In brief, the device consisted of a sacrificial chromium layer (thickness z = 60В±5 nm) deposited between two platinum electrodes. The method for micro fabricating the devices was similar to that reported previously 27. We emphasize that this work does not in any way diminish the relevance of previous efforts at realizing miniaturized reference electrodes, but instead provides an alternative, complementary approach.ДЄ nanogap sensor with two parallel planar electrodes, as shown in Fig. e) Capillary based fluid flow system consisting of two electrodes imbedded in a flow through (nano) channel. In the closed state the reference electrode (not shown) gets disconnected, as represented by the dashed lines. d) Nanogap sensor with an integrated microfluidic valve (PDMS). c) Optical microscopic image of the top view of the device before the sacrificial layer is etched. L a and z represent the active (or overlapping) length of the top and bottom electrodes and the height of the nanogap, respectively. b) Schematic diagram of the cross section of the device prior to etching (not drawn to scale). An ionic current also flows to the reference electrode. Redox molecules undergo successive oxidation and reduction reactions at the two planar electrodes while diffusing within the nanochannel. 25, 26ДЄ) Nanogap sensor in the conventional 3-electrode configuration that includes a reference electrode. Other configurations where the same general principles apply include interdigitated arrays 19, 21вЂ“ 24 and ring-disk electrodes. Another example is the incorporation of a nanogap directly in (or, more practically, in a side channel of) a capillary flow detection system ( Fig. An example application of a variant of this device in which operation without a reference electrode would be necessary is a sealable chamber allowing locally generated analyte molecules (e.g., by a catalytic reaction) to be collected and detected in a fL-level volume ( Fig. The cross sectional diagram and top views of such a device are shown in Fig.1b and 1c, respectively. Electrochemically active molecules undergo successive redox cycling by diffusing back and forth between these electrodes, generating a current that is proportional to the analyte concentration. 1a, consisting of two closely spaced parallel electrodes separated by a few tens of nanometers and forming the floor and roof of a fluid-filled nanochannel. As a prototypical system we focus on fluidic nanogap sensors, sketched in Fig. In this work we explore the underlying principles of operation of redox cycling systems without a reference electrode. While the primary purpose of redox cycling is typically to increase the magnitude of the electrochemical current or to improve selectivity 18вЂ“ 20, it also provides a pathway for electrochemically generated currents to flow through the analyte solution without the need for a reference or auxiliary electrode. One potential route for bypassing the reference electrode is offered by systems based on redox cycling, in which a current is generated by successively reducing and oxidizing an analyte at two independently biased electrodes. Realizing lab-on-a-chip devices without the use of conventional reference electrodes 12, 14вЂ“ 17 could therefore significantly improve the performance and usability of miniaturised electrochemical sensors. For example, due to downscaling, dissolution of the electrode material, leakage of the reference solution and contamination at the junction of salt bridges become difficult to avoid, rendering many sensors short-lived or unreliable. In miniaturized systems, however, the volume-to-surface area ratio of reference electrodes decreases, thereby reducing their lifetime and stability 12вЂ“ 16. Irrespective of the particular measurement method employed (amperometry, potentiometry, impedance spectroscopy, etc.), the reference electrode is a key component of electrochemical sensors. Electrochemical sensors are particularly well suited for cost effective realization and integration with fluidics due to their inherent compatibility with microfabrication 1вЂ“ 11. Motivated by the wide applications of electrochemical sensors in the biomedical domain, there is continuously rising interest in miniaturised assays suitable for integration.

Icircuit reference