Studies Of Ion Transport And Its Electroanalytical Chemistry Based On Confined Channels Of MOFs

Biological ion channels selectively regulate the transmembrane transport of substances,playing important roles in cellular processes such as maintaining osmotic pressure,signal transduction,and transmission of nerve impulses.Inspired by the structure and function of biological ion channels,researchers have utilized advanced nanofabrication techniques to prepare a variety of biomimetic nanochannels for studying the mechanisms of ion transport in confined nanoscale spaces.Based on these mechanisms,researchers have achieved precise regulated ion transport in nanochannels,and applied them in energy conversion,substance separation,analytical detection,and other fields.Analytical detection applications based on confined ion transport involve studying mass transfer processes in solution,where has the resistance of the nanochannel is the largest in the circuit.Therefore,when the analyte interacts with the nanochannel,causing changes in its effective pore size,surface charge,wettability,and other characteristics,the ion current will respond sensitively,enabling the analysis and detection of the target analyte.Metal-organic framework(MOFs)are long-range ordered porous crystalline materials composed of metal nodes and organic ligands connected by coordination bonds.MOFs possess intrinsic nanoscale or even sub-nanoscale-sized pores,and chemical properties are highly designable.In recent years,nanochannels constructed from MOFs have attracted widespread attention.Researchers have utilized the molecular-sized pores of MOFs to induce ion dehydration and rehydration processes,and the unique interactions between functional groups in MOF channels and ions,to achieve ion sensing and sieving by different energy barriers.Moreover,due to the inherent physical and chemical properties of MOFs and designable property,the ion transport behavior in MOF-based nanochannels can be regulated by various external stimuli.Therefore,ion current readout can be used for analytical of sensing external stimuli.However,there are still relatively few methods for constructing MOF-based nanochannels,and there is still much room for exploration in the ion transport mechanisms within confined MOF channels,and the analytical methods based on MOF channels are not yet fully developed.In this paper,we addressed the existing limitations and challenges by designing three types of MOF-based nanochannels.Firstly,we developed a novel strategy for the synthesis of asymmetrically grown MOF nanochannels;Then we explored new mechanisms for active ion transport in 2D MOF confined channels;Finally,we proposed a new analytical method for nanochannels with reversed ion rectification.The paper is divided into the following five chapters:Chapter 1 is the introduction section,summarizing the ion transport basic principles,regulation methods,analytical detection applications,and research on ion transport based on MOF.Chapter 2 proposed a novel strategy for the in-situ asymmetric growth of Al-TCPP MOF nanofluidic diodes.We used Al3+ ions derived from the dissociation of anodic aluminum oxide(AAO)as the metal source.Using the concentration difference of the organic ligand within the one way through AAO channels,we achieved the asymmetric growth of negatively charged AlTCPP MOF nanochannels through microwave assisted hydrothermal method.Subsequently,we etched the barrier and modified it with positively charged molecules.Thus,the nanofluidic device had asymmetric geometry and surface charge,exhibiting nanofluidic diodes with ion rectification.Additionally,since light could enhance the negative charge of Al-TCPP MOF,the rectification ratio of this nanofluidic diode could be regulated by light.Finite element simulations further confirmed the light-controlled ion rectification process.These results demonstrated that this strategy provided a universal approach for the fabrication of MOF-based nanofluidic diodes.Chapter 3 explored a new mechanism active ion transport within MOF channels.We studied the light-driven active ion transport phenomenon in a Cu-TCPP MOF membrane fabricated by filtration.We observed that the direction of active ion transport is opposite in different electrolyte concentrations,which could not be explained by the traditional ion pump mechanism.We proposed that the change in capacitance of the MOF channels after light illumination might be another driving force for active ion transport.At low electrolyte concentrations,the transmembrane ion transport resistance in the MOF confined channels was high,making it difficult to achieve ion pumping.However,at the interface between the MOF membrane and the solution,light illumination could induce opposite charges on both sides of the MOF membrane,causing ions to migrate due to electrostatic forces.Therefore,the capacitance change after light illumination could drive active ion transport.Subsequently,we used finite element simulations to simulate the phenomenon of reversed ion transport direction in low and high concentration solutions.Finally,we found that capacitance-driven active ion transport could generate a current response against concentration gradient similar to that of ion pump.This research analyzed the mechanism of the active ions transport in 2D MOF films in different electrolyte solutions.We proposed capacitance change as a new driving force for active ion transport,which is helpful to further understand the anomalous phenomena of ion transport in the confined space.In chapter 4,taking three UiO-66 type MOFs modified glass micropipettes as examples,we developed an ion rectification summation method to improve the detection sensitivity and enlarge concentration range in the nanochannels with reversed ion rectification.We synthesized UiO-66,UiO-66-NH2,and UiO-66-COOH MOFs at the tips of glass micropipettes using the counter-diffusion growth method.These three MOFs possessed functional groups responded to pH,so the ion current was related to pH.UiO-66 and UiO-66-NH2 modified glass micropipettes had reversed rectification at different pH.We proposed an ion rectification summation method,where the currents under positive and negative voltages are added together to indicate pH.This method fully utilized the pH-dependent current changes under positive and negative voltages,doubling the sensitivity of pH detection compared to UiO-66-COOH modified glass micropipettes without reversed rectification.Furthermore,the ion rectification summation method compensated for the sensitivity differences in current changes under positive and negative voltages.Thus,it significantly improved the linearity between current and pH,enabling analysis and sensing over a wider pH range.The ion rectification summation method is a universal analysis method that can be applied to other nanochannels with reversed rectification.It could significantly improve the sensitivity and expand the detection range by simple post-processing the data,without the need for additional experiments.Chapter 6 is the conclusion section,we summarize the innovative points of this thesis and provide future perspectives.