Paper TF+SE-TuM9
Catalytic Nanomotor Control: Design Techniques Using Dynamic Shadowing Growth

Tuesday, November 1, 2011, 10:40 am, Room 104

Catalytic nanomotors with nanometer-to-micrometer dimensions convert chemical energy into mechanical work via catalysis allowing for autonomous self-propulsion. They are an emerging nanotechnology field and promise important technological advances in drug delivery, transport, assembly, and other processes at the nano-scale. Catalytic nanomotors are inorganic analogues of cellular motor proteins that convert chemical energy into work through stored energy. One of the greatest challenges in this field is the manipulation and direct control of motion and swimming behaviors.

We focus upon the geometric design of catalytic nanomotors to modulate motion behaviors. To achieve this goal, a dynamic growth technique must be implemented. Most research uses template-directed electroplating (TDEP) allowing only simple geometries. We use dynamic shadowing growth (DSG) for fabrication which is a dynamic process allowing for construction of a much wider range of structures and shapes.

Depositing different materials and controlling the overlapping area is the first method of motion modulation used. To alter the swimming speed of a spherical nanomotor, Au is overlapped with the Pt-catalyst to varying degrees; the Au surface area A is changed systematically. The average moving speed u is found to follow the scaling relationship, , which agrees with the self-electrophoresis mechanism.

Swimming behaviors can also be altered by geometrical design, easily implemented by DSG making the technique useful to engineer different types of motion. Various swimming behaviors are exhibited by altering the geometry, and/or changing the location of the Pt catalyst accomplishable. Two very similar structures were fabricated and move based upon the location of the catalyst. Multi-component rotational nanomotors consisting of Pt coated TiO₂ nanoarms grown upon ~ 2.01 μm diameter silica microbeads are designed by dynamic shadowing growth. When exposed to hydrogen peroxide, H₂O₂, the structures rotate about an axis through the center of the microbead and perpendicular to the TiO₂ nanoarm at a rate of 0.15 Hz per % H₂O₂ concentration. The other nanomotors are tadpole-like structures that swim in large sweeping circular trajectories. The swimming trajectories are fine-tuned by altering the arm length and orientation exploiting geometry-dependent hydrodynamic interactions at low Reynolds number. The curvature, angular frequency, and radius of curvature of the trajectories change as a function of arm length. Simulations based on the method of regularized Stokeslets are also described and correctly capture the trends observed in the experiments.