title: Synthesis and functionalisation of zirconium Metal-Organic Frameworks for biomedical applications
Creators: name: Boyadjieva, Sophia
Publisher: name: University of Glasgow
Year: 2021
Subjects: subject: Metal-Organic Framework, MOF, zirconium, drug delivery systems, biomedicine, inorganic chemistry.

description: In recent years, the interest in Metal-Organic Frameworks (MOFs) has grown exponentially due to their properties, such as porosity, robustness, and ease of functionalisation. MOFs are hybrid structures, highly porous and crystalline, consisting of metal clusters, joined together by multidentate organic ligands via coordination bonding. The repeated nature of the bonding between the inorganic ligands leads to the formation of permanently porous structures. The possibility to tailor the framework to have specific and desired structures and properties gives the opportunity to control the characteristics and behaviour of the MOF for a variety of applications such as gas capture and separation, thin film devices, catalysis, and biomedical uses, such as imaging and drug delivery. This thesis focuses on the post-synthetic modification of UiO-based frameworks for biomedical applications. This includes attaching naphthalimides as surface reagents for their fluorescent and biocompatible character, to the Zr-based UiO-66, exploring the host-guest relationship between a series of isoreticular Zr-based UiO series and different sized anti-cancer drugs, and the study of Zr- and Hf-based hexagonal-closed pack (hcp) phase of UiO MOFs with two different linkers. In Chapter II, the focus is on the surface-functionalisation with naphthalimides of azide-modulated Zr-based UiO-66-L1, modified with L1 (p-azidomethyl benzoic acid). The modification was performed as to achieve nano-sized particles (approx. 150 nm) with higher surface stability, better dispersion, more surface control, to be able to examine potential enhancement of colloidal stability for anti-cancer drug delivery. Five carboxylic acid with varied chain lengths (M1-5), a short-chained phosphonic acid (M6), and short-chained alkyne (M7) 1,8-naphthalimides are synthesised as a library of surface reagents. The variety of molecules makes it possible to explore the effects of chain lengths, sterics, and attachment modes. The latter consists of three different attachment methods used for the different functionalities of the naphthalimides, surface ligand exchange (M1-5), soaking the MOF in an aqueous solution of the surface reagent (M6), and copper-aided click reaction (M7). Incorporation of the naphthalimides (M1-6) is at approx. 20% (w/w), however the alkyne derivative (M7) stands out at 2% (w/w), suggesting the importance of the stereochemistry effect on the loading values. Degradation profiles for both linker (BDC) and surface reagents (SRs) are carried out in phosphate-buffered saline at neutral (pH 7.4) and acidic (pH 5.5) as to mimic biological conditions of healthy and cancer cells respectively. The results show unexpected behaviour of the surface-functionalised MOFs, the organic molecules took longer (approx. 14 days) to degrade in the PBS pH 5.5, compared to the PBS pH 7.4 (approx. 14 hours), as it is seen in previous work in the group that after surface-functionalisation, UiO-66 breaks down in acidic conditions faster than in a neutral environment. These degradations highlight the importance of the stereochemistry and solubility effects on the behaviour of surface-functionalised MOFs in suspension. The biocompatibility of the new surface-functionalised MOFs are tested, showing, both MOFs and surface reagents, to be well tolerated by two healthy, human dermal fibroblasts (HDF) and human embryonic kidney 293 (HEK-293) cells, and one cancer cell lines, human breast cancer (MCF-7) cells, as determined by Alamar blue cytotoxicity assay. Confocal microscopy images show the bright fluorescence of the naphthalimides, close to the HDF cells, which suggests the potential application of the naphthalimide-functionalised MOFs in biomedical imaging. The project, outlined in Chapter III aims to apply the UiO isoreticular series of MOFs as drug delivery systems (DDS) for anti-cancer drugs, focusing on the development of reproducible protocols for these frameworks with particle size of approx. 150 nm. The Zr-based UiO MOFs, chosen for this work include frameworks with six different linkers: 1,4-biphenyl dicarboxylic acid (BDC), 2,6-naphthalene dicarboxylic acid (NDC), 4,4’-biphenyl dicarboxylic acid (BPDC), 4,4′-ethynylenedibenzoic acid (EDB), di-methyl-p-terphenyl-4,4′′-dicarboxylic acid (Me2-TPDC), and 4,4′‐[1,4‐phenylenebis(ethyne‐2,1‐diyl)]‐dibenzoic acid (BDB). Following thorough investigations of different synthesis conditions, reproducible protocols are outlined for UiO-66 with BDC as linker, Zr-NDC with NDC as linker, and UiO-67 with BPDC as linker. The host-guest relationship between the three frameworks is investigated with three different sized anti-cancer drugs, cisplatin (small), gemcitabine (medium), and doxorubicin (big) by loading each MOF separately with one of the drugs and in each case, successfully incorporating it within its structure. Pore sizes do not appear to influence the loading capacity of the frameworks, suggesting for UiO-66, Zr-NDC, and UiO-67 and all three drugs, loading occurs on the surface, which is confirmed by further analysis of the materials. The cytotoxicity of the UiO extended family linkers is tested against the healthy kidney HEK-293 cell line with Alamar blue assay, showing good tolerance of the organic molecules by the cells. UiO-66, Zr-NDC, and UiO-67 are tested against HEK-293 and breast cancer MCF-7 cells, exhibiting good biocompatibility with both cell lines. Three cancer cell lines are used to test on the naked, drug-loaded MOFs, and free drug, which are breast (MCF-7), pancreatic (PANC-1), and liver (HepG2) cancer cells. No significant difference is seen between the naked or drug-loaded UiO-66, Zr-NDC, or UiO-67. The breast cancer (MCF-7) cells do not appear to be affected much by either the naked MOFs, the cisplatin-loaded MOFs, the gemcitabine-loaded MOFs or the respective free drugs, however, show acute sensitivity towards the doxorubicin-loaded MOFs and free doxorubicin, not showing dose-dependency for the latter. The pancreatic (PANC-1) cells do not show significant sensitivity towards the naked MOFs, the gemcitabine-loaded MOFs or free gemcitabine, however, are slightly affected by the doxorubicin-loaded MOFs and free doxorubicin. Interestingly, the gemcitabine-loaded MOFs show better anti-cancer effect compared to the free gemcitabine. The liver cancer (HepG2) cells do not show a significant sensitivity toward the naked MOFs, the cisplatin-loaded MOFs, gemcitabine-loaded MOFs, their respective free drugs or doxorubicin-loaded MOFs, however at the higher concentrations, free doxorubicin exhibits a good anti-cancer effect on the cells. Chapter IV is based on the unexpectedly formed hexagonal-closed packed (hcp) phase of Zr-di-alkyne, where 4,4′‐[1,4‐phenylenebis(ethyne‐2,1‐diyl)]‐dibenzoic acid (BDB) is the ligand, from a solvothermal (120 °C) reaction, with acetic acid and water as modulators, originally intended for the purpose of the previous chapter’s work. The protocol is not reproducible and solvothermal synthesis is explored with two metal salts, zirconium and hafnium chloride, successfully yielding Hf-di-alkyne (hcp). Due to unavailability of the ligand, further investigation and characterisation is not carried out. Research is directed towards the shorter ligand, 4,4′-ethynylenedibenzoic acid (EDB), attempting to synthesise both Zr- and Hf-alkyne hcp phase with solvothermal conditions (120 °C) and acetic acid and water as modulators. The reactions are successful in yielding Hf-alkyne (hcp) by varying the concentrations of acetic acid, however even variation of water for the Zr-alkyne syntheses only forms the face-centred cubic (fcu) phase. Higher temperature (150 °C) is applied, using either acetic acid and water or formic acid and water, to yield Zr- and Hf-alkyne (hcp) reproducible synthesis protocols with the former. The formic acid modulation leads to the formation of a strange Zr-alkyne (odd) that does not appear to be either fcu or hcp phase and is also investigated further, alongside the hcp phase materials. Delamination of the three-dimensional (3D) hcp phase into the two-dimensional (2D) hexagonal nanosheets (hns) is attempted for the further understanding of the hexagonal plates and their potential applications. Sonication of the hcp phase materials does not result in the desired separation of the hexagonal plates. A different method is employed; bromination of both Zr- and Hf-alkyne (hcp) as halogenations has been successful on similar structures previously in the group. Delamination does not occur; however, bromination is successful on both frameworks, resulting in the trans isomer for Zr-alkyne (hcp) and cis and trans isomers for Hf-alkyne (hcp) MOFs.