A polymerization-powered motor.

Research into nanoand micromotors powered by catalytic reactions, or more broadly the study of autonomous motion at the microand nanoscale, has become an area of great current interest. Potential applications include the delivery of materials, self-assembly of superstructures, roving sensors, and other emerging applications. The motors described to date involve the catalytic conversion of small molecules, which typically results in a gradient of charged or neutral species that in turn drives the motor. Polymerization-powered motion has been reported in biological systems, for example, listeria has been observed to move by actin polymerization. However, there have been no reports of motion at the nanoand micrometer scale driven by polymerization. Given the large repertoire of known organometallic polymerization catalysts, the design of polymerization-driven motors would considerably increase the scope of catalytic reactions that could be employed to power autonomous motion. Furthermore, polymerization reactions offer the unique opportunity to both power motion and simultaneously allow the deposition of polymer along the motion track. Herein, we present the first motor to be powered by a polymerization reaction outside biological systems. The motor is powered by ringopening metathesis polymerization (ROMP) of norbornene. These motors show increased diffusion of up to 70 % when placed in solutions of the monomer. Furthermore, the motors were observed to display the phenomenon of chemotaxis when placed in a monomer gradient; an extremely rare example outside biology. Generating motion by polymerization has been previously suggested, although not demonstrated. We chose to employ a form of Grubbs ROMP catalyst for our initial study because of its relatively high stability and high polymerization activity with norbornene (Figure 1). The motors were fashioned by first synthesizing gold–silica Janus particles. This was performed using 0.96 mm silica particles. These particles were deposited as thin films using a published method. Then gold was deposited onto the monolayers creating the asymmetric Janus particles. The particles were then chemically modified with the Grubbs catalyst on the silica side utilizing previously published methods (Supporting Information, Figure S1). XPS confirmed that the catalyst did attach to the motor surface. Catalytic activity was then tested by adding the functionalized particles to norbornene solutions and monitoring monomer consumption by gas chromatography. The turnover frequency (TOF) was found to be proportional to monomer concentration and begins to saturate at 1m norbornene (Supporting Information, Figure S2). SEM images of these particles before and after exposure to a monomer solution shows the formation of polymer at the particle surface (Figure 2). As discussed in the