Coupled Electrochemical Reactions at Bipolar Microelectrodes and Nanoelectrodes

Here we report the voltammetric study of coupled electrochemical reactions on microelectrodes and nanoelectrodes in a closed bipolar cell. We use steady-state cyclic voltammetry to discuss the overall voltammetric response of closed bipolar electrodes (BPEs) and understand its dependence on the concentration of redox species and electrode size. Much of the previous work in bipolar electroanalytical chemistry has focused on the use of an “open” cell with the BPE located in an open microchannel. A closed BPE, on the other hand, has two poles placed in separate compartments and has remained relatively unexplored in this field. In this work, we demonstrated that carbon-fiber microelectrodes when backfilled with an electrolyte to establish conductivity are closed BPEs. The coupling between the oxidation reaction, e.g., dopamine oxidation, on the carbon disk/cylinder and the reduction of oxygen on the interior fiber is likely to be responsible for the conductivity. We also demonstrated the ability to quantitatively measure voltammetric properties of both the cathodic and anodic poles in a closed bipolar cell from a single cyclic voltammetry (CV) scan. It was found that “secondary” reactions such as oxygen reduction play an important role in this process. We also described the fabrication and use of Pt bipolar nanoelectrodes which may serve as a useful platform for future advances in nanoscale bipolar electrochemistry

Method for Low Nanomolar Concentration Analyte Sensing Using Electrochemical Enzymatic Biosensors

We introduce a new electrochemical measurement method compatible with an enzymatic biosensor that is capable of analyte sensing down to the low nanomolar concentration regime. This method is termed accumulation mode sensing and utilizes an immobilized redox polymer mediator wired to an oxidoreductase enzyme to store charge during a premeasurement charge concentration step, followed by a measurement step in which this accumulated charge is quantified. We demonstrate this new method using a model glucose sensor and show how the sensitivity of a sensor can be modified simply by adjusting the time duration of the charge concentration step. We achieve a limit of detection of 4.7 ± 1.4 nM using accumulation mode sensing, which represents a 25-fold improvement over traditional amperometry

Collision Dynamics during the Electrooxidation of Individual Silver Nanoparticles

Recent high-bandwidth recordings of the oxidation and dissolution of 35 nm radius Ag nanoparticles at a Au microelectrode show that these nanoparticles undergo multiple collisions with the electrode, generating multiple electrochemical current peaks. In the time interval between observed current peaks, the nanoparticles diffuse in the solution near the electrolyte/electrode interface. Here, we demonstrate that simulations of random nanoparticle motion, coupled with electrochemical kinetic parameters, quantitatively reproduce the experimentally observed multicurrent peak behavior. Simulations of particle diffusion are based on the nanoparticle-mass-based thermal nanoparticle velocity and the Einstein diffusion relations, while the electron-transfer rate is informed by the literature exchange current density for the Ag/Ag⁺ redox system. Simulations indicate that tens to thousands of particle–electrode collisions, each lasting ∼6 ns or less (currently unobservable on accessible experimental time scales), contribute to each experimentally observed current peak. The simulation provides a means to estimate the instantaneous current density during a collision (∼500–1000 A/cm²), from which we estimate a rate constant between ∼5 and 10 cm/s for the electron transfer between Ag nanoparticles and the Au electrode. This extracted rate constant is approximately equal to the thermal collisional velocity of the Ag nanoparticle (4.6 cm/s), the latter defining the theoretical upper limit of the electron-transfer rate constant. Our results suggest that only ∼1% of the surface atoms on the Ag nanoparticles are oxidized per instantaneous collision. The combined simulated and experimental results underscore the roles of Brownian motion and collision frequency in the interpretation of heterogeneous electron-transfer reactions involving nanoparticles