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 considerable promise for diverse practical applications.
Synthetic nanomotors, which convert chemical energy into autonomous motion, hold considerable promise for diverse applications. In this paper, we show the use of synthetic nanomotors for detecting DNA and bacterial ribosomal RNA in a fast, simple and sensitive manner. The new motion-driven DNA-sensing concept relies on measuring changes in the speed of unmodifi ed catalytic nanomotors induced by the dissolution of silver nanoparticle tags captured in a sandwich DNA hybridization assay. The concentration-dependent distance signals are visualized using optical microscopy, particularly through straight-line traces by magnetically aligned ' racing ' nanomotors. This nanomotor biodetection strategy could be extended to monitor a wide range of biomolecular interactions using different motion transduction schemes, thus providing a versatile and powerful tool for detecting biological targets.
A motion-based chemical sensing involving fuel-driven nanomotors is demonstrated. The new protocol relies on the use of an optical microscope for tracking changes in the speed of nanowire motors in the presence of the target analyte. Selective and sensitive measurements of trace silver ions are illustrated based on the dramatic and specific acceleration of bimetal nanowire motors in the presence of silver. Such nanomotor-based measurements would lead to a wide range of novel and powerful chemical and biological sensing protocols.Considerable recent efforts have been devoted to the development of artificial nanomotors. 1 In particular, fuel-driven bisegment Au-Pt nanowires exhibit autonomous self-propulsion due to electrocatalytic decomposition of hydrogen peroxide fuel. 1,2 Such autonomous motion of catalytic nanowire motors holds great promise for exciting applications ranging from drugdelivery, nanoscale assembly and transport, or motion-based biosensing. 1 This Communication reports on the first example of using catalytic nanomotors for motionbased chemical sensing, and particularly for specific detection of trace silver ions. During recent experiments in our laboratory involving electrochemically-triggered motion of catalytic nanowire motors 3 we observed unusual speed acceleration associated with silver ions NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript generated at a pseudo silver-wire reference electrode placed in the vicinity of the nanowire motors. Such unexpected specific silver effect upon the speed of catalytic nanomotors has been exploited in the present work for designing a new motion-based silver sensing protocol. The new protocol relies on the use of an optical microscope for tracking the speed of nanowire motors and offers highly selective, sensitive and simple measurements of trace silver based on direct visualization. Figure 1A displays traces of Au-Pt nanomotors (over a 3 second period), taken from videos of the nanowires in the presence of eleven different cations (100 µM each), along with the peroxide fuel. Ten of these cations caused a significant speed reduction, including a Brownian motion or a slower non-Brownian motion (with speeds ranging from 0.3 to 7.1 µm s −1 ). Such slow speed (compared to a actual speed of ~10 µm s −1 observed without these salts) is consistent with the self-electrophoresis mechanism for the propulsion of catalytic nanomotors, 4 where the speed decreases linearly with the solution conductivity. 5 In contrast, the nanomotors move over a dramatically longer path in the presence of silver (shown in the middle), displaying an average speed of 52 µm s −1 . Also shown in Figure 1 (B) is the histogram depicting the average speed of the nanomotors in the presence of the different cations tested. These data, along with the corresponding video (shown in the SI; Video 1), clearly illustrate the remarkably selective acceleration in the presence of silver. Adding other cations (e.g., Pb 2+ or K + up to 5 µM) had only slight reductions...
A simplified template-assisted layering approach for preparing catalytic conical tube microjet engines based on sequential deposition of platinum and gold on an etched silver wire template followed by dicing and dissolution of the template is described. The method allows detailed control over the tube parameters and hence upon the performance of the microengine. The recoiling bubble propulsion mechanism of the tubular microengine, associated with the ejection of internally generated oxygen microbubbles, addresses the ionic-strength limitation of catalytic nanowire motors and leads to a salt-independent movement. Similar rates of bubble generation and motor speeds are observed in salt-free and salt-rich media (at elevated ionic-strength environments as high as 1 M NaCl). Plating of an intermediate nickel layer facilitates a magnetically guided motion as well as the pickup and transport of large (magnetic) "cargo". Surfactant addition is shown to decrease the surface tension and offer a more frequent formation of dense smaller bubbles. The new and improved motor capabilities along with the simple preparation route hold great promise for using catalytic micromotors in diverse and important applications.
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