Cyclic voltammograms at coplanar and shallow recessed microdisk electrode arrays: guidelines for design and experiment

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.

Redox cycling in nanoscale-recessed ring-disk electrode arrays for enhanced electrochemical sensitivity

An array of nanoscale-recessed ring-disk electrodes was fabricated using layer-by-layer deposition, nanosphere lithography, and a multistep reactive ion etching process. The resulting device was operated in generator-collector mode by holding the ring electrodes at a constant potential and performing cyclic voltammetry by sweeping the disk potential in Fe(CN)6(3-/4-) solutions. Steady-state response and enhanced (~10×) limiting current were achieved by cycling the redox couple between ring and disk electrodes with high transfer/collection efficiency. The collector (ring) electrode, which is held at a constant potential, exhibits a much smaller charging current than the generator (disk), and it is relatively insensitive to scan rate. A characteristic feature of the nanoscale ring-disk geometry is that the electrochemical reaction occurring at the disk electrodes can be tuned by modulating the potential at the ring electrodes. Measured shifts in Fe(CN)6(3-/4-) concentration profiles were found to be in excellent agreement with finite element method simulations. The main performance metric, the amplification factor, was optimized for arrays containing small diameter pores (r < 250 nm) with minimum electrode spacing and high pore density. Finally, integration of the fabricated array within a nanochannel produced up to 50-fold current amplification as well as enhanced selectivity, demonstrating the compatibility of the device with lab-on-a-chip architectures.

Theoretical investigation of generator-collector microwell arrays for improving electroanalytical selectivity: application to selective dopamine detection in the presence of ascorbic acid

Recessed generator-collector assemblies consisting of an array of recessed disks (generator electrodes) with a gold layer (collector electrode) deposited over the top-plane insulator reportedly allow increased selectivity and sensitivity during electrochemical detection of dopamine (DA) in the presence of ascorbic acid (AA), a situation which is frequently encountered. In sensor design, the potential of the disk electrodes is set to the wave plateau of DA, whereas the plane electrode is biased at the irreversible wave plateau of AA before the onset of the DA oxidation wave. Thus, AA is scavenged but DA is allowed to enter the nanocavities to be oxidized at the disk electrodes, and its signal is further amplified by redox cycling between disk and plane electrodes. Several different theoretical approaches are elaborated herein to analyze the behavior of the system, and their conclusions are successfully tested by experiments. This reveals the crucial role of the plane-electrode area which screens access to the recessed disks (i.e. acts as a diffusional Faraday cage) and simultaneously contributes to amplification of the analyte signal through positive feedback, as occurs in interdigitated arrays and scanning electrochemical microscopy. Simulations also allow for the evaluation of the benefits of different geometries inspired by the above design and different operating modes for increasing the sensor performance.

Polycarbonate-based ordered arrays of electrochemical nanoelectrodes obtained by e-beam lithography

Ordered arrays of nanoelectrodes for electrochemical use are prepared by electron beam lithography (EBL) using polycarbonate as a novel e-beam resist. The nanoelectrodes are fabricated by patterning arrays of holes in a thin film of polycarbonate spin-coated on a gold layer on Si/Si(3)N(4) substrate. Experimental parameters for the successful use of polycarbonate as high resolution EBL resist are optimized. The holes can be filled partially or completely by electrochemical deposition of gold. This enables the preparation of arrays of nanoelectrodes with different recession degree and geometrical characteristics. The polycarbonate is kept on-site and used as the insulator that separates the nanoelectrodes. The obtained nanoelectrode arrays (NEAs) exhibit steady state current controlled by pure radial diffusion in cyclic voltammetry for scan rates up to approximately 50 mV s( - 1). Electrochemical results showed satisfactory agreement between experimental voltammograms and suitable theoretical models. Finally, the peculiarities of NEAs versus ensembles of nanoelectrodes, obtained by membrane template synthesis, are critically evaluated.

Nanocrystalline diamond nanoelectrode arrays and ensembles

In this report, the fabrication of all-nanocrystalline diamond (NCD) nanoelectrode arrays (NEAs) by e-beam lithography as well as of all-diamond nanoelectrode ensembles (NEEs) using nanosphere lithography is presented. In this way, nanostructuring techniques are combined with the excellent properties of diamond that are desirable for electrochemical sensor devices. Arrays and ensembles of recessed disk electrodes with radii ranging from 150 to 250 nm and a spacing of 10 μm have been fabricated. Electrochemical impedance spectroscopy as well as cyclic voltammetry was conducted to characterize arrays and ensembles with respect to different diffusion regimes. One outstanding advantage of diamond as an electrode material is the stability of specific surface terminations influencing the electron transfer kinetics. On changing the termination from hydrogen- to oxygen-terminated diamond electrode surface, we observe a dependence of the electron transfer rate constant on the charge of the analyte molecule. Ru(NH(3))(6)(+2/+3) shows faster electron transfer on oxygen than on hydrogen-terminated surfaces, while the anion IrCl(6)(-2/-3) exhibits faster electron transfer on hydrogen-terminated surfaces correlating with the surface dipole layer. This effect cannot be observed on macroscopic planar diamond electrodes and emphasizes the sensitivity of the all-diamond NEAs and NEEs. Thus, the NEAs and NEEs in combination with the efficiency and suitability of the selective electrochemical surface termination offer a new versatile system for electrochemical sensing.

On-chip multi-electrochemical sensor array platform for simultaneous screening of nitric oxide and peroxynitrite

In this work we report on the design, microfabrication and analytical performances of a new electrochemical sensor array (ESA) which allows for the first time the simultaneous amperometric detection of nitric oxide (NO) and peroxynitrite (ONOO(-)), two biologically relevant molecules. The on-chip device includes individually addressable sets of gold ultramicroelectrodes (UMEs) of 50 µm diameter, Ag/AgCl reference electrode and gold counter electrode. The electrodes are separated into two groups; each has one reference electrode, one counter electrode and 110 UMEs specifically tailored to detect a specific analyte. The ESA is incorporated on a custom interface with a cell culture well and spring contact pins that can be easily interconnected to an external multichannel potentiostat. Each UME of the network dedicated to the detection of NO is electrochemically modified by electrodepositing thin layers of poly(eugenol) and poly(phenol). The detection of NO is performed amperometrically at 0.8 V vs. Ag/AgCl in phosphate buffer solution (PBS, pH = 7.4) and other buffers adapted to biological cell culture, using a NO-donor. The network of UMEs dedicated to the detection of ONOO(-) is used without further chemical modification of the surface and the uncoated gold electrodes operate at -0.1 V vs. Ag/AgCl to detect the reduction of ONOOH in PBS. The selectivity issue of both sensors against major biologically relevant interfering analytes is examined. Simultaneous detection of NO and ONOO(-) in PBS is also achieved.

Electrochemical surface nanopatterning using microspheres and aryldiazonium

A multistep procedure to prepare heterogeneous structured surfaces with contrasted chemical functionalities at the nanometer scale is presented. Aryldiazonium cations are used for the nanopatterning of electrodes to create hybrid surfaces. The nanopatterning procedure involves the auto-organization of a polystyrene (PS) beads layer at gold or glassy carbon electrode surfaces. The deposited beads layer permits masking of a fraction of the surface from a first aryldiazonium electrografting process. By subsequent removal of the PS beads, the ungrafted surface areas become available for either another aryl diazonium electrografting or a metal electrodeposition, leading to hybrid nanostructured surfaces.

Lab-on-chip flow injection analysis system without an external pump and valves and integrated with an in line electrochemical detector

Surface energy in small droplets can be used to drive samples through microchannels. When a sample fluid is spontaneously driven through a solution filled microchannel with liquid droplets on its entry (sample) and exit (reservoir) ports, it is termed as a passive pumping device. A passive pump driven microfluidic system integrated with microfabricated planar electrodes or electrode arrays (e.g., interdigitated electrode arrays or microband electrode arrays) can be considered as a manifold for flow injection analysis without an external pump and injector valve. Factors affecting the passive pump driven flow rate in a polydimethylsiloxane (PDMS)-glass hybrid microfluidic system, including the volume, viscosity, and surface tension of the sample solution and the tilt of the microfluidic channel, are analyzed. By placing 2 microL of hexacyanoferrate (II) solutions at the entry port of the device, peak shaped transients were recorded. The peak heights showed linear dependence on the sample concentrations between 3 x 10(-7) and 10(-5) M.

Numerical simulation of diffusion processes at recessed disk microelectrode arrays using the quasi-conformal mapping approach

In this work, we present a theoretical analysis of diffusion processes at arrays of recessed microelectrodes and evaluate the dependence of these processes on the main geometrical parameters (distance between electrodes in the array and slope of side walls of conical recesses) of this complex system. To allow for faster computation time and excellent accuracy, numerical simulations were performed upon transforming the real space allowed for diffusion using a quasi-conformal mapping introduced for this array geometry in our previous work (Amatore, C., Oleinick, A. I. and Svir, I. J. Electroanal. Chem. 2006, 597, 77-85). The applied quasi-conformal mapping is perfectly suited to the considered microelectrode array geometry and ensures that the abrupt change of boundary conditions reflecting the contorted geometries of the considered microelectrode array are treated efficiently and precisely in the simulations.

Cyclic Voltammetry at Shallow Recessed Microdisc Electrode: Theoretical and Experimental Study

This study focuses on the cyclic voltammetry behavior at shallow recessed microdisc electrode, particularly on the transition from cottrellian behavior to steady state behavior. Diffusion to the inlaid and recessed microdisc electrode is simulated. From the shape of the CVs, for a given radius and potential scan rate, the transition time from planar diffusion to hemispherical diffusion presents a minimum as the recess increases. Theoretical prediction was confirmed by fitting the simulated CVs with experimental results. Dimensionless transition scan rate has been defined and determined by simulation for inlaid and recessed microdisc electrodes.