Microcantilevers Bend to the Pressure of Clustered Redox Centers

Abstract

The redox-activated deflection of microcantilevers has attracted interest for nanoactuation and chemical sensing. Microcantilever sensors are devices that transduce (bio)chemical reactions into a quantifiable nanomechanical motion via surface stress changes. Despite promising applications in analytical science, poor signal-to-noise ratios and a limited understanding of the molecular origins of the surface stress changes that cause the observed deflections remain obstacles to cantilever-based sensing becoming an established (bio)detection method, such as surface plasmon resonance and electrochemistry. We use phase-separated, binary self-assembled monolayers (SAMs) of ferrocenyldodecanethiolate and n-undecanethiolate as a model system to study the effect of the steric crowding of the redox centers on the surface stress change and cantilever deflection produced by the electrochemical oxidation of the surface-tethered ferrocene to ferrocenium. We correlate the measured surface stress change to the fraction of the clustered ferrocenyldodecanethiolate phase in the binary SAMs. The pairing of anions with the sterically crowded clustered ferroceniums induces a collective molecular reorientation which drives the cantilever deflection. The results provide fundamental insights into the response mechanism of microcantilever-based actuating and sensing technologies.

Tuneable Q-Factor of MEMS Cantilevers with Integrated Piezoelectric Thin Films

“…In recent years a large number of silicon based MEMS (micro electro-mechanical systems) sensors and actuators were developed. Besides a high technology readiness level, this success is based on the broad range of different application scenarios covering sensors for e.g., the detection of chemical [ 1 , 2 ] or physical quantities [ 3 , 4 ], what requests, however, an individual and an application-specific design. But, despite any differences, most approaches make use of either membranes or cantilevers as functional key components.…”

Tuneable Q-Factor of MEMS Cantilevers with Integrated Piezoelectric Thin Films

“…Based on this effort, a broad range of different application scenarios such as sensors for the detection of chemical [1,2] or physical quantities [3,4] is covered, leading to an continuously increasing number of MEMS devices which are commercially available today. Despite their individual and application-specific design most approaches make use of either membranes or cantilevers as functional key components.…”

Micromechanical Redox Actuation by Self-Assembled Monolayers of Ferrocenylalkanethiolates: Evens Push More Than Odds

“…We have previously used SAMs of ferrocenyl­alkane­thiolates (Fc­(CH 2 ) n S or FcC n S) of n = 11 or 12 chemisorbed onto gold-coated cantilevers as a model system to investigate the mechano­chemistry of a surface-confined Faradaic reaction. − FcC n SAu SAMs present a low driving voltage ( E °′ ≈ 0.3–0.4 V vs Ag/AgCl) and two stable redox states. , The electro­chemical oxidation of the SAM-bound ferrocene (Fc) to ferrocenium (Fc + ) proceeds via single-electron-transfer and ion-pairing reactions: Fc SAM + X – (aq) ⇌ (Fc + X – ) SAM + e – . , Due to physical crowding of the ferrocenes in the SAM, ion-pair formation induces a structural change in which the alkyl chains adopt a more perpendicular orientation with respect to the underlying surface and the ferrocene units rotate. , We have demonstrated that these re-orientational motions can be transformed into mechanical work (Figure ). FcC n SAu SAMs are particularly interesting for further study because the charge-density-normalized surface stress generated by these ultrathin nanometer-thick layers is at least 10-fold greater than that produced by the oxido­reduction of 100-fold thicker films of conducting polymers commonly used for electro­actuation. ,, Moreover, ion pairing is not per se required to induce a surface stress; in solid-state junctions, electro­static repulsions between the ferroceniums induce an analogous change in the SAM structure. , …”

“…Schematic illustration of the redox-induced molecular re-orientations , proposed to give rise to the cantilever bending. , …”

“…The response of cantilevers functionalized with mixed FcC 12 SAu/CH 3 C 10 SAu SAMs of varying ferrocene surface coverage and degree of phase segregation is consistent with the cantilever bending from the in-plane force exerted by the monolayer expansion resulting from the collective molecular re-orientations induced by the sterically hindered pairing of the anions with the SAM-bound ferroceniums. 10,12,27 The electrochemical oxidation of mixed SAMs consisting of isolated and non-interacting FcC 12 S molecules does not cause any bending (ion pairing is not sterically hindered). 10,12 Structural variances that impact the redox-induced molecular re-orientations could affect the force exerted on the cantilever and magnitude of the surface stress generated.…”