|Ph.D Student||Talmon Yael|
|Subject||The Effect of Water on Surface Electronchemistry|
and Binding Properties of Switchable Self-
|Department||Department of Nanoscience and Nanotechnology||Supervisors||Professor Uri Sivan|
|Professor Yoram Reiter|
|Full Thesis text|
Switchable self-assembled monolayers (SAMs) of alkanethiol on gold are designed to display two distinct redox states separated by a clear reduction/oxidation barrier. In conjunction with biomolecules that bind selectively to one of the redox states, such switchable SAMs may provide a viable route to the much desired electrical control over biological processes. Electrochemically active monolayers of quinone terminated alkanethiols are particularly attractive in that respect due to their stable redox states and reproducible electrochemistry under biologically compatible conditions. Another advantage of the quinone system is its neutral charge in both redox states. This neutrality minimizes the effect of Coulomb interactions while leaving other binding mechanisms intact. The remaining interactions are hence independent of electrolyte conditions that vary substantially in the biological environment. Finally, quinones play a central role in electron transfer processes in-vivo. Understanding their hydration and its interplay with electron transfer processes provides valuable biochemical insights regardless of our main motivation.
In the interface with biomolecules, an electrochemical signal applied to the quinone terminated monolayer changes its redox state and consequently affects protein binding to the surface. Past studies, carried out in our lab, indicated the importance of layer hydration for the modulation of protein binding to the surface and the aim of the current study was to gain deeper understanding of the mutual interplay between layer electrochemistry, its redox state dependent hydration, and the effect of hydration modulation on selective protein binding to the monolayer.
Our study established that the lower affinity of proteins to the Quinone monolayer in its reduced hydroquinone form, compared with the oxidized benzoquinone one, results from the binding of a single additional water molecule to each hydroquinone ring.
Using surfaces of quinones mixed with hydrophilic or hydrophobic inert molecules, we were able to show that the overall hydrophobicity level of the quinone affected not only protein affinity but also the properties of the quinone redox reaction itself. The redox reaction which affected layer hydration was therefore found to be influenced by the same hydration. Electrochemistry and layer hydration were linked this way in an inseparable way. The level of surface hydration was found to tilt the equilibrium balance between the reduced and oxidized forms. It was found that monolayers that mixed alkanethiol-quinone with hydrophobic methyl terminated alkanethiols drove the equilibrium towards the more hydrophobic benzoquinone state while in monolayers mixed with hydrophilic PEG molecules this preference for benzoquinone was not found. A similar trend was observed in the redox reaction kinetics as judged from cyclic voltammetry and long-term layer stability. We have also detected in this study that quinone multilayers formed readily and their hydrophobicity level increased with molecule density. In accordance with the findings described above, this increase of bulk hydrophobicity with surface density led to interesting features in the redox reaction. Finally, we found that the enhanced non-specific protein adsorption to the monolayer in its hydrophobic benzoquinone state limited the selectivity of specific antibodies to the monolayer. This finding led to the conclusion that enhanced specificity requires less hydrophobic surfaces.