MOLECULAR MUSCLE BASED NANO-ELECTRO-MECHANICAL-SYSTEMS (NEMS)

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2 1 2 2Bala Krishna Juluri, Ajeet S. Kumar, Yi Liu, Tao Ye, Ying-Wei Yang, Amar H. Flood,2 Lei Fang,3J. Fraser Stoddart, 3Paul S. Weiss,' and Tony Jun Huan 'IThe Pennsylvania State University, University Park, PA

2University of California, Los Angeles, CA3Northwestern University, Evanston, IL

ABSTRACTCreated by a bottom-up approach based on self-

assembly and molecular recognition, molecular motorsand muscles such as bistable rotaxanes are recognizedas promising actuation materials for Micro/NanoElectro-Mechanical-Systems (MEMS/NEMS). In thiswork, we report a hybrid NEMS actuation device basedon electrochemical activation of surface-bound bistablerotaxanes (molecular muscles). This NEMS actuatorconsists of a micro-cantilever coated with a self-assembled monolayer of redox-active palindromicbistable [3]rotaxane R8+ molecules, and when subjectedto oxidizing and reducing electrochemical potentialundergoes controllable and reversible bending. Thiswork represents an important step towards the practicalrealization of artificial molecular muscle basedactuators for various NEMS/MEMS applications.

INTRODUCTIONThe past two decades have witnessed huge research

interest from both academia and industry to developnumerous micro/nano electro-mechanical systems(MEMS/NEMS). Many of these systems rely on someform of actuation mechanisms and traditionallyelectrostatic and piezoelectric materials are used inthese devices.[1] However these conventional actuationmaterials require high driving voltages, and have to befabricated using photolithography and other top-downmanufacturing techniques. These fabrication methodslimit the ease of making feature sizes below 100 nm. Amore recent approach to overcome this problem has ledresearchers to fabricate hybrid NEMS/MEMS actuationdevices that are based on an integrated approach ofusing both top-down and bottom-up fabricationsmethods. This integrated approach is more favorabledue to advantages of bottom-up approach which enablesthe elegant employment of molecular machines (eitherbiological or artificial) to work coherently and performmacroscopic work at much larger scales that begins atnanoscale.

A model hybrid NEMS actuation device consists ofa top-down fabricated micro-cantilever with largesurface area and ultralow stiffness constant coated onone side with bottom-up assembled layer of molecularmachines. Various molecular machines have been usedfor hybrid NEMS actuators in the recent past [2-6]. Ourwork towards building artificial molecular machineryfor hybrid NEMS actuator applications was inspired bythe classiness with which skeleton muscles operate innature. In skeleton muscles, chemical energy isconverted to relative mechanical sliding of numerousvery well organized and interlocked myosin moleculesbetween actin filaments.[7] Coherent and cooperative

conformation changes in this system are efficientlyharnessed to obtain macroscopic muscle contraction andexpansion that starts from single molecule. Inspired andin an attempt to imitate this type of energy conversion,palindromic bistable [3]rotaxane R8+ molecule wasdesigned to undergo controlled conformational changesin the presence of external stimulus. As shown in Figure1(a), a palindromic bistable [3]rotaxane R8+ moleculesynthesized by template-directed procedure containstwo spacer separated pairs of redox activetetrathiafulvalene (TTF) and competing naphthalene(NP) stations. Two tetracationic cyclophanic, cyclobis(paraquat-para-phenylene) (CBPQT4+) rings arerecognized by each TTF site through electron donor andacceptor interactions. Under the absence of anyperturbations, the cyclobis rings are designed to stationat TTF sites (ground state) and the removal of one ortwo electrons (oxidation of TTF) from each TTF'sstation causes the rings to immediately move towardsNP stations due to columbic repulsion between the ringand the TTF station (metastable state). Reducing backthe TTF stations with a supply of electrons (reductionof TTF) causes the rings to shuttle back to TTF'sstations (ground state). Disulphide groups are tetheredto each ring of the molecule so that these molecules canform a SAM layer on gold and collective ring motionscan be harnessed efficiently to perform mechanicalwork on micro-cantilever.

There are several advantages of palindromicbistable [3]rotaxane R8+ molecule for usage as actuationmaterials in hybrid NEMS devices. First of all,rotaxanes can generate large strains up to 42%, whilethe strains generated by the gold-standard actuationmaterials - piezoelectric materials - are typically 0.1-0.2% [1]. Second, they have high force density, e.g., abistable rotaxane generates 100 pN force, while akinesin biomotor, which is much larger than a bistablerotaxane, can only generate 6 pN. Third, they canundergo controlled mechanical motion for a variety ofexternal stimuli, while traditional actuation materialsand biomotors must both rely on a single stimulus.Owing to these advantages, in our earlier work, weutilized bistable rotaxanes to build a hybrid NEMSactuator. We showed that by chemically controlling theredox state of individual surface bound [3] rotaxanemolecules attached on the gold coated micro-cantilever,it is possible to perform mechanical work on a micro-cantilever [8, 9]. In principle, the removal or supply ofelectrons to change the molecular conformation ofbistable [3] rotaxane R8+ molecule can be obtained byvarious methods which include chemical,electrochemical and photochemical methods. Incontrast to chemical method, electrochemical and

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photochemical methods have advantages of robustnessand fast operating speeds. In addition they provide away to both "read" and "write" the state of themolecular conformations and therefore helpful incontrolling and monitoring the system. Using UV-VISspectroscopy, we have previously shown that theposition of rings in liquid dispersed palindromicbistable [3]rotaxane R8+ molecules can be controlled byelectrochemical method. However the question whetherelectrochemical method can be used to activate artificialmolecular muscles attached to solid surfaces stillremains unanswered. Herein we report our investigationmade on electrochemical redox control of solid boundbistable [3]rotaxane R8+ molecules and on thepossibility of realizing a hybrid NEMS actuator basedon electrochemical activation of artificial molecularmuscles.

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Fig. 1: Molecular structures of (a) palindromic bistable[3] rotaxane R8+and (b) disulfide-tethered dumbbell D(control compound related to R89J.

EXPERIMENTALWorking Principle behind the hybrid NEMSactuator based on Electrochemical Activation of R8+

moleculesWith artificial molecular muscles attached to gold

surface of micro-cantilever, it is expected that theapplication of sufficient oxidizing potential will removeelectrons from that TTF station and make it positivelycharged. The change in the charge causes the substrateattached rings to move towards DNP station due tocolumbic repulsion between the rings and TTF stations.This in turn would build up bending moment on thesupporting micro-cantilever and bend the cantileverdownward. Applying a reducing potential consequentlyreduces the TTF station causing the rings to move backto TTF station from DNP station and hence relax thecollective bending moment built up in the previous stepand cause the cantilever reach to its original position.Therefore by applying electrochemical potentials, it is

possible to obtain control over collective buildup orrelaxation of bending moment on a micro-cantilever.

Sample Preparation and Experimental SetupRectangular Si micro-cantilevers of length 500 ptm,

width 100 ptm and thickness 1 ptm were used in all theexperiments and were obtained from commercialmanufacturer (NanoAndMore Inc, Lady's Island, SC).These micro-cantilevers were pre-coated with a 20 nmthick gold layer. Before functionalization, each micro-cantilever was thoroughly cleaned for 5 minutes withUV/Ozone process and later washed with DI water. Thepalindromic bistable [3] rotaxanes R8+ molecules andcontrol molecules were synthesized using the methodreported earlier [9]. Thoroughly cleaned micro-cantilevers were then immersed in solutions containingtarget molecules for 48 hours enabling self assembly onthe gold side of the micro-cantilever. In order to detectthe mechanical work exerted by surface bound bistable[3] rotaxane R8+ molecules on micro-cantilever duringelectrochemical activation, we combined atomic forcemicroscopy based optical deflection measurementtechnique with in-situ electrochemistry setup (PicoSPM 2500, Molecular Imaging) as shown in Figure 2.Gold coated micro-cantilever with self assembledmonolayer' s of rotaxane molecules were used asworking electrode, Ag and Pt wires were used asreference and counter electrode respectively. All theseelectrodes were then immersed in a teflon cell filledwith 0.1 M NaClO4 electrolyte. The redox state of themolecules is changed by applying a desired potentialbetween a working electrode and counter electrodeusing a three electrode potentiostat. Beam reflected bythe uncoated side of the micro-cantilever is collected bya four quadrant position sensitive photo-diode. Anydeflections in the micro-cantilever due to build-up andrelaxation of bending moment during electrochemicalactivation are readout from the variations in reflectivelaser beam position on the photodiode. Sensitivities ofeach micro-cantilever to convert deflection signal fromphotodiode in volts to nanometers were calculated byfitting the slopes of force curves.

RESULTS AND DISCUSSIONFigure 3 (a) shows the response of hybrid NEMSactuator based on molecular muscles to a triangularpotential scan. During the anodic sweep, the initialdeflection went upward and after reaching a potential of350 mV, the micro-cantilever started to deflectdownwards. The initial upward deflection could becorrelated to specific adsorption of perchlorateanions.[10] The latter downward deflection could becorrelated to the collective action of molecular musclesacting against the micro-cantilever's restoring force andthe compressive stresses originating due to specificadsorption. Consequent application of cathodic sweepcaused the micro-cantilever to restore to its neutralposition. The downward deflection

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Pt (CE)

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Fig. 2: Schematic of the experimental setup used for in situ electrochemical activation of the palindromic bistable [3]rotaxanes molecules. The inset shows the reversible electrochemical oxidation and reduction ofR8' to produce the micro-cantilever deflection. (WE: working electrode, CE: counter electrode, RE: reference electrode).

as the potential went below 28OmV indicates desorptionof perchlorate anions from the micro-cantilever. Toverify whether other affects may cause the observeddeflection signature, we conducted control experimentwith the control compound, D. As shown in Figure 1(b),D molecule is equivalent to bistable [3] rotaxane butwith no cyclobis rings and with two disulphide tetherson either ends of the molecule. This control compoundis devoid of moving elements and is expected not toperform any mechanical work on micro-cantilever. Thedeflection signature of a triangular sweep on thiscontrol compound attached to a micro-cantilever isshown in figure 3(b). It can be seen that there is a slightupward and downward deflection in the micro-cantilevers coated with control compounds and can beattributed to the perchlorate adsorption and desorptionon the gold side of micro-cantilever. The differencesbetween the direction and magnitude of deflection inR8, coated micro-cantilever and that of micro-cantilevercoated with control compound provides clear evidence

(a)

about the ability of electrochemical method to activatesolid bound molecular muscles.We further extended our studies to dynamic

performance by applying square shaped potential steps.The results with a step width of 20sec are presented inFigure 4. It can be observed that at higher potential(enough to oxidize the R8, molecules), the micro-cantilever bends downward and at lower potential, themicro-cantilever deflect upwards towards its neutralposition over a series of potential steps. The alternativebending down and bending up of the micro-cantilevercan be correlated to the alternative buildup andrelaxation of collective bending moments originatingfrom molecular muscles. Two effects should be noted inthe dynamic performance: 1) after the completion of thefirst oxidation step, there is a sudden decrease indownward deflection amplitude in the followingoxidation steps and 2) there is a gradual downwarddeflection of the micro-cantilever as the number ofoxidation and reduction cycles increase.

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The sudden decrease in the downward deflectionamplitude can be explained by the incomplete recovery

of metastable state rotaxane molecules to ground statein the first reduction cycle. The gradual downwarddeflection of micro-cantilever or creep has beenobserved in our experiments with control compound Dand in other electrochemical actuator systems [11-12].We believe that a creep mechanism arising from a

reorganization of the muscle molecules within themonolayer film plays an important role during theelectrochemical perturbation.

CONCLUSIONSIn summary, we have demonstrated that surface-boundartificial muscle molecules, when electrochemicallyactivated, cause a micro-cantilever to bend up anddown. Conversely, micro-cantilever beams that are

coated with redox-active but mechanically inert controlcompounds do not display the same bendingcharacteristics. Compared with their chemically-drivencounterparts [8,9], the electrochemically-drivenmolecular-muscle-based actuators can be operatedmuch faster, more conveniently, and with largerresponses. These results constitute a key step towardsengineering applications of NEMS based on artificialmolecular muscles.

ACKNOWLEDGEMENTSThis research was supported in part by the GraceWoodward Grants for Collaborative Research in

Engineering and Medicine, and the NSF NIRT grant(ECCS-0609128). Components of this work wereconducted at the Penn State node of the NSF-fundedNational Nanotechnology Infrastructure Network.

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