Micromachines for Microplastics Treatment

Chemotactic behavior for a self-phoretic Janus particle near a patch source of fuel

Many biological microswimmers are capable of chemotaxis, i.e., they can sense an ambient chemical gradient and adjust their mechanism of motility to move towards or away from the source of the gradient. Synthetic active colloids endowed with chemotactic behavior hold considerable promise for targeted drug delivery and the realization of programmable and reconfigurable materials. Here, we study the chemotactic behavior of a Janus particle, which converts "fuel" molecules, released at an axisymmetric chemical patch located on a planar wall, into "product" molecules at its catalytic cap and moves by self-phoresis induced by the product. The chemotactic behavior is characterized as a function of the interplay between the rates of release (at the patch) and the consumption (at the particle) of fuel, as well as of details of the phoretic response of the particle (i.e., its phoretic mobility). Among other results, we find that, under certain conditions, the particle is attracted to a stable "hovering state" in which it aligns its axis normal to the wall and rests (positions itself) at an activity-dependent distance above the center of the patch.

High removal of PS and PET microplastics from tap water by using Fe2O3 porous microparticles and photothermal irradiation with NIR light

This work reports the synthesis of Fe2O3 (FeO) microparticles using the Pechini method and their use to remove microplastics from tap water. The analysis by electronic microscopy revealed that the FeO microparticles (FeMicroPs) have a porous structure and are formed by interconnected grains with sizes of 80-120 nm. In addition, the X-ray diffraction analysis pointed out that the FeMicroPs are composed of γ- Fe2O3 and α- Fe2O3 phases. To remove the PS and PET microplastics with sizes of 0.1-3 μm from the tap water, FeO was added to the contaminated water and the mixture of FeO + microplastics was irradiated with focused NIR light (980 nm). This provoked the melting of the microplastics on the FeO surface. Later, the FeMicroPs with adsorbed microplastics was recovered with magnets. This last procedure permitted a high removal of microplastics from the tap water, and the adsorption capacity was 1000 mg/g. In the next step, the microplastics adsorbed on the FeO were irradiated with NIR light to induce its thermal decomposition by photothermal irradiation, this in turn, produced the elimination of the microplastics from the FeO surface and allowed its reuse to remove more microplastics from the tap water. The elimination of the microplastics from the FeO surface was confirmed by the FTIR and Raman techniques, since the vibrational peaks associated with the microplastics disappeared from the FeO surface after the photothermal irradiation. Thus, the results of this investigation suggest that the photothermal irradiation with NIR light not only facilitates the removal of microplastics from the tap water, but also, it was useful to degrade the microplastics definitively without producing more contamination. This technique could be used to remove microplastics in water treatment plants.

Sustainable control of microplastics in wastewater using the electrochemically enhanced living membrane bioreactor

Wastewater treatment plant (WWTP) discharges are major contributors to the release of microplastics (MPs) into the environment. This research work aimed to assess the performance of the novel living membrane bioreactor (LMBR), which utilizes a biological layer as a membrane filter for the removal of polyethylene (PE) MPs from wastewater. The impact of an intermittently applied low current density (0.5 mA/cm2) on the reduction of MPs in the electrochemically enhanced LMBR (e-LMBR) has also been examined. The reactors were also compared to a conventional membrane bioreactor (MBR) and an electro-MBR (e-MBR). 1H nuclear magnetic resonance spectroscopy (1H NMR) was implemented for the MPs detection and quantification in terms of mass per volume of sample. The LMBR and MBR achieved comparable mean PE MPs reduction at 95% and 96%, respectively. The MPs mass reduction in the e-LMBR slightly decreased by 2% compared to that achieved in the LMBR. This potentially indicated the partial breakdown of the MPs due to electrochemical processes. Decreasing and inconsistent NH4-N and PO4-P removal efficiencies were observed over time due to the addition of PE MPs in the MBR and LMBR. In contrast, the integration of electric field in the e-MBR and e-LMBR resulted in consistently high values of conventional contaminant removals of COD (99.72-99.77 %), NH4-N (97.96-98.67%), and PO4-P (98.44-100.00%), despite the MPs accumulation. Integrating electrochemical processes in the e-LMBR led to the development of a stable living membrane (LM) layer, as manifested in the consistently low effluent turbidity 0.49 ± 0.33 NTU. Despite the increasing MPs concentration in the mixed liquor, applying electrochemical processes reduced the fouling rates in the e-LMBR. The e-LMBR achieved comparable efficiencies in contaminant reductions as those observed in the e-MBR, while using a low-cost membrane material.

Biotemplated Janus Magnetic Microrobots Based on Diatomite for Highly Efficient Detection of Salmonella

Foodborne illnesses caused by Salmonella bacteria pose a significant threat to public health. It is still challenging to detect them effectively. Herein, biotemplated Janus disk-shaped magnetic microrobots (BJDMs) based on diatomite are developed for the highly efficient detection of Salmonella in milk. The BJDMs were loaded with aptamer, which can be magnetically actuated in the swarm to capture Salmonella in a linear range of 5.8 × 102 to 5.8 × 105 CFU/mL in 30 min, with a detection limit as low as 58 CFU/mL. In addition, the silica surface of BJDMs exhibited a large specific surface area to adsorb DNA from captured Salmonella, and the specificity was also confirmed via tests of a mixture of diverse foodborne bacteria. These diatomite-based microrobots hold the advantages of mass production and low cost and could also be extended toward the detection of other types of bacterial toxins via loading different probes. Therefore, this work offers a reliable strategy to construct robust platforms for rapid biological detection in practical applications of food safety.

Photophoretic MoS2-Fe2O3 Piranha Micromotors for Collective Dynamic Microplastics Removal

Microplastics are highly persistent emerging pollutants that are widely distributed in the environment. We report the use of MoS2@Fe2O3 core-shell micromotors prepared by a hydrothermal approach to explore the degradation of plastic microparticles. Polystyrene was chosen as the model plastic due to its wide distribution and resistance to degradation using current approaches. Micromotors show photophoretic-based motion at speeds of up to 6 mm s-1 and schooling behavior under full solar light spectra irradiation without the need for fuel or surfactants. During this impressive collective behavior, reactive oxygen species (ROS) are generated because of the semiconducting nature of the MoS2. Degradation of polystyrene beads is observed after 4 h irradiation because of the synergistic effect of ROS production and localized heat generation. The MoS2@Fe2O3 micromotors possess magnetic properties, which allow further cleaning and removal to be carried out after irradiation through magnetic pulling. The new micromotors hold considerable promise for full-scale treatment applications, only limited by our imagination.

Metal-Organic Framework-Based Micro-/Nanomotors for Wastewater Remediation

Micro-/nanomotors (MNMs) in water remediation have garnered significant attention over the past two decades. More recently, metal-organic framework-based micro-/nanomotors (MOF-MNMs) have been applied for environmental remediation; however, a comprehensive summary of research in this research area is yet to be reported. Herein, a review is presented to cover the recent advances in MOF-MNMs and their various applications in wastewater remediation. The review presents a comprehensive introduction to MNMs, including different propulsion approaches, fabrication, and functionalization strategies, in addition to the unique features of MOF-MNMs. The conception and various synthetic routes of MOF-MNMs are extensively covered and the implementation of MOF-MNMs in water-related applications, including adsorption, degradation, sensing, and disinfection of different pollutants, is in depth discussed. Meanwhile, the propulsion and mechanism of action behind each MOF-MNM are systematically studied. Finally, the review provides insights into the challenges and perspectives to build more effective MOF-MNMs to cover versatile applications for wastewater treatment.

Microplastics in aquatic systems: An in-depth review of current and potential water treatment processes

Plastic products, despite their undeniable utility in modern life, pose significant environmental challenges, particularly when it comes to recycling. A crucial concern is the pervasive introduction of microplastics (MPs) into aquatic ecosystems, with deleterious effects on marine organisms. This review presents a detailed examination of the methodologies developed for MPs removal in water treatment systems. Initially, investigating the most common types of MPs in wastewater, subsequently presenting methodologies for their precise identification and quantification in aquatic environments. Instruments such as scanning electron microscopy, dynamic light scattering, Fourier transform infrared spectroscopy, Raman spectroscopy, surface-enhanced Raman spectroscopy, and Raman tweezers stand out as powerful tools for studying MPs. The discussion then transitions to the exploration of both existing and emergent techniques for MPs removal in wastewater treatment plants and drinking water treatment plants. This includes a description of the core mechanisms that drive these techniques, with an emphasis on the latest research developments in MPs degradation. Present MPs removal methodologies, ranging from physical separation to chemical and biological adsorption and degradation, offer varied advantages and constraints. Addressing the MPs contamination problem in its entirety remains a significant challenge. In conclusion, the review offers a succinct overview of each technique and forwards recommendations for future research, highlighting the pressing nature of this environmental dilemma.

Nano revolution: pioneering the future of water reclamation with micro-/nano-robots

Earth's freshwater reserves are alarmingly limited, with less than 1% readily available. Factors such as industrialisation, population expansion, and climate change are compounding the scarcity of clean water. In this context, self-driven, programmable micro- and nano-scale synthetic robots offer a potential solution for enhancing water monitoring and remediation. With the aid of these innovative robots, diffusion-limited reactions can be overcome, allowing for active engagement with target pollutants, such as heavy metals, dyes, nano- and micro-plastics, oils, pathogenic microorganisms, and persistent organic pollutants. Herein, we introduced and reviewed recent influential and advanced studies on micro-/nano-robots (MNR) carried out over the past decade. Typical works are categorized by propulsion modes, analyzing their advantages and drawbacks in detail and looking at specific applications. Moreover, this review provides a concise overview of the contemporary advancements and applications of micro-/nano-robots in water-cleaning applications.

Comprehensive Understanding of Self-Propelled Janus Pt/Fe2O3 Micromotor Dynamics: Impact of Size, Morphology, and Surface Structure

The increasing use of plastics has led to the accumulation of plastic waste in the oceans, resulting in significant global environmental challenges associated with microplastic pollution. Micromotors, capable of capturing and removing microplastics from aquatic systems, have emerged as a promising solution to addressing this problem. This research aims to analyze the factors affecting the speed of micromotors, including size, morphology, and surface structure, while elucidating the underlying mechanisms governing micromotor propulsion to develop efficient and ecofriendly micromotors. In this study, we systematically manipulate various parameters by modifying the synthesis method of hematite-based micromotors, subsequently comparing their propulsion speeds and uncovering the precise role of these parameters in determining the micromotor performance. Furthermore, we shed light on the intricate interplay between drag force and propulsive force, demonstrating how these forces vary under different H2O2 conditions. These findings provide valuable insights into the design of efficient micromotors tailored for dynamic aquatic environments.

Eliminating waste with waste: transforming spent coffee grounds into microrobots for water treatment

Water pollutants such as oil spills, industrial dyes, and microplastics threaten public health and aquatic ecosystems. There are considerable challenges in removing water contaminants using traditional methods. Several studies have been conducted in recent years to develop effective water purification materials. Despite this, the mass production of most materials is extremely challenging because they involve multiple intricate steps and sophisticated equipment. Herein, we report the facile synthesis of spent coffee ground (SCG)-derived magnetic microrobots, which we dub "CoffeeBots", to remove oil, organic dyes, and microplastic pollution from contaminated seawater. In order to meet eco-friendly, high-yield and low-cost requirements, iron oxide nanoparticles (IONPs) were deposited on biodegradable SCGs using green chemistry. The IONPs on CoffeeBots facilitate magnetic navigation and recycling, microswarm assembly, and ease of retrieval after water remediation tasks. CoffeeBots' intrinsic surface hydrophobicity enables efficient on-the-fly capture and removal of oil droplets and microplastics from contaminated water with remote magnetic guidance. CoffeeBots were also functionalized with ascorbic acid (AA@CoffeeBots) to remove methylene blue (MB) dye contaminants from polluted seawater. SCGs and AA act as bioadsorbent and reducing agent, respectively, for MB dye removal whereas magnetic propulsion enhances mixing and accelerates MB decolorization. These CoffeeBots can be recycled numerous times for removing oil spills, organic dyes, and microplastics from the seawater. CoffeeBots hold considerable potential as sustainable, recyclable, and low-cost remediation agents for water treatment in the near future.

Collective Behaviors of Isotropic Micromotors: From Assembly to Reconstruction and Motion Control under External Fields

Swarms of self-propelled micromotors can mimic the processes of natural systems and construct artificial intelligent materials to perform complex collective behaviors. Compared to self-propelled Janus micromotors, the isotropic colloid motors, also called micromotors or microswimmers, have advantages in self-assembly to form micromotor swarms, which are efficient in resistance to external disturbance and the delivery of large quantity of cargos. In this minireview, we summarize the fundamental principles and interactions for the assembly of isotropic active particles to generate micromotor swarms. Recent discoveries based on either catalytic or external physical field-stimulated micromotor swarms are also presented. Then, the strategy for the reconstruction and motion control of micromotor swarms in complex environments, including narrow channels, maze, raised obstacles, and high steps/low gaps, is summarized. Finally, we outline the future directions of micromotor swarms and the remaining challenges and opportunities.

Visible-light-induced self-propelled nanobots against nanoplastics

The accumulation of plastic debris in aquatic organisms has raised serious concerns about the potential health implications of their incorporation into the food chain. However, conventional water remediation techniques are incapable of effectively removing nanoplastics (NPs) smaller than 200 nm, which can have harmful effect on animal and human health. Herein, we demonstrate the "on-the-fly" capture of NPs through their enlargement (approximately 4,100 times) using self-propelled nanobots composed of a metal-organic framework. Under visible-light irradiation, the iron hexacyanoferrate (FeHCF) nanobot exhibits fuel-free motion by electrostatically adsorbing NPs. This strategy can contribute to reducing plastic pollution in the environment, which is a significant environmental challenge. Light-induced intervalence charge transfer in the FeHCF nanobot lattice induces bipolarity on the nanobot surface, leading to the binding of negatively charged NPs. The local electron density in the lattice then triggers self-propulsion, thereby inducing agglomeration of FeHCF@NP complexes to stabilize their metastable state. The FeHCF nanobot exhibits a maximum removal capacity of 3,060 mg∙g-1 and rate constant of 0.69 min-1, which are higher than those recorded for materials reported in the literature.

Janus magnetoelastic membrane swimmers

Soft swimming microrobots have attracted considerable attention due to their potential applications in diverse fields ranging from biomedicines to environmental remediation. The locomotion control is of importance to the research of micromachines and microrobots. Inspired by the motility strategies of living microorganisms, such as flagella, cilia, and euglenoids, we focus on propulsion mechanisms with a design of Janus magnetoelastic crystalline membrane microswimmers actuated by time-varying magnetic fields. Such a Janus swimmer consists of a ferromagnetic cap completed by a magnetoelastic membrane body, where superparamagnetic particles are uniformly distributed on the surface. Under the influence of external magnetic fields, the swimmer undergoes complex shape transitions due to the interplay between the magnetic dipole-dipole interactions, the elasticity of the magnetoelastic membranes, and also the hydrodynamics of surrounding fluids. We show that those shape changes are nonreciprocal, which can generate locomotion such that the propulsion speed can be optimized by tailoring the membrane elastic properties. Besides, we also demonstrate that the Janus swimmer can be magnetically guided in a spiral trajectory. With such adequate control of locomotion in both speed and direction via non-invasive magnetic fields, this study provides another promising candidate design for the future development of microswimmers.

Extraordinary microcarriers derived from spores and pollens

Spores and pollens refer to the reproductive cells of seed plants and asexually reproducing sporophytes, exhibiting a natural core-shell structure and exquisite surface morphology. They possess extraordinary dimensional homogeneity, porosity, amphiphilicity and adhesion. Their sporopollenin exine layer endows them with chemically stable, UV resistant, and biocompatible properties, which can also be facilely functionalized due to sufficient groups on the surface. The unique characteristics of spores and pollens have facilitated a wide range of applications in drug carriers, biological imaging, food science, microrobotics, environmental purification, flexible electronics, cell scaffolds, 3D printing materials and biological detection. This review showcases the common structural composition and physicochemical properties of spores and pollens, describes the extraction and processing methods, and summarizes the recent research on their applications in various fields. Following these sections, this review analyzes the existing challenges in spores and pollen research and provides a future outlook.

Precisely Navigated Biobot Swarms of Bacteria Magnetospirillum magneticum for Water Decontamination

Hybrid biological robots (biobots) prepared from living cells are at the forefront of micro-/nanomotor research due to their biocompatibility and versatility toward multiple applications. However, their precise maneuverability is essential for practical applications. Magnetotactic bacteria are hybrid biobots that produce magnetosome magnetite crystals, which are more stable than synthesized magnetite and can orient along the direction of earth's magnetic field. Herein, we used Magnetospirillum magneticum strain AMB-1 (M. magneticum AMB-1) for the effective removal of chlorpyrifos (an organophosphate pesticide) in various aqueous solutions by naturally binding with organic matter. Precision control of M. magneticum AMB-1 was achieved by applying a magnetic field. Under a programed clockwise magnetic field, M. magneticum AMB-1 exhibit swarm behavior and move in a circular direction. Consequently, we foresee that M. magneticum AMB-1 can be applied in various environments to remove and retrieve pollutants by directional control magnetic actuation.

Microplastics in sewage sludge: Distribution, toxicity, identification methods, and engineered technologies

Microplastic pollution is becoming a global challenge due to its long-term accumulation in the environment, causing adverse effects on human health and the ecosystem. Sludge discharged from wastewater treatment plants (WWTPs) plays a critical role as a carrier and primary source of environmental microplastic contamination. A significantly average microplastic variation between 1000 and 301,400 particles kg^-1 has been reported in the sludge samples. In recent years, advanced technologies have been successfully applied to address this issue, including adsorption, advanced oxidation processes (AOPs), and membrane bioreactors (MBRs). Adsorption technologies are essential to utilizing novel adsorbents (e.g., biochar, graphene, zeolites) for effectively removing MPs. Especially, the removal efficiency of polymer microspheres from an aqueous solution by Mg/Zn modified magnetic biochars (Mg/Zn-MBC) was obtained at more than 95%. Also, advanced oxidation processes (AOPs) are widely applied to degrade microplastic contaminants, in which photocatalytic by semiconductors (e.g., TiO₂ and ZnO) is a highly suitable approach to promote the degradation reactions owing to strongly hydroxyl radicals (OH*). Biological degradation-aided microorganisms (e.g., bacterial and fungal strains) have been reported to be suitable for removing microplastics. Yet, it was affected by biotic and abiotic factors of the environmental conditions (e.g., pH, light, temperature, moisture, bio-surfactants, microorganisms, enzymes) as well as their polymer characteristics, i.e., molecular weight, functional groups, and crystallinity. Notably, membrane bioreactors (MBRs) showed the highest efficiency in removing up to 99% microplastic particles and minimizing their contamination in sewage sludge. Further, MBRs illustrate the suitability for treating high-strength compounds, e.g., polymer debris and microplastic fibers from complex industrial wastewater. Finally, this study provided a comprehensive understanding of potential adverse risks, transportation pathways, and removal mechanisms of microplastic, which full-filled the knowledge gaps in this field.

Self-driven magnetorobots for recyclable and scalable micro/nanoplastic removal from nonmarine waters

Micro/nanoplastic (MNP) contamination in nonmarine waters has evolved into a notable ecotoxicological threat to the global ecosystem. However, existing strategies for MNP removal are typically limited to chemical flocculation or physical filtering that often fails to decontaminate plastic particulates with ultrasmall sizes or ultralow concentrations. Here, we report a self-driven magnetorobot comprising magnetizable ion-exchange resin sphere that can be used to dynamically remove or separate MNPs from nonmarine waters. As a result of the long-range electrophoretic attraction established by recyclable ion-exchange resin, the magnetorobot shows sustainable removal efficiency of >90% over 100 treatment cycles, with verified broad applicability to varying plastic compositions, sizes, and shapes as well as nonmarine water samples. Our work may facilitate industry-scale MNP removal with affordable cost and minimal secondary pollution and suggests an appealing strategy based on self-propelled micro/nanorobots to sample and assess nanoplastics in aqueous environment.