(303j) On the Propulsion and Saturable Binding Kinetics of a Self-Diffusiophoretic Colloidal Motor

Colloidal engines are self-propelled particles which find applications as roving sensors and shuttles, as drug delivery methods and in the assembly of nanostructures. We have designed a prototype colloidal engine driven by the bioconjugation of a protein receptor, NeutrAvidin, to its binding ligand, biotin, using diffusiophoresis as a chemo-mechanical transduction mechanism. In this prototype, one micron diameter polystyrene colloids which have been functionalized with Neutravidin on their surfaces are coated on one hemisphere with a gold layer to block the receptors. Immersion of this Janus colloid in a solution of biotin creates a concentration gradient of biotin across the particle. The biotin in solution binds to only one side of the colloid and thus the intermolecular attraction of the biotin with the colloid is unbalanced. This imbalance causes the particle to move over the time scale required for the receptors to saturate. These motors therefore have a clock for self-propulsion.

We report experiments on this colloidal motor which demonstrate that the colloids execute motion along trajectories that are randomized by Brownian rotation but with a velocity obtained by calculating the mean square displacement. The magnitude of the velocity and the duration of the motion are determined by the concentration of the biotin fuel. The larger the fuel concentration, the greater the velocity but the shorter the time scale of the motion. A model is constructed in which the intermolecular interaction of the biotin with the colloid is assumed to be short-ranged, attractive interaction between specifically adsorbed biotin on the colloid surface and the surface NeutrAvidin. The surface adsorbed biotin is obtained by solving the unsteady mass transport equation, and the force exerted on the colloid is obtained by balancing the force of the short range attraction with the hydrodynamic resistance to the colloid motion. The results are compared successfully with the measurements of the diffusiophoretic velocity.

Self propulsion has application in a variety of far reaching technologies. As a result, the ability to manipulate colloidal particles over small length scales is desirable to enhance fields such as material science, drug delivery, and point of care diagnostics. This work expands on earlier studies in self propulsion via the mechanism of self diffusiophoresis in which asymmetry of a particle creates localized concentration gradients of a solute fuel. Here, a biologically friendly binding interaction enables particle propulsion based on the binding of ligand fuel to a target protein on one hemisphere of the particle. Through these studies, particles exhibit directed motion over a finite timescale. Once the receptors on the particles surface saturate, these motor particles will return to the random Brownian rotation and translation as seen by ordinary colloids.  Models of this saturable binding mechanism capture the finite time scale for propulsion.  As the protein receptor sites saturate, velocity decays and eventually stops. Additionally, the impact of microfluidic confinement is investigated and the resulting drag force felt by propelling particles is assessed.