Large-area Continuous Silica Isoporous Membranes For Molecular Sieving And Microfluidic Sample Purification

Increasing demands for energy-efficient separation technologies in industrial applications have stimulated a vigorous research for high-performance separation membranes.Nanoporous molecular sieving membranes are widely used or potential in water purificatio,drug delivery,micro/nanofluidics and biosensing,because of their advantages in simple operation and low energy requirements.However,conventional artificial membranes such as polymer membranes suffer a ubiquitious trade-off:highly permeable membranes lack selectivity and vice versa.Learning from nature biological membranes that own both high permeability and high selectivity have stimulated great efforts aimed at synthesis of solid-state nanoporous membranes.During this thesis work,we used the Stober solution growth approach to prepare silica isoporous membranes(SIM),and combined with poly(methyl methacrylate)(PMMA)assisted method transferred to flexible track-etched polyethylene terephthalate(PET)support membrane to fabricate large area SIM/PET hybrid membranes.These membranes can be used for high-throughput molecular sieving based on size and charge.In addition,due to the flexibility and easy-handling of the hybrid membranes,it can be integrated into the microfluidic chip for the separation and purification of complex samples.This thesis includes five chapters:In the first chapter,the mechanism of mass transfer through nanochannels or nanopores was firstly introduced,including the solvent and solute transport under different driving forces and the affecting factors,such as the steric hindrance,electrostatic interaction,hydrophobic effects and so on.Then,different kinds of nanoporous membrane such as metal oxide membrane,carbon-based membrane,polymer membrane and silicon-based membrane were summarized.Different fabrication methods,separation applications and advantages/disadvantages were discussed in details.Finally,the applications of the membrane in microfluidic chip such as chemical reagent detection,cell reaserch,electrokinetic fluid research and sample desalination and purification were briefly reviewed.In the second chapter,we consider the molecular transport across a nanoporous sieving membrane situated on a porous support using a quasi-steady state model to explore the influence of the pore size,thickness and porosity of sieving membrane.The effects of effective area,thickness of support layer and sample volume on the molecular flux and concentration equilibrium are also studied.The key factors of achieving the high permeability-selectivity combination were discussed.In th third chapter,we prepare a mesostructured silica isoporous membrane(SIM)on indium tin oxide(ITO)glass by the Stober solution growth approach.The morphology,pore size and thickness of SIM were characterized by electron microscopy.A modified PMMA-assited approach was used to transfer the SIM which detached from the ITO substrate to the PET support layer to fabricate flexible large area SIM/PET hybrid membrane(2.5 cm × 2.5 cm).Due to the uniform pore size(2.3 nm),ultrasmall thickness(90 nm)and high porosity(4.0 × 1012 pores cm-2),the hybrid membrane exihibited high-throughput molecular sieving ability based on size and charge and achieved the combination of high permeability and high selectivity.By fitting the experimental data with the theoretical formula,the steric hindrance of the sieving membrane on the molecular transport was confirmed.In addition,the molecular flux of SIM/PET is much better than that of commercial membranes.In the fourth chapter,SIM/PET was integrated into a three-layer microfluidic chip,and the separation and purification of sample was achieved based on dialysis principle.The device is simple to assemble,easy to operate and presents an excellent reproducibility.We investigated the variation of separation efficiency with the flow rate of donor and acceptor phase.Based on the size exclusion of the ultrasmall pore size of SIM,this device could fully retain the protein samples with the purification rate up to 95%.Finally,we verified that the charge effect is still significant in the microfluidic separation system.In the last chapter,the work presented in this thesis was summarized.An attempt was also made to propose the potential of SIM/PET in microfluidic and pressure-driven separation.