Encapsulated electrodes for microchip devices: microarrays and platinized electrodes for signal enhancement

A 3D-printed, multi-modal microfluidic device for measuring nitric oxide and ATP release from flowing red blood cells

In this paper, a 3D-printed multi-modal device was designed and fabricated to simultaneously detect nitric oxide (NO) and adenosine triphosphate (ATP) in red blood cell suspensions prepared from whole blood. Once a sample was injected into the device, NO was first detected (via amperometry) using a three-electrode, dual-opposed, electrode configuration with a platinum-black/Nafion coated gold working electrode. After in-line amperometric detection of NO, ATP was detected via a chemiluminescence reaction, with a luciferin/luciferase solution continuously pumped into an integrated mixing T and the resulting light being measured with a PMT underneath the channel. The device was optimized for mixing/reaction conditions, limits of detection (40 nM for NO and 30 nM for ATP), and sensitivity. This device was used to determine the basal (normoxic) levels of NO and ATP in red blood cells, as well as an increase in concentration of both analytes under hypoxic conditions. Finally, the effect of storing red blood cells in a commonly used storage solution was also investigated by monitoring the production of NO and ATP over a three-week storage time.

Fully 3D printed fluidic devices with integrated valves and pumps for flow injection analysis

The use of a PolyJet 3D printer to create a microfluidic device that has integrated valves and pumps is described. The process uses liquid support and stacked printing to result in fully printed devices that are ready to use within minutes of fabrication after minimal post-processing. A unique feature of PolyJet printing is the ability to incorporate several different materials of varying properties into one print. In this work, two commercially available materials were used: a rigid-transparent plastic material (VeroClear) was used to define the channel regions and the bulk of the device, while the pumps/valves were printed in a flexible, rubber-like material (Agilus30). The entire process, from initial design to testing takes less than 4 hours to complete. The performance of the valves and pumps were characterized by fluorescence microscopy. A flow injection analysis device that enabled the discrete injections of analyte plugs was created, with on-chip pumps being used to move the fluid streams. The injection process was found to be reproducible and linearly correlated with changes in analyte concentration. The utility was demonstrated with the injection and rapid lysis of fluorescently-labeled endothelial cells. The ability to produce a device with integrated pumps/valves in one process significantly adds to the applicability of 3D printing to create microfluidic devices for analytical measurements.

Biological applications of microchip electrophoresis with amperometric detection: in vivo monitoring and cell analysis

Microchip electrophoresis with amperometric detection (ME-EC) is a useful tool for the determination of redox active compounds in complex biological samples. In this review, a brief background on the principles of ME-EC is provided, including substrate types, electrode materials, and electrode configurations. Several different detection approaches are described, including dual-channel systems for dual-electrode detection and electrochemistry coupled with fluorescence and chemiluminescence. The application of ME-EC to the determination of catecholamines, adenosine and its metabolites, and reactive nitrogen and oxygen species in microdialysis samples and cell lysates is also detailed. Lastly, approaches for coupling of ME-EC with microdialysis sampling to create separation-based sensors that can be used for near real-time monitoring of drug metabolism and neurotransmitters in freely roaming animals are provided. Graphical abstract.

Direct embedding and versatile placement of electrodes in 3D printed microfluidic-devices

In this paper, we describe how PolyJet 3D printing technology can be used to fully integrate electrode materials into microfluidic devices during the print process. This approach uses stacked printing (separate printing steps and stage drops) with liquid support to result in devices where electrodes and a capillary fluidic connection are directly integrated and ready to use when printing is complete. A key feature of this approach is the ability to directly incorporate electrode materials into the print process so that the electrode(s) can be placed anywhere in the channel (at any height). We show that this can be done with a single electrode or an electrode array (which led to increases in signal). In both cases, we found that a middle electrode configuration leads to a significant increase in the sensitivity, as opposed to more traditional bottom channel placement. Since the electrode is embedded in the device, in situ platinum black deposition was performed to aid in the detection of nitric oxide. Finally, a generator-collector configuration with an opposed counter electrode was made by placing two working electrodes ∼750 μm apart (in the middle of the channel) and a platinum counter electrode at the bottom of the channel. The utility of this configuration was demonstrated by dual electrode detection of catechol. This 3D printing approach affords robust electrochemical detection schemes with new electrode configurations being possible in a manner that also increases the ease of use and transferability of the 3D printed devices with integrated electrode materials.

Microfluidic device using a gold pillar array and integrated electrodes for on-chip endothelial cell immobilization, direct RBC contact, and amperometric detection of nitric oxide

We describe a microfluidic device that can be used to detect interactions between red blood cells (RBCs) and endothelial cells using a gold pillar array (created by electrodeposition) and an integrated detection electrode. Endothelial cells can release nitric oxide (NO) via stimulation by RBC-derived ATP. These studies incorporate on-chip endothelial cell immobilization, direct RBC contact, and detection of NO in a single microfluidic device. In order to study the RBC-EC interactions, this work used a microfluidic device made of a PDMS chip with two adjacent channels and a polystyrene base with embedded electrodes for creating a membrane (via gold pillars) and detecting NO (at a glassy carbon electrode coated with platinum-black and Nafion). RBCs were pharmacologically treated with treprostinil in the absence and presence of glybenclamide, and ATP release was determined as was the resultant NO release from endothelial cells. Treprostinil treatment of RBCs resulted in ATP release that stimulated endothelial cells to release on average 1.8 ± 0.2 nM NO per endothelial cell (average ± SEM, n = 8). Pretreatment of RBCs with glybenclamide inhibited treprostinil-induced ATP release and, therefore, less NO was produced by the endothelial cells (0.92 ± 0.1 nM NO per endothelial cell, n = 7). In the future, this device can be used to study interactions between many other cell types (both adherent and non-adherent cell lines) and incorporate other detection schemes.

Optimization of a microchip electrophoresis method with electrochemical detection for the determination of nitrite in macrophage cells as an indicator of nitric oxide production

Nitric oxide (NO) is involved in many biological functions, including blood pressure regulation, the immune response, and neurotransmission. However, excess production of NO can lead to the generation of reactive nitrogen species and nitrosative stress and has been linked to several neurodegenerative diseases and cardiovascular disorders. Because NO is short-lived and generally difficult to detect, its primary stable degradation product, nitrite, is frequently monitored in its place. In this paper, an improved method using microchip electrophoresis with electrochemical detection (ME-EC) was developed for the separation and detection of nitrite in cell lysates. A separation of nitrite from several electroactive cell constituents and interferences was optimized, and the effect of sample and buffer conductivity on peak efficiency was explored. It was found that the addition of 10 mM NaCl to the run buffer caused stacking of the nitrite peak and improved limits of detection. A platinum black working electrode was also evaluated for the detection of nitrite and other electroactive cellular species after electrophoretic separation. The use of a modified platinum working electrode resulted in 2.5-, 1.7-, and 7.2-fold signal enhancement for nitrite, ascorbic acid, and hydrogen peroxide, respectively, and increased the sensitivity of the method for nitrite twofold. The optimized ME-EC method was used to compare nitrite production by native and lipopolysaccharide-stimulated RAW 264.7 macrophage cells.

Use of 3D Printing and Modular Microfluidics to Integrate Cell Culture, Injections and Electrochemical Analysis

Fabrication of microchip-based devices using 3-D printing technology offers a unique platform to create separate modules that can be put together when desired for analysis. A 3-D printed module approach offers various advantages such as file sharing and the ability to easily replace, customize, and modify the individual modules. Here, we describe the use of a modular approach to electrochemically detect the ATP-mediated release of nitric oxide (NO) from endothelial cells. Nitric oxide plays a significant role in the vasodilation process; however, detection of NO is challenging due to its short half-life. To enable this analysis, we use three distinct 3-D printed modules: cell culture, sample injection and detection modules. The detection module follows a pillar-based Wall-Jet Electrode design, where the analyte impinges normal to the electrode surface, offering enhanced sensitivity for the analyte. To further enhance the sensitivity and selectivity for NO detection the working electrode (100 μm gold) is modified by the addition of a 27 μm gold pillar and platinum-black coated with Nafion. The use of the pillar electrode leads to three-dimensional structure protruding into the channel enhancing the sensitivity by 12.4 times in comparison to the flat electrode (resulting LOD for NO = 210 nM). The next module, the 3-D printed sample injection module, follows a simple 4-Port injection rotor design made of two separate components that when assembled can introduce a specific volume of analyte. This module not only serves as a cheaper alternative to the commercially available 4-Port injection valves, but also demonstrates the ability of volume customization and reduced dead-volume issues with the use of capillary-free connections. Comparison between the 3-D printed and a commercial 4-Port injection valve showed similar sensitivities and reproducibility for NO analysis. Lastly, the cell culture module contains electrospun polystyrene fibers with immobilized endothelial cells, resulting in 3-D scaffold for cell culture. With the incorporation of all 3 modules, we can make reproducible ATP injections (via the 3-D printed sample injection module) that can stimulate NO release from endothelial cells cultured on a fibrous insert in the cell culture module which can then be quantitated by the pillar WJE module (0.19 ± 0.03 nM/cell, n = 27, 3 inserts analyzed each day, on 9 different days). The modular approach demonstrates the facile creation of custom and modifiable fluidic components that can be assembled as needed.

Simultaneous analysis of vascular norepinephrine and ATP release using an integrated microfluidic system

BACKGROUND

Sympathetic nerves are known to release three neurotransmitters: norepinephrine, ATP, and neuropeptide Y that play a role in controlling vascular tone. This paper focuses on the co-release of norepinephrine and ATP from the mesenteric arterial sympathetic nerves of the rat.

NEW METHOD

In this paper, a quantification technique is described that allows simultaneous detection of norepinephrine and ATP in a near-real-time fashion from the isolated perfused mesenteric arterial bed of the rat. Simultaneous detection is enabled with 3-D printing technology, which is shown to help integrate the perfusate with different detection methods (norepinephrine by microchip-based amperometery and ATP by on-line chemiluminescence).

RESULTS

Stimulated levels relative to basal levels of norepinephrine and ATP were found to be 363nM and 125nM, respectively (n=6). The limit of detection for norepinephrine is 80nM using microchip-based amperometric detection. The LOD for on-line ATP detection using chemiluminescence is 35nM.

COMPARISON WITH EXISTING METHOD

In previous studies, the co-transmitters have been separated and detected with HPLC techniques. With HPLC, the samples from biological preparations have to be derivatized for ATP detection and require collection time before analysis. Thus real-time measurements are not made and the delay in analysis by HPLC can cause degradation.

CONCLUSIONS

In conclusion, the method described in the paper can be used to successfully detect norepinephrine and ATP simultaneously and in a near-real-time fashion.

Microchip-based electrochemical detection using a 3-D printed wall-jet electrode device

Three dimensional (3-D) printing technology has evolved dramatically in the last few years, offering the capability of printing objects with a variety of materials. Printing microfluidic devices using this technology offers various advantages such as ease and uniformity of fabrication, file sharing between laboratories, and increased device-to-device reproducibility. One unique aspect of this technology, when used with electrochemical detection, is the ability to produce a microfluidic device as one unit while also allowing the reuse of the device and electrode for multiple analyses. Here we present an alternate electrode configuration for microfluidic devices, a wall-jet electrode (WJE) approach, created by 3-D printing. Using microchip-based flow injection analysis, we compared the WJE design with the conventionally used thin-layer electrode (TLE) design. It was found that the optimized WJE system enhances analytical performance (as compared to the TLE design), with improvements in sensitivity and the limit of detection. Experiments were conducted using two working electrodes - 500 μm platinum and 1 mm glassy carbon. Using the 500 μm platinum electrode the calibration sensitivity was 16 times higher for the WJE device (as compared to the TLE design). In addition, use of the 1 mm glassy carbon electrode led to limit of detection of 500 nM for catechol, as compared to 6 μM for the TLE device. Finally, to demonstrate the versatility and applicability of the 3-D printed WJE approach, the device was used as an inexpensive electrochemical detector for HPLC. The number of theoretical plates was comparable to the use of commercially available UV and MS detectors, with the WJE device being inexpensive to utilize. These results show that 3-D-printing can be a powerful tool to fabricate reusable and integrated microfluidic detectors in configurations that are not easily achieved with more traditional lithographic methods.

Alkyl-Nitrile Adlayers as Probes of Plasmonically Induced Electric Fields

Vibrational Stark shifts observed from mercaptoalkyl monolayers on surface enhanced Raman (SERS) active materials are reported to provide a direct measurement of the local electric field around plasmonic nanostructures. Adlayers of CN-, p-mercaptobenzonitrile, and n-mercaptobutylnitrile were adsorbed to a heterogeneous nanostructured Ag surface. The frequency of the CN moiety was observed to shift in a correlated fashion with the SERS intensity. These shifts are attributed to a vibrational Stark shift arising from rectification of the optical field, which gives rise to a DC potential on the surface. All three molecules showed CN Stark shifts on the plasmonic surfaces. P-mercaptobenzonitrile is observed to be a well-behaved probe of the electric field, providing a narrow spectral line, suggesting a more uniform orientation on the surface. The utility of p-mercaptobenzonitrile was demonstrated by successfully assessing the electric field between gold nanoparticles adsorbed to a monolayer of the nitrile on a flat gold surface. A model is presented where the Stark shift of the alkyl-nitrile probe can be correlated to optical field, providing an intensity independent measurement of the local electric field environment.

Sheath-flow microfluidic approach for combined surface enhanced Raman scattering and electrochemical detection

The combination of hydrodynamic focusing with embedded capillaries in a microfluidic device is shown to enable both surface enhanced Raman scattering (SERS) and electrochemical characterization of analytes at nanomolar concentrations in flow. The approach utilizes a versatile polystyrene device that contains an encapsulated microelectrode and fluidic tubing, which is shown to enable straightforward hydrodynamic focusing onto the electrode surface to improve detection. A polydimethyslsiloxane (PDMS) microchannel positioned over both the embedded tubing and SERS active electrode (aligned ∼200 μm from each other) generates a sheath flow that confines the analyte molecules eluting from the embedded tubing over the SERS electrode, increasing the interaction between the Riboflavin (vitamin B2) and the SERS active electrode. The microfluidic device was characterized using finite element simulations, amperometry, and Raman experiments. This device shows a SERS and amperometric detection limit near 1 and 100 nM, respectively. This combination of SERS and amperometry in a single device provides an improved method to identify and quantify electroactive analytes over either technique independently.

Fabrication and Characterization of All-Polystyrene Microfluidic Devices with Integrated Electrodes and Tubing

A new method of fabricating all-polystyrene devices with integrated electrodes and fluidic tubing is described. As opposed to expensive polystyrene (PS) fabrication techniques that use hot embossing and bonding with a heated lab press, this approach involves solvent-based etching of channels and lamination-based bonding of a PS cover, all of which do not need to occur in a clean room. PS has been studied as an alternative microchip substrate to PDMS, as it is more hydrophilic, biologically compatible in terms of cell adhesion, and less prone to absorption of hydrophobic molecules. The etching/lamination-based method described here results in a variety of all-PS devices, with or without electrodes and tubing. To characterize the devices, micrographs of etched channels (straight and intersected channels) were taken using confocal and scanning electron microscopy. Microchip-based electrophoresis with repetitive injections of fluorescein was conducted using a three-sided PS (etched pinched, twin-tee channel) and one-sided PDMS device. Microchip-based flow injection analysis, with dopamine and NO as analytes, was used to characterize the performance of all-PS devices with embedded tubing and electrodes. Limits of detection for dopamine and NO were 130 nM and 1.8 μM, respectively. Cell immobilization studies were also conducted to assess all-PS devices for cellular analysis. This paper demonstrates that these easy to fabricate devices can be attractive alternative to other PS fabrication methods for a wide variety of analytical and cell culture applications.

Microfluidic device with tunable post arrays and integrated electrodes for studying cellular release

In this paper, we describe the development of a planar, pillar array device that can be used to image either side of a tunable membrane, as well as sample and detect small molecules in a cell-free region of the microchip. The pores are created by sealing two parallel PDMS microchannels (a cell channel and a collector channel) over a gold pillar array (5 or 10 μm in height), with the device being characterized and optimized for small molecule cross-over while excluding a flowing cell line (here, red blood cells, RBCs). The device was characterized in terms of the flow rate dependence of analyte cross-over and cell exclusion as well as the ability to perform amperometric detection of catechol and nitric oxide (NO) as they cross-over into the collector channel. Using catechol as the test analyte, the limits of detection (LOD) of the cross-over for the 10 μm and 5 μm pillar array heights were shown to be 50 nM and 105 nM, respectively. Detection of NO was made possible with a glassy carbon detection electrode (housed in the collector channel) modified with Pt-black and Nafion, to enhance sensitivity and selectivity, respectively. Reproducible cross-over of NO as a function of concentration resulted in a linear correlation (r(2) = 0.995, 7.6-190 μM), with an LOD for NO of 230 nM on the glassy carbon/Pt-black/0.05% Nafion electrode. The applicability of the device was demonstrated by measuring the NO released from hypoxic RBCs, with the device allowing the released NO to cross-over into a cell free channel where it was detected in close to real-time. This type of device is an attractive alternative to the use of 3-dimensional devices with polycarbonate membranes, as either side of the membrane can be imaged and facile integration of electrochemical detection is possible.

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 reusable electrochemical proximity assay for highly selective, real-time protein quantitation in biological matrices

Rapid and specific quantitation of a variety of proteins over a wide concentration range is highly desirable for biosensing at the point-of-care, in clinical laboratories, and in research settings. Our recently developed electrochemical proximity assay (ECPA) is a target-flexible, DNA-directed, direct-readout protein quantitation method with detection limits in the low femtomolar range, making it particularly amenable to point-of-care detection. However, consistent quantitation in more complex matrices is required at the point-of-care, and improvements in measurement speed are needed for clinical and research settings. Here, we address these concerns with a reusable ECPA, where a gentle regeneration of the surface DNA monolayer (used to capture the proximity complex) is achieved enzymatically through a novel combination of molecular biology and electrochemistry. Strategically placed uracils in the DNA sequence trigger selective cleavage of the backbone, releasing the assembled proximity complex. This allows repeated protein quantitation by square-wave voltammetry (SWV)—as quickly as 3 min between runs. The process can be repeated up to 19 times on a single electrode without loss of assay sensitivity, and currents are shown to be highly repeatable with similar calibrations using seven different electrodes. The utility of reusable ECPA is demonstrated through two important applications in complex matrices: (1) direct, quantitative monitoring of hormone secretion in real time from as few as five murine pancreatic islets and (2) standard addition experiments in unspiked serum for direct quantitation of insulin at clinically relevant levels. Results from both applications distinguish ECPA as an exceptional tool in protein quantitation.