Separation of aminophenol isomers in polyelectrolyte multilayers modified PDMS microchip

The separation of three kinds of aminophenol isomers were achieved within 1 min in polyelectrolytes multilayers modified PDMS microchips by layer-by-layer assembly with electrochemical detection (EC). Two polyelectrolytes, poly(dially dimethyl ammonium chloride) (PDDA) and poly(sodium-4-styrene-sulfonate) (PSS) were used to form polyelectrolyte multilayers (PEMs). The surface characteristic of the modified microchip was studied by XPS. The electroosmotic flow (EOF) on PEMs modified PDMS microchips was more stable than that of the native PDMS microchips and the adsorption of samples was greatly reduced on PEMs modified PDMS microchips during the electrophoretic process. The column efficiencies on PEMs modified microchip were increased by 100 times and the signals enhanced by 2 times compared with those of native microchips. The separation conditions such as running buffer pH, running buffer concentration and separation voltage were also optimized.     

Fabrication improvements for thermoset polyester (TPE) microfluidic devices

Thermoset polyester (TPE) microfluidic devices were previously developed as an alternative to poly(dimethylsiloxane) (PDMS) devices, fabricated similarly by replica molding, yet offering stable surface properties and good chemical compatibility with some organics that are incompatible with PDMS. This paper describes a number of improvements in the fabrication of TPE chips. Specifically, we describe methods to form TPE devices with a thin bottom layer for use with high numerical aperture (NA) objectives for sensitive fluorescence detection and optical manipulation. We also describe plasma-bonding of TPE to glass to create hybrid TPE-glass devices. We further present a simple master-pretreatment method to replace our original technique that required the use of specialized equipment.     

Patterning microbeads inside poly(dimethylsiloxane) microfluidic channels and its application for immobilized microfluidic enzyme reactors

We propose a convenient and reliable approach for immobilizing microbeads on poly(dimethylsiloxane) (PDMS) microchips. It is built upon a simple fabrication procedure of PDMS chip through directly printing the master with an office laser printer which was described in our previous work (J. Chromatogr. A 2005, 1089, 270-275). On the printed toners used as the positive relief of the master, microbeads were immobilized by a thermal treatment and then transferred to the surface of the microchip by direct molding of the prepolymer on the master. With this approach, the region-selective immobilization of microbeads and the fabrication of PDMS microchips can be accomplished at the same time. Then, using these microbeads as supports, further modification with enzyme was achieved. Surface characteristics of the microbeads-modified PDMS microchannels were investigated with scanning electron microscope, atomic force microscope, and inverse fluorescence microscope. The electrokinetic properties of the native PDMS and the modified PDMS chips were also compared. Based on this approach, an immobilized glucose oxidase (GOD) reactor was constructed and the reaction using glucose as substrate was studied. All these experiments aim to show that the proposed approach may have a good potential in the study of biochemistry and other related areas.     

Ultraviolet sealing and poly(dimethylacrylamide) modification for poly(dimethylsiloxane)/glass microchips

Simple sealing methods for poly(dimethylsiloxane) (PDMS)/glass-based capillary electrophoresis (CE) microchips by UV irradiation are described. Further, we examined the possibility to modify the inner surface of separation channels, using polymethylacrylamide (PDMA) as a dynamic coating reagent. The surface properties of native PDMS, UV-irradiated PDMS, and PDMA-coated PDMS were systematically studied by atomic force microscopy (AFM), infrared absorption by attenuated total reflection infrared (ATR-IR) spectroscopy, and contact angle measurement. We found that PDMA forms a stable coating on PDMS and glass surfaces, eliminating the nonhomogeneous electroosmotic flow (EOF) in channels on PDMS/glass microchips, and improving the hydrophilicity of PDMS surfaces. Mixtures of flavin mononucleotide (FMN), flavin adenine dinucleotide (FAD), and fluorescein were separated in 35 s using PDMA-coated PDMS/glass microchips. A high efficiency of theoretical plates with at least 1365 (105 000 N/m) and a good reproducibility with relative standard deviations (RSD) below 4% in five successive separations were achieved.     

Hydrophilic biopolymer grafted on poly(dimethylsiloxane) surface for microchip electrophoresis

A novel covalent strategy was developed to modify the poly(dimethylsiloxane) (PDMS) surface. Briefly, dextran was selectively oxidized to aldehyde groups with sodium periodate and subsequently grafted onto amine-functionalized PDMS surface via Schiff base reaction. As expected, the coated PDMS surface efficiently prevented the biomolecules from adsorption. Electro-osmotic flow (EOF) was successfully suppressed compared with that on the native PDMS microchip. Moreover, the stability of EOF was greatly enhanced and the hydrophilicity of PDMS surface was also improved. To apply thus-coated microchip, the separation of peptides, protein and neurotransmitters was investigated in detail. For comparison, these analytes were also measured on the native PDMS microchips. The results demonstrated that these analytes were efficiently separated and detected on the coated PDMS microchips. Furthermore, the relative standard deviations of their migration times for run-to-run, day-to-day, and chip-to-chip reproducibilities were in the range of 0.6-2.7%. In addition, the coated PDMS microchips showed good stability within 1 month.     

Bulk modification of PDMS microchips by an amphiphilic copolymer

A simple and rapid bulk-modification method based on adding an amphiphilic copolymer during the fabrication process was employed to modify PDMS microchips. Poly(lactic acid)-poly(ethylene glycol) (PLA-PEG) was used as the additive substance. Compared to the native PDMS microchips, both the contact angle and the EOF of the bulk-modified PDMS microchips decreased. The effects of the additive loading and the pH on the EOF were investigated in detail. The bulk-modified PDMS microchips exhibited reproducible and stable EOF behavior. The application of the bulk-modified PDMS microchips was also studied and the results indicated that they could be successfully used to separate amino acids and to suppress protein adsorption.     

Influence of polymer structure on electroosmotic flow and separation efficiency in successive multiple ionic layer coatings for microchip electrophoresis

The effect of successive multiple ionic layer (SMIL) coatings on the velocity and direction of EOF and the separation efficiency for PDMS electrophoresis microchips was studied using different polymer structures and deposition conditions. To date, the majority of SMIL studies have used traditional CE and fused-silica capillaries. EOF was measured as a function of polymer structure and number of layers, in one case using the same anionic polymer and varying the cationic polymer and in the second case using the same cationic polymer and varying the anionic polymer. In both situations, the EOF direction reversed with each additional deposited polymer layer. The absolute EOF magnitude, however, did not vary significantly with layer number or polymer structure. Next, different coatings were used to compare separation efficiencies on native and SMIL-coated PDMS microchips. For native PDMS microchips, the average separation efficiency was 4105 +/- 1540 theoretical plates. The addition of two layers of polymer increased the separation efficiency anywhere from two- to five-fold, depending on the polymer structure. A maximum separation efficiency of 12 880 +/- 1050 theoretical plates was achieved for SMIL coatings of polybrene (cationic) and dextran sulfate (anionic) polymers after deposition of six total layers. It was also noted that coating improved run-to-run consistency of the peaks as noted by a reduction of the RSD of the EOF and separation efficiency. This study shows that the use of polyelectrolyte coatings, irrespective of the polymer structure, generates a consistent EOF in the current experiments and dramatically improves the separation efficiency when compared to unmodified PDMS microchips.     

Fabrication of PDMS/glass microchips by twofold replication of PDMS and its application in genetic analysis

In this paper, we describe a simple method for fabrication of high quality poly(dimethylsiloxane) (PDMS)/glass microchip by twofold replica molding of PDMS. This technique first served to transfer the negative microchannels from the glass template to the PDMS substrate as a master, and then this PDMS master with positive microchannels was used to replicate the PDMS replica with negative microchannels. Finally, the PDMS replica was bound to a glass sheet by UV radiation. The fabricated microchips were successfully applied for the detection of C677T mutation from the human methylenetetrahydrofolate reductase gene.     

Surface modification of poly(dimethylsiloxane) microfluidic devices and its application in simultaneous analysis of uric acid and ascorbic acid in human urine

A novel and simple method based on layer-by-layer (LBL) technique has been developed for the modification of the channel in PDMS electrophoresis microchip to create a hydrophilic surface with a stable EOF. The functional surface was obtained by sequentially immobilizing chitosan and deoxyribonucleic acid (DNA) onto the microfluidic channel surface using the LBL assembly technique. Compared to the native PDMS microchips, the contact angle of the chitosan-DNA modified PDMS microchips decreased and the EOF increased. Experimental conditions were optimized in detail. The chitosan-DNA modified PDMS microchips exhibited good reproducibility and long-term stability. Separation of uric acid (UA) and ascorbic acid (AA) performed on the modified PDMS microchip generated 43,450 and 46,790 N/m theoretical plates compared with 4048 and 19,847 N/m with the native PDMS microchip. In addition, this method has been successfully applied to real human urine samples, without SPE, with recoveries of 97-105% for UA and AA.     

In-channel atom-transfer radical polymerization of thermoset polyester microfluidic devices for bioanalytical applications

A new technique for polymer microchannel surface modification, called in-channel atom-transfer radical polymerization, has been developed and applied in the surface derivatization of thermoset polyester (TPE) microdevices with poly(ethylene glycol) (PEG). X-ray photoelectron spectroscopy, electroosmotic flow (EOF), and contact angle measurements indicate that PEG has been grafted on the TPE surface. Moreover, PEG-modified microchannels have much lower and more pH-stable EOF, more hydrophilic surfaces and reduced nonspecific protein adsorption. Capillary electrophoresis separation of amino acid and peptide mixtures in these PEG-modified TPE microchips had good reproducibility. Phosducin-like protein and phosphorylated phosducin-like protein were also separated to measure the phosphorylation efficiency. Our results indicate that PEG-grafted TPE microchips have broad potential application in biomolecular analysis.     

Covalent modified hydrophilic polymer brushes onto poly(dimethylsiloxane) microchannel surface for electrophoresis separation of amino acids

A new environmentally friendly method is developed for preventing nonspecific biomolecules from adsorption on poly(dimethylsiloxane) (PDMS) surface via in situ covalent modification. o-[(N-Succinimdyl)succiny]-o'-methyl-poly(ethylene glycol) (NSS-mPEG) was covalently grafted onto PDMS microchannel surface that was pretreated by air-plasma and silanized with 3-aminopropyl-triethoxysilanes (APTES). The modification processes were carried out in aqueous solution without any organic solvent. The mPEG side chains displayed extended structure and created a nonionic hydrophilic polymer brushes layer on PDMS surface, which can effectively prevent the adsorption of biomolecules. The developed method had improved reproducibility of separation and stability of electroosmotic flow (EOF), enhanced hydrophilicity of surface and peak resolution, and decreased adsorption of biomolecules. EOF in the modified microchannel was strongly suppressed, compared with those in the native and silanized PDMS microchips. Seven amino acids have been efficiently separated and successfully detected on the coated PDMS microchip coupled with end-channel amperometric detection. Relative standard deviations (RSDs) of their migration time for run-to-run, day-to-day and chip-to-chip, were all below 2.3%. Moreover, the covalent-modified PDMS channels displayed long-term stability for 4 weeks. This novel coating strategy showed promising application in biomolecules separation.     

Poly（dimethylsiloxane） Microchips with Two Sharpened Stretching Tips and Its Application to Protein Separation Using Dynamic Coating

An integrated poly（dirnethylsiloxane） （PDMS） microchip with two sharpened stretching tips for convenient sample injecting, running buffer refreshing and channel cleaning has been presented. The sample was directly introduced into the separation channel through the stretching inlet tip without complicated power switching supplies and injection cross channel. The operation of running buffer refreshing or channel cleaning was simplified by vacuuming one end of the tip and placing the other tip into the solution vial. Therefore, this fabrication method can be easily applied to most analytical laboratories economically without soft lithography and plasma bonding equipments. The attractive performance of the novel PDMS microchips has been demonstrated by using laser-induced fluorescence detection for separation of proteins. The addition of 0.04% Brij 35 in 0.04 mol/L phosphate buffer （pH 7.0） can reduce the adhesion of proteins in multienzyme tablet and make separation more easily. The electroosmotic flow （EOF） exhibits pH-independence in the range of 3-1 1 in dynamic modified microchannel.     

Thermoset polyester droplet-based microfluidic devices for high frequency generation

The vast majority of droplet-based microfluidic devices are made from polydimethylsiloxane (PDMS). Unfortunately PDMS is not suitable for high frequency droplet generation at high operating pressure due to its low shear modulus. In this paper, we report the fabrication and testing of microfluidic devices using thermoset polyester (TPE). The optical characteristics of the fabricated devices were assessed and substrate resistance to pressure also investigated. TPE devices bonded using an O(2) plasma treated PET substrate at 76 °C were shown to function efficiently at pressures up to 18 MPa. TPE material retains many of the attractive features of PDMS such as ease of fabrication but significantly, has superior mechanical properties. The improved resistance of TPE to high pressures enabled investigation of high frequency droplet generation as a function of a wide range of flow-rates with three different oils as continuous phase.     

Study on the separation of amino acids in modified poly(dimethylsiloxane) microchips

A mixture of five amino acids including arginine, histidine, phenylalanine, serine and glutamic acid was successfully separated in microchip capillary electrophoresis and detected with laser-induced fluorescence (LIF) detector. These amino acids were labeled with 5-(4, 6-dichloro-s-triazin-2-ylamino) fluorescein (DTAF). The analyses were performed on two kinds of modified poly(dimethylsiloxane) (PDMS) microchips. One kind of chip was simply treated with oxygen plasma (OP-chip), and the other was further modified by coating double layers of non-ionic polymer poly(vinyl alcohol) (PVA) after plasma oxidization (PVA-chip). The derivatization condition of amino acids by DTAF was optimized. The properties of the two modified PDMS microchips were studied and separation conditions, such as the buffer pH, buffer concentration and separation voltage, were also optimized. The column efficiencies of the two microchips were in the range of 193,000–1,370,000 plates/m. The DTAF-labeled amino acids were sufficiently separated within 50 s and 90 s in 2.5 cm channels on OP-chip and PVA-chip, respectively.     

Hot embossing of electrophoresis microchannels in PMMA substrates using electric heating wires

A simple method based on electric heating wires has been developed for the rapid fabrication of poly(methyl methacrylate) (PMMA) electrophoresis microchips in ordinary laboratories without the need for microfabrication facilities. A piece of stretched electric heating wire placed across the length of a PMMA plate along its midline was sandwiched between two microscope slides under pressure. Subsequently, alternating current was allowed to pass through the wire to generate heat to emboss a separation microchannel on the PMMA separation channel plate at room temperature. The injection channel was fabricated using the same procedure on a PMMA sheet that was perpendicular to the separation channel. The complete microchip was obtained by bonding the separation channel plate to the injection channel sheet, sealing the channels inside. The electric heating wires used in this work not only generated heat; they also served as templates for embossing the microchannels. The prepared microfluidic microchips have been successfully employed in the electrophoresis separation and detection of ions in connection with contactless conductivity detection.     

Surface modification with BSA blocking based on in situ synthesized gold nanoparticles in poly(dimethylsiloxane) microchip

A stable BSA blocking poly(dimethylsiloxane) (PDMS) microchannel was prepared based on in situ synthesized PDMS–gold nanoparticles composite films. The modified microchip could successfully suppress protein adsorption. The assembly was followed by contact angle, charge-coupled device (CCD) imaging, electroosmotic flow (EOF) measurements and electrophoretic separation methods. Contact angle measurements revealed the coated surface was hydrophilic, water contact angle for coated chips was 45.2° compared to a water contact angle for native PDMS chips of 88.5°. The coated microchips exhibited reproducible and stable EOF behavior. With FITC-labeled myoglobin incubation in the coated channel, no fluorescence was observed with CCD image, and the protein exhibited good electrophoretic effect in the modified microchip.     

Modification of the glass surface property in PDMS-glass hybrid microfluidic devices

This paper presents a simple method to change the hydrophilic nature of the glass surface in a poly(dimethylsiloxane) (PDMS)-glass hybrid microfluidic device to hydrophobic by an extra-heating step during the fabrication process. Glass substrates bonded to a native or oxygen plasma-treated PDMS chip having microchambers (12.5 mm diameter, 110 μm height) were heated at 200°C for 3 h, and then the hydrophobicity of the glass surfaces on the substrate was evaluated by measuring the contact angle of water. By the extra-heating process, the glass surfaces became hydrophobic, and its contact angle was around 109°, which is nearly the same as native PDMS surfaces. To demonstrate the usefulness of this surface modification method, a PDMS-glass hybrid microfluidic device equipped with microcapillary vent structures for pneumatic manipulation of droplets was fabricated. The feasibility of the microcapillary vent structures on the device with the hydrophobic glass surfaces are confirmed in practical use through leakage tests of the vent structures and liquid handling for the electrophoretic separation of DNA molecules.     

Improved separation efficiency of neurotransmitters on a native printed capillary electrophoresis microchip simply by manipulating electroosmotic flow

Separation and determination of dopamine and epinephrine with end-channel electrochemical (EC) detection integrated on a native printed microchip capillary electrophoresis (CE) system was investigated. Factors influencing the separation and detection were investigated and optimized. Results show manipulating EOF, which can be easily achieved by adjusting buffer pH, is a simple and effective way to achieve the baseline separation of dopamine and epinephrine in native polymeric microchips. Without surface modification of microchannel, printed microchips with advantages of low cost and easy preparation can achieve high performance like other microfluidic devices.     

A new soft lithographic route for the facile fabrication of hydrophilic sandwich microchips

Manufacturing materials are an essential element for the fabrication of microfluidic chips. PDMS, the most widely used polymeric material, is associated with apparent disadvantages such as hydrophobic nature, while other materials also suffer from some limitations. In this paper, a new soft lithographic route was proposed for the facile manufacturing of hydrophilic sandwich microchips, using bisphenol A based epoxy acrylate (BABEA) as a new patterning material. The BABEA copolymers are hydrophilic, highly transparent in visible range while highly untransparent when the wavelength is less than 290 nm, and of high replication fidelity. By combining with appropriate monomers, including glycidyl methacrylate, methylmethacrylate, and acrylic acid, the copolymers contain active functional groups, which allows for easy postmodification for desirable functional units. A fabrication procedure was proposed for manufacturing hybrid quartz/BABEA copolymer/quartz microchips. In the procedure, no micromachining equipments, wet etching, or imprinting techniques were involved, making the fabrication approach applicable in ordinary chemistry laboratories. The performance of the prepared microchips was demonstrated in terms of CIEF with UV-whole channel imaging detection. The hydrophilic microchannel ensures stable focusing while the polymeric middle layer acts as a perfectly aligned optical slit for whole channel UV absorbance detection.     

A rigid poly(dimethylsiloxane) sandwich electrophoresis microchip based on thin-casting method

We present a novel concept of glass/poly(dimethylsiloxane) (PDMS)/glass sandwich microchip and developed a thin-casting method for fabrication. Unlike the previously reported casting method for fabricating PDMS microchip, several drops of PDMS prepolymer were first added on the silanizing SU-8 master, then another glass plate was placed over the prepolymer as a cover plate, and formed a glass plate/PDMS prepolymer/SU-8 master sandwich mode. In order to form a thin PDMS membrane, a weight was placed on the glass plate. After the whole sandwich mode was cured at 80 degrees C for 30 min, the SU-8 master was easily peeled and the master microstructures were completely transferred to the PDMS membrane which was tightly stuck to the glass plate. The microchip was subsequently assembled by reversible sealing with the glass cover plate. We found that this PDMS sandwich microchip using the thin-casting method could withstand internal pressures of >150 kPa, more than 5 times higher than that of the PDMS hybrid microchip with reversible sealing. In addition, it shows an excellent heat-dissipating property and provides a user-friendly rigid interface just like a glass microchip, which facilitates manipulation of the microchip and fix tubing. As an application, PDMS sandwich microchips were tested in the capillary electrophoresis separation of fluorescein isothiocyanate-labeled amino acids.