Preparation And Properties Of The Copolymer Self-assembly Of Microporous Membranes And Their Applications In Electrochemical Sensors

Thin film technology is a hot research and development project in the 21st century. Especially, the microporous thin films with tunable configurations have received increased interest in chemistry, optoelectronics, life and material sciences, because they have been applied as membrane separation, optical filters, photoelectric equipment, microreactors, microsensors and cell culture substrates and so on. As a whole, there were two kinds of microporous thin films in existence, include polymer microporous thin films and inorganic microporous thin films. One of the most facile approaches to fabricate microporous thin films is self-assemble using copolymers. Ultramicroelectrodes (UMEs) have evoked comprehensive interest due to their intrinsic characteristics. The low iR drop enables a two-electrode system to be used and the high resistance system to be detected. The low charged currents of the nanoelectrodes increase the ratio of signal/noise and allow a high scan rate. However, an obvious challenge to successful exploration of the above benefits of individual nanoscale electrodes is their fabrication and handling; another is that single UME generates low currents, which are hardly detectable with the conventional electrochemical technique. This instrumentation problem can be circumvented by the use of UME arrays or ensembles, whereby the individual electrodes in the array operate in parallel thus amplifying the signal while retaining the beneficial characteristics of the UMEs. Thus there has been much interest in the development of collections of UMEs. Thus the many-electrode microarray can have individual sensors poised at different potentials, coated with different layers or even located within different regions of a sample matrix (e.g. fluid stream) in order to detect and pick-up different nuances of the sample matrix being investigated. In this paper, an amphiphilic block polymer, polystryene–b–polyacrylic (PS–b–PAA), and another copolymer, poly(acrylonitrile–co–acrylic acid), was using to fabricate microporous thin films by self-assembly, respectively. Some technique include in–situ atomic force microscopy, dynamic light scattering, and video optical contact angle measurement were carried out to investigate tunable surface Properties of the films and responses to external stimulus. These microporous thin films as modifiers were firstly used to fabricate to a series of UME arrays or ensembles. Electrochemical methods were used to characterize these electrodes with the typical characterization of UMEs. These fabrication approaches was testified facile, rapid and low cost. Some UMEs obtained in this paper were in application to determinations in real samples with high selectivity and sensitivity. These results will provide valuable technical reference for the UMEs fabrication and their applications. This paper includes four parts as follows:1. Fabrication of Highly Ordered Microporous Thin Film by PS–b–PAA Self-assembly and Investigation of its Tunable Surface PropertiesA facile approach was presented to fabricate high ordered microporous thin films by self-assembly via an amphiphilic block polymer (PS–b–PAA). The highly ordered microporous thin films were formed by casting PS–b–PAA tetrahydrofuran (THF) onto a glass slide under high humidity. The condensed water droplets act as sacrificed templates on the air-polymer solution interface based on thermocapillary convection. Several key influencing factors, such as the concentration of the block polymer solution, the relative humidity of the atmosphere, the properties of solvent, the spreading volume and the substrates, are investigated to control micropore size and tune film surface properties. The surface composition and wettability of the film were found to be dramatically changed in aqueous solution, and the contact angle of the film surface was interestingly reduced from nearly hydrophobic to super-hydrophilic, which was captured by optical contact angle. The influence of porosity and the ionization degree of PAA on above properties were investigated. Micropores diameters of the film determine the initial contact angle, while PAA ionization degree determine the transformation time of the wettability behavior and the last contact angle. This study is aimed at strategies for fabrication of microporous thin films with dynamically controlled micropore size and tunable surface properties. PS–b–PAA as an excellent candidate for functional materials could be promising application in fabrication of microporous thin films, smart stimuli-responsive materials, biosensors or nanofluidic devices.2. Study of Response to Environmentally Sensitive Stimuli and Tunable Surface Properties based on Microporous Thin Film by PS–b–PAA Self-assemblyThe PS–b–PAA microporous thin film exhibits different surface morphologies in response to external stimulus, especially to the environments with different pH values. The reversible evolution of the microstructures in aqueous solutions over a pH range of 2.4– 9.2 was firstly investigated by in–situ atomic force microscopy (AFM). The partial PAA chains stretched and dispersed in solutions as pH value increase, resulting in the enlargement of micropore diameter observed. Every micropore in the film was enclosed by PAA domain as a weak polyelectrolyte (pKa, 4.6), which has many carboxylic functional groups (– COOH). At low pH (pH < pKa), the–COOH groups along the polymer chains are few partially ionized and only some PAA chains disperse in water. At high pH (pH > pKa), the–COOH groups along the polymer chains are adequately ionized and more PAA chains stretch outwards and disperse in water. When the films were immerged in aqueous solution, the stimuli–responsive dynamics of micropore diameters was described by a two–stage mechanism. The micellization process for the PS–b–PAA block copolymer in dilute solution was studied to testify the dispersion of the stretched PAA chains in solutions by dynamic light scattering (DLS). This study is aimed at strategies for the functionalization of sensitive stimuli–responsive microporous thin films with pH reversible tunable surface morphology.Upon exposure the microporous thin films of PS–b–PAA in aqueous solution contained a cationic surfactant, cetyltrimethylammonium bromide (CTAB), the PAA chains in the micropores stretch outwards and lead to the sizes of the micropores increases captured by in–situ atomic force microscopy. There was the formation of"ion pairs"between the stretched PAA chains CTAB to increase the degree of the dissociation of PAA and led to the sizes of the micropores increases, resulted in the change of the morphology and structure of the PS–b–PAA microporous thin film. The interaction between PS–b–PAA block copolymer and CTAB in dilute solution was studied to testify the dispersion of the stretched PAA chains by DLS. The PS–b–PAA microporous thin films have potential applications in smart sensor materials based on CTAB–responsive behavior.3. Fabrication, Characterization and their applications of Ultramicroelectrodes Formed via Microporous Thin Film by Amphiphilic Block Copolymer (PS–b–PAA) Self-assemblyA novel one-step approach to fabricate, Au and Ag ultramicroelectrodes ensembles or array has been developed using microporous thin film by an amphiphilic block copolymer (PS–b–PAA) self-assembly as a modifier. Electrochemical methods were used to characterize these electrodes with the typical characterization of ultramicroelectrodes. Under different conditions of the microporous film fabrication, Au microelectrode array with pore radius of 800 nm were obtained. According to cycle voltammograms at different scan rates, sigmoidal-peak waveform transitions were obtained by using [Fe(CN)6]3- as a redox-active ion probe, which indicated that the redox reaction of Fe(CN)63-/ Fe(CN)64- was only controlled at low scan rate by a radial diffusion. The similar capacitances obtained from the bare electrodes and the proposed electrodes indicated a good adhesion and there was no leaking from one pore to the next in the thin polymer film. Compared with a regular Ag disc electrode, there were good reversible reaction and high signal noise ratio for a response to adenine at the Ag ultramicroelectrodes array prepared, indicated that the detection sensitivity for adenine could be logically improved. After a controlled electrochemical deposition of Au in nanopores of the film, Au nanoelectrode ensembles were fabricated with the typical characterization of ultramicroelectrodes.A new and simple method for fabricating controllable insulated nanometer-sized platinum electrodes is presented. Electrochemical etching of platinum wire is employed, and then a repeated process of cycle voltammetric deposition of electrophoretic paint and heat curing for shrink film follows which effectively controls the size of the nanoelectrodes, which is different from previous DC electrolysis deposition. This technique allows complete insulation of the whole body of the etched platinum wire, except for the very tip with the shrink film during heat curing of the film, leaving an electrochemical active area with effective diameters of nanometers. The process overcomes the pinhole formation resulting from the electrophoretic paint deposition process. The size of the platinum electrodes and the thickness of the deposed paint for insulation can be properly controlled and reproduced. The fabricated electrodes show ideal steady-state voltammetric behaviors from which the effective areas of the nanoelectrodes are measured. The effective radius of the prepared nanoelectrodes ranges from 3.1 nm to hundreds of nanometers. 4. Fabrication of Nanoporous electrode formed via PAN–co–PAA Self-Assembly and Selective Voltammetric Detection of Uric Acid in Biologic SamplesA novel approach to fabricate nanoporous glassy carbon electrode with the pores of 50 ~ 200 nm in radii has been developed using a copolymer [poly(acrylonitrile–co–acrylic acid)] self-assembly. This procedure is simple and fast, and requires only conventional, inexpensive electrochemical instrumentation. Electrochemical methods were used to characterize the nanoporous glassy carbon electrode with the characterization of an ultramicroelectrode. The nanoporous glassy carbon electrode drastically suppressed the response of ascorbic acid (AA) and resolved the overlapping voltammetric response of uric acid (UA) and AA into two well-defined peaks with a large anodic peak difference (ΔEpa) of about 330 mV. There were the weak adsorption of UA on carbon-based electrodes and a fast electron transfer reaction of UA and AA take place in the different micro-environment in the presence of the carboxylic functional groups in the nanopores. The peak current obtained from differential pulse voltammetry (DPV) was linearly dependent on the UA concentration in the range of 5.0×10-7 mol/L to 5.0×10-5 mol/L at neutral pH (PBS, pH = 6.86) with a correlation coefficient of 0.998, and the detection limit was 1.0×10-7 mol/L (S/N = 3). The nanoporous glassy carbon electrode has also been demonstrated to be applicable in the detection of UA in serum and urine samples with excellent sensitivity and selectivity. The nanoporous glassy carbon electrode will hopefully be of good application in further sensor development and biomedical analysis.