Integration of continuous-flow sampling with microchip electrophoresis using poly(dimethylsiloxane)-based valves in a reversibly sealed device

Reduction in sample injection bias using pressure gradients generated on chip

Sample injection in microchip-based capillary zone electrophoresis (CZE) frequently rely on the use of electric fields which can introduce differences in the injected volume for the various analytes depending on their electrophoretic mobilities and molecular diffusivities. While such injection biases may be minimized by employing hydrodynamic flows during the injection process, this approach typically requires excellent dynamic control over the pressure gradients applied within a microfluidic network. The current article describes a microchip device that offers this needed control by generating pressure gradients on-chip via electrokinetic means to minimize the dead volume in the system. In order to realize the desired pressure-generation capability, an electric field was applied across two channel segments of different depths to produce a mismatch in the electroosmotic flow rate at their junction. The resulting pressure-driven flow was then utilized to introduce sample zones into a CZE channel with minimal injection bias. The reported injection strategy allowed the introduction of narrow sample plugs with spatial standard deviations down to about 45 μm. This injection technique was later integrated to a capillary zone electrophoresis process for analyzing amino acid samples yielding separation resolutions of about 4-6 for the analyte peaks in a 3 cm long analysis channel.

Integrating 3D Cell Culture of PC12 Cells with Microchip-Based Electrochemical Detection

Developing in vitro cell culture models that accurately mimic in vivo processes in a manner that also enables near real-time analysis of neurotransmitters is an important research area. New technologies being developed such as 3D scaffolds for cell culture and 3D printed microfluidics provide an opportunity for such advancements. In this work, PC12 cells were used as a model system and they were immobilized onto a 3D scaffold of polystyrene (PS) fibers. These fibers were created by electrospinning onto PS sheets, which were laser cut and, after cell seeding, inserted into a 3D printed microfluidic device. The 3D printed device was designed with threads for connecting commercial fittings (to integrate automated pumps and a 4-port injection system) and a steel pin for simple coupling with PDMS/polystyrene analytical devices. A straight PDMS channel was used for simple (and continuous) flow-based detection by sealing onto a PS base containing an embedded gold array working electrode and a platinum pseudo-reference. Electrochemical detection of stimulated catecholamine release was demonstrated. The insert-based system was then integrated with a bilayer valving PDMS device (for microchip electrophoresis) sealed onto a PS base (with electrodes for electrochemical detection). This base was embedded with a Pd decoupler (for grounding the separation voltage and adsorbing hydrogen) and a 33 µm carbon fiber working electrode for in-channel detection. PC12 cells were stimulated in the 3D cell culture device, and the valving/electrophoresis microchip was able to separate and detect dopamine and norepinephrine release. This work demonstrates the ability to integrate 3D cell scaffolds with microchip-based analysis for detection of multiple analytes released from cells.

Instrumentation design for hydrodynamic sample injection in microchip electrophoresis: a review

Reproducible and representative sample injection in microchip electrophoresis has been a bottleneck for quantitative analytical applications. Electrokinetic sample injection is the most used because it is easy to perform. However, this injection method is usually affected by sample composition and the bias effect. On the other hand, these drawbacks are overcome by the hydrodynamic (HD) sample injection, although this injection mode requires HD flow control. This review gives an overview of the basic principles, the instrumentation designs, and the performance of HD sample injection systems for microchip electrophoresis.

Integration of microchip electrophoresis with electrochemical detection using an epoxy-based molding method to embed multiple electrode materials

This paper describes the use of epoxy-encapsulated electrodes to integrate microchip-based electrophoresis with electrochemical detection. Devices with various electrode combinations can easily be developed. This includes a palladium decoupler with a downstream working electrode material of either gold, mercury/gold, platinum, glassy carbon, or a carbon fiber bundle. Additional device components such as the platinum wires for the electrophoresis separation and the counter electrode for detection can also be integrated into the epoxy base. The effect of the decoupler configuration was studied in terms of the separation performance, detector noise, and the ability to analyze samples of a high ionic strength. The ability of both glassy carbon and carbon fiber bundle electrodes to analyze a complex mixture was demonstrated. It was also shown that a PDMS-based valving microchip can be used along with the epoxy-embedded electrodes to integrate microdialysis sampling with microchip electrophoresis and electrochemical detection, with the microdialysis tubing also being embedded in the epoxy substrate. This approach enables one to vary the detection electrode material as desired in a manner where the electrodes can be polished and modified as is done with electrochemical flow cells used in liquid chromatography.

Electrochemical Oscillations of Nickel Electrodissolution in an Epoxy-Based Microchip Flow Cell

We investigate the nonlinear dynamics of transpassive electrodissolution of nickel in sulfuric acid in an epoxy-based microchip flow cell. We observed bistability, smooth, relaxation, and period-2 waveform current oscillations with external resistance attached to the electrode in the microfabricated electrochemical cell with 0.05 mm diameter Ni wire under potentiostatic control. Experiments with 1mm × 0.1 mm Ni electrode show spontaneous oscillations without attached external resistance; similar surface area electrode in macrocell does not exhibit spontaneous oscillations. Combined experimental and numerical studies show that spontaneous oscillation with the on-chip fabricated electrochemical cell occurs because of the unusually large ohmic potential drop due to the constrained current in the narrow flow channel. This large IR potential drop is expected to have an important role in destabilizing negative differential resistance electrochemical (e.g., metal dissolution and electrocatalytic) systems in on-chip integrated microfludic flow cells. The proposed experimental setup can be extendend to multi-electrode configurations; the epoxy-based substrate procedure thus holds promise in electroanalytical applications that require collector-generator multi-electrodes wires with various electrode sizes, compositions, and spacings as well as controlled flow conditions.

Integration of on-chip peristaltic pumps and injection valves with microchip electrophoresis and electrochemical detection

A microfluidic approach that integrates peristaltic pumping from an on-chip reservoir with injection valves, microchip electrophoresis and electrochemical detection is described. Fabrication and operation of both the peristaltic pumps and injection valves were optimized to ensure efficient pumping and discrete injections. The final device uses the peristaltic pumps to continuously direct sample from a reservoir containing a mixture of analytes to injection valves that are coupled with microchip electrophoresis and amperometric detection. The separation and direct detection of dopamine and norepinephrine were possible with this approach and the utility of the device was demonstrated by monitoring the stimulated release of these neurotransmitters from a layer of cells introduced into the microchip. It is also shown that this pumping/reservoir approach can be expanded to multiple reservoirs and pumps, where one reservoir can be addressed individually or multiple reservoirs sampled simultaneously.

Use of a Carbon-ink Microelectrode Array for Signal Enhancement in Microchip Electrophoresis with Electrochemical Detection

In this communication, we demonstrate that a carbon ink microelectrode array, where the electrodes are held at the same potential, affords significant signal enhancement in microchip electrophoresis with amperometric detection. The ability to fabricate an array of carbon ink microelectrodes with a palladium decoupler was demonstrated and the resulting electrodes were integrated with a valving microchip design. The use of an 8 electrode array led to a significant improvement in the limits of detection at the expense of separation resolution due to the increased detection zone size. It is also shown that microdialysis sampling can be integrated with the microchip device and a multi-analyte separation achieved.

Integration of serpentine channels for microchip electrophoresis with a palladium decoupler and electrochemical detection

Although it has been shown that microchip electrophoresis (MCE) with electrochemical detection can be used to separate and detect electroactive species, there is a need to increase the separation performance of these devices so that complex mixtures can be routinely analyzed. Previous work in the MCE has demonstrated that increasing the separation channel length leads to an increase in resolution between closely eluting analytes. This paper details the use of lengthened serpentine microchannels for MCE and electrochemical detection where a palladium decoupler is used to ground the separation voltage so that the working electrodes remain in the fluidic network. In this work, palladium electrodepositions were used to increase the decoupler surface area and more efficiently dissipate hydrogen produced at the decoupler. Dopamine and norepinephrine, which only differ in structure by a hydroxyl group, were used as model analytes. It was found that increasing the separation channel length led to improvements in both the resolution and the number of theoretical plates for these analytes. The use of a bilayer valving device, where PDMS-based valves are utilized for the injection process, along with serpentine microchannels and amperometric detection resulted in a multianalyte separation and an average of 28 700 theoretical plates. It was also shown that the increased channel length is beneficial when separating and detecting analytes from a high ionic strength matrix. This was demonstrated by monitoring the stimulated release of neurotransmitters from a confluent layer of PC 12 cells.

Synchronized Current Oscillations of Formic Acid Electro-oxidation in a Microchip-based Dual-Electrode Flow Cell

We investigate the oscillatory electro-oxidation of formic acid on platinum in a microchip-based dual-electrode cell with microfluidic flow control. The main dynamical features of current oscillations on single Pt electrode that had been observed in macro-cells are reproduced in the microfabricated electrochemical cell. In dual-electrode configuration nearly in-phase synchronized current oscillations occur when the reference/counter electrodes are placed far away from the microelectrodes. The synchronization disappears with close reference/counter electrode placements. We show that the cause for synchronization is weak albeit important, bidirectional electrical coupling between the electrodes; therefore the unidirectional mass transfer interactions are negligible. The experimental design enables the investigation of the dynamical behavior in micro-electrode arrays with well-defined control of flow of the electrolyte in a manner where the size and spacing of the electrodes can be easily varied.

PMMA/PDMS valves and pumps for disposable microfluidics

Poly(methyl methacrylate) (PMMA) is gaining in popularity in microfluidic devices because of its low cost, excellent optical transparency, attractive mechanical/chemical properties, and simple fabrication procedures. It has been used to fabricate micromixers, PCR reactors, CE and many other microdevices. Here we present the design, fabrication, characterization and application of pneumatic microvalves and micropumps based on PMMA. Valves and pumps are fabricated by sandwiching a PDMS membrane between PMMA fluidic channel and manifold wafers. Valve closing or opening can be controlled by adjusting the pressure in a displacement chamber on the pneumatic layer via a computer regulated solenoid. The valve provides up to 15.4 microL s(-1) at 60 kPa fluid pressure and seals reliably against forward fluid pressure as high as 60 kPa. A PMMA diaphragm pump can be assembled by simply connecting three valves in series. By varying valve volume or opening time, pumping rates ranging from nL to microL per second can be accurately achieved. The PMMA based valves and pumps were further tested in a disposable automatic nucleic acid extraction microchip to extract DNA from human whole blood. The DNA extraction efficiency was about 25% and the 260 nm/280 nm UV absorption ratio for extracted DNA was 1.72. Because of its advantages of inexpensive, facile fabrication, robust and easy integration, the PMMA valve and pump will find their wide application for fluidic manipulation in portable and disposable microfluidic devices.

A theoretical and experimental study of the electrophoretic extraction of ions from a pressure driven flow in a microfluidic device

The electrophoretic extraction of ions from a hydrodynamic flow stream is investigated at an intersection between two microfluidic channels. A pressure gradient is used to drive samples through the main channel, while ions are electrophoretically extracted into the side channels. Hydrodynamic restrictors and a neutral coating are used to suppress bulk flow through the side channels. A theoretical model that assumes Poiseuille flow in the main channel and neglects molecular diffusion is used to calculate the extraction efficiency, eta, as a function of the ratio, R, of the average hydrodynamic velocity to the electrophoretic velocity. The model predicts complete extraction of ions (eta=1) for R<2/3 and a monotonic decrease in eta as R becomes greater than 2/3, which agrees well with the experimental results. Additionally, the model predicts that the aspect ratio of the microfluidic channel has little effect on the extraction efficiency. It is anticipated that this device can be used for on-line process monitoring, sample injection, and 2D separations for proteomics and other fields.

Integration of microdialysis sampling and microchip electrophoresis with electrochemical detection

Here we describe the fabrication, optimization, and application of a microfluidic device that integrates microdialysis (MD) sampling, microchip electrophoresis (ME), and electrochemical detection (EC). The manner in which the chip is produced is reproducible and enables the fixed alignment of the MD/ME and ME/EC interfaces. Poly(dimethylsiloxane) (PDMS)-based valves were used for the discrete injection of sample from the hydrodynamic MD dialysate stream into a separation channel for analysis with ME. To enable the integration of ME with EC detection, a palladium decoupler was used to isolate the high voltages associated with electrophoresis from micrometer-sized carbon ink detection electrodes. Optimization of the ME/EC interface was needed to allow the use of biologically appropriate perfusate buffers containing high salt content. This optimization included changes in the fabrication procedure, increases in the decoupler surface area, and a programmed voltage shutoff. The ability of the MD/ME/EC system to sample a biological system was demonstrated by using a linear probe to monitor the stimulated release of dopamine from a confluent layer of PC 12 cells. To our knowledge, this is the first report of a microchip-based system that couples microdialysis sampling with microchip electrophoresis and electrochemical detection.

Microchip-based integration of cell immobilization, electrophoresis, post-column derivatization, and fluorescence detection for monitoring the release of dopamine from PC 12 cells

In this paper, we describe the fabrication and evaluation of a multilayer microchip device that can be used to quantitatively measure the amount of catecholamines released from PC 12 cells immobilized within the same device. This approach allows immobilized cells to be stimulated on-chip and, through rapid actuation of integrated microvalves, the products released from the cells are repeatedly injected into the electrophoresis portion of the microchip, where the analytes are separated based upon mass and charge and detected through post-column derivatization and fluorescence detection. Following optimization of the post-column derivatization detection scheme (using naphthalene-2,3-dicarboxaldehyde and 2-beta-mercaptoethanol), off-chip cell stimulation experiments were performed to demonstrate the ability of this device to detect dopamine from a population of PC 12 cells. The final 3-dimensional device that integrates an immobilized PC 12 cell reactor with the bilayer continuous flow sampling/electrophoresis microchip was used to continuously monitor the on-chip stimulated release of dopamine from PC 12 cells. Similar dopamine release was seen when stimulating on-chip versus off-chip yet the on-chip immobilization studies could be carried out with 500 times fewer cells in a much reduced volume. While this paper is focused on PC 12 cells and neurotransmitter analysis, the final device is a general analytical tool that is amenable to the immobilization of a variety of cell lines and analysis of various released analytes by electrophoretic means.

Fabrication and evaluation of a 3-dimensional microchip device where carbon microelectrodes individually address channels in the separate fluidic layers

A fabrication method that results in a 3-dimensional fluidic device containing poly(dimethylsiloxane) (PDMS) -embedded microelectrodes that individually address each layer is described. The two electrode-containing layers and the polycarbonate membrane are reversibly sealed together, eliminating the need for plasma oxidation during device assembly, while enabling simultaneous amperometric detection in membrane-separated fluidic channels. The electrodes were characterized using microchip-based flow analysis. It was found that PDMS-embedded electrodes have a limit of detection (400 nM for catechol) that is 5-fold lower than that reported for microchip-based flow analysis with similar electrodes in a hybrid PDMS-glass device. The selectivity of the carbon ink microelectrodes can be tuned by a simplified modification procedure; this was demonstrated by the selective detection of nitric oxide over possible interferents. Finally, the ability to monitor processes occurring in separate layers of a 3-dimensional device was shown by the simultaneous detection of catechol on either side of the polycarbonate membrane. The electrode response in each fluidic channel was found to be linear as a function of concentration and the transport between layers could be controlled by varying the linear velocities of each fluidic channel. The ability to fabricate and operate this type of 3-dimensional device will be useful for the development of cell-based in vivo mimics that involve the transport of molecular messengers and/or pharmaceuticals across layers of immobilized cells.