Electrochemical Behavior Of Isoporous Silica-micelle Membrane

Nanochannel is a tubular structure with the aperture of 1?100 nm and the channel length deeper than the aperture.Currently,nanochannels mainly include protein nanochannels and solid-state nanochannels.Arising from mechanical/chemical stability and tunable structure/surface properties,solid-state nanochannels have aroused enormous attention and shown unique superiority in the field of biomimetic ion channel design,DNA sequencing,single-molecule analysis,energy conversion,drug controlled delivery and biosensing.Mesoporous silica,whose pore size has a narrow distribution between 2 and 50 nm,has received considerable attention because of its accessibility,high specific surface area,ordered mesostructure,tunable pore size and easy functionalization.As an important class,mesoporous silica film has attracted significant attention from the electrochemists owing to their highly ordered and perpendicular orientated nanochannels.Isoporous silica-micelle membrane(iSMM),consisting of silica nanochannels and cylindrical micelles,has characteristics of ultrasmall pore size,high porosity,vertically-aligned orientation,easy modification and uniform/tunable thickness.The current thesis aims at studying the permselectivity of iSMM and exploring its applications in electrochemical sensors and biosensors.It is divided into five chapters:In first chapter,according to different output signal,principle of nanochannel based analysis was classified.Subsequently various kinds of nanochannels were introduced,including protein nanochannels and solid-state nanochannels.Their different fabrication methods and advantages/disadvantages of solid-state nanochannels were discussed in details.Finally,electrochemical applications of single solid nanochannel/solid nanochannel arrays were reviewed.In second chapter,iSMM was prepared in situ on indium tin oxide(ITO)by Stober method.The morphology,aperture and thickness of iSMM were characterized by electron microscopy.The vertically-oriented nanochannels are in favor of the mass transport to and exchange electron with underlying ITO surface.Therefore electrochemical methods,i.e.,cyclic voltammetry and electrochemical impedance spectroscopy,were employed to investigate permselectivity of isoporous silica membrane(iSM,after micelle removal from iSMM)and iSMM(micelle retained).It was found that iSM possesses size/charge selectivity,and the permeability to co-ions(the ion charge is the same as that of the nanochannels)can be modulated by ion strength of supporting electrolyte.Meanwhile iSMM possesses size/charge/lipophilicity selectivity.Only small-sized/neutral/hydrophobic molecules can permeate micelle of iSMM and have electrochemical reaction at the micelle/ITO interface.Finally,electrochemical behavior of FcMeOH at different electrodes was investigated by electrochemical impedance spectroscopy and the corresponding equivalent circuits were proposed for the first time.The minimum charge transfer resistance at the micelle/ITO interface implied pre-concentration effect of micelle phase towards FcMeOH leading to favored electrochemical process.In the third chapter,direct electrochemical detection in human whole blood remains challenging due to electrode surface fouling and passivation by abundant biological substances.Based on size sieving capacity,charge/liphophilicity selectivity,and pre-concentration ability,only small and neutral/lipophilic analytes can permeate iSMM,be concentrated,and subsequently be detected at the underlying ITO electrode.It is however impermeable to cells,lipid,protein,carbohydrate,ions etc.in body fluids.iSMM can act as excellent barrier,preventing underlying electrode surface from fouling and passivation.Additionally,thanks to pre-concentrating ability of micelle,quantitative analysis of trace analyte can be achieved.As a proof-of-concept experiment,lipophilic drug molecules,chloramphenicol,was electrochemically determined in human whole blood directly.In the fourth chapter,we report a label-free and immobilization-free electrochemical biosensor based on 3,3',5,5'-tetramethylbenzidine(TMB)responsive iSMM.TMB has been frequently used as an indicator in G-quadruplex/hemin DNAzyme(G4zyme)-based chemical and biochemical analysis,and its oxidation products are usually monitored by electrochemical or optical methods to quantify G4zyme formation-related analytes.Herein,based on its charge/lipophilicity permselectivity,selective response of iSMM towards TMB was investigated.Since depletion of TMB was catalyzed by G4zymes formed in the presence of specific analyte,decreased electrochemical signal enabled quantitative detection of this analyte.As proof-of-concept experiments,four substances,i.e.,potassium cation,adenosine triphosphate,thrombin and nucleic acid,were detected.In the last chapter,work presented in this article was summarized.An attempt was also made to propose future research trend of iSMM in electrochemical sensors.