In an article recently published in the journal ACS Nano, the authors prepared cubic and walnut-shaped light-driven hematite/platinum (Pt) microrobots, followed by their transformation into Janus structures by Pt deposition.
Study: Shape-Controlled Self-Assembly of Light-Powered Microrobots into Ordered Microchains for Cells Transport and Water Remediation. Image Credit: DreamStockIcons/Shutterstock.com
They also reported the self-assembly of cubic hematite/Pt microrobots to form microchains by their magnetic dipole moment distributed asymmetrically in the crystal.
The self-assembly of a biological system in a synchronized manner to execute desired tasks is an authentic behavior of nature. Further, desired goal accomplishments without external stimuli have superior advantages in making the process efficient and robust. This behavior of nature fascinated researchers working with micro/nanorobotics.
Micro/nanorobots can harvest energy from the surroundings (chemical fuels/light/magnetic felt, ultrasound) and convert it into an autonomous movement to perform various tasks. Light is an abundant, powerful, and efficient energy source for microrobots.
Actively moving particles have the asymmetric orientation of dipole moment, driving the microrobot’s movement. In this context, the two-faced Janus microrobots with the photolytic semiconductor and an asymmetrically deposited metal layer can be efficient light-powered self-propelled microrobots.
Light-Powered Self-Motile Microrobots
In the present work, the authors prepared walnut and cubic-shaped hematite microparticles using a cost-effective hydrothermal process, followed by their coating with a 30-nanometer thick Pt layer to attribute asymmetric orientation of dipole moment. They demonstrated a fascinating self-assembly behavior of prepared cubic hematite/Pt microrobots but not in walnut-like hematite/Pt microrobots.
All the as-prepared microrobots used ultraviolet (UV) light for their self-propulsion in water. The authors observed faster locomotion in cubic microrobots than in walnut-shaped counterparts. Additionally, the cubic microrobots exhibited self-assemblage to form microchains. These self-assembled cubic microrobots can accomplish multiple tasks in a challenging environment. They can perform cargo transportation to pollutant degradation in water with contaminants or photolytically degraded products of polymer chains.
Further, the prepared hematite microparticle crystalline structure and cubic/walnut-like microrobots were assessed using X-ray diffraction (XRD), scanning electron microscopy (SEM) images, and the energy-dispersive X-ray spectroscopy (EDX).
The facile hydrothermal reaction for hematite microparticles and a subsequent sputtered coating to deposit a Pt layer of 30-nanometer thickness led to the preparation of Janus microrobots. The SEM images and EDX mapping revealed two hematite shapes, and showed that walnut-like hematite microparticles had a diameter of 2 – 3 nanometers. It also showed that their hierarchical porous structure with a diameter of 100 nanometers was formed by the random aggregation of microparticles.
The SEM and EDX images also showed 2-micrometer sized cubed hematite microparticles with a rough surface. Moreover, the distribution of iron and oxygen was uniform for both the hematite microparticles in EDX images. The uneven Pt distribution in EDX images suggested distinct asymmetrical Janus microrobots obtained after deposition of the Pt layer. The obtained peaks in XRD agree with the hexagonal structure containing the rhombohedral center of hematite microparticles with α-Fe2O3 crystals.
Further, the walnut-like microrobot showed a higher speed than the cubic hematite in pure water. However, in the catalytic H2O2 (0.1%), cubic hematite microrobots showed a two-fold speed increase over walnut-like microrobots in water under UV irradiation. Increasing the H2O2 concentration (1%) resulted in an eight-fold increase in speed in cubic microrobots and a three-fold increase in speed in walnut-like microrobots, compared to their speed in pure water.
The SEM images of cubic hematite microrobots showed their self-assembly to form a micro chain containing a 4–8-microrobot configuration with an approximate length of 6-10 micrometers. Further, these self-assembled microchains revealed three main motion modes in water with 0.1% H2O2 and UV light irradiation, which is associated with self-propulsion in parallel, perpendicular, and rotational directions.
Microchains exhibit reconfigurable capability under the influence of an external magnetic field. Measuring magnetic hysteresis loops of hematite microrobots using a vibrating sample magnetometer (VSM) revealed its ferromagnetic behavior. Further, under the transversal rotating magnetic field of 3 milliTesla and 10 hertz, the hematite rotated and moved in an upside direction along the magnetic field. When this field was absent, these microrobots were reconfigured as microchains.
When polyethylene glycol (PEG) was mixed with cubic microrobots (leading to microchains) and exposed to UV irradiation, the results from the matrix-assisted laser desorption/ionization (MALDI) spectra showed the disappearance of the peak that corresponds to the PEG macromolecule. This peak disappearance is due to the photocatalytic generation of reactive oxygen species (ROS) that attacked the carbon-oxygen (C-O) bond in the PEG backbone via the photo-Fenton reaction.
In conclusion, the authors demonstrated the structural effects of hematite/Pt microrobots on their self-assembling behavior. Due to the asymmetric orientation of magnetic dipole moment in cubic hematite/Pt microrobots, they exhibit self-assembly behavior forming microchains, while the walnut-like microrobots, instead, underwent random aggregation.
The authors further exploited this behavior of cubic hematite/Pt microrobots and developed the light-driven microchains with self-propulsion ability in low concentrated H2O2 solution. Based on the microrobot’s mutual orientation during self-assembly and reconfiguration under an external magnetic field, the authors observed different types of autonomous movements.
The authors also confirmed the advantage of synchronized behavior of microchains over individual microrobots by employing them to clean suspended matter in water originating from personal care products and light-assisted polymer degradation products.
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Peng, X., Urso, M., Ussia, M., and Pumera, M. Shape-Controlled Self-Assembly of Light-Powered Microrobots into Ordered Microchains for Cells Transport and Water Remediation. ACS nano. (2022). https://pubs.acs.org/doi/full/10.1021/acsnano.1c11136