Oscillations of the Local pH Reverses Silver Micromotors in H2O2

Asymmetric chemical reactions on the surfaces of colloidal particles are known to propel them into directional motion. The dynamics of such chemical micromotors are sensitive to their local chemical environments, which also continually evolve with the reactions on motor surfaces. This two-way coupling between the motor dynamics and the local environment may result in complex nonlinear behaviors. As an example, we report that Janus Ag microspheres, which self-propel in hydrogen peroxide (H2O2), spontaneously reverse their direction of motion two or more times. We hypothesize that two distinct chemical reactions between Ag and H2O2 drive the micromotor in opposite directions, and which reaction dominates depends on the local pH. Interestingly, the local pH near a Ag micromotor oscillates spontaneously in H2O2, likely due to a complex interplay between the kinetics of the reaction between Ag and H2O2 and the diffusion of chemical species. Consequently, the pH-sensitive Ag micromotor reverses its direction of motion in response to these pH oscillations. This study introduces a new mechanism for regulating the speed and directionality of micromotors, highlights the potential of Ag micromotors in chemical sensing, and sheds new light on the interplay between chemical kinetics and micromotor dynamics.