A mechanical actuator driven electrochemically by artificial molecular muscles

Experimental demonstration of a single-molecule electric motor

For molecules to be used as components in molecular machines, methods that couple individual molecules to external energy sources and that selectively excite motion in a given direction are required. Significant progress has been made in the construction of molecular motors powered by light and by chemical reactions, but electrically driven motors have not yet been built, despite several theoretical proposals for such motors. Here we report that a butyl methyl sulphide molecule adsorbed on a copper surface can be operated as a single-molecule electric motor. Electrons from a scanning tunnelling microscope are used to drive the directional motion of the molecule in a two-terminal setup. Moreover, the temperature and electron flux can be adjusted to allow each rotational event to be monitored at the molecular scale in real time. The direction and rate of the rotation are related to the chiralities of both the molecule and the tip of the microscope (which serves as the electrode), illustrating the importance of the symmetry of the metal contacts in atomic-scale electrical devices.

Surface-enhanced Raman spectroscopy to probe reversibly photoswitchable azobenzene in controlled nanoscale environments

We apply in situ surface-enhanced Raman spectroscopy (SERS) to probe the reversible photoswitching of azobenzene-functionalized molecules inserted in self-assembled monolayers that serve as controlled nanoscale environments. Nanohole arrays are fabricated in Au thin films to enable SERS measurements associated with excitation of surface plasmons. A series of SERS spectra are recorded for azobenzene upon cycling exposure to UV (365 nm) and blue (450 nm) light. Experimental spectra match theoretical calculations. On the basis of both the simulations and the experimental data analysis, SERS provides quantitative information on the reversible photoswitching of azobenzene in controlled nanoscale environments.

The switching of rotaxane-based motors

Linear molecular motors have recently attracted considerable interest. In this paper we use molecular dynamics simulations to investigate the structural and energetic properties of neutral and oxidized [2]rotaxane motors. We first consider a neutral structure to identify the stable configuration of a bistable [2]rotaxane whose ring component is on the tetrathiafulvalene (TTF) recognition site, followed by a study of the dynamic switching process of an oxidized [2]rotaxane. The study shows that for a neutral structure the ring component stays at the TTF station in both free and constrained situations. When the station is oxidized, the ring is pushed to the other station and the dynamic switching process finishes in about 10 ns. By comparing the results for the cases of free and fixed ends, the simulations show that structural deformation plays an important role during the switching process and can significantly affect the displacement output of molecular motors.

Fullerenol-based electroactive artificial muscles utilizing biocompatible polyetherimide

Two essential functional requirements for electroactive artificial muscles, which can be used for biomedical active devices, are biocompatibility and sufficient range of motion. Fullerenol nanoparticles and their derivatives have been validated as potential candidates to be used for nanobiomaterials and biomedical applications because of their excellent proton conductivity, hydrophilicity, and biocompatibility. We developed fullerenol-based electroactive artificial muscles utilizing biocompatible polyetherimide. By using a solvent recasting method, present ionic networking membranes have been successfully synthesized with homogeneous dispersion of polyhydroxylated fullerene (PHF) nanoparticles into a sulfonated polyetherimide (SPEI) matrix. In comparison with pure SPEI membranes, the PHF-SPEI nanocomposite membranes show much higher water uptake and proton conductivity, which are both essential characteristics for high-performance ionic polymer actuators. The developed PHF-SPEI actuator shows over three times larger motion ranges and two times higher blocking forces than the pure SPEI actuator. The excellent biocompatibility of PHF and SPEI makes these actuators promising candidate materials for biomedical devices such as active stents and catheters.

High-performance, low-voltage, and easy-operable bending actuator based on aligned carbon nanotube/polymer composites

In this work, we show that embedding super-aligned carbon nanotube sheets into a polymer matrix (polydimethylsiloxane) can remarkably reduce the coefficient of thermal expansion of the polymer matrix by two orders of magnitude. Based on this unique phenomenon, we fabricated a new kind of bending actuator through a two-step method. The actuator is easily operable and can generate an exceptionally large bending actuation with controllable motion at very low driving DC voltages (<700 V/m). Furthermore, the actuator can be operated without electrolytes in the air, which is superior to conventional carbon nanotube actuators. Proposed electrothermal mechanism was discussed and confirmed by our experimental results. The exceptional bending actuation performance together with easy fabrication, low-voltage, and controllable motion demonstrates the potential ability of using this kind of actuator in various applicable areas, such as artificial muscles, microrobotics, microsensors, microtransducers, micromanipulation, microcantilever for medical applications, and so on.

Switching around two axles: controlling the configuration and conformation of a hydrazone-based switch

A hydrazone-based rotary switch, having a quinolinyl stator and a pyridine ring as part of the rotor, can be induced using pH to undergo a four-step switching sequence. This process yields three stable isomers and a fourth "metastable" one that can all be addressed separately based on the sequence of acid and base added. The switching process proceeds via conformational and/or configurational changes, allowing the molecule to rotate around two different axles.

Protein thin film machines

We report the first example of microcantilever beams that are reversibly driven by protein thin film machines fueled by cycling the salt concentration of the surrounding solution. We also show that upon the same salinity stimulus the drive can be completely reversed in its direction by introducing a surface coating ligand. Experimental results are throughout discussed within a general yet simple thermodynamic model.

Changing stations in single bistable rotaxane molecules under electrochemical control

We have directly observed electrochemically driven single-molecule station changes within bistable rotaxane molecules anchored laterally on gold surfaces. These observations were achieved by employing molecular designs that significantly reduced the mobility and enhanced the assembly of molecules in orientations conducive to direct measurement using scanning tunneling microscopy. The results reveal molecular-level details of the station changes of surface-bound bistable rotaxane molecules, correlated with their different redox states. The mechanical motions within these mechanically interlocked molecules are influenced by their interactions with the surface and with neighboring molecules, as well as by the conformations of the dumbbell component.

Bilability is defined when one electron is used to switch between concerted and stepwise pathways in Cu(I)-based bistable [2/3]pseudorotaxanes

Supramolecular switches operate as simple machines by using a stimulus to turn stations off and on, generating thermodynamic differences that define bistability and enable motion. What has not been previously investigated, yet is required to gain further control over molecular movements for complex operations, is an understanding of how the same stimulus can also switch pathways off and on, thus, defining the kinetic property of bilability. To address this challenge, the mechanisms of the forward and return reactions of redox-switchable Cu(I)-based [2/3]pseudorotaxanes have been quantitatively characterized utilizing mechanistic cyclic voltammetry and employing a series of isosteric bis-bidentate ligands. First, the bistability of the switch is retained across the series of ligands: Reduction of the ligand drives the reaction forward where a [2]pseudorotaxane switches into a reduced [3]pseudorotaxane and reoxidation drives the switching cycle back to the beginning. Second, the switch is bilabile with the forward reaction following an association-activated interchange pathway (concerted), whereas the reverse reaction follows a different dissociation-based dethreading pathway (stepwise). The forward reaction is more sensitive to denticity (bidentate tetrazinyl ligand, k(2) = 12,000 M(-1) s(-1), versus the monodentate pyrazinyl ligand, k(2) = 1500 M(-1) s(-1)) than to electronics (k(2) = 12,000 M(-1) s(-1) for methyl and trifluoromethyl substituents). The rate of return with the pyrazinyl ligand is k(1) = 50 s(-1). Consequently, both the mechanism and the thermodynamics of switching are stimuli dependent; they change with the oxidation state of the ligand. These findings have implications for the future design of molecular motors, which can be built from systems displaying allosterically coupled bistability and bilability.

Approaches to increasing surface stress for improving signal-to-noise ratio of microcantilever sensors

Microcantilever sensor technology has been steadily growing for the last 15 years. While we have gained a great amount of knowledge in microcantilever bending due to surface stress changes, which is a unique property of microcantilever sensors, we are still in the early stages of understanding the fundamental surface chemistries of surface-stress-based microcantilever sensors. In general, increasing surface stress, which is caused by interactions on the microcantilever surfaces, would improve the S/N ratio and subsequently the sensitivity and reliability of microcantilever sensors. In this review, we will summarize (A) the conditions under which a large surface stress can readily be attained and (B) the strategies to increase surface stress in case a large surface stress cannot readily be reached. We will also discuss our perspectives on microcantilever sensors based on surface stress changes.

Double-layer mediated electromechanical response of amyloid fibrils in liquid environment

Harnessing electrical bias-induced mechanical motion on the nanometer and molecular scale is a critical step toward understanding the fundamental mechanisms of redox processes and implementation of molecular electromechanical machines. Probing these phenomena in biomolecular systems requires electromechanical measurements be performed in liquid environments. Here we demonstrate the use of band excitation piezoresponse force microscopy for probing electromechanical coupling in amyloid fibrils. The approaches for separating the elastic and electromechanical contributions based on functional fits and multivariate statistical analysis are presented. We demonstrate that in the bulk of the fibril the electromechanical response is dominated by double-layer effects (consistent with shear piezoelectricity of biomolecules), while a number of electromechanically active hot spots possibly related to structural defects are observed.

Direct observation of bifunctional electrocatalysis during CO oxidation at Ru(theta=0.37)/Pt{111} surfaces via surface stress measurements

The surface stress response during the electrooxidation of CO at Pt{111}, Ru{0001}, and Ru(theta=0.37)/Pt{111} textured electrodes was studied in 0.1 M HClO(4) electrolytes. The surface stress signal resolves for the first time the adsorption of OH(-) at the CO-covered Ru{0001} surface prior to significant CO oxidation, a phenomenon that is not discernible in the voltammetry. The surface stress signal shows that significant tensile surface stress occurs upon oxidation of the adsorbed CO and occurs at nearly the same potential on Ru{0001} and Ru/Pt{111} surfaces. These observations demonstrate that the mechanism of bifunctionality is the OH(ads) provided to the Pt surface sites via Ru sites.

A metal-organic framework replete with ordered donor-acceptor catenanes

A metal-organic framework was constructed from struts in which donor-acceptor [2]catenane units become integrated, leading to a high density of these molecular machinery modules positioned precisely in well-defined layered (2D) structures.

Mechanically bonded macromolecules

Mechanically bonded macromolecules constitute a class of challenging synthetic targets in polymer science. The controllable intramolecular motions of mechanical bonds, in combination with the processability and useful physical and mechanical properties of macromolecules, ultimately ensure their potential for applications in materials science, nanotechnology and medicine. This tutorial review describes the syntheses and properties of a library of diverse mechanically bonded macromolecules, which covers (i) main-chain, side-chain, bridged, and pendant oligo/polycatenanes, (ii) main-chain oligo/polyrotaxanes, (iii) poly[c2]daisy chains, and finally (iv) mechanically interlocked dendrimers. A variety of highly efficient synthetic protocols--including template-directed assembly, step-growth polymerisation, quantitative conjugation, etc.--were employed in the construction of these mechanically interlocked architectures. Some of these structures, i.e., side-chain polycatenanes and poly[c2]daisy chains, undergo controllable molecular switching in a manner similar to their small molecular counterparts. The challenges posed by the syntheses of polycatenanes and polyrotaxanes with high molecular weights are contemplated.