| Recently, microfluidic analytical systems aiming at miniaturization, integration, portability of analytical instruments have been growing rapidly. The bulky and complex equipments required by laser-induced fluorescence (LIF) and mass-spectrum (MS) impede the integration of LIF or MS with the microchips. Moreover, the cost of LIF and MS is so expensive that they are usually not available in ordinary analytical laboratory. Electrochemical detection in amperometric mode (AD) is an attractive alternative to utilize for microchip analysis owing to its such impressive advantages as high sensitivity, wide range of application, ideal compatibility with integration and miniaturization.microfluidic chips are mostly fabricated with glass and quartz substrates. However, chips made of glass or quarts are fragile and expensive. They are difficult in mass production. Polymeric substrates have such advantages as less expensive than glass and quartz, not fragile as glass, easy to be mass produced, and a variety range of materials can be explored. Thus, they are most suitable to be used for fabrication of disposable microfluidic chips.The present work is aimed to fabricate integrated micro electrodes on polymeric microchips used for capillary electrophoresis-amperometric detection (CE-AD)..The thesis is composed of five parts:In chapter 1, the recent advances in both microchip capillary electrophoresis with amperometric detection and polymer microfluidic chips were reviewed.In chapter 2, a novel technique was developed for fabrication of gold or copper film microelectrode on polycarbonate (PC) sheets by coupling of UV-photochemically modification of the selective area on PC surface with electroless plating. Low-pressure mercury lamp and chromium coated quartz photomask withmicroelectrode pattern were employed for selective photochemical modification of PC surface. Hydrophilic, reactive groups were formed on the UV-irradiated PC surface. After a series of chemical reaction, gold nanoparticles, the catalysts needed for electroless plating, were formed on the UV-irradiated area. Electroless placing was then occurred in the activated area of the PC sheet when it was immerged in gold or copper plating bath, forming gold or copper microfilm electrode on the irradiated area. Factors affecting the UV-photochemical modification including lamp types, and irradiation time were examined. Several analytical methods were used to characterize the modified PC surface. The problematic non-selective plating, termed as overplating, was overcome by cleaning the activated PC sheet in an ultra sonication bath of KSCN solution. The gold microelectrodes fabricated with this technique have smooth edges and sharp angles, and its electrochemical property is similar to that of gold disk electrode.In chapter 3, CE-AD polycarbonate microchips were fabricated by thermal bonding a microelectrode-integrated PC sheet with a channel-embossed PC sheet, and the fabricated chips were used to separate and detect dopamine (DA) and catechol (CA). The PC substrate containing the separation channel was laid on the cover plate containing the gold microelectrode. With the help of an optical microscope, the microelectrode was aligned to the exit of the separation channel, yielding an electrode-to-channel distance of 20 ± 5 urn. Dichloromethane was applied to fix temporarily the two PC pieces in the correct position. The aligned PC pieces were pinched and thermally bonded at 150 °C for 20 mins. The formed CE-AD chip was used to separate and detect DA and CA with MES as separation buffer. The separation efficiencies of DA and CA were 1.4><104and 3.4x104/m, respectively, and detection limits achieved were 0.65 and 1.03 umol/L, respectively. Electropherograms for fiveconsecutive runs of DA and CA showed that the RSDs of migration time were 1.5% and 5.4%, respectively. Compared to the hybrid chips composed of PDMS and glass, the analytical performances of PC CE-AD chip were significantly improved. The plated gold film electrode can be used for about 4 hours, bearing usually more than 100 runs before failure.In chapter 4, a novel photomask composed of a fused silica substrate and an AZ photoresist coating was developed for the UV-photochemical modification of PC surface. Spectroscopic study reveals that a positive AZ photoresist film thicker than 10 urn can hardly be transmitted by the UV lights of the wavelength ranging 200-285 nm. Based on this observation, the AZ photoresist films was substituted for the chromium layer that usually used as the light-masking layer deposited on the fused silica substrate. After the pattern was transferred onto the photomasks by photolithography and wet etching, the photomask composed of photoresist film/fused silica substrate had almost the same pattern accuracy as that made of chromium layer/fused silica substrate. By using a low-pressure mercury lamp that mainly emits 254 nm line as the radiation source, the developed photomask has been applied for selectively photochemical modification of PC surface, followed by chemical modification of the non-masked (irradiated) area, and finally gold electroless plating therein. With this procedure, metal film microstructures as fine as 35 urn in width could be fabricated on the non-masked PC surface. By using the novel photomasks,gold microelectrode and microelectrode array for CE-AD have been fabricated.In chapter 5, the modification of the gold film microelectrode fabricated on PC microchips with conductive polymer-polyaniline was studied. Cyclic voltametrywas conducted to electro-deposit a layer of polyaniline on the sensing part of gold microelectrode. The effect of pH on the electrochemical activity of polyaniline was examined. Cyclic voltametric test in various pH medium showed that polyaniline-modified gold film electrode has reversibility, electrochemical activity and quite good stability in media of pH< 5. When used for cyclic voltametry of dopamine, the polyaniline-modified gold film microelectrode, compared to the bare gold film microelectrode, shifted anodic peak potential towards the negative by 70 milli-volts, and generated a ~ 100-fold oxidation current of the analyte. These preliminary results showed the great potential of the polyaniline-modified gold microelectrode to be used in CE-AD PC microchip.The main novelty of the present work is summarized as:1. Developed a novel technique by coupling UV-photochemical surface modificationto electroless plating for fabrication of metal film microelectrode on specified area of polycarbonate sheets. Based on this novel technique, CE-AD microchips with integrated micro metal working electrode was fabricated and successfully applied for separation and detection of dopamine and catechol. The analytical performances of the fabricated chip were in general superior to those obtained with PDMS/glass hybrid CE-AD chips.2. Invented a simple photomask by replacing the chromium layer deposited on fusedsilica substrate for light-masking by a AZ photoresist film coated the same substrate for light-absorption. PC sheets can be selectively surface modified with this developed photomask.3. Developed a polyaniline-modified gold film microelectrode on a polycarbonatesheet and revealed its promising potential to be used as an integrated micro electrode in CE-AD microchips. |
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