Towards the nanomechanical actuation and controlled assembly of nanomaterials using charge-transfer reactions in electroactive self-assembled monolayers

Surface-functionalized microcantilevers provide an ideal platform for nano- and micro-mechanical actuation and highly sensitive sensing technologies.  The basic principle of operation is that a chemical or physical event occurring at the functionalized surface of one side of the cantilever generates a surface stress difference (between the active functionalized and passive non-functionalized sides) that causes the cantilever to bend away from its resting position.  However, the factors and phenomena contributing to both the nature and magnitude of the surface stress are not well understood.  To this end, the first part of this thesis focused on investigating the potential-controlled actuation and surface stress properties of free-standing gold-coated microcantilevers functionalized with a redox-active self-assembled monolayer (SAM).   A ferrocenylundecanethiolate (FcC11SAu) SAM on a gold-coated cantilever was used as a model system to investigate the surface stress generated by faradaic chemistry. The data obtained clearly demonstrates that the electrochemical transformation of a ferrocene moiety in a monomolecular organic film can generate a surface stress change of sufficient magnitude to deflect a microcantilever.  In fact, depending on the flexibility of the microcantilever, the mechanical deflection resulting from the redox transformation of the surface-tethered ferrocene can range on the order of nanometers to micrometers.  The oxidation of the FcC11SAu SAM in perchlorate electrolyte generates a compressive surface stress change.  The microcantilever deflection is driven by the lateral tension resulting from molecular reorientation/volume expansion accompanying the charge-transfer and ion-pairing events.  To verify this hypothesis, mixed SAM-modified microcantilevers, in which the electroactive ferrocenes are isolated from one another by an inert n-alkylthiolate matrix, were investigated.  Under an applied potential, a Faradaic current was measured, but no microcantilever beam deflection was observed.   This finding confirms that the cantilever responds to the lateral pressure exerted by an ensemble of re-orienting ferrocenium-bearing alkylthiolates upon each other rather than to individual anion pairing events.  Changes in molecular structure and anion type can also be used to modulate the extent of micromechanical motion. In the next part of the dissertation, electrochemical measurements and surface plasmon resonance spectroscopy were combined to present a description of the adsorption and aggregation of n-alkyl sulfates at the FcC11SAu/electrolyte interface.  At all bulk solution concentrations, the surfactant moieties packed perpendicular to the electrode surface in the form of an interdigitated condensed film.  However, the density of the specifically adsorbed film was found to be affected by the organizational state of the surfactants in solution.  At low concentrations, where the surfactant molecules are present as solvated monomers, the monomers can readily adapt to the changing ferrocenium concentration with the potential potential scan.  However, when the molecules are present as micellar structures in solution, a lower surfactant packing density was found because of the inability to respond effectively to the dynamically generated surface ferroceniums.  This research demonstrates the potential utility of charge-transfer interactions for organizing materials at solid interfaces and effecting micromechanical actuation using an electrifical stimulus.

Keyphrases: Surface stress, Microcantilever, Ferrocene, Self-assembled monolayer, Electrochemistry, Surfactant aggregation, Surface plasmon resonance