Siliica Isoporous Membrane With Molecular Selectivity For Electrochemical Analysis And Energy Conversion

Nanoscopic silica isoporous membranes(abbreviated as SIM)are a class of porous silica materials with vertically ordered channel and uniform channel size on the nano-scale.SIM possesses perpendicular channels,favoring the transfer of molecules or ions inside channels.Moreover,due to the small channel size,the molecular transport in SIM maybe remarkably different from that in large micrometer-and millimeter-scale structure.The mass transport will become more and more controlled by molecular properties,e.g.molecular size,charge,lipophilicity and chirality.In addition,modification of the SIM with suitable functional groups can further modulate and/or improve these behaviors.All these characteristics are of importance and usefulness,particularly in designing new methodologies for electrochemical sensing,molecular separation,nanofluidics and drug delivery.In this thesis,SIM with lipophilicity-selectivity,size-selectivity and ion-selectivity was employed for the applications of direct electrochemical analysis of redox-active small analytes in complex samples,construction of ionic rectification,and salinity-gradient-driven energy conversion.In the first chapter,the basic theories of nanochannels were presented,including preconcentration effect in ultrasmall volume,electrical double layer effect(charge selective effect),nanochannel conductance and ionic transport properties(eg.ionic selectivity,ionic gate and ionic rectification)inside nanochannels.Then,recent progress on the applications of nanochannels(mainly the SIM materials)in electrochemical sensing and molecular separation was summarized,in terms of synthesis of nanomaterials and permselective effects(size,charge and lipophilicity selectivity of the SIM).Finally,the application of nanochannels for salinity-gradient-driven energy conversion was reviewed,in terms of homogeneous membrane and heterogeneous membrane.In the second chapter,the indium tin oxide(ITO)electrodes modified with lipophilicity-selective and size-selective membranes consisting of vertically ordered micelles and silica nanochannels(micelles@SIM/ITO)were prepared by using the Stober-solution growth approach.Each micelle confined in SIM formed a hydrophobic core,which acted as an excellent adsorbent for rapid extraction and preconcentration of trace small,lipophilic redox-active molecules from aqueous solutions via the hydrophobic effect.Furthermore,the extracted molecules can difuse freely to the underlying electrode surface and be determined therein by valtammetry with a high sensitivity.Firstly,nitroaromatic organophosphate pesticides(including paraoxon,methyl parathion and fenitrothion)were chosen as a proof-of-concept.Using the micelles@SIM/ITO sensor,electrochemical analysis of paraoxon was obtained with a high sensitivity and low detection limit of 3.53 ppb.Moreover,SIM with a channel diameter of 2-3 nm prevents the underlying electrode surface from fouling and contamination by large compounds/particles(>3 nm),displaying excellent anti-interference and anti-fouling ability in the complex samples.Based on the hydrophobic extraction ability of micelles and simultaneously the size-exclusion effect of SIM,the micelles@SIM/ITO sensor had been successfully applied to detect small,lipophilic redox-active molecules in complex samples(e.g.soil dispersion,human serum and milk)with a high sensitivity,a wide detection range and a low limit of detection,without sample pre-treatment.Finally,lipophilic and hydrophilic antioxidants were selectively detected in commercial fruit juices by modulating the permeability of the SIM layer modified on the electrode surface.SIM filled with micelles are only permeable to lipophilic antioxidants,while bare ones(SIM/ITO)are only permeable to hydrophilic antioxidants.Compared with the Chinese national standard method,the ascorbic acid concentration in orange juice can be accurately and reliably determined by the SIM/ITO electrode without sample pretreatments.And the retinol response detected by the micelles@SIM/ITO electrode might be useful in the evaluation of total activity of hydrophobic antioxidants,although the retinol quantity in carrot juice was not precisely determinedIn the third chapter,perforative SIM with asymmetric surface charge(bipolar SIM)was achieved by PMMA-assited transfer method combined with silylation reaction.Bipolar SIM consists of two kinds of SIM,namely positively charged SIM(p-SIM)and negatively charged SIM(n-SIM).p-SIM(pore size 4.5-5.5 nm,thickness?59 nm)was prepared by using the biphase stratification growth method and silylation reaction.And SIM(pore size?2-3 nm,thickness?90 nm)was obtained by using the Stober-solution growth approach.Based on the ionic selectivity of bipolar SIM,the proposed bipolar SIM exhibited ion current rectification phenomenon with the maximal rectification ratio of ca.3.The fourth chapter presented perforative ultrathin SIM with ultrasmall and uniform nanochannels and negative surface charge(n-SIM)was employed as cation-selective membrane for converting the Gbbis free energy stored in salinity gradients to electricity.Due to the ultrasmall channel size,n-SIM shows ionic selectivity,favoring the transport of cations and suppressing that of anions.Thus,salinity-gradient-dependent ion current rectification in n-SIM with a relatively high rectification ratio(23.8)was achieved.Moreover,n-SIM possesses high porosity(16.7%),vertically aligned channels,and ultrasmall thickness(-90 nm),leading to rapid mass transport and low membrane resistance.Therefore,the obtained maximum power density output on external electrical load was up to 212.7 W/m2 by mixing the artificial seawater(0.5 M NaCl)and river water(0.01 M NaCl).Moreover,p-SIM was also used to investigate ion current rectification based on salinity gradients and energy harvesting performance.The maximal rectification ratio obtained at p-SIM was 3.1.Last chapter summarized the work presented in this thesis and ended with an outlook and perspective on the possible applications of SIM.