Tuning metal-organic frameworks for smart sensing of toxins through fluorometry toward device implementation
Creators: name: Olorunyomi, Joseph
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 340301
subject: Inorganic materials (incl. nanomaterials)
classificationCode: 340305
subject: Physical properties of materials
classificationCode: 401605
subject: Functional materials

description: Metal-Organic Frameworks (MOFs) have demonstrated potential for many applications, particularly chemical sensing, due to their large surface areas, regular pore structures and propensity for chemical functionalization that enables them to interact with different absorbates. The combination of responsive MOFs and electronic devices has attracted significant interest in the last decades due to the simplicity of fabricating the MOF films and their easy integration with electronic components. MOF applications have always been hindered by the inherent limited electronic conductivity that MOFs possess, resulting in  poor interfaces with electronic devices and reduced sensor sensitivity. Alternatively, by leveraging the optical properties of fluorescent materials that are incorporated into MOFs, it is possible to achieve desired sensitivity through fluorimetry. To date, most fluorescent sensing studies with MOFs have demonstrated performance using bulk materials instead of films that could be more easily coupled with devices. To compensate for the lack of fully developed optical systems in handheld devices compared to benchtop analytical spectrofluorometers, materials modification is  key to attaining high fluorometric response.
 
In this thesis, three approaches were developed through the incorporation of different light-controlling materials with fluorescent-responsive MOF composite films to form viable solid-state sensing platforms: (i) colloidal crystal ordering of MOF composite films for room temperature organic vapour detection, (ii) plasmonic gold nanoparticles with carbon dot/MOF composites that were deposited as films for aqueous-based detection of metal ions and (iii) deposition of a biomineralized MOF at the surface of direct laser written polymer microprints of microscale dimension having photonic crystal capabilities for fluorescent detection of glucose in aqueous solutions. The following are the motivations for these studies and the general conclusions drawn.

Polymer and silica-based colloidal crystals can enhance fluorescence, but they have limited porosity that results in low chemical sensitivity. The strategy that was employed to address this issue was to control the morphology and particle size of MOFs to obtain monodisperse colloids that could themselves be self-assembled into colloidal crystals. Monodisperse zeolitic imidazolate framework-8 (ZIF-8) particles that have been dyed with fluorescent Nile Red (NR) were self-assembled to form a colloidal crystal (NR~ccZIF-8) film. ZIF-8 is a hydrophobic MOF that has a large surface area and is chemically stable to organic vapours. The NR~ccZIF-8 film showed a 200-fold fluorescent intensity amplification when compared to the disordered film (NR~ZIF-8). The NR~ccZIF-8 film further showed a response to 100 ppm of acetone vapour at room temperature with a detection limit of 60 ppm. Also, more than 40 times higher sensitivity to toluene vapour at 770 ppm was demonstrated by the NR~ccZIF-8 film compared to the polymer-based system that was fabricated through the adsorption of NR on non-porous polystyrene particles that formed a colloidal crystal film.

In the second thesis project, a composite material, HS-C/Au(x)/UiO-66 that was synthesised by incorporating plasmonic gold nanoparticles and thiol-functionalized carbon quantum dots with a zirconium terephthalate MOF, was deposited as films and applied to the aqueous detection of metal ions. The HS-C/Au(x)/UiO-66 films gave fluorescent emission that was tuned by varying the Au nanoparticle wt. % (x). The HS-C/Au(x)/UiO-66 films (x = 0.5 and 1.4) demonstrated a linear Stern-Volmer (SV) fluorescent quenching response to sub-10 parts per billion concentrations of Pb2+ in water. The SV constant, KSV, for the HS-C/Au(1.4)/UiO-66 and HS-C/Au(0.5)/UiO-66 films were 1.5 ×108 M-1 and 6.4 ×107 M-1, respectively; significantly higher than the 5.0 ×106 M-1 obtained for the HS-C/UiO-66 film without Au nanoparticles. Limits of detection of 80 and 100 parts per trillion Pb2+ were attained for the HS-C/Au(1.4)/UiO-66 and HS-C/Au(0.5)/UiO-66 films, respectively. Furthermore, the HS-C/Au(1.4)/UiO-66 film showed selectivity to Pb2+ over Ca2+, Cu2+ and Zn2+, while the non-thiolated C/Au(1.4)/UiO-66 film showed a less selective response. The HS-C/Au(1.4)/UiO-66 film could be reused for detection of Pb2+ as demonstrated through 5 cycles of sensing and regeneration without any performance decline.

In the third and final project, a fluorescent sensing platform with a microscale dimension was developed. The motivation for this project is to reduce the dimension of MOF films so that they could be integrated with handheld devices, without sacrificing the sensing performance. 3D microstructures were fabricated through a direct laser writing approach based on two-photon polymerization. A microfabricated hexagonal pattern with an area of 1.4 mm2 was functionalized with a biocatalytic cascade reactor consisting of glucose oxidase and horseradish peroxidase encapsulated in ZIF-8 particles and subsequently applied to fluorescent detection of glucose. A linear response range of 10-200 μM glucose was obtained from the sensing response, which is consistent with the salivary glucose level in healthy humans. The sensing system explored in this study is promising for the development of non-invasive and painless optical glucose monitoring device.

Overall, by integrating photonic materials with MOF composites for improved fluorescent sensitivity, the research in this thesis offers a novel approach towards developing viable MOF-based optical devices for chemical sensing applications.