Biomolecules in metal-organic frameworks: From optical studies and devices of metal-loaded adeninate MOFs to adsorptive separation of amino acids.

Metal-organic frameworks (MOFs) are a class of hybrid solid materials, built up by coordinative bonds between a wide variety of inorganic metal ion or cluster nodes and organic linking molecules. Their highly ordered three-dimensional architectures are often crystalline and porous, with pore sizes ranging from a few Ångström to ten nanometers. Owing to their large chemical versatility and highly porous architectures, MOFs have been thoroughly investigated over the last two decades as highly tailorable materials with unique structure-property relations for a wide scope of applications. This partly organic nature of MOFs involves the possibility of photoluminescence, which can be applied in lighting applications or can be an easy read-out mechanism for advanced sensing devices. Even more, biomolecules that play key roles in nature can be applied as building blocks for MOFs: for instance amino acids and nucleobases. This dissertation mainly resolves around the changing optical properties of adenine-containing MOFs, so-called bio-MOFs, by introduced silver and copper species and their applications in LED devices. Additionally, the interactions of amino acids in the pores of water-stable MOFs were studied for adsorption and recovery purposes from protein-rich waste streams.

In a first part, the mixed-linker carboxylate- and adenine-containing bio-MOF-1 framework is introduced. It was proven that the presence of silver ions greatly influences the crystalline structure of this material, which is transformed to a purely carboxylate framework MOF-69A. With this MOF-to-MOF structural transformation, the photoluminescent properties also change remarkably, as luminescent silver-adeninate species are formed under the correct reducing environment provided by a ratio of ethanol and water in the silver nitrate solution. A wide range of spectroscopic and microscopy techniques were applied to visualize this process for bio-MOF-1. In the second part, the structure of a new mixed-linker adeninate MOF, BDC bioMOF, was elucidated based on diffraction-based techniques and solid-state nuclear magnetic resonance spectroscopy. The cation-loading of BDC bioMOF is further discussed, along with its optical properties. In contrast to bio-MOF-1, this BDC bioMOF remains stable in the presence of silver ions, but the optical properties again change drastically when silver is loaded on this material from a reducing environment in the form of alcohol solutions.

In the second and third part, the latter two photoluminescent MOF materials were investigated for their electroluminescent properties as primary light source in lighting devices (LEDs). The investigation of MOF electroluminescence is a very recent research topic, but possibly of large interest for applications in LED lighting. Various silver- and lanthanide-loaded versions of bio-MOF-1 and BDC bioMOF were tested. It was observed that the introduction of silver ions and the presence of defects on the inorganic node of the MOF greatly improved the electroluminescent behavior of these materials compared to the as synthesized versions. The role of (defects at) the inorganic node is also crucial, as they localize the electron-hole recombination and electroluminescent emission, while the presence of silver enhances the materials’ electrical conductivity.

In the next part, cuprous (I) iodide clusters were successfully introduced by sublimation at 250 °C into various microporous zeolites and MOFs, including adenine-containing frameworks. The strong and distinct red luminescence from these CuII clusters is very peculiar, as it could possibly replace rare earths as efficient red phosphors in lighting applications. Similar to the case of silver-loading from solution, cupper (I) introduction through sublimation strongly influences the structure of bio-MOF-1, while BDC bioMOF remains stable. Thermally stable Al3+- and Zr4+-MOFs performed even better than the adeninate MOFs, displaying the strong red CuII cluster emission. Zeolites with pores consisting of ring structures larger than 10- and 12-membered T-atom rings also showed strong CuII cluster emission, while the zeolites with small 8-membered ring pores showed mass transfer limitations and no CuII emission.

In the final part, water-stable MOFs were probed for the adsorption and separation of another type of biomolecules from aqueous solutions, namely amino acids. In particular aromatic amino acids (l-tryptophan, l-phenylalanine and l-tyrosine) were investigated, as they are expected to have strong dispersive interactions through their aromatic ring structures with the aromatic linkers present in MOFs. The Zr-MOF MIL-140C was found to display the largest affinity for l-tryptophan and l-tyrosine over l-phenylalanine by additional hydrogen bonding of the functional groups in the former two aromatic amino acids with the inorganic unit of MIL-140C. This was visualized with FT-IR and vibrational analysis. Finally, these aromatic amino acids, along with l-glutamic acid and l-aspartic acid, were successfully separated from a complex mixture of all 20 amino acids present in protein. With this result it was proven that MOFs could potentially be used as selective adsorbents in the recovery of high-value and important essential amino acids from protein waste streams.