Executive Summary Final - the Center for Nanoscale Science - an ...

IRG2: Powered Motion at the Nanoscale
A third area of research has been the optical
control of motor behavior: the combined
operation of cyclic chemical reactions and
propulsive motion in coupled populations of Ag/
AgCl particles that engage in periodic clumping
and explosion in a complex non-linear fashion,
with nearby clusters dynamically triggering the
explosions of nearby clusters in novel reaction
waves of a mixed character: part homogeneous
solution and part particle-surface mediated. The
figure at right – which depicts the probability that
a cluster will explode given that a cluster has
previously exploded a distance R away at the
specified time in the past, provides proof of
cluster-cluster interaction: the peak for a 0.2 s delay at around 20 microns clearly indicates
cluster-cluster correlations. The overlap of ion gradients of nearby particles produces a
particularly strong motor-motor coupling in these systems, with enticing prospects to exhibit
collective behaviors including particle-reaction waves, bistability, etc.
In the catalytic motor are as a whole, in 2010 we have discovered the first examples of nano/
micro-objects outside living systems that move autonomously by converting chemical energy
into mechanical forces. With very little “information” input (in the form of gradients), these
objects begin to display emergent collective behavior that were thought to lie solely in the realm
of biology. Freed of usual biological constraints, we now have the unprecedented opportunity to
probe the ultimate limits of self-organization in these dynamic systems that operate far from
equilibrium. Our eventual goal is to develop “intelligent” nano/microbots that can sense their
environment, communicate the information to other bots, and induce swarming behavior in
heterogeneous populations of these bots to produce pre-determined functional outcomes.
At smaller length scales, molecular motors designed to exhibit controlled mechanical motions
on the molecular scale present promising possibilities of building novel functional materials and
to extend ideas of catalytically-powered motion – already developed in solution on the singlemolecule
level in the work described above – to molecular systems that are harnessed to
substrates or within active materials. Our research in molecular motors aims towards assembling
various single-molecule motors in well-defined nanoscale assemblies on surfaces to induce
controlled motion with light, electrons, and ions, and to harness these motions towards
applications in nano-electro-mechanical systems and nanophotonics.
As with catalytic motors, we have had several significant accomplishments in molecular motors
project over the past year. First, we have discovered that the structure of specially designed
rotaxane molecules can be changed reversibly between two (or more) stable states by employing
a redox reaction. We used the nanoscale motion induced within the molecule to generate
microscale motions by arranging them to work in unison. In order to control the motion and
orientation, we also studied the conformational changes in rotaxanes at the single-molecule level
and observed that conformational changes correlate with the known redox states of rotaxanes.