Study On The Surface Modification Of PDMS Microchip And Its Application In The Separation Of Biomaterials

Microchip capillary electrophoresis (MCE) system is a newly rapidly developed research technology which was based on the routine capillary electrophoresis (CE) principle and has extensive application perspective. MicroChannel networks on the microchip are manufactured by the technology of Micro Electro Mechanical System (MEMS) on the substrate of glass, fused silica, and polymers, etc. For its small dimension and high thermal conductivity, MCE system can realize more rapid analysis. PDMS has become a popular material for building microfluidic devices mainly due to its excellent optical transparency, easy sealing with other materials, nontoxicity, low electrical conductivity, low cost, increasing versatility, relatively low curing temperature, and biocompatibility. However, PDMS microfluidic devices employed for electrophoresis show some defects that need to be overcome. These disadvantages include the unstable electroosmotic flow (EOF), extreme hydrophobicity and easy adsorption of samples onto the channel surface, etc. Through modification of appropriate substance on the PDMS surface, the adsorption on the PDMS fabricated microchip can be suppressed. The thesis was composed of four parts:1. In chapter 1, the working principles, characteristics, evaluation standards and recent developments of microchip capillary electrophosis were reviewed. We introduced the materials and fabrication techniques of microchip and some detection techniques. The advantages and disadavantages of PDMS microchip were mentioned, and the modification techniques on PDMS surface were discussed, which included modification by exposure to energy, dynamic coating, bulk-modification and layer-by-layer (LBL) technique, and so on. Non-covalent interactions were often used to construct various films, and the most effective non-covalent driving force was electrostatic interaction which was widely used in LBL technique between polyions. In this thesis, we assembled different substances via electrostatic interactions through LBL technique, and studied the surface property of modified microchips.2. A new fabrication of hydrophilic and biologically active PDMS microchip channel based on surface modification with chitosan and DNA using the LBL technique was proposed. The properties of the modifiers were investigated by Fourier transformed infrared adsorption by total attenuated reflection (ATR-FT-IR) spectra of the surface and the contact angle measurement. The results showed that after modification, EOF was more stable and the surface hydrophilicity was improved. Uric acid and ascorbic acid as a group of separation models were used to evaluate the effect of the functional PDMS microfluidic devices. On the chitosan-DNA modified PDMS microchip, the separation time was obviously decreased, and the sensitivity and separation efficiency were greatly enhanced. The separation time for uric acid and ascorbic acid was dramatically decreased from 200 to 85 s on native and chitosan-DNA modified microchips, respectively. The theoretical plate numbers were 43450 and 46790 N/m at the separation voltage of 1300 V for UA and AA, respectively. In addition, this method has been successfully applied to real human urine samples with satisfactory results.3. TiO 2 NPs were employed to construct a nano-structure functional film on the PDMS microchip channel surface through LBL assembly technique on a pre-layer of polycation PDDA. Results showed that on the PDDA-TiO₂ NPs coated microchip, the apparent mobilities of target analytes as well as EOF can be altered, which led to enhanced separation efficiencies. Dopamine and epinephrine served as a model system to evaluate the impact of TiO₂ NPs on EOF and separation. The analytes were well separated on the modified microchip, and it was clearly evident that TiO₂ NPs modification improved the separation efficiencies of dopamine and epinephrine, and the resolution for them was largely enhanced from 0.61 on native PDMS microchip to 1.55 on coated PDMS microchip in 40 mM PBS. The theoretical plate numbers were 1.2534×10⁵ N/m and 9.5757×10⁴ N/m at the separation voltage of 1300 V for dopamine and epinephrine, respectively. Linear responses of them were obtained both from 25 to 600 uM with detection limits of 2.1μM for dopamine and 3.2μM for epinephrine, respectively. Moreover, the modified PDMS channels have a long-term stability and an excellent reproducibility within two weeks.4. Amino acids as the main components in organism play an essential role in physiological procedures such as transfer nerve information, regulation metabolic activity, and biosynthesis protein and peptide. Therefore, to establish a rapid and simple method for the analysis of amino acids is very important. However, the strong interactions between PDMS surface and amino acids resulted in unavoidable adsorption on channel surface and poor separation efficiency. In this paper, five amino acids have been detected with an end-channel amperometric detection mode at a copper microdisk electrode on the PDDA-TiO₂ NPs modified microchip. Here, the copper microdisk electrode was used as a working electrode. Compared with the native PDMS microchip, EOF on the PDDA-TiO₂ NPs modified microchip was decreased and more stable, which was favorable for the separation of amino acids since they had similar migration times in the short channels. As a result, the phenomenon of adsorption was well suppressed, and arginine, proline, histidine, valine and serine were successfully separated within 90 s.