R. Scott Martin

Microchip capillary electrophoresis/electrochemistry.

Microfabricated fluidic devices have generated considerable interest over the past ten years due to the fact that sample preparation, injection, separation, derivatization, and detection can be integrated into one miniaturized device. This review reports progress in the development of microfabricated analytical systems based on microchip capillary electrophoresis (CE) with electrochemical (EC) detection. Electrochemical detection has several advantages for use with microchip electrophoresis systems, for example, ease of miniaturization, sensitivity, and selectivity. In this review, the basic components necessary for microchip CEEC are described, including several examples of different detector configurations. Lastly, details of the application of this technique to the determination of catechols and phenols, amino acids, peptides, carbohydrates, nitroaromatics, polymerase chain reaction (PCR) products, organophosphates, and hydrazines are described.

Recent developments in amperometric detection for microchip capillary electrophoresis.

The interest in microfluidic devices has increased considerably over the past decade due to the numerous advantages of working within a miniature, microfabricated format. This review focuses on recent advances in coupling amperometric detection with microchip capillary electrophoresis (CE). Advances in electrochemical cell design, isolation of the detector from the separation field, and integration of both pre‐ and postseparation reaction chambers are discussed. The use of microchip CE with amperometric detection for enzyme/immunoassays, clinical and environmental assays, and the determination of neurotransmitters is described.

3D printed microfluidic devices with integrated versatile and reusable electrodes

We report two 3D printed devices that can be used for electrochemical detection. In both cases, the electrode is housed in commercially available, polymer-based fittings so that the various electrode materials (platinum, platinum black, carbon, gold, silver) can be easily added to a threaded receiving port printed on the device; this enables a module-like approach to the experimental design, where the electrodes are removable and can be easily repolished for reuse after exposure to biological samples. The first printed device represents a microfluidic platform with a 500 × 500 μm channel and a threaded receiving port to allow integration of either polyetheretherketone (PEEK) nut-encased glassy carbon or platinum black (Pt-black) electrodes for dopamine and nitric oxide (NO) detection, respectively. The embedded 1 mm glassy carbon electrode had a limit of detection (LOD) of 500 nM for dopamine and a linear response (R(2) = 0.99) for concentrations between 25-500 μM. When the glassy carbon electrode was coated with 0.05% Nafion, significant exclusion of nitrite was observed when compared to signal obtained from equimolar injections of dopamine. When using flow injection analysis with a Pt/Pt-black electrode and standards derived from NO gas, a linear correlation (R(2) = 0.99) over a wide range of concentrations (7.6-190 μM) was obtained, with the LOD for NO being 1 μM. The second application showcases a 3D printed fluidic device that allows collection of the biologically relevant analyte adenosine triphosphate (ATP) while simultaneously measuring the release stimulus (reduced oxygen concentration). The hypoxic sample (4.8 ± 0.5 ppm oxygen) released 2.4 ± 0.4 times more ATP than the normoxic sample (8.4 ± 0.6 ppm oxygen). Importantly, the results reported here verify the reproducible and transferable nature of using 3D printing as a fabrication technique, as devices and electrodes were moved between labs multiple times during completion of the study.

A 3D printed fluidic device that enables integrated features

Fluidic devices fabricated using conventional soft lithography are well suited as prototyping methods. Three-dimensional (3D) printing, commonly used for producing design prototypes in industry, allows for one step production of devices. 3D printers build a device layer by layer based on 3D computer models. Here, a reusable, high throughput, 3D printed fluidic device was created that enables flow and incorporates a membrane above a channel in order to study drug transport and affect cells. The device contains 8 parallel channels, 3 mm wide by 1.5 mm deep, connected to a syringe pump through standard, threaded fittings. The device was also printed to allow integration with commercially available membrane inserts whose bottoms are constructed of a porous polycarbonate membrane; this insert enables molecular transport to occur from the channel to above the well. When concentrations of various antibiotics (levofloxacin and linezolid) are pumped through the channels, approximately 18-21% of the drug migrates through the porous membrane, providing evidence that this device will be useful for studies where drug effects on cells are investigated. Finally, we show that mammalian cells cultured on this membrane can be affected by reagents flowing through the channels. Specifically, saponin was used to compromise cell membranes, and a fluorescent label was used to monitor the extent, resulting in a 4-fold increase in fluorescence for saponin treated cells.

Fabrication and evaluation of a carbon‐based dual‐electrode detector for poly(dimethylsiloxane) electrophoresis chips

The first carbon‐based dual‐electrode detector for microchip capillary electrophoresis (CE) is described. The poly(dimethylsiloxane) (PDMS)‐based microchip CE devices were constructed by reversibly sealing a PDMS layer containing separation and injection channels to another PDMS layer containing carbon fiber working electrodes. End‐channel amperometric detection was employed and the performance of the chip was evaluated using catechol. The response was found to be linear between 1 and 600 μM with an experimentally determined limit of detection (LOD) of 500 nM and a sensitivity of 30 pA/μM. Collection efficiencies for catechol ranged from 36.0 to 43.7% at field strengths of 260—615 V/cm. The selectivity that can be gained with these devices is demonstrated by the first CE‐based dual‐electrode detection of a Cu(II) peptide complex. These devices illustrate the potential for a rugged and easily constructed microchip CE system with an integrated carbon‐based detector of similar scale.

Carbon paste-based electrochemical detectors for microchip capillary electrophoresis/electrochemistry

The first reported use of a carbon paste electrochemical detector for microchip capillary electrophoresis (CE) is described. Poly(dimethylsiloxane) (PDMS)-based microchip CE devices were constructed by reversibly sealing a PDMS layer containing separation and injection channels to a separate PDMS layer that contained carbon paste working electrodes. End-channel amperometric detection with a single electrode was used to detect amino acids derivatized with naphthalene dicarboxaldehyde. Two electrodes were placed in series for dual electrode detection. This approach was demonstrated for the detection of copper(II) peptide complexes. A major advantage of carbon paste is that catalysts can be easily incorporated into the electrode. Carbon paste that was chemically modified with cobalt phthalocyanine was used for the detection of thiols following a CE separation. These devices illustrate the potential for an easily constructed microchip CE system with a carbon-based detector that exhibits adjustable selectivity.

Microchip electrophoretic separation systems for biomedical and pharmaceutical analysis

The application of microchip capillary electrophoresis (CE) systems to biomedical and pharmaceutical analysis is described and reviewed. Fabrication, instrumentation, and operation of the systems are discussed. An overview of applications is presented, covering four main areas: DNA sequencing, genetic analysis, immunoassays, and protein and peptide analysis. These systems have the potential to dramatically change the way that biochemical analyses are performed.

Detecting thiols in a microchip device using micromolded carbon ink electrodes modified with cobalt phthalocyanine

This paper describes the fabrication and evaluation of a chemically modified carbon ink microelectrode to detect thiols of biological interest. The detection of thiols, such as homocysteine and cysteine, is necessary to monitor various disease states. The biological implications of these thiols generate the need for miniaturized detection systems that enable portable monitoring as well as quantitative results. In this work, we utilize a microchip device that incorporates a micromolded carbon ink electrode modified with cobalt phthalocyanine to detect thiols. Cobalt phthalocyanine (CoPC) is an electrocatalyst that lowers the potential needed for the oxidation of thiols. The CoPC/carbon ink composition was optimized for the micromolding method and the resulting microelectrode was characterized with microchip-based flow injection analysis. It was found that CoPC lowers the overpotential for thiols but, as compared to direct amperometric detection, a pulsed detection scheme was needed to constantly regenerate the electrocatalyst surface, leading to improved peak reproducibility and limits of detection. Using the pulsed method, cysteine exhibited a linear response between 10-250 microM (r(2) = 0.9991) with a limit of detection (S/N = 3) of 7.5 microM, while homocysteine exhibited a linear response between 10-500 microM (r(2) = 0.9967) with a limit of detection of 6.9 microM. Finally, to demonstrate the ability to measure thiols in a biological sample using a microchip device, the CoPC-modified microelectrode was utilized for the detection of cysteine in the presence of rabbit erythrocytes.

Thick-Film Electrochemical Detectors for Poly(dimethylsiloxane)-based Microchip Capillary Electrophoresis

A new poly(dimethylsiloxane) (PDMS)-based microchip capillary electrophoresis (CE) device, with a thick-film electrochemical detector, is described. The end-column design relies on screen-printing the amperometric carbon working electrode on the base plate of a PDMS microchip (opposite to the exit of the microchannel). Since the channel depth and electrode height are quite similar, this is a flow-onto/flow-by hybrid arrangement. The influence of relevant experimental variables, such as the separation and detection potentials, is reported along with the attractive analytical performance. Flat baselines and extremely low noise levels are observed even at high separation fields (approaching 700 V/cm), reflecting the effective electrical isolation of the detector. The resulting detection limits (150 nM for epinephrine and 280 nM for catechol) compare favorably with those obtained by other PDMS-based electrochemical detectors. Such coupling of low-cost and versatile PDMS chips and thick-film electrochemical detectors holds great promise for high-volume production of disposable microfluidic analytical devices.

3D-printed microfluidic devices: fabrication, advantages and limitations—a mini review

A mini-review with 79 references. In this review, the most recent trends in 3D-printed microfluidic devices are discussed. In addition, a focus is given to the fabrication aspects of these devices, with the supplemental information containing detailed instructions for designing a variety of structures including: a microfluidic channel, threads to accommodate commercial fluidic fittings, a flow splitter; a well plate, a mold for PDMS channel casting; and how to combine multiple designs into a single device. The advantages and limitations of 3D-printed microfluidic devices are thoroughly discussed, as are some future directions for the field.