Using electrochemical coupling between parallel microbands for in situ monitoring of flow rates in microfluidic channels

Quasi-One-Dimensional Generator-Collector Electrochemistry in Nanochannels

Mass transport in fluidic channels under conditions of pressure-driven flow is controlled by a combination of convection and diffusion. For electrochemical measurements the height of a channel is typically of the same order of magnitude as the electrode dimensions, resulting in complex two- or three- dimensional concentration distributions. Electrochemical nanofluidic devices, however, can have such a low height-to-length ratio that they can effectively be considered as one-dimensional. This greatly simplifies the modeling and quantitative interpretation of analytical measurements. Here we study mass transport in nanochannels using electrodes in a generator-collector configuration. The flux of redox molecules is monitored amperometrically. We observe the transition from diffusion-dominated to convection-dominated transport by varying both the flow velocity and the distance between the electrodes. These results are described quantitatively by the one-dimensional Nernst–Planck equation for mass transport over the full range of experimentally accessible parameters.

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.

Effect of Electrode Shape and Flow Conditions on the Electrochemical Detection with Band Microelectrodes

In this work, we report the analysis of the electrochemical detection of electroactive species with band microelectrodes that operate under controlled convection. The study focuses on the determination of the collection efficiency of the analyte as a function of inlet flow velocity and microband geometry (inlaid, bumped and recessed), also providing a straightforward method for the theoretical determination of the lower detection limit. The analysis has been carried out by simulating the dimensionless mass transport with the finite element method, delivering the stationary limiting current density. Simulations have been performed on systems consisting of single and double band electrodes to investigate the trail effect on the electrochemical detection. We show that the obtained dimensionless results can be easily turned into dimensional data, providing a tool for the design of devices. The proposed method is general and can easily be extended to systems with different geometry.

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.

Feedback-amplified electrochemical dual-plate boron-doped diamond microtrench detector for flow injection analysis

An electrochemical flow cell with a boron‐doped diamond dual‐plate microtrench electrode has been developed and demonstrated for hydroquinone flow injection electroanalysis in phosphate buffer pH 7. Using the electrochemical generator‐collector feedback detector improves the sensitivity by one order of magnitude (when compared to a single working electrode detector). The diffusion process is switched from an analyte consuming “external” process to an analyte regenerating “internal” process with benefits in selectivity and sensitivity.

Electrochemistry in hollow-channel paper analytical devices

In the present article we provide a detailed analysis of fundamental electrochemical processes in a new class of paper-based analytical devices (PADs) having hollow channels (HCs). Voltammetry and amperometry were applied under flow and no flow conditions yielding reproducible electrochemical signals that can be described by classical electrochemical theory as well as finite-element simulations. The results shown here provide new and quantitative insights into the flow within HC-PADs. The interesting new result is that despite their remarkable simplicity these HC-PADs exhibit electrochemical and hydrodynamic behavior similar to that of traditional microelectrochemical devices.

Mass transport at infinite regular arrays of microband electrodes submitted to natural convection: theory and experiments

Mass transport at infinite regular arrays of microband electrodes was investigated theoretically and experimentally in unstirred solutions. Even in the absence of forced hydrodynamics, natural convection limits the convection-free domain up to which diffusion layers may expand. Hence, several regimes of mass transport may take place according to the electrode size, gap between electrodes, time scale of the experiment, and amplitude of natural convection. They were identified through simulation by establishing zone diagrams that allowed all relative contributions to mass transport to be delineated. Dynamic and steady-state regimes were compared to those achieved at single microband electrodes. These results were validated experimentally by monitoring the chronoamperometric responses of arrays with different ratios of electrode width to gap distance and by mapping steady-state concentration profiles above their surface through scanning electrochemical microscopy.

Dual-electrode microfluidic cell for characterizing electrocatalysts

In this paper we introduce a microelectrochemical cell configured for generation-collection experiments and designed primarily for examining the kinetics of electrocatalysts. The heart of the device consists of two, closely spaced, pyrolyzed photoresist microband electrodes enclosed within a microchannel. The cell is suitable for evaluating the efficiency of electrocatalysts under an unprecedented range of conditions. Specifically, compared to the gold-standard rotating ring-disk electrode (RRDE), this device offers four major advantages. First, collection efficiencies of 97% are easily achieved, compared to values of 20-37% that are characteristic of RRDEs. Second, mass transfer coefficients of 0.5 cm s(-1) are accessible for typical redox species, which is significantly higher than RRDEs (up to 0.01 cm s(-1)). Third, we show that the device can operate effectively at temperatures up to 70 °C, which is important for measuring electrochemical kinetics that are relevant to fuel cell catalysts. Finally, much less catalyst and much smaller volumes of electrolyte solution are required to make kinetic measurements using the microelectrochemical device compared to the RRDE. Here, we present the simple procedure used to fabricate the device, fundamental electroanalytical characterization, and electrocatalytic measurements relevant to the oxygen reduction reaction.

High-sensitivity electrochemical enzyme-linked assay on a microfluidic interdigitated microelectrode

A novel enzyme-linked DNA hybridization assay on an interdigitated array (IDA) microelectrode integrated into a microfluidic channel is demonstrated with sub-nM detection limit. To improve the detection limit as compared to conventional electrochemical biosensors, a recyclable redox product, 4-aminophenol (PAP) is used with an IDA microelectrode. The IDA has a modest and easily fabricated inter-digit spacing of 10 μm, yet we were able to demonstrate 97% recycling efficiency of PAP due to the integration in a microfluidic channel. With a 70 nL sample volume, the characterized detection limit for PAP of 1.0 × 10⁻¹⁰ M is achieved, with a linear dynamic range that extends from 1.0 × 10⁻⁹ to 1.0 × 10⁻⁵ M. This detection limit, which is the lowest reported detection limit for PAP, is due to the increased sensitivity provided by the sample confinement in the microfluidic channel, as well as the increased repeatability due to perfectly static flow in the microchannel and an additional anti-fouling step in the protocol. DNA sequence detection is achieved through a hybridization sandwich of an immobilized complementary probe, the target DNA sequence, and a second complementary probe labeled with β-galactosidase (β-GAL); the β-GAL converts its substrate, 4-aminophenyl-d-galactopyranoside (PAPG), into PAP. In this report we present the lowest reported observed detection limit (1.0 × 10⁻¹⁰ M) for an enzyme-linked DNA hybridization assay using an IDA microelectrode and a redox signaling paradigm. Thus, we have demonstrated highly sensitive detection of a targeted DNA sequence using a low-cost easily fabricated electrochemical biosensor integrated into a microfluidic channel.

Pumping-induced perturbation of flow in microfluidic channels and its implications for on-chip cell culture

We study the rate of response to changes in the rate of flow and the perturbations in flow in polydimethylsiloxane (PDMS) microfluidic chips that are subjected to several common flow-control systems. We find that the flow rate of liquid delivered from a syringe pump equipped with a glass syringe responds faster to the changes in the conditions of flow than the same liquid delivered from a plastic syringe; and the rate of flow delivered from compressed air responds faster than that from a glass syringe. We discover that the rate of flow that is driven by a syringe pump and regulated by an integrated pneumatic valve responds even faster, but this flow-control method is characterized by large perturbations. We also examine the possible effects of these large perturbations on NIH 3T3 cells in microfluidic channels and find that they could cause the detachment of NIH 3T3 cells in the microchannels.

Signal amplification in a microchannel from redox cycling with varied electroactive configurations of an individually addressable microband electrode array

Amperometric detection at microelectrodes in lab-on-a-chip (LOAC) devices lose advantages in signal-to-background ratio, reduced ohmic iR drop, and steady-state signal when volumes are so small that diffusion fields reach the walls before flux becomes fully radial. Redox cycling of electroactive species between multiple, closely spaced microelectrodes offsets that limitation and provides amplification capabilities. A device that integrates a microchannel with an individually addressable microband electrode array has been used to study effects of signal amplification due to redox cycling in a confined, static solution with different configurations and numbers of active generators and collectors. The microfabricated device consists of a 22 microm high, 600 microm wide microchannel containing an array of 50 microm wide, 600 microm long gold microbands, separated by 25 microm gaps, interspersed with an 800 microm wide counter electrode and 400 microm wide passive conductor, with a distant but on-chip 400 microm wide pseudoreference electrode. Investigations involve solutions of potassium chloride electrolyte containing potassium ferrocyanide. Amplification factors were as high as 7.60, even with these microelectrodes of fairly large dimensions (which are generally less expensive, easier, and more reproducible to fabricate), because of the significant role that passive and active (instrumentally induced) redox cycling plays in confined volumes of enclosed microchannels. The studies are useful in optimizing designs for LOAC devices.

Generator-collector experiments at a single electrode: exploring the general applicability of this approach by comparing the performance of surface immobilized versus solution phase sensing molecules

We demonstrate proof-of-concept that generator-collector experiments can be performed at a single macroelectrode and used to determine mechanistic information. The practical advantages of such a system over conventional generator-collector techniques are also outlined. The single-electrode generator-collector technique is applied to study the known mechanism of oxygen reduction in aqueous conditions as a model system. We seek to demonstrate that the single-electrode generator-collector approach is capable of detecting local pH changes, immediately adjacent to the electrode surface during a redox reaction. Experiments are performed using a molecular pH probe attached to the electrode surface. Comparison of experimental data with numerical simulations verifies that the reduction of oxygen at pH 6.8 proceeds via a two-electron, two-proton mechanism. Experiments were also performed with a molecular pH probe dissolved in the electrolyte solution in order to explore the feasibility of this approach, which is potentially applicable to a much wider range of electrochemical systems.

Theory and experiments of transport at channel microband electrodes under laminar flows. 2. Electrochemical regimes at double microband assemblies under steady state

The development of any particular analytical or preparative applications using electrochemical techniques in microfluidic devices requires integration of microelectrodes. This involves detailed predictions for optimizing the design of devices and selecting the best hydrodynamic conditions. For this purpose, we undertook a series of works aimed at a precise investigation of mass transport near electrodes with focus on analytical measurements. Part I of this series (Anal. Chem. 2007, 79, 8502-8510) evaluated the common case of a single microband electrode embedded within a microchannel under laminar flow. The present work (Part 2) investigated the case of a pair of microband electrodes operating either in generator-generator or generator-collector modes. The influence of the confining effect and flow velocity on the amperometric responses was examined on the basis of numerical simulations under steady-state regime. Several situations were identified, each of them corresponding to specific interactions taking place between the electrodes. Related conditions were extracted to establish a zone diagram describing all the situations. These predictions were systematically validated by experimental measurements. The results show that amperometric detections within microchannels can be performed at dual electrodes with higher analytical performances than at single ones.

In Situ and On-Line Monitoring of Hydrodynamic Flow Profiles in Microfluidic Channels Based on Microelectrochemistry: Optimization of Channel Geometrical Parameters for Best Performance of Flow Profile Reconstruction

A theoretical approach for flow profile reconstruction in a rectangular microfluidic channel equipped with one or two microband electrodes working in generator-collector and generator-generator regimes was proposed by us previously (ChemPhysChem 2005, 6, 1581-1589; ChemPhysChem 2006, 7, 482-487). The purpose of the current study is to determine the ranges of dimensionless parameters corresponding to the highest sensitivity of the minimized functional to the shape of the flow profile. By application of a cubic spline to approximate the flow profile and analysis of the least-squares functional, which can then be represented as a function of one variable, we derive the area of optimal method performance. Thus, mathematical confirmation of our previous theoretical physical predictions could be obtained.

In situ and online monitoring of hydrodynamic flow profiles in microfluidic channels based upon microelectrochemistry: optimization of electrode locations

Herein, we extend our previous approach concerning the reconstruction of profiles of pressure-driven hydrodynamic flow in microfluidic channels based on current measurements at band electrode(s) [see the preceding paper ChemPhysChem 2005, 6, 1581]. We address the central issue of optimization of geometrical parameters describing the electrode(s) assembly (a single band and two bands working in generator-collector mode) within the channel flow cell to enhance the speed and precision of the flow profile reconstruction method.

Microfluidic Channel Flow Cell for Simultaneous Cryoelectrochemical Electron Spin Resonance

A novel microfluidic electrochemical channel flow cell has been constructed for in situ operation in a cylindrical TE011 resonant ESR cavity under variable temperature conditions. The cell has a U-tube configuration, consisting of an inlet and outlet channel which run parallel and contain evaporated gold film working, pseudo-reference, and counter electrodes. This geometry was employed to permit use in conjunction with variable temperature apparatus which does not allow a flow-through approach. The cell is characterized qualitatively and quantitatively using the one-electron reduction of p-bromonitrobenzene in acetonitrile at room temperature as a model system, and the ESR signal-flow rate response is validated by use of three-dimensional digital simulation of the concentration profile for a stable electrogenerated radical species under hydrodynamic conditions. The cell is then used to obtain ESR spectra for a number of radical species in acetonitrile at 233 K, including the radical anions of m- and p-iodonitrobenzene, o-bromonitrobenzene, and m-nitrobenzyl chloride, the latter three being unstable at room temperature. Spectra are also presented for the radical anion of 2-chloranthraquinone and the crystal violet radical, which display improved resolution at low temperatures.

Developing a tissue perfusion sensor

The development of a electrochemical tissue perfusion sensor is presented. The sensor is a platinum/platinum ring-disc microelectrode that relies on the principle of collector-generator to monitor mass transport within its vicinity. Tissue perfusion is a mass transport mechanism that describes the movement of respiratory gases, nutrients and metabolites in tissue. The sensor's capability of detecting perfusion at the cellular level in a continuous fashion is unique. This sensor will provide insight into the way nutrients and metabolites are transported in tissue especially in cases were perfusion is low such as in wounds or ischemic tissue. We present experimental work for the development and testing of the sensors in vitro. Experimental flow recordings in free steam solutions as well as the flow through tissue-like media are shown. Tests on post operative human tissue are also presented. The sensor's feature such as the continuous recoding capacities, spatial resolution and the measurement range from ml/min to microl/min are highlighted.

Voltammetric Monitoring of Transient Hydrodynamic Flow Profiles in Microfluidic Flow Cells

We consider the transition to steady-state flow in the inlet region of a hydrodynamic channel cell and show that a microelectrode positioned within this inlet region allows chronoamperometric results to be recorded, from which information about the extent of the development of the flow profile may be deduced as well as information about the precise dimensions of the microfluidic channel.