Recent Advances in Biosensing for Neurotransmitters and Disease Biomarkers using Microelectrodes

Developing Microelectrode Arrays for the Point-of-Care Multiplex Detection of Metabolites

DNA-aptamer-functionalized electrode arrays can provide an intriguing method for detecting pathogen-derived exometabolites. This work addresses the limitations of previous aptamer-based pathogen detection methods by introducing a novel surface design that bridges the gap between initial efforts in this area and the demands of a point-of-care device. Specifically, the use of a diblock copolymer coating on a high-density microelectrode array and Cu-mediated cross coupling reactions that allow for the exclusive functionalization of that coating by any electrode or set of electrodes in the array provides a device that is stable for 1 year and compatible with the multiplex detection of small-molecule targets. The new chemistry developed allows one to take advantage of a large number of electrodes in the array with one experiment described herein capitalizing on the use of 960 individually addressable electrodes.

Microelectrode Arrays, Electrocatalysis, and the Need for Proper Characterization

Indirect electrochemical methods are a powerful tool for synthetic chemistry because they allow for the optimization of chemical selectivity in a reaction while maintaining the advantages of electrochemistry in terms of sustainability. Recently, we have found that such methods provide a handle for not only the synthesis of complex molecules, but also the construction of complex, addressable molecular surfaces. In this effort, the indirect electrochemical methods enable the placement or synthesis of molecules by any electrode or set of electrodes in a microelectrode array. The success of these surface-based reactions are typically evaluated with the use of fluorescence labelling studies. However, these fluorescence-based evaluations can be misleading. While they are excellent for determining that a reaction has occurred in a site-selective fashion on an array, they do not provide information on whether that reaction is the one desired or how well it worked. We describe here how the use of a "safety-catch" linker strategy allows for a more accurate assessment of reaction quality on an array, and then use that capability to illustrate how the use of transition metal mediated cross-coupling reactions on an array prevent unwanted background reactions that can occur on a polymer-coated electrode surface. The method enables a unique level of quality control for array-based transformations.

Microelectrode arrays, electrosynthesis, and the optimization of signaling on an inert, stable surface

Microelectrode arrays are powerful tools for monitoring binding interactions between small molecules and biological targets. In most cases, molecules to be studied using such devices are attached directly to the electrodes in the array. Strategies are in place for calibrating signaling studies utilizing the modified electrodes so that they can be quantified relative to a positive control. In this way, the relative binding constants for multiple ligands for a receptor can potentially be determined in the same experiment. However, there are applications of microelectrode arrays that require stable, tunable, and chemically inert surfaces on the electrodes. The use of those surfaces dictate the use of indirect detection methods that are dependent on the nature of the stable surface used and the amount of the binding partner that is placed on the surface. If one wants to do a quantitative study of binding events that involve molecules on such a stable surface, then once again a method for calibrating the signal from a positive control is needed. Fortunately, the electrodes in an array are excellent handles for conducting synthetic reactions on the surface of an array, and those reactions can be used to tune the surface above the electrodes and calibrate the signal from a positive control. Here, we describe how available Cu-based electrosynthetic reactions can be used to calibrate electrochemical signals on a polymer-coated electrode array and delineate the factors to be considered when choosing a polymer surface for such a study.

Capitalizing on Mediated Electrolyses for the Construction of Complex, Addressable Molecular Surfaces

Synthetic organic chemists are beginning to exploit electrochemical methods in increasingly creative ways. This is leading to a surge in productivity that is only now starting to take advantage of the full-potential of electrochemistry for accessing new structures in novel, more efficient ways. In this perspective, we provide insight into the potential of electrochemistry as a synthetic tool gained through studies of both direct anodic oxidation reactions and more recent indirect methods, and highlight how the development of new electrochemical methods can expand the nature of synthetic problems our community can tackle.

Electrochemical Detection of Dopamine and Riboflavine on a Screen-Printed Carbon Electrode Modified by AuNPs Derived from Rhanterium suaveolens Plant Extract

A AuNP-modified screen-printed carbon electrode (AuNP/SPCE) for monitoring important biomolecules, such as dopamine (DA) and riboflavin (RF), having a high potential for personalized medicine and for continuous monitoring of human health is here proposed. AuNPs were synthesized using the extract of Rhanterium suaveolens as a reducing medium and were characterized by UV-vis spectroscopy, dynamic light scattering (DLS), and scanning and transmission electron microscopy (SEM and TEM). The synthesized AuNPs appear spherical and present a bimodal size distribution with a maximum centered at around 30-50 nm. Cyclic voltammetry (CV) experiments demonstrated that the modified AuNP/SPCE sensor exhibits superior electrochemical performances to the bare SPCE. Low limits of detection (LODs) of 0.2 and 0.07 μM at S/N = 3 and sensitivities of 550.4  and 2399 μA mM^-1 cm^-2 were registered for DA and RF detection, respectively. Results demonstrate the promising electrochemical characteristics of the synthesized AuNPs and developed AuNP/SPCE electrochemical sensor for the determination of these important biomolecules.

Microelectrode Arrays, Dihydroxylation, and the Development of an Orthogonal Safety-Catch Linker

Construction of larger molecular libraries on an addressable microelectrode array requires a method for recovering and characterizing molecules from the surface of any electrode in the array. This method must be orthogonal to the synthetic strategies needed to build the array. We report here a method for achieving this goal that employs the site-selective dihydroxylation reaction of a simple olefin.

Recent Trends in Electrochemical Sensors for Vital Biomedical Markers Using Hybrid Nanostructured Materials

This work provides a succinct insight into the recent developments in electrochemical quantification of vital biomedical markers using hybrid metallic composite nanostructures. After a brief introduction to the biomarkers, five types of crucial biomarkers, which require timely and periodical monitoring, are shortlisted, namely, cancer, cardiac, inflammatory, diabetic and renal biomarkers. This review emphasizes the usage and advantages of hybrid nanostructured materials as the recognition matrices toward the detection of vital biomarkers. Different transduction methods (fluorescence, electrophoresis, chemiluminescence, electrochemiluminescence, surface plasmon resonance, surface-enhanced Raman spectroscopy) reported for the biomarkers are discussed comprehensively to present an overview of the current research works. Recent advancements in the electrochemical (amperometric, voltammetric, and impedimetric) sensor systems constructed with metal nanoparticle-derived hybrid composite nanostructures toward the selective detection of chosen vital biomarkers are specifically analyzed. It describes the challenges involved and the strategies reported for the development of selective, sensitive, and disposable electrochemical biosensors with the details of fabrication, functionalization, and applications of hybrid metallic composite nanostructures.

Direct Electrochemical Detection of Glutamate, Acetylcholine, Choline, and Adenosine Using Non-Enzymatic Electrodes

Non-electroactive neurotransmitters such as glutamate, acetylcholine, choline, and adenosine play a critical role in proper activity of living organisms, particularly in the nervous system. While enzyme-based sensing of this type of neurotransmitter has been a research interest for years, non-enzymatic approaches are gaining more attention because of their stability and low cost. Accordingly, this focused review aims to give a summary of the state of the art of non-enzymatic electrochemical sensors used for detection of neurotransmitter that lack an electrochemically active component. In place of using enzymes, transition metal materials such as those based on nickel show an acceptable level of catalytic activity for neurotransmitter sensing. They benefit from fast electron transport properties and high surface energy and their catalytic activity can be much improved if their surface is modified with nanomaterials such as carbon nanotubes and platinum nanoparticles. However, a general comparison reveals that the performance of non-enzymatic biosensors is still lower than those that use enzyme-based methods. Nevertheless, their excellent stability demonstrates that non-enzymatic neurotransmitter sensors warrant additional research in order to advance them toward becoming an acceptable replacement for the more expensive enzyme-based sensors.

The Use of Electrochemical Biosensors in Food Analysis

Rapid and accurate analysis of food produce is essential to screen for species that may cause significant health risks like bacteria, pesticides and other toxins. Considerable developments in analytical techniques and instrumentation, for example chromatography, have enabled the analyses and quantitation of these contaminants. However, these traditional technologies are constrained by high cost, delayed analysis times, expensive and laborious sample preparation stages and the need for highly-trained personnel. Therefore, emerging, alternative technologies, for example biosensors may provide viable alternatives. Rapid advances in electrochemical biosensors have enabled significant gains in quantitative detection and screening and show incredible potential as a means of countering such limitations. Apart from demonstrating high specificity towards the analyte, these biosensors also address the challenge of the multifactorial food industry of providing high analytical accuracy amidst complex food matrices, while also overcoming differing densities, pH and temperatures. This (public and Industry) demand for faster, reliable and cost-efficient analysis of food samples, has driven investment into biosensor design. Here, we discuss some of the recent work in this area and critique the role and contributions biosensors play in the food industry. We also appraise the challenges we believe biosensors need to overcome to become the industry standard.