Continuously Variable Regulation of the Speed of Bubble-Propelled Janus Microcapsule Motors Based on Salt-Responsive Polyelectrolyte Brushes.

Engineering self-propelled micro-/nanomotors (MNMs) with continuously variable speeds, like macroscopic automobiles equipped a continuously variable transmission, is still a huge challenge. In this study, through grafting with salt-responsive poly[2-(methacryloyloxy)ethyltrimethylammonium chloride] (PMETAC) brushes, bubble-propelled Janus microcapsule motors with polyelectrolyte multilayers exhibit adjustable motion speeds upon changing the type and concentration of counterions. The reversible switching between low-speed and high-speed states has been realized through modulating PMETAC brushes between hydrophobic and hydrophilic configurations by exchanging anions between ClO4- and polyphosphate. Such continuously variable regulation enables the speed control in an accurate and predictable manner and autonomous response to the local chemical environment. This study suggests that the integration of polymer brushes with precisely adjustable responsiveness renders a promising way for the motion control, towards smart MNMs acting like their counterparts in living systems.