A Chip-Based Electrophoresis System with Electrochemical Detection and Hydrodynamic Injection

Abstract

A simple capillary electrophoresis system in the planar format that uses a new, hydrodynamic injection principle is described. The system was realized with poly(dimethylsiloxane)-glass chips and microdisk electrodes for amperometric detection. Using a double-tee injector, no precise voltage control of the electrolyte reservoirs was needed, thus making the microchip CE system more user-friendly. The analytical characteristics of chip-based CE-EC were evaluated using ascorbic acid as the model analyte. The reproducibility of migration time and signal height was expressed by relative standard deviations of 1.2% and 5.1%, respectively (n = 5). The limit of detection for ascorbic acid was ~5 μM at a signal-to-noise ratio of 3. Practical application concerning the determination of physiologically relevant compounds such as noradrena-line and L-dopa are discussed.In recent years, there has been a significant interest in miniaturized analytical systems. The downscale in electrophoresis results in a decrease in analysis time and a potential increase in efficiency while the instrument becomes more compact due to integration of essential parts onto a single substrate.1,2 Another attractive feature is the possibility of parallel analysis using multiple separation systems residing on one chip. Capillary electrophoresis (CE) in the planar format has been applied to a variety of analytes, e.g., DNA,3,4 neurotransmitters,5,6 explosives,7 and the bioassay of clinically relevant compounds.8 Fabrication of microfluidic devices in poly(dimethylsiloxane) (PDMS) by soft lithography provides a fast, inexpensive route to devices that can handle aqueous solutions.9 These soft lithographic methods are based on rapid prototyping and replica molding and are easily accessible to chemists and biologists working under benchtop conditions. Crucial in chip-based CE is the reproducible introduction of well-defined sample zones in the separation channel and the detection of narrow bands. While detection schemes, such as electrochemical detection (EC) or laser-induced fluorescence (LIF), can be adapted from conventional CE, sample introduction requires a completely new approach.In microchip CE, injection is usually performed by electro-kinetic methods. The introduction of a well-defined sample plug is achieved through the channel network, i.e., a sample-guiding channel that intersects the separation channel. The first CE chips employed a tee-injector design.10 Due to difficulties in the control of the sample plug, other injector layouts were developed. Improved control of the sample plug can be gained by using a cross11 or double-tee injector,12 which works well for the analysis of DNA using polymer sieving media.3,4 For injection, an electric field is applied across the sample reservoir and the sample waste reservoir. The resulting analyte stream is perpendicular to the separation channel. In the separation phase, an electric field is applied across the separation channel. The sample plug, residing in the channel intersection, is transpor...

Soft lithography for micro- and nanoscale patterning

“…When combined with selective etching and lift-off, these nanostructures have been transferred into metals and then used as substrates for biosensing. The methods of detection have included optical diffraction 38 , surface plasmon resonance 39 , surface-enhanced Raman scattering 40 and electrochemistry 41 .…”

Collection, focusing, and metering of DNA in microchannels using addressable electrode arrays for portable low-power bioanalysis

“…Stacking at the interface between the sample and a background buffer has also been performed in T-injector geometries (13)(14)(15)(16)(17). Other techniques investigated include employing bulk flow (18), capillary (19), diffusion-based (20), pressure-driven (21), hydrodynamic (22), and dielectrophoretic trapping (23)(24)(25) processes. The use of nanocapillary array interconnects (26), as well as designs exploiting opposing electrophoretic and electroosmotic processes in the vicinity of a microchannel-nanochannel intersection (27,28), have also been explored to achieve concentration of DNA and proteins.…”

Study of injection bias in a simple hydrodynamic injection in microchip CE

“…The process was also monitored using 50-μM FITC in separation buffer in the SR as shown in Figure 6. SR and SW can essentially be exchanged [16]. Considering the convenience of the gated process, the reservoir assignment is as shown Figure 6a.…”

“…To solve the sample injection problems associated with the electrokinetic injection modes, researchers have focused on hydrodynamic injection approaches widely applied in traditional CE, and several schemes have been developed for μCE [6,[15][16][17][18]. Among these, the injection driving force includes external pressure or hydrostatic force, and the flow control in microchip networks is carried out by valveless gated flow or a valve integrated on the microchip.…”

“…Lin et al [6] used a syringe pump to push the sample flow into the intersection of a single-T microchip and used the voltage-gated scheme to prevent the sample solution from entering the separation channel except during the sample injection step. Backofen et al [16] used hydrostatic pressure by adding a little more sample solution into the sample reservoir (SR) relative to the buffer reservoir (BR) and the sample waste reservoir (SW) on a double-T microchip. They also used gated flow to push back the sample solution and prevent its entry into the separation channel.…”