Individually addressable gel-integrated voltammetric microelectrode array for high-resolution measurement of concentration profiles at interfaces

Tip-porous microneedle: A highly stable sensing platform for direct determination of labile metals in natural seawater

Nanomaterial-functionalized voltammetric microsensors are promising tools for detecting trace metals at low concentrations in the complex environment of natural seawater. However, the sensitivity reduction caused by the loss of modified nanomaterials in the detection process has always been a major problem. Herein, to fabricate a highly stable sensing platform, a microneedle electrode with a hierarchical porous tip was prepared through electrochemical etching technology to firmly embed nanoparticles. Using copper (Cu) as a model trace metal, a micro-cluster needle sensor based on a gold nanoparticle (AuNP)-embedded tip-porous microneedle electrode (P-MNE) was fabricated for the direct voltammetric determination of labile Cu in natural seawater. The porous structure of P-MNE not only provided a larger specific surface area and active sites for AuNPs which had excellent electrocatalytic performance for Cu²⁺ determination, but also protected from their loss during the detection process in seawater. Therefore, this novel micro-cluster needle sensor exhibited significantly improved stability with a relative standard deviation (RSD) of 1.5% for 30 detections. The linear range of Cu²⁺ on this micro-cluster needle sensor was from 0.1 to 1000 nM with a detection limit of 0.03 nM. More importantly, this micro-cluster needle sensor was successfully used for directly detecting labile Cu in natural seawater samples without any preaccumulation treatment or reagent addition to obtain the contribution proportions of the labile fraction in total dissolved Cu. Furthermore, this sensing platform might also be extended to the reliable determination of other labile metals in seawater by changing the functional nanoparticles embedded in the nanopores.

Two approaches for addressing electrochemical electrode arrays with reduced external connections

Although patterning hundreds or thousands of electrochemical electrodes on lab-on-a-chip devices is straightforward and cost-effective using photolithography, easily making connections between hundreds of electrodes and external amplifiers remains a bottleneck. Here we describe two electrode addressing approaches using multiple fluid compartments that can potentially reduce the number of external connections by ~100-fold. The first approach enables all compartments on the device to be filled with solution at the same time, and then each fluid compartment is sequentially electrically activated to make the measurements. The second approach achieves lower measurement noise by sequentially filling recording chambers with solution. We propose an equivalent circuit to explain measurement noise in these recording configurations and demonstrate application of the approaches to measure quantal exocytosis from individual cells. A principle advantage of using these approaches is that they reduce the fraction of the microchip area that needs to be dedicated to making external connections and therefore reduces the cost per working electrode.

Droplet array on local redox cycling-based electrochemical (LRC-EC) chip device

We have previously reported a local redox cycling-based electrochemical (LRC-EC) system for the incorporation of many electrochemical sensors into a small chip device. In the present study, a new type of LRC-EC chip device was fabricated for the detection of a droplet array. To detect electrochemically redox compounds in droplets, Pt pseudo-reference/counter electrodes were incorporated into the individual sensors of the LRC-EC chip device. Cyclic voltammetry for the LRC-EC chip device with internal Pt pseudo-reference electrodes indicated well-defined voltammograms based on redox cycling for the individual sensor points. The device was successfully applied to the addressable detection of alkaline phosphatase (ALP) activity of HeLa cells in single droplets on the sensor points. Therefore, the LRC-EC chip device is considered to be a useful device for the bioanalysis of droplet systems.

Addressable electrode array device with IDA electrodes for high-throughput detection

An electrochemical device is proposed for high-throughput electrochemical detection that consists of 32 row and 32 column electrodes on a single glass substrate. The row and column electrodes are connected to interdigitated array (IDA) electrodes to form 1024 (32 × 32) addressable sensor points in the device. Electrochemical responses from each of the 1024 sensors were successfully acquired on the device within 1 min using redox cycling at individual IDA electrodes, which ensures application of the device to comprehensive, high-throughput electrochemical detection for enzyme-linked immunosorbent assay (ELISA), reporter gene assay for monitoring gene expressions, and DNA analysis.

Fluorimetric lead detection in a microfluidic device

A microfabricated device has been developed for the selective detection of lead in water. It is based on the use of a selective and sensitive fluorescent molecular sensor for lead (Calix-DANS4) which contains a calix[4]arene bearing four dansyl groups. The microchip-based lead sensor contains a Y-shape microchannel equipped with a passive mixer and moulded on a glass substrate. An optimization of the microcircuit length has been performed in order to have a full complexation of the Calix-DANS4. The detection is performed by using a configuration in which the sensing molecules are excited by two optical fibres, each one connected to a 365 nm UV LED, and the light collection is made by another optical fibre with a photomultiplier. By using this configuration we have shown the possibility to detect lead with a detection limit of 5 ppb. The effect of interfering cations such as calcium has been evaluated. The obtained measurements have been validated by an alternative method (ASV).

Cadmium speciation assessed by voltammetry, ion exchange and geochemical calculation in soil solutions collected after soil rewetting

Analytical techniques and speciation models have been developed to characterize the speciation of Cd in soil solution. They provide an estimate of operationally defined species of Cd that need to be compared, especially for soil solutions highly concentrated in organic matter as are the solutions collected after soil rewetting. This work deals with the comparison between the speciation of Cd measured by anodic stripping voltammetry (ASV) and ion exchange and the speciation of Cd calculated using Visual MINTEQ. The aim of this study was to quantify and explain the differences in Cd speciation observed between the three approaches. Cd speciation was assessed in soil solutions collected 4, 8, 24, 48, 96 and 144h after the rewetting of an air-dried contaminated soil. To optimize the computed speciation of Cd, other physico-chemical parameters were followed (e.g. pH, ionic strength and the concentrations of major anions, major cations and dissolved organic carbon) and a brief characterisation of dissolved organic matter (DOM) was performed. The discrepancy between model predictions and analytical measurements highlighted the need for caution in the interpretation of geochemical speciated data for Cd. The major result of this study was that a characterization of DOM based on its specific UV-absorbance at 254 nm improved the accuracy of model predictions. Another finding is that labile Cd complexes, even organic, may have been included in the electrochemically labile fraction of Cd measured by ASV.

Microelectrode arrays for electrochemistry: approaches to fabrication

Microelectrode arrays have unique electrochemical properties such as small capacitive-charging currents, reduced iR drop, and steady-state diffusion currents. These properties enable the use of microelectrode arrays and have captured much interest in the field of electrochemistry. Techniques for the fabrication of such arrays are reviewed. The relative features and merits of different techniques are also discussed.