Protein Adsorption Kinetics Under an Applied Electric Field
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The dimensions of the largest naturally occurring molecular species and those of the smallest manmade features converge at the nanoscale. Proteins, the basic building blocks of living organisms, are among the larger molecules in nature with dimensions ranging from 1 to 100 nm. Materials incorporating proteins are therefore true nanomaterials. Of particular importance are thin films of proteins immobilized at a solid substrate. Biosensing, tissue engineering, enzymatic catalysis, and bioelectronics are just a few of the areas in which immobilized layers of proteins play a key role.
The tendency of proteins to attach to interfacial regions is well documented. Ionic, van der Waals, solvation, and donor–acceptor interactions all play important roles in rendering the interfacially adsorbed state to be thermodynamically favored over the solution state. Proteins are colloidal objects, possessing a distribution of surface charge and, in an electrolytic solution, a distribution of weakly associated counterions. Their interaction with a solid substrate is thus expected to be sensitive to the substrate's charge distribution. By controlling the polarization of an adsorbing surface (i.e., applying an electric field), one alters this charge distribution and therefore the surface–protein interaction. This possibility is understandably appealing to those wishing to control the adsorption process, perhaps desiring adsorbed layers of preferred orientation or spatial distribution. However, the interaction between proteins and surfaces is complex and predicting adsorbed layer properties by considering the contributions from the interaction modes listed above remains a significant challenge. Adding an electric field makes the problem even more complex. Thus while influencing an adsorbed protein layer with an electric field is both possible and desirable, the outcome is as yet poorly understood.
In this contribution, we review the field of protein adsorption kinetics under an applied electric field. By focusing on kinetics, we limit ourselves to studies where adsorbed layer properties are measured in situ during the electroformation process. We begin with a brief presentation of certain basic theoretical considerations. We then introduce the methods employed to measure protein adsorption kinetics under an applied electric field. Next, we introduce some of the key results, grouping our presentation by investigator. A perspective on future directions is then given and this is followed by a conclusion. By summarizing some of the key accomplishments and open questions, we hope to guide future efforts to produce nanoscale devices employing adsorbed protein layers formed under an electric field.