| Microfluidic analysis, the technical key component of the field of micro total analysis systems (μTAS), has been always aimed at integrating various fundamental functions such as sample introduction, pretreatment, mixing/reaction, separation and detection on a microfluidic chip to fulfill the miniaturization, integration and portability of the whole analytical systems. Manipulation of microfluids in a microchip is the basis of microfluidic analysis. However, it is also one of major bottle-neck in the development of microfluidic chips. Many efforts are currently made to develop low-cost devices with simple operating procedures to achieve the manipulation of microfluids.Poly (N-isopropylacrylamide) (PNIPAAm) is a thermal-responsive polymer that undergoes a rapid phase transition at its lower critical solution temperature (LCST,32℃), leading to reversible change in volume and hydrophobicity/hydrophilicity. Taking advantage of the sharp volume change property of a PNIPAAm gel plug that is immobilized inside micro channel, one can not only control the microfluids easily, but also manipulate the electroosmotic flow and ion electrophoresis within microchannels. The topic of this dissertation is focused on the development of on-chip integrated thermal-switchable valves for manipulation of microfluids on micro-channels, and for sample preconcentration and separation.The thesis is composed of four parts:In chapter 1, the stimulus-sensitive mechanism of PNIPAAm and its synthetic methods and applications were introduced. The recent progress in the flow control techniques of microfluidic analysis based on PNIPAAm and sample preconcentration based on ion bridges was reviewed. Based on different actuation principles, the flow regulation via PNIPAAm was developed in the form of active or passive microvalves, complex microvalves (micropumps). In the section of sample preconcentration, a few types of the ion bridges with a variety of structures such as nanochannels, porous membranes and monolith were presented.In chapter 2, a thermal switchable microvalve made of temperature-sensitive PNIPAAm was integrated into microfluidic channels and applied for simple flow injection analysis. With 2-hydroxy-2-methyl propiophenone as photo-initiator, selectively photo-polymerization of the N-isopropylacrylamide monomer in water/ethanol was carried out within the channels of a glass chip. The polymerization conditions for the thermally actuated microfluidic valve and its fluid-manipulating behavior were systematically investigated. The polymerization time was only 1 min after systematic optimizations. Based on these investigations, using syringe pumps to propel solutions and micro Peltier devices to control the open/close status of PNIPAAm microvalves, a protocol of on-chip micro-flow injection analysis (μFIA) system equipped with a single thermally actuated microfluidic valve was established. The analytical performance of theμFIA system was demonstrated with the chelating reaction between the magnesium (II) and the fluorescent agent o,o'-dihydroxyazobenzene (DHAB). Experimental results showed that the mirofluidic valve behaved well for the flow control and a precision of 3.4% was achieved for theμFIA fluorescence detection of magnesium (Ⅱ).In chapter 3, based on the fabrication technique developed by Chapter 2, a microfluidic glass chip integrated with multi PNIPAAm valves was developed to perform volume-based sample injection for micro flow injection chemilluminescence analysis. Recently, some reports have developed PNIPAAm-based thermally actuated microfluidic valves in microfluidic chip, but in their system, only one or two such valves were usually fabricated to achieve simple microflow control. In this chapter, using two micro Peltiers to actively regulate the temperatures at two adjacent temperature zones on a micro chip, two groups of such prepared PNIPAAm valves separately located on the two adjacent temperature zones can be alternately opened or closed in opposite close/open phases, and the opening/closing time were 3 s and 7 s respectively. Thus, sampling loops can be switched between loading and injection without any mechanical moving parts to perform volume-based sample injection. Cooperating with syringe pumps, the microfluidic chip with the integrated sample injector has been demonstrated for micro flow injection chemilluminescence detection of hydrogen peroxide based on chemilluminescence reaction of hydrogen peroxide with luminol under the catalysis of K3Fe(CN)6. For a sampling volume of 6 nL, linear response was observed over the H2O2 concentration range of 0-2 mmol L-1, and a precision of 0.6% (RSD, n=11) was achieved for a standard H2O2 solution 2 mmol L-1.In chapter 4, a novel ion-enrichment and elution protocol was developed based on thermally responsive swell and shrink behaviors of PNIPAAm. Mechanism of the electrophoresis-based mass transport phenomenon of PNIPAAm monolith for temperature controlled enrichment and elution of fluorescein isothiocyanate (FITC) was experimentally studied. It is concluded that the electrokinetic concentration of FITC was resulted from ion-permselectivity mechanism at low temperature, while the following elution of previously enriched zone was due to the electroosmotic flow within the hydrogel plug at the high temperature. Based on the temperature controlled ion enrichment and elution effect of PNIPAAm monolithic plug, we developed a microfluidic glass chip with a single microchannel inside which a thermo-actuated PNIPAAm monolithic plug was integrated to perform on-line preconcentration and electrophoresis separation of FITC labeled amino acids. Additionally, the effect of the different elution direction of enriched zone on electrophoresis was examined. For a preconcentraton time of 3 min using the PNIPAAm plug, a maximum enrichment factor of about 26 was achieved for FITC. With the temperature controlled switchable PNIPAAm monolithic plug, on-line preconcentration and electrophoresis separation of two different FITC labeled amino acids were achieved.
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