Electrochemical Sensing via Redox Cycling in Nanoporous Dual-Electrode Devices

Redox cycling is a powerful tool for the electrical detection of chemical electrode reactions. Owing to repeated analyte oxidation and reduction, this technique enables extremely sensitive and highly selective identification of redox-active molecules. Further benefits are provided when implementing the principle of redox cycling in nanoporous devices. Here, a thin non-conducting layer separates two individually addressable electrodes. One of these electrodes faces the analyte containing medium directly, while the other one can be accessed through apertures within the first electrode and the spacing layer. Due to the modular layout according sensors are strongly coupled to the reservoir. Additionally, the lateral sensor's dimensions can freely be chosen during fabrication. Within this work, three major contributions are made regarding the detailed understanding, the convenient fabrication, and the comprehensive theoretical analysis of nanoporous dual-electrode sensors. First, the fundamental characteristics of redox cycling in nanopores are investigated. To this purpose, a sensor-chip holding an array of 32 dual-microelectrodes is fabricated. Each of the sensors features up to 209,000 nanopores with radii of 50 nm and interpore distances of 200 nm. The efficiency for electrochemical sensing is proven by screening a concentration range of three orders of magnitude. The densely integrated pores and the close electrode spacing of about 100 nm lead to per-area sensitivities of up to 9 mA/mM cm2. Furthermore, as supported by numerical in-depth analyses, every parameter involved in the redox cycling process leaves a unique footprint in the current recorded at the sensor. Consequently, the specific nanoporous geometry allows detailed studies on the heterogeneous electron transfer reactions of an analyte. In this context, a highly asymmetric transfer coefficient is revealed for the Fc(MeOH)20/1+-couple. As a second part of this work, a dual-electrode sensor is presented that is based on nanoporous alumina membranes. Novel processing steps are developed to enable high-throughput fabrication. These steps include a selective passivation of structured electrode surfaces by polymers and processes for the selective etching of titania. Combining the techniques with the on-chip anodization of aluminum films, large-area nanoporous redox cycling sensors are fabricated on the wafer scale. Eventually, each of the sensors comprises more than one billion pores on a basal area of 9 mm2. With interpore distances of approximately 100 nm and pore radii of 20 nm sensitivities of up to 330 µA/mM are reached. Furthermore, the specific coupling of the extended nanopore array is found to have a direct impact on the redox cycling currents of analytes, such as the Fe(CN)64-/3--couple, which feature distinct diffusion coefficients for the reduced and the oxidized species. The issue of accurately simulating nanopatterned redox cycling sensors is addressed in the third part of this work. To describe effects similar to those investigated at the microelectrode sensor, an according model has to represent the detailed nano-geometry of a patterned interface. As shown by the large-area alumina sensor, however, the model must additionally include the coupling between a sensor and its surrounding volume. To this end, a novel approach is developed that can quickly and precisely calculate a sensor's comprehensive behavior. While other methods might even fail to predict basic current characteristics, errors of the presented approach are commonly below the percentage range

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[thin space (1/6-em)]000 well defined nanopores with minimal pore radii of less than 40 nm and an electrode separation of [similar]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

Transistor Functions Based on Electrochemical Rectification

Electronic elements built from organic semiconductors, such as molecular transistors, have significant potential for technical innovations and mass use owing to their flexible mechanical properties, cost-effective processability, and tuneable optical properties.1 Self-assembled monolayer field-effect transistors (SAMFET) are among the latest developments of novel microelectronic devices.2 In SAMFETs, charge carriers laterally pass through a molecular layer (parallel to substrate) and exhibit transistor behavior, which can be modulated by an electrostatic gate. The charge transport properties of the SAMFETs are affected by the length of the channel.3, 4 Herein, we present a novel chip-based molecular transistor concept, which is based on charge-transfer processes across a monolayer of molecules perpendicular to the sample surface plane. To this end, redox active molecules are adsorbed to a collector electrode (CE) acting as electron-transfer mediators. A two-step charge transfer between solid interdigitated array electrodes (IDA), charge transfer mediator, and liquid-phase redox probe yields a unidirectional current response. An adsorbate-free generator electrode (GE) can be used to modulate the unidirectional currents, resulting in a transistor-like behavior. The electrochemical transistor-like system exhibits high current outputs at a low-voltage operation, high on/off switching current ratios, and is operated as a 24-bit code generator

Ein elektrochemischer Gleichrichter ermöglicht Transistorfunktionen

Elektronischen Bauelementen, die auf organischen Halbleitern basieren, wird aufgrund ihrer flexiblen mechanischen Eigenschaften sowie ihrer preiswerten Herstellung und Verarbeitung großes Potenzial für eine Massennutzung zugeschrieben.1 Zu den vielversprechendsten organischen Bauelementen zählen Feldeffekttransistoren (FET), die mithilfe von selbstorganisierten Monoschichten (SAMs) funktionaler Moleküle hergestellt werden.2 In diesen SAMFETs werden Ladungsträger lateral durch eine Molekülfilm transportiert. Die Strom-Spannungs-Kennlinien der Bauelemente kann durch eine Gatterelektrode gesteuert werden. Der Ladungstransport in den SAMFETs hängt zudem kritisch von der Kanallänge ab.3, 4 Hier stellen wir ein neuartiges, chipbasiertes Transistorkonzept vor, das auf einem vertikalen Ladungstransfer durch eine Molekülschicht beruht. Dafür werden redoxaktive Moleküle als Redoxvermittler auf einer Kollektorelektrode (CE) gebunden. Ein zweistufiger Ladungstransfer zwischen ineinandergreifenden Mikroelektroden (IDA), Redoxvermittler und gelösten Redoxionen im Elektrolyten führen zu einem unidirektionalen Stromsignal. Mithilfe einer nichtmodifizierten Generatorelektrode (GE) kann der unidirektionale Stromfluss gesteuert werden, was das Ausführen von Transistorfunktionen ermöglicht. Das elektrochemische Bauelement weist hohe Stromsignale bei geringen Betriebsspannungen sowie hohe Ein-Aus-Signalverhältnisse auf. Weiterhin zeigen wir, dass das System zum Kodieren von Informationen genutzt werden kann