(499h) On the Diffusiophoretic Motion of Biocolloids with Saturable Kinetics

Phoretic transport is the process by which colloidal particles migrate due to a gradient of a thermodynamic variable, such as electric potential, temperature, or concentration, which is applied across the particle. Due to the difficulties associated with imposing an external gradient to move a particle over small length scales, self-propulsion is a promising alternative. One mechanism for autonomous motion is self-diffusiophoresis, in which a concentration gradient of a solute across the particle gives rise to unbalanced Van der Waals forces on the particle which move the particle forward. These self-propelled colloidal particles (“swimmers”) can function as motors to transport cargo over small scale  landscapes.

A self-diffusiophoresic motor has been designed previously in which hydrogen peroxide reacts on a platinum catalyst on one side of the colloid. Consumption of the fuel and production of oxygen generates concentration gradients that drive  the diffusiophoretic motion. As a result of the caustic reaction involved, it is imperative to find alternatives to the peroxide system in order to use colloidal motors in a broader range of applications.

In this presentation we develop a diffusiophoretically swimming colloid fueled by bio-conjugation. These are a class of new, biologically compatible engines which find application in emerging technologies requiring motors to transport cargo as in cellular drug delivery, microfluidic “lab-on-a-chip” platforms for point of care diagnostics and the bottom-up fabrication of nanoparticle assemblies. Colloids with one functionalized surface (Janus particles) displaying protein receptors are immersed in a solution containing a ligand which conjugates to the receptor. Conjugation on the active side of the colloid creates a concentration gradient of the ligand that drives the diffusiophoretic motion.  As the receptors become saturated the motion decreases to zero and, as a result, this scheme can be used to propel colloids over a prescribed time interval.  Using optical microscopy and particle tracking, we compare the randomized Brownian motion of colloids to the directed motion of Janus motors to verify the concept of self-diffusiophoretic swimming driven by asymmetric bioconjugation and the timed nature of the motion.

Experimental measurements of the self-propulsion velocity compare favorably to an asymptotic theory in which the length scale of the Van der Waals interaction is much smaller than the particle radius. The concentration field driving the motion is determined by the unsteady binding of the ligand to the surface receptor and hence captures the fact that the velocity can only be sustained for a finite time.