Chemistry in Motion: Tiny Synthetic Motors

Abstract

Diffusion is the principal transport mechanism that controls the motion of solute molecules and other species in solution; however, the random walk process that underlies diffusion is slow and often nonspecific. Although diffusion is an essential mechanism for transport in the biological realm, biological systems have devised more efficient transport mechanisms using molecular motors. Most biological motors utilize some form of chemical energy derived from their surroundings to induce conformational changes in order to carry out specific functions. These small molecular motors operate in the presence of strong thermal fluctuations and in the regime of low Reynolds numbers, where viscous forces dominate inertial forces. Thus, their dynamical behavior is fundamentally different from that of macroscopic motors, and different mechanisms are responsible for the production of useful mechanical motion.There is no reason why our interest should be confined to the small motors that occur naturally in biological systems. Recently, micron and nanoscale motors that use chemical energy to produce directed motion by a number of different mechanisms have been made in the laboratory. These small synthetic motors also experience strong thermal fluctuations and operate in regimes where viscous forces dominate. Potentially, these motors could be directed to perform different transport tasks, analogous to those of biological motors, for both in vivo and in vitro applications. Although some synthetic motors execute conformational changes to effect motion, the ma- * To whom correspondence should be addressed jority do not, and, instead, they use other mechanisms to convert chemical energy into directed motion.In this Account, we describe how synthetic motors that operate by self-diffusiophoresis make use of a self-generated concentration gradient to drive motor motion. A description of propulsion by selfdiffusiophoresis is presented for Janus particle motors comprising catalytic and noncatalytic faces. The properties of the dynamics of chemically powered motors are illustrated by presenting the results of particle-based simulations of sphere-dimer motors constructed from linked catalytic and noncatalytic spheres. The geometries of both Janus and sphere-dimer motors with asymmetric catalytic activity support the formation of concentration gradients around the motors. Because directed motion can occur only when the system is not in equilibrium, the nature of the environment and the role it plays in motor dynamics are described. Rotational Brownian motion also acts to limit directed motion, and it has especially strong effects for very 1 arXiv:1407.6338v2 [cond-mat.soft] 3 Nov 2014 small motors. We address the following question: how small can motors be and still exhibit effects due to propulsion, even if only to enhance diffusion? Synthetic motors have the potential to transform the manner in which chemical dynamical processes are carried out for a wide range of applications.

Self-diffusiophoresis of chemically active colloids

“…References [38,40] show that this result is the zeroth order term of the solution for surface films of non-vanishing thickness. This means that corrections in δ/R become important as the radius of the colloid decreases [84]. Reference [37] has shown that accounting for chemical kinetics which are not completely reaction-limited, i.e., Da = 0, also leads to a dependence of the phoretic velocity on the particle size; this theoretically predicted dependence agrees well with the experimental observations.…”

Nanomotors

“…One might therefore also assume that "nanoswimmers" are described by continuum hydrodynamics, although this is an assumption, as there is currently no complete theoretical description of (the hydrodynamics) of a catalytically active nanoparticle. A theoretical treatment that considers the scaling behaviour of diffusiophoretic particles has recently appeared [5]. These authors conclude on the basis of particle-based simulations of chemically powered Janus ∼ nm motors that the principle of self-diffusiophoresis still operates at the nanoscale, even though the motion is subject to strong fluctuations [5].…”

“…A theoretical treatment that considers the scaling behaviour of diffusiophoretic particles has recently appeared [5]. These authors conclude on the basis of particle-based simulations of chemically powered Janus ∼ nm motors that the principle of self-diffusiophoresis still operates at the nanoscale, even though the motion is subject to strong fluctuations [5].…”

“…Whether enzymes do indeed "swim" when they are catalytically active is a question that a number of research groups are beginning to address [3][4][5]. This minireview does not consider motor proteins or "anchored" nanomotors, but introduces aspects related to the study of "freely" diffusing and actively moving macromolecules and artificial nanoparticles and nanomotors.…”

Communication: Mechanochemical fluctuation theorem and thermodynamics of self-phoretic motors

“…Self-propulsion is achieved by the generation of local gradients of chemical concentrations, electrochemical potential, or temperature, which produce the force driving the motor [6][7][8][9][10][11]. This is the case in particular for Janus motors with catalytic and chemically-inactive hemispheres, moving by diffusiophoresis in a solution with out-of-equilibrium concentrations of fuel and product [11][12][13][14]. The propulsion mechanism is based on the mechanochemical coupling between the fluid velocity around the motor and the concentration fields induced by the reaction taking place on the catalytic hemisphere.…”