Redox Actuation of a Microcantilever Driven by a Self-Assembled Ferrocenylundecanethiolate Monolayer: An Investigation of the Origin of the Micromechanical Motion and Surface Stress

Abstract

The electrochemically induced motion of free-standing microcantilevers is attracting interest as micro/nanoactuators and robotic devices. The development and implementation of these cantilever-based actuating technologies requires a molecular-level understanding of the origin of the surface stress that causes the cantilever to bend. Here, we report a detailed study of the electroactuation dynamics of gold-coated microcantilevers modified with a model, redox-active ferrocenylundecanethiolate self-assembled monolayer (FcC11SAu SAM). The microcantilever transducer enabled the observation of the redox transformation of the surface-confined ferrocene. Oxidation of the FcC11SAu SAM in perchlorate electrolyte generated a compressive surface stress change of −0.20 ± 0.04 N m−1, and cantilever deflections ranging from ∼0.8 μm to ∼60 nm for spring constants between ∼0.01 and ∼0.8 N m−1. A comparison of the charge-normalized surface stress of the FcC11SAu cantilever with values published for the electrochemical oxidation of polyaniline- and polypyrrole-coated cantilevers reveals a striking 10- to 100-fold greater stress for the monomolecular FcC11SAu system compared to the conducting polymer multilayers used for electroactuation. The larger stress change observed for the FcC11SAu microcantilever is attributable to steric constraints in the close-packed FcC11SAu SAM and an efficient coupling between the chemisorbed FcC11S− monolayer and the Au-coated microcantilever transducer (vs physisorbed conducting polymers). The microcantilever deflection vs quantity of electrogenerated ferrocenium obtained in cyclic voltammetry and potential step/hold experiments, as well as the surface stress changes obtained for mixed FcC11S−/C11SAu SAMs containing different populations of clustered vs isolated ferrocenes, have permitted us to establish the molecular basis of stress generation. Our results strongly suggest that the redox-induced deflection of a FcC11SAu microcantilever is caused by a monolayer volume expansion resulting from collective reorientational motions induced by the complexation of perchlorate ions to the surface-immobilized ferroceniums. The cantilever responds to the lateral pressure exerted by an ensemble of reorienting ferrocenium-bearing alkylthiolates upon each other rather than individual anion pairing events. This finding has general implications for using SAM-modified microcantilevers as (bio)sensors because it indicates that the cantilever responds to collective in-plane molecular interactions rather than reporting individual (bio)chemical events