Micromotors for Environmental Applications

[eng] Scarce supply of clean water and rising water pollution are key global challenges for water sustainability. Much of the wastewater generated by human agricultural and industrial activity is left untreated. Nanotechnological materials and systems have emerged as new tools for improving the efficiency of water treatment. Among those, self-propelled micromotors have shown several advantageous characteristics. Micromotors are autonomously propelled systems which either use chemical energy present in their environment or are propelled via external force fields. Diverse designs, materials composition and mechanisms of propulsion are reported for micromotors found in the literature. Among them, bubble-propelled micromotors, which move due to the generation and release of gas bubbles from their surface, are the main type of motors used for water remediation applications. In addition to the motion in fluids, the bubbles generated by the motors, also contribute with additional mixing of the fluid and enhance the mass transfer between active material and pollutant at the microscale. Additionally, the structure of micromotors can be modified to target a wide variety of pollutants, almost on demand. The micromotors that we synthesized during the research work for this thesis can remove organic and heavy metal pollutants, as well as exhibit bactericidal activity. We studied Iron/Platinum (Fe/Pt) micromotors for their reusability, effect of sizes, swimming behaviors and catalytic properties. These micromotors were fabricated by spontaneous roll-up of iron and platinum nanomembranes, deposited on the pre-fabricated patterns of a photoresist substrate. The iron layer present as the outer surface of these micromotors can degrade organic pollutants via Fenton-like reaction and the inner platinum layer acts as the engine decomposing hydrogen peroxide to oxygen for bubble propulsion. We observed that Fe/Pt micromotors can swim continuously for hours, and can be stored for weeks before reuse, without sacrificing much of their activity. The results suggested that Fe/Pt micromotors act as a heterogeneous catalyst due to in situ generated iron oxide species on the surface, without leaching high concentration of iron in the media. We developed graphene oxide-based micromotors (GOx-micromotors) for heavy metal removal, consisting of nanosized multilayers of graphene oxide, nickel, and platinum. These micromotors can capture, transfer, and remove heavy metals (i.e. lead) from contaminated water. GOx-micromotors are synthesized by electrodepositions of electro-reducible graphene oxide, nickel and platinum layers in the polycarbonate porous templates. The outer layer of graphene oxide captures lead on their surface, and the inner layer of platinum provides self-propulsion in hydrogen peroxide, while the middle layer of nickel enables external magnetic control of the micromotors. We observed that the mobile GOx-micromotors can remove lead 10 times more efficiently than non-motile GOx-micromotors, cleaning water from 1000 ppb down to below 50 ppb. We have demonstrated control of their motion and directionality in a proof of concept microfluidic system. Silver nanoparticles (AgNPs) decorated Magnesium Janus micromotors were designed for disinfection and remove of Escherichia coli (E. coli) bacteria from contaminated water. Magnesium present in the micromotors functions as both, the template for the spherical shape and propulsion source by producing hydrogen bubbles while in contact with water. The inner layer of iron provides functionality for the magnetic remote guidance, and an outer AgNP coated gold layer facilitates adhesion of bacteria and gives bactericidal properties to the micromotors. We observed that the AgNPs-coated Au cap of the micromotors shows dual capabilities, capturing bacteria and killing them. In our efforts to develop multifunctional micromotors and scalable synthesis methods, we developed two types of micromotors. (i) Mesoporous silica-based micromotors with manganese dioxide (MnO2) layer on the inner surface and coated with γ-Fe2O3 nanoparticles (FeSiMnOx micromotors). These micromotors can remove both organic and heavy metal pollutants, and they are synthesized using only template-assisted chemical methods. (ii) Cobalt ferrite micromotors (CFO micromotors) synthesized by template-free chemical synthesis approach. They are made up of aggregated cobalt ferrite nanoparticles, which act as the catalyst for propulsion and for Fenton-like reactions. We qualitatively measured the generation of hydroxyl radicals by CFO micromotors and studied the effect of surfactants on the degradation efficiency of CFO micromotors. We hope that such approach of synthesizing micromotors via relatively facile methods will push the use of micromotors towards commercially practical solutions for water treatment. Overall, our results show that the multifunctional self-propelled micromotors have potential to become an effective tool for water remediation in the near future.[spa] El escaso suministro de agua limpia y el aumento de la contaminación del agua son desafíos globales clave para la sostenibilidad del agua, sobre todo teniendo en cuenta que gran parte del agua residual generada por la actividad agrícola e industrial humana no se trata. Los materiales y sistemas nanotecnológicos han surgido como nuevas herramientas para mejorar la eficiencia del tratamiento de aguas. Entre ellos, los micromotores autopropulsados han mostrado varias características ventajosas. Los micromotores son sistemas de propulsión autónoma que utilizan energía química presente en su entorno o pueden también ser propulsadas a través de campos de fuerzas aplicadas externamente. Varios diseños, composición de materiales y mecanismos de propulsión se han sido reportados en el campo de los micromotores. Entre ellos, principalmente los micromotores propulsados por burbujas, los cuales se mueven debido a la generación y liberación de burbujas de gas de su superficie, se utilizan como una herramienta para aplicaciones de remediación de aguas. Esto se debe a la eficacia añadida de la transferencia de masa a la microescala, que se origina a partir de su movimiento y el movimiento de las burbujas liberadas. Además, la estructura de los micromotores se puede modificar para dirigirse a una amplia variedad de contaminantes, según los requerimientos. Los micromotores que sintetizamos durante el trabajo de investigación para esta tesis pueden eliminar contaminantes orgánicos y metales pesados, así como exhibir actividad anti bactericida. Estudiamos micromotores de hierro / platino (Fe / Pt) por su reutilización, efecto de tamaños, su comportamiento durante su movimiento y propiedades catalíticas. Estos micromotores se fabricaron mediante enrollamiento espontáneo de nanomembranas de hierro y platino, depositadas en los patrones prefabricados definidos en una capa sacrificial fotorresistente. La capa de hierro presente como superficie externa de estos micromotores puede degradar los contaminantes orgánicos a través de la reacción tipo Fenton y la capa interna de platino actúa como el motor, siendo el catalizador que descompone el peróxido de hidrógeno en oxígeno para generar una propulsión por burbujas. Observamos que los micromotores Fe / Pt pueden nadar continuamente durante horas y pueden almacenarse durante semanas antes de volver a ser usados, sin que esto repercuta de manera significativa en su actividad. Los resultados de nuestros experimentos sobre el análisis de superficie de micromotores, estudio de nanoindentación y liberación de hierro sugirieron que los micromotores Fe / Pt actúan como un catalizador heterogéneo debido a las especies de óxido de hierro generadas in situ en la superficie, sin lixiviación de alta concentración de hierro en los medios. Desarrollamos micromotores basados en óxido de grafeno (micromotores GOx) para la eliminación de metales pesados que consisten en multicapas nanométricas de óxido de grafeno, níquel y platino. Estos micromotores pueden capturar, transferir y eliminar metales pesados (es decir, plomo) del agua contaminada. Los micromotores GOx se sintetizan mediante electrodeposiciones de capas de óxido de grafeno, níquel y platino, los cuales son electroreducidos en la parte interior de membranas de policarbonato porosas. La capa externa de óxido de grafeno captura el plomo en su superficie, y la capa interna de platino proporciona autopropulsión en presencia de peróxido de hidrógeno, mientras que la capa intermedia de níquel permite el control magnético externo de los micromotores. Observamos que los micromotores móviles GOx pueden eliminar el plomo hasta 10 veces más que los micromotores GOx no móviles (Figura 1B), limpiando el plomo en agua de 1000 ppb a menos de 50 ppb en menos de 60 min. Hemos demostrado el control de su movimiento y direccionalidad en un sistema microfluídico como prueba de concepto. Diseñamos también micromotores tipo Janus decorados con nanopartículas de plata (AgNP) para la desinfección y eliminación de la bacteria Escherichia coli (E. coli) en agua contaminada. Los micromotores Janus se sintetizaron recubriendo un lado de una micro-partícula de magnesio con capas de hierro y oro, las cuales posteriormente se funcionalizaron con AgNP. El magnesio presente en los micromotores funciona no sólo como estructura principal para conseguir una forma esférica, sino también como fuente de propulsión mediante la producción de burbujas de hidrógeno al entrar en contacto con el agua. La capa interna de hierro proporciona la funcionalidad requerida para el posterior control magnético externo, mientras que la capa de oro externa decorada con AgNPs promueve la adhesión de bacterias y dota de propiedades bactericidas a los micromotores. En nuestro esfuerzo por desarrollar micromotores multifuncionales y métodos de síntesis escalables, desarrollamos dos tipos de micromotores. (i) Micromotores mesoporosos basados en sílice con una capa de dióxido de manganeso (MnO2) en la superficie interna y recubiertos con nanopartículas γ-Fe2O3 (micromotores FeSiMnOx). Estos micromotores pueden eliminar contaminantes orgánicos y metales pesados, y se sintetizan utilizando solo métodos químicos asistidos por un molde (por ejemplo, una membrana porosa). (ii) Los micromotores de ferrita de cobalto (micromotores CFO) fueron sintetizados sin necesidad de utilizar ningún molde. Están formados por nanopartículas de ferrita de cobalto agregadas, que actúan como catalizadores para la propulsión y para reacciones tipo Fenton. Medimos cualitativamente la generación de radicales hidroxilos por micromotores CFO y estudiamos el efecto de los tensioactivos sobre la eficiencia de degradación de los micromotores CFO. Esperamos que la síntesis de micromotores a través de métodos relativamente fáciles empuje la implementación de micromotores en soluciones comercialmente prácticas para el tratamiento del agua. En general, nuestros resultados muestran que los micromotores autopropulsados multifuncionales tienen el potencial de convertirse en una herramienta efectiva para la limpieza de aguas en el futuro

Nano-photocatalysts in microfluidics, energy conversion and environmental applications

Extensive studies have been carried out on photocatalytic materials in recent years as photocatalytic reactions offer a promising solution for solar energy conversion and environmental remediation. Currently available commercial photocatalysts still lack efficiency and thus are economically not viable for replacing traditional sources of energy. This article focuses on recent developments in novel nano-photocatalyst materials to enhance photocatalytic activity. Recent reports on optofluidic systems, new synthesis of photocatalytic composite materials and motile photocatalysts are discussed in this article.Postprint (published version

Platinum-free cobalt ferrite based micromotors for antibiotic removal

Self-propelled micromotors have previously shown to enhance pollutant removal compared to non-motile nano-micro particles. However, these systems are expensive, difficult to scale-up and require surfactant for efficient work. Efficient and inexpensive micromotors are desirable for their practical applications in water treatment technologies. We describe cobalt-ferrite based micromotors (CFO micromotors) fabricated by a facile and scalable synthesis, that produce hydroxyl radicals via Fenton-like reaction and take advantage of oxygen gas generated during this reaction for self-propulsion. Once the reaction is complete, the CFO micromotors can be easily separated and collected due to their magnetic nature. The CFO micromotors are demonstrated for highly efficient advanced oxidative removal of tetracycline antibiotic from the water. Furthermore, the effects of different concentrations of micromotors and hydrogen peroxide on the antibiotic degradation were studied, as well as the generation of the highly reactive hydroxyl radicals responsible for the oxidation reaction

Micro- and Nanomotors as Active Environmental Microcleaners and Sensors

© 2018 American Chemical Society. The quest to provide clean water to the entire population has led to a tremendous boost in the development of environmental nanotechnology. Toward this end, micro/nanomotors are emerging as attractive tools to improve the removal of various pollutants. The micro/nanomotors either are designed with functional materials in their structure or are modified to target pollutants. The active motion of these motors improves the mixing and mass transfer, greatly enhancing the rate of various remediation processes. Their motion can also be used as an indicator of the presence of a pollutant for sensing purposes. In this Perspective, we discuss different chemical aspects of micromotors mediated environmental cleanup and sensing strategies along with their scalability, reuse, and cost associated challenges

Metal-oxide-based microjets for the simultaneous removal of organic pollutants and heavy metals

Water contamination from industrial and anthropogenic activities is nowadays a major issue in many countries worldwide. To address this problem, efficient water treatment technologies are required. Recent efforts have focused on the development of self-propelled micromotors that provide enhanced micromixing and mass transfer by the transportation of reactive species, resulting in higher decontamination rates. However, a real application of these micromotors is still limited due to the high cost associated to their fabrication process. Here, we present Fe2O3-decorated SiO2/MnO2 microjets for the simultaneous removal of industrial organic pollutants and heavy metals present in wastewater. These microjets were synthesized by low-cost and scalable methods. They exhibit an average speed of 485 ± 32 μm s–1 (∼28 body length per s) at 7% H2O2, which is the highest reported for MnO2-based tubular micromotors. Furthermore, the photocatalytic and adsorbent properties of the microjets enable the efficient degradation of organic pollutants, such as tetracycline and rhodamine B under visible light irradiation, as well as the removal of heavy metal ions, such as Cd2+ and Pb2+

Medical imaging for the tracking of micromotors

Micro/nanomotors are useful tools for several biomedical applications, including targeted drug delivery and minimally invasive microsurgeries. However, major challenges such as in vivo imaging need to be addressed before they can be safely applied on a living body. Here, we show that positron emission tomography (PET), a molecular imaging technique widely used in medical imaging, can also be used to track a large population of tubular Au/PEDOT/Pt micromotors. Chemisorption of an iodine isotope onto the micromotor’s Au surface rendered them detectable by PET, and we could track their movements in a tubular phantom over time frames of up to 15 min. In a second set of experiments, micromotors and the bubbles released during self-propulsion were optically tracked by video imaging and bright-field microscopy. The results from direct optical tracking agreed with those from PET tracking, demonstrating that PET is a suitable technique for the imaging of large populations of active micromotors in opaque environments, thus opening opportunities for the use of this mature imaging technology for the in vivo localization of artificial swimmers

Core-shell microspheres for the ultrafast degradation of estrogen hormone at neutral pH

In the past few years there has been growing concern about human exposure to endocrine disrupting chemicals. This kind of pollutants can bioaccumulate in aquatic organisms and lead to serious health problems, especially affecting child development. Many efforts have been devoted to achieving the efficient removal of such refractory organics. In this regard, a novel catalyst based on the combination of α-FeOOH and MnO2@MnCO3 catalysts has been developed by up-scalable techniques from cheap precursors and tested in the photo-Fenton-like degradation of an endocrine disruptor. Almost total degradation of 17α-ethynylestradiol hormone was achieved after only 2 min of simulated solar irradiation at neutral pH. The outstanding performance of FeOOH@MnO2@MnCO3 microspheres was mainly attributed to a larger generation of hydroxyl radicals, which are the primary mediators of the total oxidation for this hormone. This work contributes to the development of more cost-effective systems for the rapid and efficient removal of persistent organic pollutants present in sewage plant effluents under direct solar light

Metal-oxide-based microjets for the simultaneous removal of organic pollutants and heavy metals

Water contamination from industrial and anthropogenic activities is nowadays a major issue in many countries worldwide. To address this problem, efficient water treatment technologies are required. Recent efforts have focused on the development of self-propelled micromotors that provide enhanced micromixing and mass transfer by the transportation of reactive species, resulting in higher decontamination rates. However, a real application of these micromotors is still limited due to the high cost associated to their fabrication process. Here, we present Fe2O3-decorated SiO2/MnO2 microjets for the simultaneous removal of industrial organic pollutants and heavy metals present in wastewater. These microjets were synthesized by low-cost and scalable methods. They exhibit an average speed of 485 ± 32 μm s–1 (∼28 body length per s) at 7% H2O2, which is the highest reported for MnO2-based tubular micromotors. Furthermore, the photocatalytic and adsorbent properties of the microjets enable the efficient degradation of organic pollutants, such as tetracycline and rhodamine B under visible light irradiation, as well as the removal of heavy metal ions, such as Cd2+ and Pb2+