Catalytic nanomotors: remote-controlled autonomous movement of striped metallic nanorods.

Powering nanoscale machinery by nanosized motors that move by in situ conversion of stored chemical energy is one of the most interesting challenges facing nanotechnology. Such a procedure would circumvent the need for an external macroscopic power source. In nature, nanoscale motors typically operate by energy derived from the enzymatic catalysis of spontaneous reactions. At the nanoscale, interfacial forces dominate over inertia and, in principle, can be harnessed to move nanoobjects. Indeed, we have recently described the autonomous, non-Brownian movement of platinum/gold (Pt/ Au) nanorods with spatially defined zones that catalyze the spontaneous decomposition of hydrogen peroxide (H2O2) to oxygen (O2) in aqueous solutions at the platinum end of the rods. These rods are moved by an interfacial tension gradient resulting from the dissolution of the less polar oxygen in the medium around the platinum end. The sustained catalytic reaction results in the interfacial tension gradient being continuously re-established as the rod moves. While the above system displayed autonomous movement, the direction of movement was subject to random fluctuations. Directed motion is needed for catalytic nanomotors to be useful in a number of potential applications. Herein, we present a method for controlling the directionality of nanorods by using an external magnetic field. Striped nanorods with platinum, nickel, gold, nickel, and gold segments were employed (Figure 1). The platinum segment serves as a catalyst for the decomposition of hydrogen peroxide, whereas the ferromagnetic nickel segments can be magnetized and used to control the direction of rod movement. Whitesides and coworkers have shown that electroplating nickel segments shorter than the diameter of the rod, results in a rod that could be magnetized transversely rather than longitudinally. 5] Furthermore, the nickel segments in our striped nanorods are single domains because their lengths are less than the critical domain size (150 nm). After fabrication and magnetization of the nickel segments, the rods were imaged by scanning electron microscopy (SEM; Figure 1). Dark-field microscopy was used to observe rod movement in dilute solutions of hydrogen peroxide. We remotely controlled the direction in which the rods moved with a NeFeB magnet that had a field strength of approximately 550 G with respect to the sample, if it is assumed there is a constant field strength for the very small area of observation under the microscope objective (ca. 100 mm). When a magnetic field is applied, a majority of the rods orient themselves perpendicular to the magnetic field lines and move the platinum end forward (Figure 2). This ability to