Magnetically Powered Flexible Metal Nanowire Motors Wei Gao, † Sirilak Sattayasamitsathit, † Kalayil Manian Manesh, † Daniel Weihs, ‡ and Joseph Wang* ,† Department of Nanoengineering, UniVersity of California San Diego, La Jolla, California 92093, and Faculty of Aerospace Engineering, Technion, Haifa, 32000, Israel Received August 11, 2010; E-mail: josephwang@ucsd.edu Abstract: Fuel-free magnetically driven propulsion of ﬂexible Au/ Ag/Ni nanowires, with a gold ‘head’ and nickel ‘tail’, linked by a partially dissolved and weakened silver bridge, is described. The ﬂexible bridge facilitates the cyclic mechanical deformations under an external rotating magnetic ﬁeld. Under such a ﬁeld the nickel segment starts to rotate, facilitating the rotation of the gold segment at a different amplitude, hence breaking the system symmetry and inducing the movement. Forward (‘pushing’) and backward (‘pulling’) magnetically powered locomotion and a precise On/Off motion control are achieved by tailoring the length of the nickel and gold segments and modulating the magnetic ﬁeld, respectively. Efﬁcient locomotion in urine samples and in high-salt media is illustrated. The new magnetic nanowire swimmers can be prepared in large scale using a simple template electrodeposition protocol and offer con- siderable promise for diverse practical applications. The use of nanomotors to power nanomachines and nanofactories is currently a research area of intense activity due to numerous potential applications. 1 Most attention in the development of artiﬁcial nanomo- tors has been given to catalytic nanowire motors that exhibit autono- mous self-propulsion in the presence of a hydrogen peroxide fuel. 1 However, numerous potential applications of future nanomachines, particularly biomedical ones, would require elimination of the fuel requirements. Efforts in this direction have recently led to the propulsion of fuel-free nanowire diodes under external electric ﬁelds. 2 Magnetically controlled motion, inspired by the motility of prokary- otic and eukaryotic microorganisms, 3 represents another attractive route for addressing the challenge of nanoscale propulsion and accomplishing a fuel-free locomotion. Such nanoscale propulsion requires breaking the system symmetry by deforming the motor shape. 4 For example, E. coli bacteria use rotating helical ﬂagella to propel in viscous media. 3b Magnetic force has been widely used to provide the mechanical deformation essential for breaking the symmetry. The controlled beating motion of ﬂexible DNA-linked assemblies of paramagnetic microparticles was illustrated by Dreyfus et al. 5 Nelson 6 and Ghosh 7 reported recently the fabrication and magnetically controlled motion of efﬁcient artiﬁcial ﬂagella consisting of helical tails. These cork- screw swimmers offer attractive propulsion but require specialized ‘top down’ self-scroll or shadow-growth preparation routes along with advanced microfabrication facilities. Here we demonstrate a simple new approach for addressing the fuel requirement and for creating magnetically driven propulsion based on easily prepared ﬂexible metal nanowire swimmers. As illustrated in SI Scheme 1A, the new three-segment nanowire motors (∼6 μm long, 200 nm in diameter) are readily prepared using a template electrodeposition approach. Such preparation involves the sequential deposition of the Au, Ag, and Ni segments into the alumina membrane micropores. Subsequent dissolution of the template and release of the nanowires are followed by partial dissolution of the central silver segment in hydrogen peroxide to create the ﬂexible thinner joint (linking the Au ‘head’ and Ni ‘tail’) essential for the controlled mechanical deformation. The ﬂexibility of thin Ag nanowires has been discussed earlier. 8 Flexible nanorods based on a polyelectrolyte bridge were also described by Ozin’s team but not in connection to directed motion. 9 The new template electrochemical synthesis of ﬂexible metal nanowires greatly simpliﬁes the preparation of magnetic swimmers compared to the fabrication of cork-screw or helical magnetic propellers 6,7 and offers reproducible preparation that reﬂects the precise charge control. The resulting fuel-free ﬂexible nanowire swimmers offer great promise for diverse biomedical applications as indicated below from their efﬁcient propulsion in urine medium. SI Video 1 clearly illustrates the rapid and yet incomplete dissolution of the central Ag segment and formation of a ﬂexible joint within 10-15 s in the peroxide solution. A SEM image of the resulting ﬂexible Ag section (SI Figure 2) indicates a rough porous segment of a slightly smaller diameter compared to its nonporous metal neighbors. The dissolution of silver in hydrogen peroxide leads to hydroxyl products that chemisorb on the Ag surface and results in AgOH and Ag 2 O surface products. 10,11 Such formation of surface byproducts in the presence of hydrogen peroxide has been shown to hinder further silver dissolution. 10,11 To illustrate the critical role of the nanowire ﬂexibility in achieving the magnetic propulsion we compared the motion of Au/Ag/Ni nanowires before and after the partial silver dissolution. Figure 1A displays the motion trajectories of conventional Au/Ag/Ni nanowires † University of California San Diego. ‡ Technion. Figure 1. Comparison of the ‘forward’ (A) and ‘backward’ (B) movement of rigid Au/Ag/Ni (a) and ﬂexible Au/Ag ﬂex /Ni (b) nanowires under a rotating magnetic ﬁeld (5 G, f ) 15(A); 10 (B) Hz) over a 15 s period. The lengths of the Au, Ag, and Ni segments are as follows: (A) 3, 3, and 0.5 μm; (B) 2, 3.5, and 1 μm, respectively. (See corresponding SI Video 2 and SI Figure 1) (c) Schematic of the magnetic swimmer moving ‘forward’ (A) and ‘backward’ (B). 2Ag(s) + H 2 O 2 f 2Ag + (aq) + 2OH - 10.1021/ja1072349  XXXX American Chemical Society J. AM. CHEM. SOC. XXXX, xxx, 000 9 A