Study On The Surface Modification For PDMS Microfluidic Chip And Their Application

Microchip capillary electrophoresis (MCE) has been a branch of micro-total analysis system. Poly (dimethylsiloxane) (PDMS) microchip has great potential in constructing low-cost, disposable and multifunctional microfluidic devices instead of glass and quartz. However, PDMS is inert and analytes are easily adsorbed onto it, which hinders the further application of PDMS microchips in life science. In order to eliminate the adsorption on PDMS fabricated microchip, we have focused on the modification of the inner surface of this elastomeric polymer channel and made some achievements as follows.1. PDMS microfluidic channels alternately modified by poly (diallyldimethylammonium chloride) (PDDA) and poly (sodium 4-styrenesulfonate) (PSS) were successfully used to separate uric acid and ascorbic acid. When the microchannel was coated alternately by PDDA and PSS, the EOF enhances greatly and the surface of the channel was very hydrophilic. Results show that uric acid and ascorbic acid can be well separated and detected simultaneously in modified microchips. Under the optimal conditions, the linear ranges of uric acid and ascorbic acid were both from 25 to 600μM, respectively. The detection limits were 8μM for uric acid and 5μM for ascorbic acid. Factors influencing separation and detection, including buffer solution, detection potential and separation voltage, were investigated and optimized. In addition, the dependences of the current response on sensitivity, reproducibility were studied, and the stability of the device was also evaluated in detail. This method was successfully used to determine uric acid and ascorbic acid in human urine.2. A method for the rapid separation of dopamine and epinephrine was presented in PDMS microchip electrophoresis integrated with amperometric detection. The PDMS channels dynamically modified by 2-morpholinoethanesulfonic acid (MES) show decteased EOF, less adsorption and more enhanced the separation efficiency than that of unmodified ones. Parameters influencing the separation efficiency and resolution, including separation voltage, detection potential, and the concentration of additive, are assessed and optimized.3. DNA (calf thymus) is employed to construct a functional film on the PDMS microfluidic channel surface and applied to perform electrophoresis coupled with electrochemical detection. The functional film was formed by sequentially immobilizing Chitosan (Chit) and DNA to the PDMS microfluidic channel surface via layer-by-layer assembly. ATR-FT-IR experiment was conducted to demonstrate Chit and DNA that had been coated on PDMS surface. Contact angle experiments were used to estimate the surface hydrophilicity. Aminophenol isomers (p-, o-, and m-aminophenol) served as a separation model to evaluate the effect of the functional PDMS microfluidic chips. The results clearly showed that these analytes were efficiently separated within 60 s in a 3.7 cm long separation channel and successfully detected on the modified microchip coupled with in-channel amperometric detection mode at a single carbon fiber electrode. The theoretical plate numbers were 74021, 92658 and 60552 Nm^-1at the separation voltage of 900 V with the detection limits of 1.6,4.7and 2.5μM (S/N = 3) for p-, o-, and m-aminophenol, respectively.4. A hydrophilic PDMS microchip with stable EOF was prepared by a simple and reproducible coating procedure with TiO₂ nanoparticles. The microchannel wall of PDMS chip was coated with a layer of PDDA and then collected TiO₂ nanoparticles. Electroosmotic flow measurements, contact angle, and electrophoretic separation experiments were used to estimate the effect and stability of the modified surfaces. Experiments show that the EOF is more stable at pH 6～10, the contact angle is 58.5°, perfect resistance to nonspecific adsorption of analytes and improved resolution.