Metal-Organic Frameworks Interfaced with Biological Systems
- Ji, Zhe
- Advisor(s): Yaghi, Omar M.
Chapter 1: Nature is sophisticated in organizing constituents of life across all size regimes, from molecules to cells and tissues, forming a hierarchy of well-defined structures where at each level chemical entities in specific sequences cooperate under the guidance of their architectural backbone. The challenge in approaching this level of complexity and performance with synthetic materials is being actively addressed by the development of reticular chemistry, particularly, metal-organic frameworks (MOFs). This new class of porous highly crystalline solids are made by linking molecular building units with strong bonds. The knowledge gained from the construction of extended backbones and the installation of functionalities promises the chance to take the design principle from nature and build framework materials with rich chemical sequences and functional biological interfaces.
Chapter 2: Metal-sulfur clusters are employed by many proteins to facilitate their charge transport pathways and active sites. The connection of these highly functional clusters into extended structures remains largely unexplored, in contrast to the use of metal-oxide clusters prevalent in MOF chemistry. Mo-S clusters, which have shown great promise in catalyzing hydrogen evolution, were connected by organic thiolate linkers into ordered dimers, cages, and chains. The resulting structures allow for controlled spatial arrangement of these catalytic units on electrode for achieving high turnover frequency and high current density, outperforming other Mo-S catalysts (both molecular and solids-state). To a greater senses, this report extends the scope of MOFs beyond the metal-carboxylate coordination chemistry and effectively introduces more polarizable components into framework structures.
Chapter 3: One fascinating element of life is the programmable sequences encoded in protein and DNA. The incorporation of multiple types of species into one structure can be realized in MOFs, where the variation of building units does not compromise the overall ordered architecture. The heterogeneous spatial arrangements of these units thereby form rich chemical sequences, each creating a unique chemical environment of the pore. However, deciphering such chemical sequence encoded in framework structures requires atomic resolution and remains aaracterization challenge. Here, atom probe tomography was employed to map the position of metal ions in the single crystals of mixed-metal MOF-74. The obtained real space information and the customized data analysis revealed that, depending on the specific metal combination and the reaction temperature, four types of metal sequences are observed: random, inserted, short-duplicate, and long-duplicate. The success of this work paves the way to reading and writing sequences from and into synthetic structures.
Chapter 4: Linking molecular building units into extended frameworkallows to perform exquisite chemical control in a large space. This character makes M20promising candidates to be directly interfaced with biological objects, such as living cells. The molecular definitiveness of both MOFs and cellular surfaces enables precisely designed interface and a productive interplay between framework chemistry and cell metabolism. In light of this, a MOF monolayer was used to uniformly wrap Morella thermoacetica bacteria through the direct coordination bonding between the phosphate units dangling from the cell surface and the zirconium clusters on MOF monolayer. The catalytic activity of the MOF enclosure towards decomposition of reactive oxygen species protects the wrapped strictly anaerobic bacteria against oxidative stress, thus promoting their prolonged photosynthesis. Additionally, the ultrathin MOF wrapping and the dynamic coordination bonding allow for cell elongation and separation without inhibiting the expansion of cell volume.
Chapter 5: There is plenty of room to further increase the structural complexity and the level of function of framework materials. One target is to build hierarchy by using frameworks to make superframeworks; thus a crystal of the framework at one level is employed as the building block to make that at a higher level. This strategy, termed augmented reticular chemistry, can potentially provide access to well-defined structures in all size regimes, superordinate pores in size of hundreds of nanometers, magnified dynamics at the macroscopic level, and reaction networks integrated in one crystal. Another consideration is to examine how the relative position of functionalities determines the way they operate. Here I summarize the functionalization of MOFs into three categories: site isolation, site coupling, and site cooperation. The particular emphasis on the synergy between functionalities, as illustrated by the application in guest binding and catalysis, encourages the crafting of MOF pores in a similar way that protein pockets are decorated with rich sequences of amino acid residues.