Fabrication of microchip electrophoresis devices and effects of channel surface properties on separation efficiency

https://doi.org/10.1016/j.snb.2004.12.069Get rights and content

Abstract

This paper describes the influence of chip material on the separation efficiency in microchip electrophoresis using quartz, glass, polydimethylsiloxane (PDMS), polymethylmethacrylate (PMMA), and glass hybrid as the microchip material. The fabrication processes for each microchip are described in detail. We introduce a new fabrication method for a glass chip using amorphous silicon on the glass surface as an etch mask and a bonding interface for anodic bonding. The microchannel structures in each microchip were compared using a scanning electron microscope or profilometer. We investigated the mobilities and bandwidths of electroosmotic flow (EOF) and analytes in microchip electrophoresis. Significant band broadening was observed in a microchip composed of a hybrid material (glass and silicon dioxide membrane) compared with a microchip of single material due to the nonuniformity of the surface charge density.

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.

References (27)

  • A. Manz et al.

    Miniaturized total chemical analysis systems: a novel concept for chemical sensing

    Sens. Actuators B

    (1990)
  • N. Bao et al.

    Electrochemical detector for microchip electrophoresis of poly(dimethylsiloxane) with a three-dimensional adjustor

    J. Chromatogr. A

    (2004)
  • S.C. Jacobson et al.

    Microchip structures for submillisecond electrophoresis

    Anal. Chem.

    (1998)
  • N.A. Lacher et al.

    Microchip capillary electrophoresis/electrochemistry

    Electrophoresis

    (2001)
  • S. Shoji

    Micro-total analysis system (μTAS)

    Electron. Commun. JPN 2

    (1999)
  • R.S. Martin et al.

    Dual-electrode electrochemical detection for poly(dimethylsiloxane)-fabricated capillary electrophoresis microchips

    Anal. Chem.

    (2000)
  • V. Dolnik et al.

    Capillary electrophoresis on microchip

    Electrophoresis

    (2000)
  • S.C. Jacobson et al.

    Fused quartz substrates for microchip electrophoresis

    Anal. Chem.

    (1995)
  • Q.-S. Pu et al.

    Comparison of capillary zone electrophoresis performance of powder-blasted and hydrogen fluoride-etched microchannels in glass

    Electrophoresis

    (2003)
  • Y.S. Shin et al.

    PDMS-based micro-PCR chip with Parylene coating

    J. Micromech. Microeng.

    (2003)
  • J. Kameoka et al.

    A polymeric microfluidic chip for CE/MS determination of small molecules

    Anal. Chem.

    (2001)
  • R.-H. Horng et al.

    PMMA-based capillary electrophoresis electrochemical detection microchip fabrication

    J. Micromech. Microeng.

    (2005)
  • S. Lai et al.

    Design of a compact disk-like microfluidic platform for enzyme-linked immunosorbent assay

    Anal. Chem.

    (2004)
  • Cited by (36)

    • Single-step electrohydrodynamic separation of 1–150 kbp in less than 5 min using homogeneous glass/adhesive/glass microchips

      2020, Talanta
      Citation Excerpt :

      In contrast to capillaries, which present homogeneous surfaces, microchip devices are in general heterogeneous in composition, for instance with polymer/glass or silicon/glass walls. Yet, some reports have shown the degradation of separation performances in microchips made out of hybrid materials [16]. We therefore set out to perform our experiments in conventional silicon-glass microchips as well as in specifically developed microfluidic devices with homogeneous surfaces.

    • Microfluidic platform for cell analysis using through-polydimethylsiloxane micro-tip electrode array

      2019, Microelectronic Engineering
      Citation Excerpt :

      In the case of an electrode chemical application, the sensing part consists of microelectrodes. When a microelectrode is located inside the microfluidic channel, the target material injected into the channel can be sorted and analyzed using electrophoresis and dielectric electrophoresis [14–17]. However, these in-plane electrodes present some problems.

    • A simple method using two-step hot embossing technique with shrinking for fabrication of cross microchannels on PMMA substrate and its application to electrophoretic separation of amino acids in functional drinks

      2016, Talanta
      Citation Excerpt :

      Micrometer sized fluidic channels have been employed to perform electrophoretic separation in various applications for biochemical, pharmaceutical, clinical and environmental fields. Although there are various platforms to produce microchannels on different materials for electrophoretic separation, the typical configuration to perform electrophoretic separation on microfluidics devices is only a simple cross-shape [1–3]. There are also many fabrication technologies for producing microfluidic devices for electrophoretic separations of chemical and biochemical substances in bioanalytical applications.

    • A polydimethylsiloxane electrophoresis microchip with a thickness controllable insulating layer for capacitatively coupled contactless conductivity detection

      2012, Electrochemistry Communications
      Citation Excerpt :

      The thickness of PDMS films is easily controlled down to submicrometers by diluting PDMS with toluene. Moreover, the homogeneous microchannel is desirable for electrophoresis separations [13]. The PDMS electrophoresis microchip consists of a glass slide (76 × 38 × 1 mm) with microelectrodes, a PDMS film as the insulating layer and a PDMS channel plate (76 × 25 × 2 mm) (Fig. 1).

    View all citing articles on Scopus

    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.

    View full text