| Amino acids are essential building blocks of proteins, and play key roles in neurochemical response, metabolism regulation, protein synthesis, as well as in providing energy and strength for muscle and brain. Therefore, the development of simple and efficient methods for the analysis of amino acids is of great importance for life science. Microchip capillary electrophoresis effectively integrates the laboratory functions such as sample pretreatment, reactor, separation and detector, etc. on just a credit card-sized microchip, which has the advantages of reduced time for analytical process, high sample throughout, low consumptions of samples and reagents, automatic control and easy integration. Polydimethylsiloxane (PDMS) has become a popular material for building microfluidic devices mainly due to its excellent optical transparency, easy sealing with other materials, nontoxicity, low cost, increasing versatility and relatively low curing temperature. However, PDMS microfluidic devices employed for electrophoresis also show some defects including the unstable electroosmotic flow (EOF), extreme hydrophobicity and easy adsorption of amino acids onto the channel surface, etc., which would result in peak broadening and low separation efficiency. To overcome these defects, surface modification techniques were applied to improve the surface properties of PDMS, which were as follows.1. A hydrophilic and biologically active PDMS microchip channel was fabricated based on surface modification with PDDA and TiO2 NPs using the layer-by-layer technique. Compared with the native PDMS microchip, the PDDA/TiO2 NPs modified PDMS microchip yielded a reduced EOF which kept stable at a wide pH range, and greatly depressed the adsorption of amino acids. Arginine, phenylalanine, serine and threonine were successfully separated within 100 s, and the theoretical plate numbers were 9.85×103,7.03×104,6.18×104 and 7.96×104 plates/m, respectively. The resolution of phenylalanine and serine was 1.82, serine and threonine was 1.68. These four amino acids were detected with an in-channel indirect amperometric detection mode at a carbon cylindrical microelectrode, with the linear ranges all from 50 to 600μM, and detection limits of 9.5,11.8,12.6 and 11.2μM (S/N=3), respectively.2. Most of amino acids are nonelectroactive on the conventional carbon electrode, but they can be detected directly at the copper electrode surface. At a constant detection potential (+0.6 V) in the less alkaline borate medium (pH 9.2), the detection of amino acids on the copper electrode is based on a complexation reaction between amino acids and the oxidized Cu(I) or Cu(II), which results in the increase of the anodic cuurents. Arginine, proline, histidine, valine and serine were successfully separated on the PDDA/TiO2 NPs modified PDMS microchip, and then detected at a copper microdisk electrode which was placed at 10μm away from the separation channel outlet. Since the cross-sectional area of the disk-shaped copper electrode (127μm) was much larger than the separation channel outlet (50μm), it was anticipated that efficient electrode/channel alignment would be easier to achieve. In this case, the electrode response was sufficiently stable, with the response differences in the range of only 2-10% during two weeks. This detection method showed high sensitivity and low detection limits. The detection limits of arginine, proline, histidine, valine and serine 7.1,6.2,5.3,6.0 and 4.6μM (S/N=3), respectively.3. Chirality is one of the most significant attributes in the nature. As the model analytes in the chiral analysis, the separation of amino acid enantiomers has become the focus in the researching area. Here, a simple and facile method was developed to realize fast chiral separation of D,L-tryptophan inside PDMS channel by combining the advantages of the fast responsivity of magnetic nanoparticles and the excellent separation ability of microchip electrophoresis. First, magnetic Fe3O4@Au NPs were synthesized. Then, BSA was conjugated with magnetic Fe3O4@Au NPs via the interaction of Au and amine. Finally, the prepared magnetic conjugate was tightly packed inside PDMS microchannel by the application of magnets. Compared with the native PDMS microchip, the Fe3O4@Au NPs-BSA conjugate modified PDMS microchip yielded an enhanced and stable EOF, and greatly depressed the adsorption of amino acids. Efficient chiral separation of D,L-tryptophan was achieved within 70 s with the resolution of 1.25. The theoretical plate numbers of D-tryptophan and L-tryptophan were 1.7×105 and 5.8×105 plates/m, respectively. This modification technique has facilitated the easy manipulation of magnetic nanoparticles, and the coating demonstrated excellent stability and reproducibility.
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