Programmable artificial phototactic microswimmer

Abstract

Phototaxis is commonly observed in motile photosynthetic microorganisms. For example, green algae are capable of swimming towards a light source (positive phototaxis) to receive more energy for photosynthesis, or away from a light source (negative phototaxis) to avoid radiation damage or to hide from predators. Recently, with the aim of applying nanoscale machinery to biomedical applications, various inorganic nanomotors based on different propulsion mechanisms have been demonstrated. The only method to control the direction of motion of these self-propelled micro/nanomotors is to incorporate a ferromagnetic material into their structure and use an external magnetic field for steering. Here, we show an artificial microswimmer that can sense and orient to the illumination direction of an external light source. Our microswimmer is a Janus nanotree containing a nanostructured photocathode and photoanode at opposite ends that release cations and anions, respectively, propelling the microswimmer by self-electrophoresis. Using chemical modifications, we can control the zeta potential of the photoanode and program the microswimmer to exhibit either positive or negative phototaxis. Finally, we show that a school of microswimmers mimics the collective phototactic behaviour of green algae in solution.

Device and programming abstractions for spatiotemporal control of active micro-particle swarms

“…For other swarm systems, the stimuli set will vary to achieve these commands. 1,18,22,25,35,36 However, the commands themselves are general methods for manipulating matter, and thus are broadly applicable. Finally, system-level commands require the system to be able to monitor its state (i.e.…”

“…magnetic or light fields) to control directionality of many agents, but this precludes local control over subgroups of agents. 1,27,30,[34][35][36] Other setups capitalize on local stimuli (e.g. projection of images 31,37 ) to control individual Fig.…”

Chemical Nanomotors at the Gram Scale Form a Dense Active Optorheological Medium

“…[12,13] Examples of such large scale collective behaviors in synthetic active colloids have been experimentally demonstrated in electrically powered Quincke "rollers" [9,14] and metal-coated colloidal particles [8,15] or by magnetic colloids; [16,17] however it has been challenging to realize this in large volumes and at high density with artificial chemically active systems. While several model synthetic active systems are known, including catalytically active Janus particles, [23][24][25][26][27][28][29][30][31][32] it has thus far been difficult to obtain truly large numbers of chemical motors to form an active medium and operate them stably for an extended period of time. [18][19][20][21][22] Moreover, most of the conventional active colloidal systems are either confined to two dimensions or are operated at low densities, and hence are unable to drive changes in the bulk.…”

“…Here, we show that inorganic chemically-active nanomotors can be used to prepare large volumes of an active medium, whose bulk viscosity can be controlled by the activity of its constituents. [25,26,31,35] However, since both the processes are limited to a monolayer of particles, the overall yield is always low. Due to the asymmetric distribution of catalyst near the colloid, the reaction gives rise to a concentration gradient of product (and educt) molecules or ions across the particle.…”

Nonlinear self-action of light through biological suspensions

“…While the linear optical properties of biological media have been well studied [4,5], little is known about their nonlinear properties. Recently, there has been an increasing interest in light controlled motion of microorganisms and their hosting flows [6,7], but these controls are based on phototaxis in bacterial suspensions rather than optical nonlinearity. To efficiently propagate light through highly scattering media, it is important to study the nonlinear optical properties of soft-matter systems [8][9][10][11][12].…”