(2ge) Investigation of Metal-Organic Frameworks (MOFs) As Thin Films, and Polymer-MOF Gels and Hybrids for Drug-Delivery and Carbon Capture Applications

Research Interests: Thin films of metal-organic frameworks (MOFs), polymer-MOF gels and hybrids for drug delivery and carbon capture

Metal-organic frameworks (MOFs) are porous crystalline materials composed of inorganic metal clusters and organic linkers. The exceptionally high specific surface area and tunable porosity of MOFs make them excellent candidates for applications in a variety of fields, including but not limited to catalysis, separation, sensing, and drug delivery. Since most applications require MOF composites such as MOF thin films and polymer-MOF hybrids, optimizing the synthesis conditions to prepare MOF composites with desired properties is of great importance.

This work presents the synthesis of MOF thin films with strong adhesion and excellent orientation, and in-situ formation of polymer-MOF gels and hybrids with the potential to utilize them in drug delivery and carbon capture applications. We synthesize films of NU-1000 MOF using the self-assembled monolayer method which provides strong adhesion of NU-1000 crystals to the substrate. Strong adhesion is required such that MOF particles don’t come off the surface. Additionally, we present a unique way to alter the density of NU-1000 nuclei to control the orientation of NU-1000 crystals to the substrate.

Despite the usefulness of MOF thin films, the minimal hydrophilicity of MOFs results in the poor solubility and poor solution processibility of MOFs in an aqueous medium thus limiting their biomedical applications. On the other hand, polymers are solution processible and show exceptional compatibility with aqueous media. Therefore, Combining MOFs and polymers to create polymer-MOF gels and hybrids may overcome the limitations of MOFs. To date, limited research has been done on the synthesis and properties of polymer-MOF gels and a generalizable way to prepare polymer-MOF gels is still unknown. We approach this problem by understanding the formation mechanism of polymer-MOF gels. We hypothesize that introduction of chemical interactions between MOF and polymer would result in polymer chains cross-linked by MOF particles thus rendering gel structures. We test this hypothesis using poly(vinyl alcohol) (PVA) which interacts with metal clusters through non-covalent chemical interactions. We prepare a solution of PVA and organic linker followed by the addition of metal cluster solution which results in a self-standing gel at room temperature after 24 h. We confirm the formation of MOF particles in gel using X-ray diffraction and transmission electron microscopy. Additionally, we synthesize polymer-MOF gels using PVA and 4 different MOFs thus demonstrating the generalizability of the synthesis process. To demonstrate the utility of gels for drug delivery in an aqueous medium, we show the release of small drug molecules for up to 2 weeks from polymer-MOF gels.

Another exciting application we are working on is the utilization of polymer-MOF hybrids for carbon capture. The rising amount of CO₂ in the atmosphere has instigated the development of new materials for carbon capture. Commercially, CO₂ is captured using amine-functionalized polymers such as polyethyleneimine (PEI) supported on a porous substrate. As porous support, MOFs provide high specific surface area and tunable functionality. Particularly, UiO-66-NH₂ MOF has shown excellent CO₂ uptake due to the presence of amine groups on MOF. However, the powder nature of UiO-66-NH₂ inhibits its industrial implementation. Here, we combine PEI and UiO-66-NH₂ to create a PEI-UiO-66-NH₂ hybrid at room temperature. The X-ray diffraction pattern confirms the formation of UiO-66-NH₂ and Fourier Transform Infrared Spectroscopy (FTIR) demonstrates the presence of PEI in the hybrid. To utilize the hybrid for CO₂ capture, we put PEI-UiO-66-NH₂ under a pure CO₂ environment for 24 hours followed by dissolution in water. The dissolution causes a sharp increase in the CO₂ concentration as measured by the CO₂ sensor which indicates the CO₂ desorption during the dissolution of PEI-UiO-66-NH₂. Furthermore, the PEI-UiO-66-NH₂ hybrid shows excellent CO₂ adsorption up to 5 cycles.