Fabrication of microchip electrophoresis devices and effects of channel surface properties on separation efficiency
Introduction
Microchip electrophoresis is a promising analytical technique that has generated a great deal of interest [1], [2]. The advantages of microchip electrophoresis include: rapid analysis; miniscule consumption of the sample; the possibility of electrokinetic control of the fluids; the use of high separation field strength; and simple coupling of various channels for sample concentration, mixing, dilution, reaction, etc. [3], [4], [5]. Various separation modes and sample manipulation methods have been developed for microchip electrophoresis, increasing its usefulness [6]. Moreover, microchip electrophoresis devices can be fabricated using a variety of materials, such as quartz [7], glass [8], and plastics [9], [10], [11], [12]. Therefore, diverse and complicated designs for microchip electrophoresis devices can be constructed easily by selecting a suitable material.
In general, the device for microchip electrophoresis is fabricated in two steps. First, a substrate with the network of microchannels is constructed using photolithography and etching techniques in a glass or quartz substrate. Depending on the chip material, however, diverse fabrication techniques can be employed including molding [9], hot embossing [10], and machining [13]. Second, the other substrate is fabricated to define reservoirs. Then two substrates are then aligned and bonded to create a closed network of microchannels [14], [15]. In particular, the bonding process is very important in integrating the varied components. Various bonding methods, such as anodic bonding, silicon fusion bonding, and thermal bonding, have been developed [16]. The thermal bonding method is used often in quartz and glass chips, although the channel deformations are possible due to the high temperature required. Anodic bonding uses electrostatic attraction to attach a glass substrate to a silicon substrate and forms covalent bonds between the two substrates [16]. Compared with thermal bonding, anodic bonding has the advantage of using a lower temperature with lower residual stress and less stringent requirements for the surface quality of the substrates [16]. The use of anodic bonding is limited, however, because it is applicable only to glass and silicon substrates.
In microchip electrophoresis, a buffer solution and analytes usually move through a capillary under the influence of an electric field. This phenomenon is termed electroosmotic flow (EOF). Because the electric double layer on the channel surface is very thin, EOF is considered to be a wall-driven phenomenon. Consequently, the separation efficiency depends heavily upon the characteristics of substrate material or channel surface in microchip electrophoresis [17], [18], [19]. For example, the band of a sample will be dispersed if surface charges on a microchannel wall are nonuniform. Increased dispersion will degrade the separation performance by reducing the efficiencies and resolution of close eluted peaks. The chip material also affects the ability to control the movement of discrete samples within a microchannel [17]. Therefore, it is essential to understand the effects of chip material and fabrication methods on the performance of a microchip.
In this paper, we investigate the effects of microchip material on electrophoretic separation efficiency in microchip electrophoresis. We fabricated microfluidic devices for microchip electrophoresis using various substrates, quartz, glass, polydimethylsiloxane (PDMS), and polymethylmethacrylate (PMMA). We calculated the separation efficiency from the migration time and bandwidth of EOF and the analytes in each microchip. In addition, we studied band broadening effects by comparing the bandwidth of the analytes in the hybrid microchips composed of PDMS/glass and glass/silicon dioxide membrane.
Section snippets
Chemicals
2,7-Dichlorofluorescein (DCF) was purchased from Merck (Darmstadt, Germany) and sodium fluorescein from Junsei Chemicals (Tokyo, Japan). All reagents were of analytical grade and were used without further purification. The 1 mM stock solution of fluorescein and DCF was prepared in a 10% (v/v) aqueous ethanol solution. A few drops of 0.1 M NaOH were added when the solution was diluted to the desired concentration. Deionized water was obtained from a NANOpure® purification system (Barnstead,
Results and discussion
The channel profile in the microchip is an important parameter influencing the performance of the microchip electrophoresis. Depending on the fabrication method, different channel shapes are obtained. First, the cross-sections of a microchannel in a quartz and glass microchip were compared using a scanning electron microscope (SEM). Because the same etching method was used, the profile of both channels looks the same as that shown in Fig. 6. A trapezoidal shape instead of a rectangular shape
Conclusions
A comparison of silica and polymer microfluidic devices is useful to researchers of microchip electrophoresis. We fabricated various microchips composed of a single material, such as quartz, glass, PDMS, and PMMA, as well as hybrid microchannels composed of different materials. The bonding technique for each microchip is described in detail. In particular, anodic bonding was very convenient because it is a lower-temperature process and requires less stringent quality of substrate surface than
Acknowledgement
This work was supported by a grant of the International Mobile Telecommunications 2000 R&D Project (Ministry of Information and Communication).
Min-Su Kim received his BS and MS degrees at the School of Electrical Engineering in Seoul National University in 2001 and 2003, respectively. He is a PhD student studying BIO-MEMS and micro-fluidics in the Lab. for Micro-Sensors and Actuators. He is currently working on the design and fabrication of microchip electrophoresis devices and CE-ESI microchip.
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Min-Su Kim received his BS and MS degrees at the School of Electrical Engineering in Seoul National University in 2001 and 2003, respectively. He is a PhD student studying BIO-MEMS and micro-fluidics in the Lab. for Micro-Sensors and Actuators. He is currently working on the design and fabrication of microchip electrophoresis devices and CE-ESI microchip.
Seung Il Cho received his BS (1992), MS (1994), and PhD (2001) degrees in chemistry at Seoul National University. He worked as a visiting scholar in Brigham and Women's Hospital, affiliate of Harvard Medical School (1998–1999). He has been a postdoctoral research associate in the School of Electrical Engineering and Computer Science at Seoul National University (2002–2003). He is currently a postdoctoral research associate in the Department of Chemistry at University of Maryland. His current research interests include electrochemistry, chromatography, and nanotechnology.
Kook-Nyung Lee is postdoctoral researcher at Nano Bioelectronics and Systems Research Center in Seoul National University. He received his BS, MS and PhD degrees at the School of Electrical Engineering and Computer Science of Seoul National University in 1998, 2000 and 2003, respectively. Since 1998, he has been working on the design and fabrication of micro-optical devices for biochip application. He is currently studying on the modeling, design, fabrication and testing of optical MEMS devices, especially for the miniaturizaton of fluorescence detection system for biochip.
Yong-Kweon Kim received his BS and MS degrees in electrical engineering from Seoul National University in 1983 and 1985, respectively, and Dr. Eng. degree from the University of Tokyo in 1990. His doctoral dissertation was on modeling, design, fabrication and testing of micro-linear actuators in magnetic levitation using high critical temperature superconductors. In 1990, he joined the Central Research Laboratory of Hitachi Ltd. in Tokyo as a researcher and worked on actuators of hard disk drives. In 1992, he joined Seoul National University, where he is currently a Professor in the School of Electrical Engineering. His current research interests are modeling, design, fabrication and testing of electric machines, especially micro-electro-mechanical systems, micro-sensors and actuators.