Vapor-Driven Propulsion of Catalytic Micromotors

Abstract

Chemically-powered micromotors offer exciting opportunities in diverse fields, including therapeutic delivery, environmental remediation, and nanoscale manufacturing. However, these nanovehicles require direct addition of high concentration of chemical fuel to the motor solution for their propulsion. We report the efficient vapor-powered propulsion of catalytic micromotors without direct addition of fuel to the micromotor solution. Diffusion of hydrazine vapor from the surrounding atmosphere into the sample solution is instead used to trigger rapid movement of iridium-gold Janus microsphere motors. Such operation creates a new type of remotely-triggered and powered catalytic micro/nanomotors that are responsive to their surrounding environment. This new propulsion mechanism is accompanied by unique phenomena, such as the distinct off-on response to the presence of fuel in the surrounding atmosphere, and spatio-temporal dependence of the motor speed borne out of the concentration gradient evolution within the motor solution. The relationship between the motor speed and the variables affecting the fuel concentration distribution is examined using a theoretical model for hydrazine transport, which is in turn used to explain the observed phenomena. The vapor-powered catalytic micro/nanomotors offer new opportunities in gas sensing, threat detection, and environmental monitoring, and open the door for a new class of environmentally-triggered micromotors.

Figure 1

(A) Scanning electron microscopy (SEM) image of the Au/Ir motor. (B,C) Energy-dispersive X-ray (EDX) spectroscopy images illustrating the distribution of the gold inner core and iridium catalytic patch, respectively.

Figure 2. Catalytic micromotors powered by a remote fuel source.

(A) Propulsion of Ir-Au micromotors by external hydrazine diffusing from the fuel source droplet through the surrounding atmosphere into the motor droplet. (B) Simulation of the hydrazine concentration gradient produced by the hydrazine source (left) and the hydrazine concentration gradient generated within the sample droplet (right) based on a theoretical model for hydrazine transport. (C,D) Track lines of the motion of Ir-Au motors over 2 seconds, taken from SI Video 2, before (C) and 15 seconds after (D) placing the 20% hydrazine droplet (1 μL; diameter: 2.5 mm) 0.5 cm away from the micromotor droplet (1 μL; diameter: 2.5 mm). Scale bar, 10 μm.

Figure 3. Spatio-temporal speed dependence of vapor-powered micromotors.

(A) Dependence of the motor speed upon time and fuel concentration at a fixed separation distance of 0.5 cm. (a–c) Track lines of the micromotor motion, over a 2 sec period, taken from SI Video 3, 5 min after placing droplets of 10%, 20%, and 30% hydrazine, respectively, 0.5 cm apart. (B) Dependence of the motor speed upon the time and separation distance using a source droplet containing 20% hydrazine. (a–c) Track lines of the micromotor motion, over 2 second periods, taken from SI Video 4, 5 min after placing the fuel droplet containing 20% hydrazine at separation distances of 1, 2, and 3 cm, respectively. (d) Corresponding 3D plots. Scale bar, 10 μm. Droplet diameter and volume: 2.5 mm and 1 μL, respectively.

Figure 4. Profiles of the hydrazine level and the motor speed within the motor droplet.

(A) Motor speeds at several radial distances from the center within the sample droplet after a one min exposure to the 20% hydrazine source droplet (the negative distance being closest to the source, and positive being furthest). Droplets (2.5 mm diameter, 1 μL) separated by 0.5 cm. (B) 3D simulated plot of the normalized hydrazine concentration within the sample droplet 1 min after exposure. (C) Normalized hydrazine concentration profile as a function of the radial position (ρ = r/R) within the sample droplet for different times after placing the hydrazine source: 0, 0.5, 1, 2.5, and 5 min (a–e).