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Drug delivery systems and other medical usages

Typical treatment modalities for cancer (malignant tumors) in contemporary society include chemotherapy (using antitumor agents), radiotherapy, and surgical intervention; these have been combined more recently by immunotherapy and gene therapy. However, they are imperfect: taking chemotherapy for example, many antitumor agents are associated with severe side effects, and there are reasons to doubt that the drugs actually efficiently act on tumor cells, their primary target. These concerns have accelerated R&D into a new therapeutic modality in recent years: the drug delivery system (DDS). Systems being investigated as potential carriers of antitumor drugs include inorganic porous silica, quantum dots, and metal nanoparticles as well as organic polymer micelles, liposomes, and dendrimers.

Yet many likewise suffer from drawbacks: only small quantities of drug can be embedded in micelles, liposomes, and dendrimers, for example, while many inorganic porous silicas are toxic or prone to accumulate in the body. To circumvent these issues, scientists are exploring approaches to apply MOFs as carriers of anticancer drugs. The use of MOFs—many of which are biodegradable—could allow greater dosages of drugs to be released into the bloodstream, with greater control over release profiles. Thanks to their diversity in shape, size, and chemical composition compared with other carriers, MOFs have greater potential for precision engineering of multifunctional and stimulus-responsive DDSs. MOF-DDSs could be triggered to release their load based on changes in any number of physical properties such as pH, magnetism, temperature, light, and pressure, or combinations thereof.

R&D is progressing on how to embed cisplatin, caffeine, and other drugs in MOF-DDSs, in both in vitro and in vivo settings. Photo-responsive DDSs offer yet another delivery method, through the use of quantum dots (QD) that react to light in the infrared spectrum. If exposed to infrared light shone from outside the body, QDs localized in malignant tissue react by emitting intense heat, attacking adjacent tumor cells. We are exploring new methods based on infrared-responsive QDs, a product already fabricated at Green Science Alliance.

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Separation, adsorption, and storage of a wide variety of gases and gaseous molecules

MOFs can be engineered to capture specific gas molecules due to their ultra-porous nature, which also makes them ideal materials for separating and purifying various gaseous mixtures. The surface area available for gas adsorption in one gram of MOF is incredible—comparable to a regulation soccer field—indicating that huge quantities of gas can be captured. Scientists are exploring how to exploit this property for the recovery and storage of carbon dioxide and methane, major causes of global warming. Other applications include hydrogen storage in fuel cells and fuel-cell electric vehicles, and the separation and storage of ethylene gas in food packaging. New usages continue to be proposed for MOCs in the adsorption, separation, and storage of a variety of gases, ranging from moisture to hazardous sulfur- and nitrogen-based gases.

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Water extraction from deserts and other dry environments

The adsorptive properties of MOFs can be exploited to collect atmospheric moisture in deserts and other arid environments, where the air contains very little of it. R&D is urgently needed on the matter of which MOFs can most efficiently adsorb moisture from air. Green Science Alliance will work on moisture collection devices includes searching for candidate MOFs and hybrid MOF composites.

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Recovery of hazardous metals and metal ions from wastewater

The heavy metals present in water and wastewater are not biodegradable and thus destroy the environment and seriously harm organisms as they accumulate in soil and biological tissue. This makes R&D into technology capable of removing heavy metals from contaminated water a matter of critical importance. It is well known that MOFs can adsorb not only gases, but also various metals and metal ions owing to their morphology and porosity. Scientists are exploring ways to exploit this high adsorptivity to remove toxic metal ions such as lead, gadolinium, and mercury from water and solutions.

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Solid catalysts

R&D is being conducted on solid catalysts (also known as “heterocatalysts”), a class of cutting-edge materials with high potential for reusability in industrial chemical processes. The use of liquid catalysts such as sulfuric and hydrochloric acid is associated with heavy burdens, both on fabrication equipment and the environment. Solid catalysts, in comparison, can be treated by filtration or other means and removed after a reaction cycle is complete for repeated use. Scientists have high hopes for MOF-based solid-catalyst systems, given its expected effectiveness in this domain. Research is being done into their catalytic activity in a variety of reactions, including replacement (e.g., carbon–carbon to oxygen– or nitrogen–carbon bonds), addition, hydrogenation, and esterification, as well as how to minimize performance loss over repeated uses. The properties of these systems are being researched for reactions not only in aqueous solutions but also in gases and solid phases and at boundaries between phases. Their catalytic activity is dependent on a number of factors, including topology, surface functional group(s), and pore size; stability under expected pH and temperature conditions is another vital consideration.

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Electrodes and electrolytes in batteries and capacitors

Scientists are also exploring potential applications for MOFs as electrodes in electrochemical fuel cells and metal–air batteries. The reduction of oxygen at the cathode in such cells, usually accomplished by platinum-on-carbon (Pt/C), is critical to their performance. However, reliance on platinum and other precious metals is associated with numerous drawbacks such as resource depletion and high cost. One solution currently under active R&D is carbonized MOFs, prepared by pyrolysis, which have been reported to achieve catalytic activity similar to that of Pt/C cathodes. Scientists are also exploring potential applications for MOF systems, including carbonized MOFs, MOF-derived oxides, and unaltered MOFs, as electrode materials for lithium-ion batteries and as solid-state electrolytes. They are also being investigated as electrode material for use in electrochemical capacitors. Scientists have reported excellent performance for capacitors that utilize MOFs as electrode material, also called “ultra-capacitors”, thanks to their enormous surface area.

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In solid-state sensors, MOF technology could also be applied to detect various toxic gases, including sarin and other neurotoxins, as well as hazardous compounds in gaseous mixtures such as aldehydes, alkanes, trimethylamine, ammonia, hydrogen sulfide, and nitric oxide. MOFs’ potential is not limited to gas detection; systems to detect compounds in water should also be feasible. Such sensors could function through the detection of changes in luminescence, electrochemistry, and color.

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Artificial photosynthesis and photocatalysts

Most MOFs are insulators, implying that they can neither conduct electricity nor absorb visible light. Today, chip manufacturers are pursuing the development of MOFs having the properties typically desired of semiconductor materials: namely, high conductivity and (light) absorbance. Novel MOFs exhibiting these properties would permit new applications as catalysts, exploiting their large relative surface area and as photovoltaics in solar cells and other devices. A given MOF’s ability to absorb light is primarily dependent on the organic-ligand component(s), as opposed to the metal ion(s). These linkers can be fully customized, engineering and synthesizing any number of organic groups with finely tuned electronic states — to ultimately fabricate an MOF with the desired properties, such as absorbance and redox potential (changes) in response to photonic stimulation. In addition, it should be feasible to engineer systems with unique optical functionality, arising from electron transfer between the different types of organic linkers and metal-oxide clusters bound to one another at the molecular level. Research is progressing into ways to apply such properties to develop systems capable of artificial photosynthesis, using MOFs as photocatalytic material in combination with sacrificial electron donors such as triethanolamine.

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Adsorption and separation of dyes and pigments from solutions

Studies have shown MOF technology’s utility in separating and removing specific gas molecules from air, as well as metal ions in water, as mentioned above. Furthermore, early reports indicate MOFs as promising candidate materials for removing organic pigments and dyes from aqueous solutions. Both the micro- and meso-/macrostructures of MOF-based systems—the picometer-scale porosity characteristic of such frameworks and the 3D matrix, respectively—contribute to such systems’ excellent adsorption of and selectivity against a variety of organic pigments and mixtures. Success has already been reported for methylene blue.

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