Preparation Of Polymeric Microfluidic Chins Via UV-photochemical Reaction And Applications Thereof

Microfluidic chips, the core of the micro total analysis systems (μ-TAS), are micro-chips integrated with micro channels, micro reaction cells, micro detectors and other micro components to perform the operations in traditional laboratories such as sampling, dilution, reaction, seperation, detection and so on. With the help of a few external equipments, the microfluidic chips can lead to fast, automated chemical or biochemical total analyses featuring the so-called "sample in and data out". Therefore, the preparation of microfluidic chips is the key to realize the micro total analyses.Preparation of microfluidic chips with traditional micro electro-mechanical systems (MEMS) needs expensive equipments, clean rooms, and skilled operational technicians. Therefore, it is hardly possible to prepare microfluidic chips via MEMS technology in ordinary chemistry or biochemistry laboratories.Surface treatment of various substrates via UV-photochemical reaction has recently attracted the attention of micrfluidic chip researchers due to its advantages such as pollution-free, easiness to perform highly-accurately region-selective surface treatment, and needless of expensive equipments but a simple UV light source. So far, however, the UV-based surface treatment of microfluidic chips reported by literatures is mainly focused on modifying the surface properties of the chip channels, in-turn, to improve their analytical performance. The present work intends to develop new UV-photoreaction-based methods and techniques used to fabricate micro/nano fluidic chips, to prepare on-chip integrated micro-devices, and to study the possible applications of developed chips in various chemical or biochemical analyses.This thesis is composed of six chapters:In chapter1, a brief introduction to the mechanisms of photochemical reactions between UV lights and polymers was made, and the recent progress in the application of UV lights for fabrication of micrfluidic chips was reviewed.In chapter2, a novel method was developed for preparing of micro-electrodes on polystyrene (PS), one of the most biocompatible polymer materials, by electroless gold plating after UV-induced regional activation of the PS surface. First, several types of UV light soucres were investigated into their performance in hydrophilic modification of PS surface. The high-pressure mercury lamp that mainly emits365nm UV lights did almost no effect on hydrophilic modification of PS, while the low-pressure mercury UV-lamp that mainly emits254nm UV lights could effectively improve the wettability of PS surface. Based on this observation, a low-pressure mercury lamp was selected as the UV light source for photo-modification of PS surface. Micro gold film devices of micro-heater and micro electrochemical sensing electrodes were electrolessly plated on the UV-exposed PS surface after the regionally UV-exposed PS surface was subjected to a series of chemical reactions. The gold films plated with developed method possessed strong adhesion, good resistance to acid and alkali, and excellent elctrochemical properties.The PS cover sheet with micro-electrodes and PS substrate with micro-channels were bonded after plasma treatment, forming a full-PS capillary electrophoresis chip with integrated amperometric detector. With dopamine (DA) and catechol (CA) as model analytes, the separation efficiency obtained at an electric field of238V/cm was1.3×104plates/m for DA. The detection limits were0.5(DA) and0.7(CA)μmol/L respectively. Five consecutive runs of a standard solution containing100μmol/L DA and100μmol/L CA showed RSDs of0.5%(DA) and0.4%(CA) in migration time and RSDs of1.1%(DA) and2.7%(CA) in peak height of signals, respectivly. Compared with the analytical performances previously reported by using a PDMS-glass hybrid electrophoresis chip, the present full PS chip possessed higher separation efficiency, better signal precision and shorter analysis time.Polymethylmethacrylate (PMMA) is one of the most widely used polymer material for microfluidic chips. However, it is hardly possible to prepare micro metal devices on the surface of PMMA with the electroless plating method similar to the protocol developed in chapter2. Thus, in chapter3, an investigation into the photochemical reaction between UV lights and PMMA surface was carried out. Deep UV-lights, ozone, and deep UV-lights in cooperation with ozone (UV/O3) were compared with respect to the efficiency of hydrophilization of the PMMA surface. It was observed that only UV/O3can generate oxygen-containing moieties on the PMMA surface, consequently, turn the PMMA surface from hydrophobic to hydrophilic effectively. Based on the observation, a novel method for preparation of micro metal devices on the PMMA surface was established by using UV/O3as the UV-light source. The prepared micro metal devices showed accurate size, strong adhesion to PMMA substrate, good electric and electrochemical properties.In chapter4, a novel and facile method was developed for preparation of PMMA fluidic chips with1-dimension (1-D) nano-channels. First,1-D nano-or sub-micro channels were etched by photoresist-free UV-lithography based on the UV photodegradation of the PMMA. Then the channels were sealed by UV-assisted low temperature bonding. Different UV light sources were compared with respect to the efficiency of etching of PMMA. It was found that UV/O3has a faster etch rate than UV, so UV/O3was selectived as the UV light source. The etched depth increased with the UV-exposed time, with the depth-time profile fitting into a quadric polynomial curve. The depth of the etched channel increased by around20%after vigorously flushed with water. To bond the chip with nano-channels, the PMMA substrate with open nano-channels and a flat PMMA cover sheet were firstly treated with UV/O3, forming hydrophilic moieties such as hydroxyl and carboxylic acid groups. Then the two UV-exposed PMMA were brought into intimate contact under running water. The bonding was completed under a temperature of45℃and a pressure of1.2×10Pa for35min. The bonding protocol worked well for nanochanels whose aspect ratio was greater than1:1000, and the depth of the bonded nanochannels was decreased by (13+9)%in comparison to that afore-bonded. The bonding strength was very strong, the bonded chips can bear a tensile of6.71±2.50MPa. A PMMA chip with hybrid micro-and nano-fluidic channels was prepared with the developed method, and it was demonstrated for the electrokinetically-driven enrichment and depletion of fluorescein isothiocyanate isomer (FITC) dye anions. In some circumstances, hybrid microfluidic chips composed of elastic, gas-permeable polydimethylsiloxane (PDMS) and rigid plastics are needed. However, It is very difficult to irreversiblely bond PDMS with plastics such as PS. In chapter5, a novel method for bonding PDMS to PS was proposed based on silanization of PS surface in combination of UV-activating of the to-be-bonded surfaces. Firstly, the PS substrate was exposed to UV/O3to produce hydroxyl and carboxylic acid moieties on its surface. The UV/O3treated PS was silanized with (3-aminopropyl)triethoxysilane (APTES) through the reaction between the hydroxyl and carboxylic acid moieties on the PS surface and the molecules of APTES. Then both the silanized-PS and PDMS surfaces were treated with UV/O3to generate silicon hydroxyl moieties on both surfaces. Finally, the UV-activated PDMS was intimately contacted to the UV-activated PS surface, and irreversibly bonding occurred between PS and PDMS after pressed and stayed for one hour due to the condensation reaction between the silicon hydroxyl moieties on the PS surface and those on PDMS surface. A hybrid PDMS-PS microfluidic chip composed of gas-permeable PDMS substrate with channel network and excellently biocompatible PS cover sheet was fabricated for cell culture. The results showed that the cells cultured in the hybrid PDMS-PS chip grew significantly better than those cultured in a full-PS chip or in a full-PDMS chip.In chapter6, a novel on-chip-integrated micro electrochemical detection system for the determination of electro-active species contained in aqueous droplets of water in oil (W/O, n-octanol as the oil) two-phase flow was developed and studied. Three micro-electrodes for amperometric detection were prepared on the PS chip with W/O droplet generator network by using the protocol developed by chapter2. With H2O2as the model analyte, the electrochemical behavior of the detector towards the W/O droplet-based system was studied. It was found that the recorded current-time profile of the amperometric signal consisted of peak-shaped front and platform-shaped trailing edge. The attributions of both the charge current and Farady current to signal profiles of either the phosphate blank droplets or H2O2analyte droplets were characterized. The current signals generated by H2O2-containing W/O droplet flow were much lower than that produced by H2O2-containing continuous flow. This could be ascribed to the fact that the oil film packaging the water droplet prevented the droplet from direct contac to the sensing electrodes. The effect of the flow rates, phase ratio of the water to oil, configuration of the electrode system and the width of the electrode on the current signals of the droplets was examined. After optimization, a detection sensitivity of22.2pA/(mol/L) for H2O2concentration ranging from10to500μmol/L was achieved. The RSDs of the current signals observed with W/O droplets containing250μmol/L H2O2were3.2%for inter-day (n=9) and8.9%for intra-day (n=3), respectively.The main novelties of the present work are summarized as:1. Two novel approaches have been established for preparation of micro metal film devices on polymeric chips via UV-directed region-selective electroless metal plating, one for PS chips and the other for PMMA chips. It has been revealed that the low pressure mercury lamp emitting254nm UV-light without generation of ozone is effective for photochemical formation of carboxyl moieties on PS surface but it does not work for PMMA. To produce the same moieties on PMMA surface, a low pressure mercury lamp emitting254nm UV-light in company of ozone generation is required. With the carboxyl moieties serving as the anchor, gold nano-particle catalysts for the followed region-selectively electroless plating can be deposited onto the UV-exposed area.2. A novel approach has been established for fabrication of nanofluidic PMMA chips. The approach includes preparation of1-D nanochannels on PMMA substrates by means of UV/O3lithography and bonding of PMMA substrates with nanochannels at low temperature and low pressure after UV/03-photochemical modification of the to-be-bonded PMMA surfaces. Without needs of expensive equipments, the established approach provides chemists or biochemists with a facile and innovative technical platform for fabricating PMMA nano-or hybrid nano/micro-fluidic chips in common chemistry laboratories.3. A novel and easy-to-do method has been developed for irreversibly bonding of an elastic PDMS slab to a PS substrate based on UV-assisted silanization of the PS surface followed by UV-photochamical activation of both the PDMS slab and silanized-PS substrate immediately before bonding. With the developed method, hybrid PDMS-PS microchips possessing both the elastic, gas-permeable properties of PDMS and the rigid, biocompatible properties of PS can be fabricated in chemistry or biochemistry laboratories.4. A novel PS microfluidc chip has been fabricated, which possesses the functions of on-chip generation of droplet-based two-phase flow and on-chip, real-time electrochemical detection of the electroactive species contained inside the W/O droplets. The electrochemical behaviour of the on-chip integrated amperometric detector for in-droplet-contained H2O2has been comprehensively studied, and some phenomenons specifically relevant to the two-phase droplet system revealed.