ZnO-based micromotors fueled by CO2: the first example of self-reorientation-induced biomimetic chemotaxis

Abstract

Synthetic chemotactic micro/nanomotors are envisioned to actively 'seek out' targets by following specific chemicals, but they are mainly powered by bioincompatible fuels and only show pseudochemotaxis (or advanced chemokinesis) due to their weak self-reorientation capabilities. Here we demonstrate that synthetic ZnO-based Janus micromotors can be powered by the alternative biocompatible fuel of CO₂, and further provide the first example of self-reorientation-induced biomimetic chemotaxis using them. The ZnO-based micromotors are highly sensitive to dissolved CO₂ in water, which enables the corrosion of ZnO to continuously occur by providing H⁺ through hydration. Thus, they can autonomously move even in water exposed to air based on self-diffusiophoresis. Furthermore, they can sense the local CO₂ gradient and perform positive chemotaxis by self-reorientations under the phoretic torque. Our discovery opens a gate to developing intelligent micro/nanomotors powered by, and sensitive to, biocompatible atmospheric or endogenous gaseous chemicals for biomedical and environmental applications.

Keywords: carbon dioxide; chemotaxis; micro/nanomotors; self-propulsion; zinc oxide.

Figure 1.

Characterization and self-propulsions of the ZnO-based MMs. (A) SEM image of the ZnO/SiO₂ MMs. Scale bar, 1 μm. The inset shows the schematic illustration of their Janus structure. (B) EDX mapping of Zn and Si elements in a ZnO/SiO₂ MM. Scale bar, 500 nm. (C) Self-propulsions of ZnO-based MMs in different water media. (i and ii) Trajectories of ZnO/SiO₂ MMs in 9 s in the (i) air-exposed water (Air) and (ii) CO₂-eliminated water, including the freshly boiled water (Bo), Ar-gassed water (Ar), O₂-gassed water (O₂) and N₂-gassed water (N₂). Scale bars, 10 μm. (iii) Trajectories of a ZnO/Pt MM in the air-exposed water without (red curve) and with (green curve) UV irradiation. Scale bars, 5 μm. (D) MSD of the MMs in different water media. Solid color lines are fitting curves using a quadratic function. (E) Self-propulsion of a typical ZnO/SiO₂ MM within a lifetime of 90 min. Red curves depict its trajectories in 9 s at different time intervals. Scale bar, 5 μm.

Figure 2.

The self-propulsion mechanism of the ZnO/SiO₂ MMs. (A) Numerical simulations of the electric potential (ϕ, the color background) and the electro-osmotic flow (EOF, black streamlines with arrows) around a ZnO/SiO₂ MM. The EOF is initiated by the surface electro-osmotic slip (EOS) in its electric double layer. (B) Schematic illustration of the propulsion mechanism of the ZnO/SiO₂ MMs. (C) Trajectories of the MMs in freshly boiled water at different time intervals when exposed to air. Scale bar, 5 μm. (D) Time-dependent normalized v of the MMs in a film of freshly boiled water when exposed to air (red dots) and the air-exposed water (black dots). Solid red and black curves are fitting results using exponential functions. (E) The simulated C_CO2 as a function of time in a CO₂-eliminated water film (160 μm in thickness) when exposed to air. (F) The v of the MMs at different C_CO2 in water. The red line is the linear fitting line of the experimental data in the low C_CO2 regime (0–8.54 μM), and the red curve in the inset is the fitting result according to the Michaelis-Menten kinetics in the high C_CO2 regime (8.54 μM to 20.5 mM).

Figure 3.

Chemotaxis of the ZnO/SiO₂ MMs. (A) Schematic illustration of the positive chemotaxis of the ZnO/SiO₂ MMs. The color background depicts numerical simulations of C_CO2 gradient () near a micropipette (inner d, 20 μm) filled with a CO₂ solution (C_CO2 = 93.7 μM). (B) Trajectories of the MMs moving toward the CO₂ source. Scale bar, 20 μm. (C) The color trajectory of a slightly etched ZnO/SiO₂ MM approaching the CO₂ source, in which the colors represent the normalized magnitude of v of the MM. Scale bar, 5 μm. (D) The reorientations of the ZnO/SiO₂ MM shown in (C) at different times by clockwise (CW) or counterclockwise (CCW) rotations. Scale bar, 2 μm. (E) Numerical simulations of the EOSs (purple arrows) on a ZnO/SiO₂ MM and the resulted flow fields (black streamlines with black arrows) when it aligns with (θ = 0, the left panel) and deviates with it in a θ of π/4 (the right panel), respectively. The color background depicts the magnitude of the flow velocity. The green arrow represents the phoretic torque (M_p) induced by the unbalanced EOSs. (F) The calculated M_p at different θ. (G) Time-lapse images displaying the change of N near the CO₂ source. Scale bar, 20 μm.