(200d) Electrostatic Charge Shuttles As Chemically Powered Colloidal Motors

As nanotechnology progresses from the synthesis of diverse structures to the design of functional systems, there is an increasing demand for methods by which to power and propel nanoscale machines.  The sustained operation of such machines requires a steady input of energy which must be delivered remotely (chemical fuels) and converted efficiently into useful mechanical motions.  Despite impressive demonstrations of chemically powered, nanoscale propulsion, existing systems remain limited by poor efficiencies (typically ~1E-09) which prohibits their operation in low fuel environments (e.g., biological media).  Here, we describe a new strategy by which electrochemical processes are coupled to mechanical motion through electrostatic forces acting on colloidal "charge shuttles".  With the use of high-speed optical microscopy and electrical measurements we show that this approach is ca. one billion times more efficient than existing nanomotors.  Moreover, careful electrode design and surface modification via self-assembled monolayers affords low-voltage operation (~1V), thereby allowing power to be sourced electrochemically.   Our experimental results are explained by a simple model which accounts for particle charging at an electrode and its movement in the applied field, and further indicates that the underlying forces scale favorably down to molecular dimensions.  In closing, we demonstrate rectification of the system's oscillatory behavior for linear actuation or directed propulsion, and highlight the intriguing dynamics of multi-particle systems.