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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 goldcoated microcantilevers modified with a model, redox-active ferrocenylundecanethiolate self-assembled monolayer (FcC 11 SAu SAM). The microcantilever transducer enabled the observation of the redox transformation of the surface-confined ferrocene. Oxidation of the FcC 11 SAu 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 chargenormalized surface stress of the FcC 11 SAu 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 FcC 11 SAu system compared to the conducting polymer multilayers used for electroactuation. The larger stress change observed for the FcC 11 SAu microcantilever is attributable to steric constraints in the close-packed FcC 11 SAu SAM and an efficient coupling between the chemisorbed FcC 11 S-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 FcC 11 S-/C 11 SAu 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 FcC 11 SAu 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.

and Regroupement qu eb ecois sur les mat eriaux de pointe, C.P. 6128 succursale Centre-ville, Montr eal, QC H3C 3J7 Canada b S Supporting InformationABSTRACT: The redox-activated deflection of microcantilevers has attracted interest for chemical sensing and nanoactuation. However, the development and optimization of this type of microcantilever transduction requires a better understanding of the effect of the particular system parameters on the surface stress changes that cause the measured bending response. We investigate here the effects of the adsorbate structure and electrolyte anion on the surface stress generated by the electrochemical oxidation of ferrocene-terminated self-assembled monolayers (SAMs) chemisorbed to the surface of gold-coated microcantilevers. Ferrocenylalkanethiolate-modified cantilevers are an interesting system for study as the electroactive monolayer can induce charge-normalized surface stress changes that are at least 10-fold greater than those generated by multilayers of the conducting polymers commonly used for electroactuation. The resonance angle shifts measured by surface plasmon resonance spectroscopy in the presence of ClO 4 -suggest that the extent of the oxidationinduced SAM reorganization is the same for short (n = 6) and long (n = 12) chain ferrocenylalkanethiolate (FcC n SAu) SAMs and for a long chain carbonyl derivative (Fc(CO)C 11 SAu). The magnitude of the measured cantilever deflection is however not the same for the different SAMs, reflecting differences in the tensile contributions to the overall surface stress of the SAM elasticity and wettability versus the compressive lateral pressure generated by the collective molecular reorientations induced by the pairing of anions to the surface-confined, oxidized ferrocenium cations. The hydrophobic anions PF 6 -and ClO 4 -, which form 1:1 contact ion pairs with the SAM-bound ferrocenium cations, give reversible cantilever deflections and the largest compressive surface stress changes. By contrast, oxidation of the FcC 11 SAu SAM in NO 3 -and F -, which exhibit the poorest ion-pairing abilities of the anions investigated, results in an irreversible deformation of the cantilever bending and smaller stress changes. The surface phenomena that give rise to the observed differences in the surface stress as a function of the ion pairing ability are discussed.