Redox cycling (RC) is a unique electrochemical technique to selectively detect redox-active species in presence of molecules with similar oxidation potential but whose electrochemical reactions display different electron transfer kinetics. Pairs of interdigitated electrodes (IDEs) working as generator-collector devices have been widely applied in RC. The diffusion of species that takes place between the two IDEs has to be minimized in order to enhance the device sensitivity. Therefore, scaling down the IDE dimensions is carried out to detect very low concentrations of the molecules under study. The application of interdigitated nanoelectrode structures (nano-IDEs) is currently limited by low-yield production and packaging processes. Here, we describe a fabrication approach that enables robust mass production of gold nano-IDEs at wafer scale, along with a packaging strategy to easily operate with the electrodes. The theoretical and experimental detection of dopamine in presence of ascorbic acid demonstrates the superior analytical performance of the resulting devices. Nano-IDEs with dimensions of 150 nm width and 300 nm pitch showed faradaic steady-state currents related to the oxidation of dopamine that were 16.7 times larger than those recorded by standard cyclic voltammetry. By contrast, if the dimensions of the nano-IDEs are increased by a factor of 10, the amplification factor of the redox reaction is decreased to less than one fourth.

Gold interdigitated nanoelectrodes as a sensitive analytical tool for selective detection of electroactive species via redox cycling

A simple fabrication process for disposable interdigitated electrode arrays with nanogaps for lab-on-a-chip applications

Global food production needs to increase in order to meet the demands of an ever growing global population. As resources are finite, the most feasible way to meet this demand is to minimize losses and improve efficiency. Regular monitoring of factors like animal health, soil and water quality for example, can ensure that the resources are being used to their maximum efficiency. Existing monitoring techniques however have limitations, such as portability, turnaround time and requirement for additional reagents. In this work, we explore the use of micro- and nano-scale electrode devices, for the development of an electrochemical sensing platform to digitalize a wide range of applications within the agri-food sector. With this platform, we demonstrate the direct electrochemical detection of pesticides, specifically clothianidin and imidacloprid, with detection limits of 0.22 ng/mL and 2.14 ng/mL respectively, and nitrates with a detection limit of 0.2 µM. In addition, interdigitated electrode structures also enable an in-situ pH control technique to mitigate pH as an interference and modify analyte response. This technique is applied to the analysis of monochloramine, a common water disinfectant. Concerning biosensing, the sensors are modified with bio-molecular probes for the detection of both bovine viral diarrhea virus species and antibodies, over a range of 1 ng/mL to 10 µg/mL. Finally, a portable analogue front end electronic reader is developed to allow portable sensing, with control and readout undertaken using a smart phone application. Finally, the sensor chip platform is integrated with these electronics to provide a fully functional end-to-end smart sensor system compatible with emerging Agri-Food digital decision support tools.

https://www.mdpi.com/1424-8220/21/9/3149/pdf

Advanced Solid State Nano-electrochemical Sensors and System for Agri 4.0 Applications

Abstract Microelectrode arrays-based double electrode systems possess the property of diffusion layer overlapping between the two sets of electrodes. They have received increased attention in recent years in the field of electrochemistry and electroanalytical chemistry. This review introduces the fabrication, characterization, and applications of microelectrode arrays-based double electrode systems. The progress of photolithography enables and benefits microfabrication of the electrode systems with various geometries. As an essential step before electrochemical applications, various characterization methods are addressed to monitor the quality of the fabricated electrodes. Following that, applications as electrochemical sensors in generator-collector mode are discussed. Then, electrochemical sensors in bipolar mode, an interesting phenomenon existing in double electrode systems, are also introduced. Finally, applications of double electrode systems to the measurement of fundamentally electrochemical parameters are demonstrated.

Double electrode systems with microelectrode arrays for electrochemical measurements

We present a process flow for wafer-scale fabrication of a surface phase grating with sub-micron feature sizes from a single semiconductor material. We demonstrate this technique using a 110-oriented GaP semiconductor wafer with second-order nonlinearity to obtain a nanostructured device (800 nm lateral feature size and a 245 nm height modulation) with applications relevant to near-infrared optical diffraction and time-resolved terahertz (THz) technologies. The fabrication process involves a plasma-enhanced chemical deposition of a SiO2 layer on the wafer followed by contact photolithography and inductively coupled plasma reactive ion etching (ICP-RIE). We discuss the required radiation dosage, exposure times, temperatures and other key parameters to achieve high-quality nanogratings in terms of filling ratio, edge profile, and overall shape. The phase-grating properties, such as the pitch, spatial homogeneity, and phase retardation, are characterized with an atomic force microscope, scanning electron microscope and a non-invasive optical evaluation of the optical diffraction efficiency into different orders. We demonstrate an application of this device in a time-domain THz spectroscopy scheme, where an enhanced THz spectral bandwidth is achieved by optical rectification of near-infrared laser pulses incident on the grating and efficiently diffracted into the first orders. Finally, the reported process flow has the potential to be applied to various materials by considering only slight adjustments to the ICP-RIE etching steps, paving the way to scalable fabrication of sub-micron patterns on a large range of substrates.

/pdf/scalable-fabrication-of-nanogratings-on-gap-for-efficient-1mtrr3bb.pdf

Scalable Fabrication of Nanogratings on GaP for Efficient Diffraction of Near-Infrared Pulses and Enhanced Terahertz Generation by Optical Rectification

Interdigitated electrode arrays (IDAs) are often used for electrochemical detection using redox cycling for signal amplification. Its geometry, especially the gap width, is an important factor defining its electrochemical performance (sensitivity and collection efficiency). Minimal defect density on a wafer is a must, as a single shortcut prevents the function of the sensor chip. Therefore, the fabrication of interdigitated electrode arrays is a fundamental step in the integration of electrochemical sensors. This paper presents a novel fabrication approach that allows electrode spacing in the sub-micrometer region by using standard equipment such as UV Mask Aligner, physical vapor deposition and diffusion furnace. The fabrication procedure is a combination of dry etching, thermal oxidation and wet etching. This approach has potential to realize electrode distances down to 140 nm or even less. The proposed method enables the fabrication of sub-micron IDAs with the use of a conventional Mask Aligner and standard silicon technology. Moreover, it allows the electrode spacing adjustment without changing the mask design. The presented results demonstrate high sensitivity electrochemical sensors with amplification factors more than 12 without the need of e-beam lithography. Hence, this fabrication method offers a low-cost alternative to sub-micron e-beam written IDAs.

Novel fabrication process for sub-micron interdigitated electrode arrays for highly sensitive electrochemical detection

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Novel fabrication process for sub-micron interdigitated electrode arrays for highly sensitive electrochemical detection