Impact of surface charge on the motion of light-activated Janus micromotors

Abstract

Control over micromotors’ motion is of high relevance for lab-on-a-chip and biomedical engineering, wherein such particles encounter complex microenvironments. Here, we introduce an efficient way to influence Janus micromotors’ direction of motion and speed by modifying their surface properties and those of their immediate surroundings. We fabricated light-responsive Janus micromotors with positive and negative surface charge, both driven by ionic self-diffusiophoresis. These were used to observe direction-of-motion reversal in proximity to glass substrates for which we varied the surface charge. Quantitative analysis allowed us to extract the dependence of the particle velocity on the surface charge density of the substrate. This constitutes the first quantitative demonstration of the substrate’s surface charge on the motility of the light-activated diffusiophoretic motors in water. We provide qualitative understanding of these observations in terms of osmotic flow along the substrate generated through the ions released by the propulsion mechanism. Our results constitute a crucial step in moving toward practical application of self-phoretic artificial micromotors. Graphic abstract

Electrokinetic Janus micromotors moving on topographically flat chemical patterns

“…We fabricate photocatalytic blue lightactivated Janus particles consisting of a 60-nm-thick AgCl film prepared on polystyrene (PS) microspheres with a diameter of about 2 µm. In contrast to prior reports on plasmonic Ag/AgCl Janus micromotors (24,25), we utilize Ag/AgCl/PS Janus particles which are coated with β-FeOOH nanocrystals (12). Fig.…”

“…Once suspended in deionized water and dropped onto the surface of glass, Janus particles quickly sediment and experience thermal fluctuations. When illuminating them with blue light, Janus particles are brought into motion (12), driven by several parallel photocatalytic processes. The light initiates the decomposition of AgCl and results in an asymmetric electric field around the Janus particle (12,13,24,25,27).…”

“…When illuminating them with blue light, Janus particles are brought into motion (12), driven by several parallel photocatalytic processes. The light initiates the decomposition of AgCl and results in an asymmetric electric field around the Janus particle (12,13,24,25,27). Due to the positive charge of the particle, it sticks to a negatively charged substrate and moves along a positively charged substrate in the direction towards the catalytic cap (Fig.…”

“…This task calls for the investigation of the interaction between the channel potential and macro-ions and propagation of the particles through microfabricated pores. Charged colloidal particles (10,11) and spherical Janus micromotors (12) driven by the ionic self-electrophoresis (13,14) can serve as a model system of macro-ions to carry out fundamental studies of the ionic transport at the microscale.…”

Editorial: Motile active matter

“…More information about these aspects can be found in Refs. [4,14,[18][19][20][21][22][23][24][25][26][27][28][29] of this Topical Issue.…”