Transistor Functions Based on Electrochemical Rectification

High-Current-Density Organic Electrochemical Diodes Enabled by Asymmetric Active Layer Design

Owing to their outstanding electrical/electrochemical performance, operational stability, mechanical flexibility, and decent biocompatibility, organic mixed ionic-electronic conductors have shown great potential as implantable electrodes for neural recording/stimulation and as active channels for signal switching/amplifying transistors. Nonetheless, no studies exist on a general design rule for high-performance electrochemical diodes, which are essential for highly functional circuit architectures. In this work, generalizable electrochemical diodes with a very high current density over 30 kA cm^-2 are designed by introducing an asymmetric active layer based on organic mixed ionic-electronic conductors. The underlying mechanism on polarity-sensitive balanced ionic doping/dedoping is elucidated by numerical device analysis and in operando spectroelectrochemical potential mapping, while the general material requirements for electrochemical diode operation are deduced using various types of conjugated polymers. In parallel, analog signal rectification and digital logic processing circuits are successfully demonstrated to show the broad impact of circuits incorporating organic electrochemical diodes. It is expected that organic electrochemical diodes will play vital roles in realizing multifunctional soft bioelectronic circuitry in combination with organic electrochemical transistors.

Redox Cycling in Individually Encapsulated Attoliter-Volume Nanopores

Redox cycling electrochemistry in arrays of individually encapsulated attoliter-volume ( V ∼ 10 aL) nanopores is investigated and reported here. These nanopore electrode array (NEA) structures exhibit distinctive electrochemical behaviors not observed in open NEAs, which allow free diffusion of redox couples between the nanopore interior and bulk solution. Confined nanopore environments, generated by sealing NEAs with a layer of poly(dimethylsiloxane), are characterized by enhanced currents-up to 250-fold compared with open NEAs-owing to effective trapping of the redox couple inside the nanopores and to enhanced mass transport effects. In addition, electrochemical rectification ( ca. 1.5-6.3) was observed and is attributed to ion migration. Finite-element simulations were performed to characterize the concentration and electric potential gradients associated with the disk electrode, aqueous medium, and ring electrode inside the nanopores, and the results are consistent with experimental observations. The additional signal enhancement and redox-cycling-based rectification behaviors produced in these self-confined attoliter-volume nanopores are potentially useful in devising ultrasensitive sensors and molecular-based iontronic devices.

Asymmetric Nafion-Coated Nanopore Electrode Arrays as Redox-Cycling-Based Electrochemical Diodes

Inspired by the functioning of cellular ion channels, pore-based structures with nanoscale openings have been fabricated and integrated into ionic circuits, for example, ionic diodes and transistors, for signal processing and detection. In these systems, the nonlinear current responses arise either because asymmetric nanopore geometries break the symmetry of the ion distribution, creating unequal surface charge across the nanopore, or by coupling unidirectional electron transfer within a nanopore electrode. Here we develop a high-performance redox-cycling-based electrochemical diode by coating an asymmetric ion-exchange membrane, that is, Nafion, on the top surface of a nanopore electrode array (Nafion@NEA), in which each pore in the array exhibits one or more annular electrodes. Nafion@NEAs exhibit highly sensitive and charge-selective electroanalytical measurements due to efficient redox-cycling reaction, the permselectivity of Nafion, and the strong confinement of redox species in the nanopore array. In addition, the top electrode of dual-electrode Nafion@NEAs can serve as a voltage-controlled switch to gate ion transport within the nanopore. Thus Nafion@NEAs can be operated as a diode by switching voltages applied to the top and bottom electrodes of the NEA, leading to a large rectification ratio, fast response times, and simplified circuitry without the need for external electrodes. By taking advantage of closely spaced and individually addressable electrodes, the redox-cycling electrochemical diode has the potential for application to large-scale production and electrochemically controlled circuit operations, which go well beyond conventional electronic diodes or transistors.

Improving Single-Carbon-Nanotube-Electrode Contacts Using Molecular Electronics

We report the use of an electroactive species, acetaminophen, to modify the electrical connection between a carbon nanotube (CNT) and an electrode. By applying a potential across two electrodes, some of the CNTs in solution occasionally contact the electrified interface and bridge between two electrodes. By observing a single CNT contact between two microbands of an interdigitated Au electrode in the presence and absence of acetaminophen, the role of the molecular species at the electronic junction is revealed. As compared with the pure CNT, the current magnitude of the acetaminophen-modified CNTs significantly increases with the applied potentials, indicating that the molecule species improves the junction properties probably via redox shuttling.

Programmable Electrochemical Rectifier Based on a Thin-Layer Cell

A programmable electrochemical rectifier based on thin-layer electrochemistry is described here. Both the rectification ratio and the response time of the device are programmable by controlling the gap distance of the thin-layer electrochemical cell, which is easily controlled using commercially available beads. One of the electrodes was modified using a ferrocene-terminated self-assembled monolayer to offer unidirectional charge transfers via soluble redox species. The thin-layer configuration provided enhanced mass transport, which was determined by the gap thickness. The device with the smallest gap thickness (∼4 μm) showed an unprecedented, high rectification ratio (up to 160) with a fast response time in a two-terminal configuration using conventional electronics.

Electrochemical Rectification of Redox Mediators Using Porphyrin-Based Molecular Multilayered Films on ITO Electrodes

Electrochemical charge transfer through multilayer thin films of zinc and nickel 5,10,15,20-tetra(4-ethynylphenyl) porphyrin constructed via copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) "click" chemistry was examined. Current rectification toward various outer-sphere redox probes is revealed with increasing numbers of layers, as these films possess insulating properties over the neutral potential range of the porphyrin, then become conductive upon reaching its oxidation potential. Interfacial electron transfer rates of mediator-dye interactions toward [Co(bpy)3](2+), [Co(dmb)3](2+), [Co(NO2-phen)3](2+), [Fe(bpy)3](2+), and ferrocene (Fc), all outer-sphere redox species, were measured by hydrodynamic methods. The ability to modify electroactive films' interfacial electron transfer rates, as well as current rectification toward redox species, has broad applicability in a number of devices, particularly photovoltaics and photogalvanics.

A label-free and enzyme-free system for operating various logic devices using poly(thymine)-templated CuNPs and SYBR Green I as signal transducers

For the first time by integrating fluorescent polyT-templated CuNPs and SYBR Green I, a basic INHIBIT gate and four advanced logic circuits (2-to-1 encoder, 4-to-2 encoder, 1-to-2 decoder and 1-to-2 demultiplexer) have been conceptually realized under label-free and enzyme-free conditions. Taking advantage of the selective formation of CuNPs on ss-DNA, the implementation of these advanced logic devices were achieved without any usage of dye quenching groups or other nanomaterials like graphene oxide or AuNPs since polyA strands not only worked as an input but also acted as effective inhibitors towards polyT templates, meeting the aim of developing bio-computing with cost-effective and operationally simple methods. In short, polyT-templated CuNPs, as promising fluorescent signal reporters, are successfully applied to fabricate advanced logic devices, which may present a potential path for future development of molecular computations.

Electrochemical current rectification with cross reaction at a TEMPO/viologen-substituted polymer thin-layer heterojunction

The electrochemical large current rectification was achieved in the bilayer system composed of the TEMPO- and viologen-containing polymer thin layers.

Multi-level logic gate operation based on amplified aptasensor performance

Conventional electronic circuits can perform multi-level logic operations; however, this capability is rarely realized by biological logic gates. In addition, the question of how to close the gap between biomolecular computation and silicon-based electrical circuitry is still a key issue in the bioelectronics field. Here we explore a novel split aptamer-based multi-level logic gate built from INHIBIT and AND gates that performs a net XOR analysis, with electrochemical signal as output. Based on the aptamer-target interaction and a novel concept of electrochemical rectification, a relayed charge transfer occurs upon target binding between aptamer-linked redox probes and solution-phase probes, which amplifies the sensor signal and facilitates a straightforward and reliable diagnosis. This work reveals a new route for the design of bioelectronic logic circuits that can realize multi-level logic operation, which has the potential to simplify an otherwise complex diagnosis to a "yes" or "no" decision.

Electrochemical current rectification-a novel signal amplification strategy for highly sensitive and selective aptamer-based biosensor

Electrochemical aptamer-based (E-AB) sensors represent an emerging class of recently developed sensors. However, numerous of these sensors are limited by a low surface density of electrode-bound redox-oligonucleotides which are used as probe. Here we propose to use the concept of electrochemical current rectification (ECR) for the enhancement of the redox signal of E-AB sensors. Commonly, the probe-DNA performs a change in conformation during target binding and enables a nonrecurring charge transfer between redox-tag and electrode. In our system, the redox-tag of the probe-DNA is continuously replenished by solution-phase redox molecules. A unidirectional electron transfer from electrode via surface-linked redox-tag to the solution-phase redox molecules arises that efficiently amplifies the current response. Using this robust and straight-forward strategy, the developed sensor showed a substantial signal amplification and consequently improved sensitivity with a calculated detection limit of 114nM for ATP, which was improved by one order of magnitude compared with the amplification-free detection and superior to other previous detection results using enzymes or nanomaterials-based signal amplification. To the best of our knowledge, this is the first demonstration of an aptamer-based electrochemical biosensor involving electrochemical rectification, which can be presumably transferred to other biomedical sensor systems.

Redox cycling in nanoporous electrochemical devices

Nanoscale redox cycling is a powerful technique for detecting electrochemically active molecules, based on fast repetitive oxidation and reduction reactions. An ideal implementation of redox cycling sensors can be realized by nanoporous dual-electrode systems in easily accessible and scalable geometries. Here, we introduce a multi-electrode array device with highly efficient nanoporous redox cycling sensors. Each of the sensors holds up to 209,000 well defined nanopores with minimal pore radii of less than 40 nm and an electrode separation of ~100 nm. We demonstrate the efficiency of the nanopore array by screening a large concentration range over three orders of magnitude with area-specific sensitivities of up to 81.0 mA (cm(-2) mM(-1)) for the redox-active probe ferrocene dimethanol. Furthermore, due to the specific geometry of the material, reaction kinetics has a unique potential-dependent impact on the signal characteristics. As a result, redox cycling experiments in the nanoporous structure allow studies on heterogeneous electron transfer reactions revealing a surprisingly asymmetric transfer coefficient.

Nanoporous dual-electrodes with millimetre extensions: parallelized fabrication and area effects on redox cycling

Novel fabrication techniques lead to highly sensitive electrochemical sensors (left). The large-area characteristics of redox-cycling within the sensor's nanopores further cause potential-dependent variations of the overall analyte concentration (right).