Theory and Experiments of Transport at Channel Microband Electrodes Under Laminar Flow. 3. Electrochemical Detection at Electrode Arrays under Steady State

Enhancement of faradaic current in an electrochemical cell integrated into silicon microfluidic channels

Implantable electrochemical sensors enable fast and sensitive detection of analytes in biological tissue, but are hampered by bio-foulant attack and are unable to be recalibrated in-situ. Herein, an electrochemical sensor integrated into ultra-low flow (nL/min) silicon microfluidic channels for protection from foulants and in-situ calibration is demonstrated. The small footprint (5 μm radius channel cross-section) of the device allows its integration into implantable sampling probes for monitoring chemical concentrations in biological tissues. The device is designed for fast scan cyclic voltammetry (FSCV) in the thin-layer regime when analyte depletion at the electrode is efficiently compensated by microfluidic flow. A 3X enhancement of faradaic peak currents is observed due to the increased flux of analytes towards the electrodes. Numerical analysis of in-channel analyte concentration confirmed near complete electrolysis in the thin-layer regime below 10 nL/min. The manufacturing approach is highly scalable and reproducible as it utilizes standard silicon microfabrication technologies.

Electrochemical assessments of droplet contents in microfluidic channels. Application to the titration of heterogeneous droplets

Series of aqueous droplets containing redox species were generated on-demand in a microfluidic channel and detected downstream by an electrochemical cell. Depending on the cell geometry, amperometric detections were performed to simultaneously determine the velocity, volume and content of circulating droplets in oil. Volumes and velocities were estimated from specific transition times on the chronoamperometric responses, while charge were evaluated from current integration. The results showed that the total charge within droplets was controlled by the geometry of the electrochemical cell and droplet velocity, leading to accurate determinations of droplet content under specific operating conditions. An active merging of droplets with titrating solutions was tested for analytical purposes. The results demonstrated that even if the mixing was not complete during detection, the assessment of droplet content was still valid. The performance of electrochemical detection was thus evidenced to determine the content of heterogeneous droplets. This property is pertinent since the design of sophisticated circuits is no longer required to fully homogenize the droplet content before characterization, opening broader perspectives in droplet-based microfluidics.

Electrochemical Performance of Micropillar Array Electrodes in Microflows

The microchip-based electrochemical detection system (μEDS) has attracted plenty of research attention due to its merits including the capability in high-density integration, high sensitivity, fast analysis time, and reduced reagent consumption. The miniaturized working electrode is usually regarded as the core component of the μEDS, since its characteristic directly determines the performance of the whole system. Compared with the microelectrodes with conventional shapes such as the band, ring and disk, the three-dimensional (3D) micropillar array electrode (μAE) has demonstrated significant potential in improving the current response and decreasing the limits of detection due to its much larger reaction area. In this study, the numerical simulation method was used to investigate the performance of the μEDS, and both the geometrical and hydrodynamic parameters, including the micropillars shape, height, arrangement form and the flow rate of the reactant solution, were taken into consideration. The tail effect in μAEs was also quantitatively analyzed based on a pre-defined parameter of the current density ratio. In addition, a PDMS-based 3D μAE was fabricated and integrated into the microchannel for the electrochemical detection. The experiments of cyclic voltammetry (CV) and chronoamperometry (CA) were conducted, and a good agreement was found between the experimental and simulation results. This study would be instructive for the configuration and parameters design of the μEDS, and the presented method can be adopted to analyze and optimize the performance of nanochip-based electrochemical detection system (nEDS).

Pencil Lead as a Material for Microfluidic 3D-Electrode Assemblies

We present an electrochemical, microfluidic system with a working electrode based on an ordered 3D array of pencil leads. The electrode array was integrated into a plexiglass/PDMS channel. We tested the setup using a simple redox probe and compared the results with computer simulations. As a proof of concept application of the device we showed that the setup can be used for determination of dopamine concentration in physiological pH and ultrasensitive, although only qualitative, detection of p-nitrophenol with a limit of detection below 1 nmol L⁻¹. The observed limit of detection for p-nitrophenol is not only much lower than achieved with similar methods but also sufficient for evaluation of exposure to pesticides such as methyl parathion through urinalysis. This low cost setup can be fabricated without the need for clean room facilities and in the future, due to the ordered structure of the electrode could be used to better understand the process of electroanalysis and electrode functionalization. To the best of our knowledge it is the first application of pencil leads as 3D electrochemical sensor in a microfluidic channel.

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.

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

In this paper, we present two new methodologies of improving the performance of microchip-based electrochemical detection in microfluidic devices. The first part describes the fabrication and characterization of epoxy-embedded gold microelectrode arrays that are evenly spaced and easily modified. Electrodepositions using a gold plating solution can be performed on the electrodes to result in a 3D pillar array that, when used with microchip-based flow injection analysis, leads to an eightfold increase in signal (when compared to a single electrode), with the LOD for catechol being 4 nM. For detecting analytically challenging molecules such as nitric oxide (NO), platinization of electrodes is commonly used to increase the sensitivity. It is shown here that microchip devices containing either the pillar arrays or more traditional glassy carbon electrodes can be modified with platinum black (Pt-black) for NO detection. In the case of using glassy carbon electrodes for NO detection, integration of the resulting platinized electrode with microchip-based flow analysis resulted in a ten times signal increase relative to use of a bare glassy carbon electrode. In addition, it is demonstrated that these electrodes can be coated with Nafion to impart selectivity toward NO over interfering species such as nitrite. The LOD for NO when using the Pt-black /Nafion-coated glassy carbon electrode was 9 nM. These electrodes can also be embedded in a polystyrene substrate, with the applicability of these sensitive and selective electrodes being demonstrated by monitoring the adenosine triphosphate-mediated release of NO from endothelial cells immobilized in a microfluidic network without any adhesion factor.

Convective delivery of electroactive species to annular nanoband electrodes embedded in nanocapillary-array membranes

Significant technological drivers motivate interest in the use of reaction sites embedded within nanometer-scale channels, and an important class of these structures is realized by an embedded annular nanoband electrode (EANE) in a cylindrical nanochannel. In this structure, the convective delivery of electroactive species to the nanoelectrode is tightly coupled to the electrochemical overpotential via electroosmotic flow. Simulation results indicate that EANE arrays significantly outperform comparable microband electrode/microchannel structures, producing higher conversion efficiencies at low Peclet number. The results of this in-depth analysis are useful in assessing possible implementation of the EANE geometry for a wide range of electrochemical targets within microscale total analysis systems.

Optimisation of a microfluidic analysis chamber for the placement of microelectrodes

The behaviour of droplets entering a microfluidic chamber designed to house microelectrode detectors for real time analysis of clinical microdialysate is described. We have designed an analysis chamber to collect the droplets produced by multiphase flows of oil and artificial cerebral spinal fluid. The coalescence chamber creates a constant aqueous environment ideal for the placement of microelectrodes avoiding the contamination of the microelectrode surface by oil. A stream of alternating light and dark coloured droplets were filmed as they passed through the chamber using a high speed camera. Image analysis of these videos shows the colour change evolution at each point along the chamber length. The flow in the chamber was simulated using the general solution for Poiseuille flow in a rectangular chamber. It is shown that on the centre line the velocity profile is very close to parabolic, and an expression is presented for the ratio between this centre line velocity and the mean flow velocity as a function of channel aspect ratio. If this aspect ratio of width/height is 2, the ratio of flow velocities closely matches that of Poiseuille flow in a circular tube, with implications for connections between microfluidic channels and connection tubing. The droplets are well mixed as the surface tension at the interface with the oil dominates the viscous forces. However once the droplet coalesces with the solution held in the chamber, the no-slip condition at the walls allows Poiseuille flow to take over. The meniscus at the back of the droplet continues to mix the droplet and acts as a piston until the meniscus stops moving. We have found that the no-slip conditions at the walls of the chamber, create a banding effect which records the history of previous drops. The optimal position for sensors is to be placed at the plane of droplet coalescence ideally at the centre of the channel, where there is an abrupt concentration change leading to a response time ≪16 ms, the compressed frame rate of the video. Further away from this point the response time and sensitivity decrease due to convective dispersion.