Chemical Auxiliary for Photocatalytic Active Colloids.

Active colloids with the ability to self-propel and collectively organize are emerging as indispensable elements in microrobotics and soft matter physics. For chemically powered colloids, their activity is often induced by gradients of chemical species in the particle's vicinity. The direct manipulation of these gradients, however, presents a considerable challenge, thereby limiting the extent to which active colloids can be controlled. Here, we introduce a series of rationally designed molecules, denoted as chemical auxiliary (CA), that intervene with specific chemical gradients and thus unveil new capabilities for regulating the behaviors of photocatalytic active colloids. We show that CA can alter the diffusiophoretic and osmotic interactions between active colloids and their subsequent self-organization. Also, CA can tune the self-propulsion of active particles, enabling a record high propulsion speed of over 100 μm/s and endowing high salt tolerance. Furthermore, CA is instrumental in establishing dynamic, competing gradients around active particles, which signifies an in situ, noninvasive, and reversible strategy for reconfiguring between modes of colloidal activity.