volume of maximum activity can be small in the multidimensional state space of a protein on a solid surface. This state space has at least the hydrophobicity and electrochemical potential of the electrode and the properties of the solvent (ion concentration and type, pH) as relevant dimensions.
Electrochemical surface termination of diamond electrodes provides the technological framework for the realization of the solid protein surface.
This framework allows valuable insight in the biophysics of protein-solid interaction.
At least for some proteins, the nanofractional control over the electrode hydrophobicity allows generation of a protein-mimetic electrode surface with optimized protein activity.