Oxygen Microbubble Generator Enabled by Tunable Catalytic Microtubes

Bridging the gap: harnessing liquid nanomachine know-how for tackling harmful airborne particulates

The emergence of "nanomotors", "nanomachines", and "nanorobotics" has transformed dynamic nanoparticle research, driving a transition from passive to active and intelligent nanoscale systems. This review examines two critical fields: the investigation of airborne particles, significant contributors to air pollution, and the rapidly emerging domain of catalytic and field-controlled nano- and micromotors. We examine the basic concepts of nano- and micromachines in motion and envision their possible use in a gaseous medium to trap and neutralize hazardous particulates. While past studies described the application of nanotechnology and nanomotors in various scenarios, airborne nano/micromachine motion and their control have yet to be thoroughly explored. This review intends to promote multidisciplinary research on nanomachines' propulsion and task-oriented applications, highlighting their relevance in obtaining a cleaner atmospheric environment, a critical component to consider for human health.

Electrocatalytic Reaction Induced Colloidal Accumulation: The Role of Dielectrophoresis

A surface-driven flow is generated during the electrocatalytic reaction of a platinum-gold bielectrode within hydrogen peroxide. This flow can be experimentally visualized and quantified using micrometer-sized particles that are transported by a flow field. Tracer particles, which possess an inherent surface charge, also interact with the induced electric field and exhibit a collective behavior at the surface of the electrodes where they accumulate. The underlying mechanism for the accumulation dynamics demonstrated by these catalytic pump systems has so far been lacking. In this work, the accumulation dynamics and kinetics were experimentally investigated. With use of numerical simulations, we demonstrate that the self-driven particle accumulation is controlled by a positive dielectrophoretic force, mediated by the reaction-induced electric and flow field. These results contribute to the fundamental knowledge on immobilized bimetallic systems.

Electrocatalytic Reaction Driven Flow: Role of pH in Flow Reversal

Immobilized bimetallic structures generate fluid flow during electrocatalytic reactions with hydrogen peroxide, which is typically driven from the anodic metal to the cathodic metal similar to an electroosmotic flow. However, under low reactive regimes, the generated flow becomes fully reversed, which cannot be explained by the classical electroosmotic theory. This work aims at unraveling the origin and dynamics of this flow hysteresis through a combined experimental and numerical approach. The key electrocatalytic parameters that contribute to flow reversal are analyzed experimentally and numerically under low reactive regimes induced by bulk pH variations. The proton gradient that initiates chemomechanical actuation is probed with the use of fluorescence lifetime imaging. The fluid flow dynamics under reactive regimes are visualized by the use of particle tracking. Our numerical simulations elucidate the role of pH variations and additional ionic species (counterions) toward flow reversal. The combination of these techniques highlights the interplay between electrocatalytic and electrokinetic phenomena on the occurrence of flow reversal.

Fluorescence Detection of miRNA-21 Using Au/Pt Bimetallic Tubular Micromotors Driven by Chemical and Surface Acoustic Wave Forces

In this study, surface acoustic wave (SAW) systems are described for the removal of molecules that are unbound to micromotors, thereby lowering the detection limit of the cancer-related biomarker miRNA-21. For this purpose, in the first step, mass production of the Au/Pt bimetallic tubular micromotor was performed with a simple membrane template electrodeposition. The motions of catalytic Au/Pt micromotors in peroxide fuel media were analyzed under the SAW field effect. The changes in the micromotor speed were investigated depending on the type and concentration of surfactants in the presence and absence of SAW streaming. Our detection strategy was based on immobilization of probe dye-labeled single-stranded probe DNA (6-carboxyfluorescein dye-labeled-single-stranded DNA) to Au/Pt micromotors that recognize target miRNA-21. Before/after hybridization of miRNA-21 (for both w/o SAW and SAW streaming conditions), the changes in the speed of micromotors and their fluorescence intensities were studied. The response of fluorescence intensities was observed to be linearly varied with the increase of the miRNA-21 concentration from 0.5 to 5 nM under both w/o SAW and with SAW. The resulting fluorescence sensor showed a limit of detection of 0.19 nM, more than 2 folds lower compared to w/o SAW conditions. Thus, the sensor and behaviors of Au/Pt tubular micromotors were improved by acoustic removal systems.

Oxygen Generation Using Catalytic Nano/Micromotors

Gaseous oxygen plays a vital role in driving the metabolism of living organisms and has multiple agricultural, medical, and technological applications. Different methods have been discovered to produce oxygen, including plants, oxygen concentrators and catalytic reactions. However, many such approaches are relatively expensive, involve challenges, complexities in post-production processes or generate undesired reaction products. Catalytic oxygen generation using hydrogen peroxide is one of the simplest and cleanest methods to produce oxygen in the required quantities. Chemically powered micro/nanomotors, capable of self-propulsion in liquid media, offer convenient and economic platforms for on-the-fly generation of gaseous oxygen on demand. Micromotors have opened up opportunities for controlled oxygen generation and transport under complex conditions, critical medical diagnostics and therapy. Mobile oxygen micro-carriers help better understand the energy transduction efficiencies of micro/nanoscopic active matter by careful selection of catalytic materials, fuel compositions and concentrations, catalyst surface curvatures and catalytic particle size, which opens avenues for controllable oxygen release on the level of a single catalytic microreactor. This review discusses various micro/nanomotor systems capable of functioning as mobile oxygen generators while highlighting their features, efficiencies and application potentials in different fields.

Parameters Optimization of Catalytic Tubular Nanomembrane-Based Oxygen Microbubble Generator

A controllable generation of oxygen gas during the decomposition of hydrogen peroxide by the microreactors made of tubular catalytic nanomembranes has recently attracted considerable attention. Catalytic microtubes play simultaneous roles of the oxygen bubble producing microreactors and oxygen bubble-driven micropumps. An autonomous pumping of peroxide fuel takes place through the microtubes by the recoiling microbubbles. Due to optimal reaction–diffusion processes, gas supersaturation, leading to favorable bubble nucleation conditions, strain-engineered catalytic microtubes with longer length produce oxygen microbubbles at concentrations of hydrogen peroxide in approximately ×1000 lower in comparison to shorter tubes. Dynamic regimes of tubular nanomembrane-based oxygen microbubble generators reveal that this depends on microtubes’ aspect ratio, hydrogen peroxide fuel concentration and fuel compositions. Different dynamic regimes exist, which produce specific bubble frequencies, bubble size and various amounts of oxygen. In this study, the rolled-up Ti/Cr/Pd microtubes integrated on silicon substrate are used to study oxygen evolution in different concentrations of hydrogen peroxide and surfactants. Addition of Sodium dodecyl sulfate (SDS) surfactants leads to a decrease of bubble diameter and an increase of frequencies of bubble recoil. Moreover, an increase of temperature (from 10 to 35 °C) leads to higher frequencies of oxygen bubbles and larger total volumes of produced oxygen.

Catalytic/magnetic assemblies of rolled-up tubular nanomembrane-based micromotors

Nano/-micromotors self-assembling into static and dynamic clusters are of considerable promise to study smart, interactive, responsive, and adaptive nano/-micromaterials that can mimic spatio-temporal patterns, swarming, and collective behaviors widely observed in nature. Previously, the dynamic self-assembly of bubble-propelled catalytic micromotors initiated by capillary forces has been reported. This manuscript shows novel self-assembly modes of magnetic/catalytic Ti/FeNi/Pt tubular micromotors. When chemical fuel (hydrogen peroxide) is added it is decomposed on contact with Pt catalyst into oxygen and water. Here, the non-bubbling motion and autonomous assembly of catalytic/magnetic nanomembranes, i.e. without nucleation/generation of oxygen bubbles, are shown. Moreover, magnetic Ti/FeNi/Pt micromotors are spun using an external magnetic field and they form dynamic clusters balanced by attractive magnetic and repulsive hydrodynamic interactions. Micromotors form dynamic clusters, undergo precession and rapidly propagate through the solution.

Ti/FeNi/Pt tubular micromotors self-assemble into static and dynamic clusters during catalytic decomposition of hydrogen peroxide and using an external rotational magnetic field.

Light-controlled two-dimensional TiO2 plate micromotors

In this work, UV light-controlled two-dimensional (2D) TiO2 plate micromotors are demonstrated for the first time. The 2D TiO2 micromotors are produced by the well-known anodic oxidation method in combination with a cracking and separation process. When the motor is placed in H2O2 aqueous solution under UV irradiation, oxygen bubbles are generated in the holes of the TiO2 membrane. The 2D micromotor thus moves upon O2 bubbles under its own weight. In contrast to bubble-propelled micromotors, which require an addition of surfactants to chemical fuels, the 2D micromotor is capable of moving in aqueous H2O2 solution without surfactants. Moreover, speed of the 2D TiO2 micromotor can be controlled by the intensity of the UV light. Such surfactant-free micromotors and their facile fabrication hold considerable promise for diverse practical applications in the biomedical and energy fields, for example, and in new materials.