Ultrathin Silica Nanoporous Membrane With Molecular Selectivity And Its Electroosmotic Flow

Silica nanoporous membrane(SNM)is a kind of artificial nanoporous membrane with highly vertically ordered channels,nano-scaled pore diameter and thickness,and uniform pore size.Typically,due to the vertical channels which are beneficial to the transfer of molecules inside the channels,this kind of nanoporous membranes have been applied in the fields of molecular sieving,drug delivery,catalysis,nanofluidics and so on.Moreover,thanks to the nano-scaled pore diameter,SNM is promising to be an excellent molecular siever.The molecular selectivity of SNM can be further improved by asymmetric interface modification.In addition to diffusion,deverse transport behavior can be realized if an external voltage is applied across the membrane,which is very meaningful to mimic the transport behavior of biological ion channels.This thesis focuses on the mass transfer behavior of SNM,aiming at studying the effects of size,charge and asymmetric modification on the molecular transmembrane behavior and the electroosmotic flow of SNM under an applied electric field.In the first chapter,the basic theories of nanochannels are presented,including electrical double layer,the distribution of potential inside nanochannels,Debye-Hiickel approximation,Nernst-Planck equation,Smoluchoeski equation and the second kind of electroosmotic flow.Then,the molecular selectivity of homogeneous nanoporous membrane and the transmembrane behavior of heterogeneous nanoporous membrane under the concentration gradient are summarized.Finally,an overview of the electroosmotic flow of nanoporous membrane with highly ordered nanochannels is presented,and the applications of electroosmotic flow based nanoporous membrane for separation and detection,micromixer,drug delivery,electroosmotic flow rectification,self-electroosmotic pump and microfluidics are also summarized.In the second chapter,the free-standing SNM was prepared by PMMA-assisted transfer method.The molecular filtration by SNM using a U-shaped cell and spectrophotometric detection was performed,focusing on the quantitative evaluation of permeability and selectivity of SNM.Thanks to the ultrasmall channel size,namely ca.2?3 nm,and the negatively charged channel surface arising from the deprotonation of silanol groups,the SNM displayed excellent size and charge selectivity for molecular filtration.The selectivity coefficient for separation of small methyl viologen from large cytochrome c is as high as 273,resulted from the uniform pore/channel size.The charge-based filtration can be modulated by the electrolyte concentration and solution pH,which control the overlap of radial electrical double layer and surface charge polarity/density,respectively.Owing to the high porosity,namely 16.7%,and the straight and vertical channel orientation,the SNM is highly permeable,displaying a molecular flux much higher than commercially available dialysis membrane and others reported previously.In addition,we demonstrated that,by biasing a small voltage across the SNM,both the flux and separation selectivity could be significantly enhanced.In the third chapter,a novel molecular check valve was fabricated by asymmetric modification of SNM.Asymmetric modification refers to the thermal deposition of hydrophobic polydimethylsiloxane(PDMS)only on one side of the SNM to generate hydrophobic nanoorifices.Such an asymmetric nanostructure,designated as PDMS-SNM,could synergistically exert a hydrophobic force on the molecules by PDMS nanoorifices and an electrostatic force by naked silica nanochannels,leading to unidirectional molecular transport under specific circumstances.Typically,only positively charged molecules were able to transport across the PDMS-SNM from the PDMS nanoorifice side,while backward transport from the other side was prohibited.In the former case,positively charged molecules were subject to electrostatic attraction from naked silica channels,which could exceed the hydrophobic rejection from PDMS nanoorifices to pull the molecule across the PDMS-SNM.However,in the latter case the electrostatic attraction is no longer a driving force to overcome the hydrophobic rej ection from PDMS nanoorifices to promote the molecular transport.On the other hand,the PDMS-SNM based molecular check valve can be shut down to prevent any molecular transport from either side of the PDMS-SNM under certain conditions,such as a high salt concentration or an appropriate pH(e.g.,pH=3).The fourth chapter presents an efficient electroosmotic pump(EOP)based on the ultrathin silica nanoporous membrane(u-SNM),which can drive the motion of fluid under the operating voltage as low as 0.2 V.Thanks to the ultrathin thickness of u-SNM(?75 nm),the effective electric field strength across u-SNM could be as high as 8.27 x 105 V m-1 in 0.4 M KC1 when 1.0 V of voltage was applied.The maximum normalized electroosmotic flow(EOF)rate was as high as172.90 mL min-1 cm-2 V-1,which was larger than most of other nanoporous membrane based EOPs.In addition to the ultrathin thickness,the high porosity of this membrane(with a pore density of 4 × 1012 cm-2,corresponding to a porosity of 16.7%)also contributed to such a high normalized EOF rate.The EOF rate was proportional to both the applied voltage and the concentration of electrolyte due to the small electrokinetic radius of u-SNM(Ka<10).The EOF rate reached the maximum when increasing the electrolyte concentration up to 0.4 M,as a result of the change of zeta potential and the electrokinetic radius.The last chapter summarizes the work presented in this thesis and ends with an outlook and perspective on the possible applications of SNM.