Electrochemical redox cycling devices

Nanoelectrochemistry is a fascinating research discipline at the interface between nanotechnology and electrochemistry. Alongside proceeding technological advances in micro- and nanofabrication, the miniaturization of classic electrochemical setups was enabled during the last two decades. By this means, new effects were observed, which allowed the development of novel concepts for nanoelectrochemical sensors. The presented work investigates the comparably young concept of nanofluidic redox-cycling sensors in regard to their fabrication, application, and theoretical description. These sensors have been first introduced by the group of Serge Lemay in 2007 and usually comprise two parallel electrodes that are incorporated underneath and above a nanofluidic channel, while electrodes can be biased individually. Redox-active molecules can then participate in fast, repeated reactions at the electrodes. By this means, a current is formed across the gap, which is significantly amplified in comparison to conventional electrochemical sensors. Therefore, the sensitivity of such devices often exceeds the sensitivity of classic sensors by orders of magnitude and even enables sensing at molecular resolution. Both presented devices are highly-integrated on-chip sensors for the spatiotemporal detection of redox-active molecules. The first chip features an array of nanofluidic sensors, while individual sensors can be operated in parallel. The chip is characterized in detail and is used for the spatiotemporal detection of concentration gradients inside a microfluidic system. The second chip is developed particularly for high spatial resolutions and can be employed for electrochemical imaging. By utilizing a crossbar-architecture, the degree of integration is drastically increased and the overall number of sensors on the chip as well as the sensor density is significantly improved. The small size and the high sensitivity of nanofluidic sensors further lead to a variety of mesoscopic effects that can partly be observed in the sensor's current noise. In order to investigate these effects, a comprehensive simulation framework for the modeling of the sensor's noise is described, which already was employed in various studies. The model is based on the description of the redox-active molecules' Brownian movement through random walks, which allows simulations of noise phenomena that cannot be modeled by finite elements approaches. By this means, experimental data can be closely reproduced and predicted. Based on these results, new sensor concepts are suggested for the detection of biological macromolecules in nano- and micro pores and for the measurement of the average adsorption times of redox-active molecules in common electrochemical measurement setups

Parallel On-Chip Analysis of Single Vesicle Neurotransmitter Release

Real-time investigations of neurotransmitter release provide a direct insight on the mechanisms involved in synaptic communication. Carbon fiber microelectrodes are state-of-the-art tools for electrochemical measurements of single vesicle neurotransmitter release. Yet, they lack high-throughput capabilities that are required for collecting robust statistically significant data across multiple samples. Here, we present a chip-based recording system enabling parallel in vitro measurements of individual neurotransmitter release events from cells, cultured directly on planar multielectrode arrays. The applicability of this cell-based platform to pharmacological screening is demonstrated by resolving minute concentration-dependent effects of the dopamine reuptake inhibitor nomifensine on recorded single-vesicle release events from PC12 cells. The experimental results, showing an increased half-time of the recorded events, are complemented by an analytical model for the verification of drug action

Understanding single enzyme activity via the nano-impact technique.

To evaluate the possible detection of single enzyme activity via electrochemical methods, a combined finite difference and random walk simulation is used to model individual enzyme-electrode collisions where such events are monitored amperometrically via the measurement of products formed by the enzyme in solution. It is found that the observed signal is highly sensitive to both the enzyme turnover number, the size of the electrode and the bandwidth of the electronics. Taking single catalase impacts as an example, simulation results are compared with experimental data. Our work shows the requirement for the detection of electrochemically active product formed by individual enzymes and gives guidance for the design of experiments

Use of artificial intelligence in electrode reaction mechanism studies: Predicting voltammograms and analyzing the dissociative CE reaction at a hemispherical electrode

Artificial intelligence (AI) is used to learn the key voltammetric characteristics of the dissociative CE mechanism via training from multiple simulations using bespoke code. This allows first for the prediction of voltammograms without the need for further simulations, given knowledge of the relevant experimental parameters (rate and equilibrium constants, electrode geometry, and diffusion coefficients). Second, it is applied to analyze noisy experimental voltammetry to characterize the mechanistic type and to successfully extract the key kinetic and thermodynamic parameters

Nanocavity crossbar arrays for parallel electrochemical sensing on a chip

We introduce a novel device for the mapping of redox-active compounds at high spatial resolution based on a crossbar electrode architecture. The sensor array is formed by two sets of 16 parallel band electrodes that are arranged perpendicular to each other on the wafer surface. At each intersection, the crossing bars are separated by a ca. 65 nm high nanocavity, which is stabilized by the surrounding passivation layer. During operation, perpendicular bar electrodes are biased to potentials above and below the redox potential of species under investigation, thus, enabling repeated subsequent reactions at the two electrodes. By this means, a redox cycling current is formed across the gap that can be measured externally. As the nanocavity devices feature a very high current amplification in redox cycling mode, individual sensing spots can be addressed in parallel, enabling high-throughput electrochemical imaging. This paper introduces the design of the device, discusses the fabrication process and demonstrates its capabilities in sequential and parallel data acquisition mode by using a hexacyanoferrate probe