James Guo Sheng Moo

Chemical Energy Powered Nano/Micro/Macromotors and the Environment

The rise of miniaturized artificial self-powered devices, demonstrating autonomous motion, has brought in new considerations from the environmental perspective. This review addresses the interplay between these nano/micro/macromotors and the environment, recent advances, and their applications in pollution management. Such self-propelled devices are able to actuate chemical energy into mechanical motion in situ, adding another powerful dimension towards solving environmental problems. Use of synthetic nano/micro/macromotors has demonstrated potential in environmental remediation, both in pollutant removal and contaminant degradation, owing to motion-induced mixing. At the same time, the chemical environment exerts influence on the locomotion of the motors. These sensitized self-powered devices demonstrate capabilities for being deployed as sensors and their chemotactic behaviors show efficacy to act as first responders towards a chemical leakage. Thus, the notion of a self-propelling entity also entails further investigation into its inherent toxicity and possible implications as a pollutant. Future challenges and outlook of the use of these miniaturized devices are discussed, with specific regard to the fields of environmental remediation and monitoring, as we move towards their wider acceptance. We believe that these tiny machines will stand up to the task as solutions for environmental sustainability in the 21st century.

Biomimetic Artificial Inorganic Enzyme‐Free Self‐Propelled Microfish Robot for Selective Detection of Pb2+ in Water

The availability of drinking water is of utmost importance for the world population. Anthropogenic pollutants of water, such as heavy-metal ions, are major problems in water contamination. The toxicity assays used range from cell assays to animal tests. Herein, we replace biological toxicity assays, which use higher organisms, with artificial inorganic self-propelled microtubular robots. The viability and activity of these robots are negatively influenced by heavy metals, such as Pb(2+) , in a similar manner to that of live fish models. This allows the establishment of a lethal dose (LD50 ) of heavy metal for artificial inorganic microfish robots. The self-propelled microfish robots show specific response to Pb(2+) compared to other heavy metals, such as Cd(2+) , and can be used for selective determination of Pb(2+) in water. It is a first step towards replacing the biological toxicity assays with biomimetic inorganic autonomous robotic systems.

Photochromic Spatiotemporal Control of Bubble-Propelled Micromotors by a Spiropyran Molecular Switch.

Controlling the environment in which bubble-propelled micromotors operate represents an attractive strategy to influence their motion, especially when the trigger is as simple as light. We demonstrate that spiropyrans, which isomerize to amphiphilic merocyanines under UV irradiation, can act as molecular switches that drastically affect the locomotion of the micrometer-sized engines. The phototrigger could be either a point or a field source, thus allowing different modes of control to be executed. A whole ensemble of micromotors was repeatedly activated and deactivated by just altering the spiropyran-merocyanine ratio with light. Moreover, the velocity of individual micromotors was altered using a point irradiation source that caused only localized changes in the environment. Such selective manipulation, achieved here with an optical microscope and a photochromic additive in the medium, reveals the ease of the methodology, which can allow micro- and nanomotors to reach their full potential of not just stochastic, but directional controlled motion.

From Nanomotors to Micromotors: The Influence of the Size of an Autonomous Bubble-Propelled Device upon Its Motion

Synthetic autonomously moving nano and micromotors are in the forefront of nanotechnology. Different sizes of nano and micromotors have been prepared, but the systematic study of the influence of their sizes on motion is lacking. We synthesized different sizes of tubular micro/nanomotors by membrane template-assisted electrodeposition. The influence of dimensions on the dynamics of micro/nanotubes was studied at a significantly reduced scale than rolled-up microtubes, down to the nanometer regime. Both the geometric parameters and the chemical environment can affect the dynamics of micro/nanotubes. The bubble size and ejection frequency were investigated in correlation with the velocity of micro/nanotubes. The comparison between different sizes of micro/nanotubes showed that geometric parameters of micro/nanotubes will influence the velocity of micro/nanotubes at moderate fuel concentrations. Furthermore, it also affects the activity of micro/nanotubes at low fuel concentrations and imposes limitations on the velocity at very high fuel concentrations. Nanotubes with nanometer-sized openings need a higher concentration of H2O2 to be activated. Larger tubes can possess a higher absolute value of velocity than smaller tubes, but do not necessarily have a higher velocity by body lengths per unit time. Insight into bubble ejection/propulsion cycle is also provided. The results presented here provide important implications for the consideration of dimensions in the fabrication of tubular micro/nanomotors.

Influence of pH on the Motion of Catalytic Janus Particles and Tubular Bubble-Propelled Micromotors.

Self-propelled miniaturized machines harness the chemical potential of their environment for movement. Locomotion of chemically powered micromotors have been hugely dependent on the surroundings. The use of pH to alter the mobility of micromotors is demonstrated in this work through the manipulation of hydrogen peroxide chemistry in different acidity/alkalinity. The sequential addition of sodium hydroxide to increase the pH of the solution led to a consequent increase in activity of micromotors. Meanwhile, addition of hydrochloric acid compromised the structural integrity of the microstructures, culminating in locomotive changes. Such dramatic changes in activity and velocities of the micromotors allow the usage of this behavior for pH detection. This concept was illustrated with Janus silver micromotors and tubular bimetallic Cu/Pt micromotors. Alteration of pH serves as a useful general strategy for increasing hydrogen peroxide decomposition for enhanced oxygen-bubble propulsion in catalytic micromotors.

Graphene Nanobubbles Produced by Water Splitting

Graphene nanobubbles are of significant interest due to their ability to trap mesoscopic volumes of gas for various applications in nanoscale engineering. However, conventional protocols to produce such bubbles are relatively elaborate and require specialized equipment to subject graphite samples to high temperatures or pressures. Here, we demonstrate the formation of graphene nanobubbles between layers of highly oriented pyrolytic graphite (HOPG) with electrolysis. Although this process can also lead to the formation of gaseous surface nanobubbles on top of the substrate, the two types of bubbles can easily be distinguished using atomic force microscopy. We estimated the Youngs modulus, internal pressure, and the thickness of the top membrane of the graphene nanobubbles. The hydrogen storage capacity can reach ∼5 wt % for a graphene nanobubble with a membrane that is four layers thick. The simplicity of our protocol paves the way for such graphitic nanobubbles to be utilized for energy storage and industrial applications on a wide scale.

Electrochemical properties of carbon nanodiscs

Carbon nanodiscs are new bottom-up prepared nanomaterials with well-defined diameter and thickness (of 1–2 μm and 20–50 nm, respectively). We investigate their electrochemical performance towards oxidation and reduction of small inorganic molecules as well as biomarkers. We have found that carbon nanodiscs exhibit faster observable heterogeneous electron transfer rates than graphite. However, this can be due to nanographitic impurities within the nanodiscs sample.

Detecting the complex motion of self-propelled micromotors in microchannels by electrochemistry

Autonomous self-propelled nano/micromotors are new frontiers in micro- and nanotechnology, with a plethora of possible applications in environmental remediation and biomedicine. However, key challenges remain, one of which is the monitoring of motion in these self-propelled nano- and microdevices. Tracking of these miniaturized objects is typically done by optical microscopy. Such a manual methodology has several inherent challenges, ranging from demanding computational power for optical image analysis to following objects in opaque or non-transparent environments. Here we developed a monitoring system for an autonomous self-propelled micromotor in a microfluidic channel via the placement of electrodes in the pathways. The electrochemical detection methodology, based on the disturbances in the electrical double layer of an electrode surface in our devised instrumentation technique, allows for different modes of motion in micromotors in channel environments to be recognized. This ability to detect the motion of autonomous self-powered micromotors in opaque/non-transparent channels will find widespread applications in the future.