Dual-electrode microfluidic cell for characterizing electrocatalysts

Development of a Redox-Polymer-Based Electrochemical Glucose Biosensor Suitable for Integration in Microfluidic 3D Cell Culture Systems

The inclusion of online, in situ biosensors in microfluidic cell cultures is important to monitor and characterize a physiologically mimicking environment. This work presents the performance of second-generation electrochemical enzymatic biosensors to detect glucose in cell culture media. Glutaraldehyde and ethylene glycol diglycidyl ether (EGDGE) were tested as cross-linkers to immobilize glucose oxidase and an osmium-modified redox polymer on the surface of carbon electrodes. Tests employing screen printed electrodes showed adequate performance in a Roswell Park Memorial Institute (RPMI-1640) media spiked with fetal bovine serum (FBS). Comparable first-generation sensors were shown to be heavily affected by complex biological media. This difference is explained in terms of the respective charge transfer mechanisms. Under the tested conditions, electron hopping between Os redox centers was less vulnerable than H₂O₂ diffusion to biofouling by the substances present in the cell culture matrix. By employing pencil leads as electrodes, the incorporation of these electrodes in a polydimethylsiloxane (PDMS) microfluidic channel was achieved simply and at a low cost. Under flow conditions, electrodes fabricated using EGDGE presented the best performance with a limit of detection of 0.5 mM, a linear range up to 10 mM, and a sensitivity of 4.69 μA mM^-1 cm^-2.

Microfluidics for Electrochemical Energy Conversion

Electrochemical energy conversion is an important supplement for storage and on-demand use of renewable energy. In this regard, microfluidics offers prospects to raise the efficiency and rate of electrochemical energy conversion through enhanced mass transport, flexible cell design, and ability to eliminate the physical ion-exchange membrane, an essential yet costly element in conventional electrochemical cells. Since the 2002 invention of the microfluidic fuel cell, the research field of microfluidics for electrochemical energy conversion has expanded into a great variety of cell designs, fabrication techniques, and device functions with a wide range of utility and applications. The present review aims to comprehensively synthesize the best practices in this field over the past 20 years. The underlying fundamentals and research methods are first summarized, followed by a complete assessment of all research contributions wherein microfluidics was proactively utilized to facilitate energy conversion in conjunction with electrochemical cells, such as fuel cells, flow batteries, electrolysis cells, hybrid cells, and photoelectrochemical cells. Moreover, emerging technologies and analytical tools enabled by microfluidics are also discussed. Lastly, opportunities for future research directions and technology advances are proposed.

Tutorial review: Enrichment and separation of neutral and charged species by ion concentration polarization focusing

Ion concentration polarization focusing (ICPF) is an electrokinetic technique, in which analytes are enriched and separated along a localized electric field gradient in the presence of a counter flow. This field gradient is generated by depletion of ions of the background electrolyte at an ion permselective junction. In this tutorial review, we summarize the fundamental principles and experimental parameters that govern selective ion transport and the stability of the enriched analyte plug. We also examine faradaic ICP (fICP), in which local ion concentration is modulated via electrochemical reactions as an attractive alternative to ICP that achieves similar performance with a decrease in both power consumption and Joule heating. The tutorial covers important challenges to the broad application of ICPF including undesired pH gradients, low volumetric throughput, samples that induce biofouling or are highly conductive, and limited approaches to on- or off-chip analysis. Recent developments in the field that seek to address these challenges are reviewed along with new approaches to maximize enrichment, focus uncharged analytes, and achieve enrichment and separation in water-in-oil droplets. For new practitioners, we discuss practical aspects of ICPF, such as strategies for device design and fabrication and the relative advantages of several types of ion selective junctions and electrodes. Lastly, we summarize tips and tricks for tackling common experimental challenges in ICPF.

Electrochemical Generation and Detection of Transient Concentration Gradients in Microfluidic Channels. Theoretical and Experimental Investigations

Transient concentration gradients generated and detected electrochemically in continuous flow microchannels were investigated by numerical simulations and amperometric measurements. Operating conditions including device geometry and hydrodynamic regime were theoretically delineated for producing gradients of various profiles with tunable characteristics. Experiments were carried out with microfluidic devices incorporating a dual-channel-electrode configuration. Under these conditions, high electrochemical performance was achieved both to generate concentration gradients and to monitor their dynamics along linear microchannels. Good agreement was observed between simulated and experimental data validating predictions between gradient properties and generation conditions. These results demonstrated the capability of electrochemical microdevices to produce in situ tunable concentration gradients with real-time monitoring. This approach is versatile for the active control in microfluidics of microenvironments or chemical gradients with high spatiotemporal resolution.

Polycaprolactone-enabled sealing and carbon composite electrode integration into electrochemical microfluidics

Combining electrochemistry with microfluidics is attractive for a wide array of applications including multiplexing, automation, and high-throughput screening. Electrochemical instrumentation also has the advantage of being low-cost and can enable high analyte sensitivity. For many electrochemical microfluidic applications, carbon electrodes are more desirable than noble metals because they are resistant to fouling, have high activity, and large electrochemical solvent windows. At present, fabrication of electrochemical microfluidic devices bearing integrated carbon electrodes remains a challenge. Here, a new system for integrating polycaprolactone (PCL) and carbon composite electrodes into microfluidics is presented. The PCL : carbon composites have excellent electrochemical activity towards a wide range of analytes as well as high electrical conductivity (∼1000 S m-1). The new system utilizes a laser cutter for fast, simple fabrication of microfluidics using PCL as a bonding layer. As a proof-of-concept application, oil-in-water and water-in-oil droplets are electrochemically analysed. Small-scale electrochemical organic synthesis for TEMPO mediated alcohol oxidation is also demonstrated.

Design and Finite Element Model of a Microfluidic Platform with Removable Electrodes for Electrochemical Analysis

A microfluidic platform for hydrodynamic electrochemical analysis was developed, consisting of a poly(methyl methacrylate) chip and three removable electrodes, each housed in 1/16" OD polyether ether ketone tube which can be removed independently for polishing or replacement. The working electrode was a 100-μm diameter Pt microdisk, located flush with the upper face of a 150 μm × 20 μm × 3 cm microchannel, smaller than previously reported for these types of removable electrodes. A commercial leak-less reference electrode was utilized, and a coiled platinum wire was the counter electrode. The platform was evaluated electrochemically by oxidizing a potassium ferrocyanide solution at the working electrode, and a typical limiting current behavior was observed after running linear sweep voltammetry and chronoamperometry, with flow rates 1-6 μL/min. While microdisk channel electrodes have been simulated before using a finite difference method in an ideal 3D geometry, here we predict the limiting current using finite elements in COMSOL Multiphysics 5.3a, which allowed us to easily explore variations in the microchannel geometry that have not previously been considered in the literature. Experimental and simulated currents showed the same trend but differed by 41% in simulations of the ideal geometry, which improved when channel and electrode imperfections were included.

Selective assembly and functionalization of miniaturized redox capacitor inside microdevices for microbial toxin and mammalian cell cytotoxicity analyses

We report a novel strategy for bridging information transfer between electronics and biological systems within microdevices. This strategy relies on our "electrobiofabrication" toolbox that uses electrode-induced signals to assemble biopolymer films at spatially defined sites and then electrochemically "activates" the films for signal processing capabilities. Compared to conventional electrode surface modification approaches, our signal-guided assembly and activation strategy provides on-demand electrode functionalization, and greatly simplifies microfluidic sensor design and fabrication. Specifically, a chitosan film is selectively localized in a microdevice and is covalently modified with phenolic species. The redox active properties of the phenolic species enable the film to transduce molecular to electronic signals (i.e., "molectronic"). The resulting "molectronic" sensors are shown to facilitate the electrochemical analysis in real time of biomolecules, including small molecules and enzymes, to cell-based measurements such as cytotoxicity. We believe this strategy provides an alternative, simple, and promising avenue for connecting electronics to biological systems within microfluidic platforms, and eventually will enrich our abilities to study biology in a variety of contexts.

Microfluidic Surface Titrations of Electroactive Thin Films

We report the use of microfluidic surface titrations (MSTs) for studying electroactive self-assembled monolayers (eSAMs) and other thin films. The technique of MST utilizes a microfluidic generation-collection dual channel electrode (DCE) configuration to quantify the charge associated with electroactive thin films that might or might not be in direct contact with an electrode surface. This technique allows for quantitative measurement of surface coverages, Γ, as low as 30 pmol cm^-2 for electrodeposited Cu thin films. Additionally, we show that it is possible to quantify Γ for ferrocene (Fc)-terminated alkylthiols in mixed-monolayer eSAMs. Interestingly, MSTs sometimes reveal a two-fold higher eSAM concentration compared to direct electrochemical measurements. This finding suggests that in these instances not all the constituent Fc-moieties of the eSAM are in sufficiently close proximity to the surface to be addressable via direct electrochemistry.

Electrochemical flow injection analysis of hydrazine in an excess of an active pharmaceutical ingredient: achieving pharmaceutical detection limits electrochemically

The quantification of genotoxic impurities (GIs) such as hydrazine (HZ) is of critical importance in the pharmaceutical industry in order to uphold drug safety. HZ is a particularly intractable GI and its detection represents a significant technical challenge. Here, we present, for the first time, the use of electrochemical analysis to achieve the required detection limits by the pharmaceutical industry for the detection of HZ in the presence of a large excess of a common active pharmaceutical ingredient (API), acetaminophen (ACM) which itself is redox active, typical of many APIs. A flow injection analysis approach with electrochemical detection (FIA-EC) is utilized, in conjunction with a coplanar boron doped diamond (BDD) microband electrode, insulated in an insulating diamond platform for durability and integrated into a two piece flow cell. In order to separate the electrochemical signature for HZ such that it is not obscured by that of the ACM (present in excess), the BDD electrode is functionalized with Pt nanoparticles (NPs) to significantly shift the half wave potential for HZ oxidation to less positive potentials. Microstereolithography was used to fabricate flow cells with defined hydrodynamics which minimize dispersion of the analyte and optimize detection sensitivity. Importantly, the Pt NPs were shown to be stable under flow, and a limit of detection of 64.5 nM or 0.274 ppm for HZ with respect to the ACM, present in excess, was achieved. This represents the first electrochemical approach which surpasses the required detection limits set by the pharmaceutical industry for HZ detection in the presence of an API and paves the wave for online analysis and application to other GI and API systems.

Spatially distributed current oscillations with electrochemical reactions in microfluidic flow cells

The formation of spatiotemporal patterns is investigated by using a chemical reaction on the surface of a high-aspect-ratio metal electrode positioned in a flow channel. A partial differential equation model is formulated for nickel dissolution in sulfuric acid in a microfluidic flow channel. The model simulations predict oscillatory patterns that are spatially distributed on the electrode surface; the downstream portion of the metal surface exhibits large-amplitude, nonlinear oscillations of dissolution rates, whereas the upstream portion displays small-amplitude, harmonic oscillations with a phase delay. The features of the dynamical response can be interpreted by the dependence of local dynamics on the widely varying surface conditions and the presence of strong coupling. The patterns can be observed for both contiguous and segmented metal surfaces. The existence of spatially distributed current oscillations is confirmed in experiments with Ni electrodissolution in a microfluidic device. The results show the impact of a widely heterogeneous environment on the types of patterns of chemical reaction rates.

Electrochemical properties of metal-oxide-coated carbon electrodes prepared by atomic layer deposition

Here we report on the electrochemical properties of carbon electrodes coated with thin layers of Al2O3 and SnO2. These oxide films were deposited using atomic layer deposition (ALD) and range in thickness from 1 to 6 nm. Electrochemical experiments show that the thinnest oxide layers contain defects that penetrate to the underlying carbon electrode. However, oxygenation of the carbon surface prior to ALD increases the surface concentration of nucleation sites for oxide growth and suppresses the defect density. Films of Al2O3 just ∼3-4 nm in thickness are free of pinholes. Slightly thicker coatings of SnO2 are required for equivalent passivation. Both Al2O3 and SnO2 films are stable in both neutral and acidic electrolytes even after repeated voltammetric scanning. The results reported here open up the possibility of studying the effect of oxide supports on electrocatalytic reactions.

Tunable electrochemical pH modulation in a microchannel monitored via the proton-coupled electro-oxidation of hydroquinone

Electrochemistry is a promising tool for microfluidic systems because it is relatively inexpensive, structures are simple to fabricate, and it is straight-forward to interface electronically. While most widely used in microfluidics for chemical detection or as the transduction mechanism for molecular probes, electrochemical methods can also be used to efficiently alter the chemical composition of small (typically <100 nl) microfluidic volumes in a manner that improves or enables subsequent measurements and sample processing steps. Here, solvent (H2O) electrolysis is performed quantitatively at a microchannel Pt band electrode to increase microchannel pH. The change in microchannel pH is simultaneously tracked at a downstream electrode by monitoring changes in the i-V characteristics of the proton-coupled electro-oxidation of hydroquinone, thus providing real-time measurement of the protonated forms of hydroquinone from which the pH can be determined in a straightforward manner. Relative peak heights for protonated and deprotonated hydroquinone forms are in good agreement with expected pH changes by measured electrolysis rates, demonstrating that solvent electrolysis can be used to provide tunable, quantitative pH control within a microchannel.

Ultrasensitive detection of dopamine using a carbon nanotube network microfluidic flow electrode

The electrochemical measurement of dopamine (DA), in phosphate buffer solution (pH 7.4), with a limit of detection (LOD) of ∼5 pM in 50 μL (∼ 250 attomol) is achieved using a band electrode comprised of a sparse network of pristine single-walled carbon nanotubes (SWNTs), which covers <1% of the insulating substrate. The SWNT electrodes are deployed as amperometric (anodic) detectors in microfluidic cells, produced by microstereolithography, designed specifically for flow injection analysis (FIA). The flow cells, have a channel (duct) geometry, with cell height of 25 μm, and are shown to be hydrodynamically well-defined, with laminar Poiseuille flow. In the arrangement where solution continuously flows over the electrode but the electrode is only exposed to the analyte for short periods of time, the SWNT electrodes do not foul and can be used repeatedly for many months. The LOD for dopamine (DA), reported herein, is significantly lower than previous reports using FIA-electrochemical detection. Furthermore, the SWNT electrodes can be used as grown, i.e., they do not require chemical modification or cleanup. The extremely low background signals of the SWNT electrodes, as a consequence of the sparse surface coverage and the low intrinsic capacitance of the SWNTs, means that no signal processing is required to measure the low currents for DA oxidation at trace levels. DA detection in artificial cerebral fluid is also possible with a LOD of ∼50 pM in 50 μL (∼2.5 fmol).

Effect of mass transfer on the oxygen reduction reaction catalyzed by platinum dendrimer encapsulated nanoparticles

Here we report on the effect of the mass transfer rate (k(t)) on the oxygen reduction reaction (ORR) catalyzed by Pt dendrimer-encapsulated nanoparticles (DENs) comprised of 147 and 55 atoms (Pt(147) and Pt(55)). The experiments were carried out using a dual-electrode microelectrochemical device, which enables the study of the ORR under high k(t) conditions with simultaneous detection of H(2)O(2). At low k(t) (0.02 to 0.12 cm s(-1)) the effective number of electrons involved in ORR, n(eff), is 3.7 for Pt(147) and 3.4 for Pt(55). As k(t) is increased, the mass-transfer-limited current for the ORR becomes significantly lower than the value predicted by the Levich equation for a 4-electron process regardless of catalyst size. However, the percentage of H(2)O(2) detected remains constant, such that n(eff) barely changes over the entire k(t) range explored (0.02 cm s(-1)). This suggests that mass transfer does not affect n(eff), which has implications for the mechanism of the ORR on Pt nanoparticles. Interestingly, there is a significant difference in n(eff) for the two sizes of Pt DENs (n(eff) = 3.7 and 3.5 for Pt(147) and Pt(55), respectively) that cannot be assigned to mass transfer effects and that we therefore attribute to a particle size effect.