Introduction to Microelectrode Arrays, the Site-Selective Functionalization of Electrode Surfaces, and the Real-Time Detection of Binding Events

Light-harvesting microelectronic devices for wireless electrosynthesis

High-throughput experimentation (HTE) has accelerated academic and industrial chemical research in reaction development and drug discovery and has been broadly applied in many domains of organic chemistry1,2. However, application of HTE in electrosynthesis-an enabling tool for chemical synthesis-has been limited by a dearth of suitable standardized reactors3-7. Here we report the development of microelectronic devices, which are produced using standard nanofabrication techniques, to enable wireless electrosynthesis on the microlitre scale. These robust and inexpensive devices are powered by visible light and convert any traditional 96-well or 384-well plate into an electrochemical reactor. We validate the devices in oxidative, reductive and paired electrolysis and further apply them to achieve the library synthesis of biologically active compounds and accelerate the development of two electrosynthetic methodologies. We anticipate that, by simplifying the way electrochemical reactions are set up, this user-friendly solution will not only enhance the experience and efficiency of current practitioners but also substantially reduce the barrier for nonspecialists to enter the field of electrosynthesis, thus allowing the broader community of synthetic chemists to explore and benefit from new reactivities and synthetic strategies enabled by electrochemistry8-12.

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.

Lessons from an Array: Using an Electrode Surface to Control the Selectivity of a Solution-Phase Chemical Reaction

Electrochemistry offers a variety of novel means by which selectivity can be introduced into synthetic organic transformations. In the work reported, it is shown how methods used to confine chemical reactions to specific sites on a microelectrode array can also be used to confine a preparative reaction to the surface of an electrode inserted into a bulk reaction solution. In so doing, the surface of a modified electrode can be used to introduce new selectivity into a preparative reaction that is not observed in the absence of either the modified electrode surface or the effort to confine the reaction to that surface. The observed selectivity can be optimized in the same way that confinement is optimized on an array and is dependent on the nature of the functionalized surface.

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.

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.

From Molecules to Molecular Surfaces. Exploiting the Interplay Between Organic Synthesis and Electrochemistry

For many years, we have been looking at electrochemistry as a tool for exploring, developing, and implementing new synthetic methods for the construction of organic molecules. Those efforts examined electrochemical methods and mechanisms and then exploited them for synthetic gain. Chief among the tools utilized was the fact that in a constant current electrolysis the working potential at the electrodes automatically adjusted to the oxidation (anode) or reduction (cathode) potential of the substrates in solution. This allowed for a systematic examination of the radical cation intermediates that are involved in a host of oxidative cyclization reactions. The result has been a series of structure-activity studies that have led to far greater insight into the behavior of radical cation intermediates and in turn an expansion in our capabilities of using those intermediates to trigger interesting synthetic reactions. With that said, the relationship between synthetic organic chemistry and electrochemistry is not a "one-way" interaction. For example, we have been using modern synthetic methodology to construct complex addressable molecular surfaces on electroanalytical devices that in turn can be used to probe biological interactions between small molecules and biological receptors in "real-time". Synthetic chemistry can then be used to recover the molecules that give rise to positive signals so that they can be characterized. The result is an analytical method that both gives accurate data on the interactions and provides a unique level of quality control with respect to the molecules giving rise to that data. Synthetic organic chemistry is essential to this task because it is our ability to synthesize the surfaces that defines the nature of the biological problems that can be studied. But the relationship between the fields does not end there. Recently, we have begun to show that work to expand the scope of microelectrode arrays as bioanalytical devices is teaching us important lessons for preparative synthetic chemistry. These lessons come in two forms. First, the arrays have taught us about the on-site generation of chemical reagents, a lesson that is being used to expand the use of paired electrochemical strategies for synthesis. Second, the arrays have taught us that reagents can be generated and then confined to the surface of the electrode used for that generation. This has led to a new approach to taking advantage of molecular recognition events that occur on the surface of an electrode for controlling the selectivity of a preparative reaction. In short, the confinement strategy developed for the arrays is used to ensure that the chemistry in a preparative electrolysis happens at the electrode surface and not in the bulk solution. This Account details the interplay between synthetic chemistry and electrochemistry in our group through the years and highlights the opportunities that interplay has provided and will continue to provide in the future.

Electrochemical strategies for C-H functionalization and C-N bond formation

Conventional methods for carrying out carbon-hydrogen functionalization and carbon-nitrogen bond formation are typically conducted at elevated temperatures, and rely on expensive catalysts as well as the use of stoichiometric, and perhaps toxic, oxidants. In this regard, electrochemical synthesis has recently been recognized as a sustainable and scalable strategy for the construction of challenging carbon-carbon and carbon-heteroatom bonds. Here, electrosynthesis has proven to be an environmentally benign, highly effective and versatile platform for achieving a wide range of nonclassical bond disconnections via generation of radical intermediates under mild reaction conditions. This review provides an overview on the use of anodic electrochemical methods for expediting the development of carbon-hydrogen functionalization and carbon-nitrogen bond formation strategies. Emphasis is placed on methodology development and mechanistic insight and aims to provide inspiration for future synthetic applications in the field of electrosynthesis.

Organic Electrochemistry and a Role Reversal: Using Synthesis To Optimize Electrochemical Methods

Diblock copolymers are excellent coatings for microelectrode arrays because they provide a stable surface that can support both synthetic and analytical electrochemistry. However, the surfaces that are optimal for synthetic studies are not the same as the surfaces that are optimal for analytical studies. Hence, no one surface provides an ideal platform for both building and analyzing a molecular library. Fortunately, the synthetic chemistry available on a microelectrode array allows a surface that is ideal for synthesis can be converted into one that is ideal for signaling studies; a scenario that allows for the use of an optimized synthetic and analytical surface on a single microelectrode array.

New Methods for the Site-Selective Placement of Peptides on a Microelectrode Array: Probing VEGF-v107 Binding as Proof of Concept

Cu(I)-catalyzed "click" reactions cannot be performed on a borate ester derived polymer coating on a microelectrode array because the Cu(II) precursor for the catalyst triggers background reactions between both acetylene and azide groups with the polymer surface. Fortunately, the Cu(II)-background reaction can itself be used to site-selectively add the acetylene and azide nucleophiles to the surface of the array. In this way, molecules previously functionalized for use in "click" reactions can be added directly to the array. In a similar fashion, activated esters can be added site-selectively to a borate ester coated array. The new chemistry can be used to explore new biological interactions on the arrays. Specifically, the binding of a v107 derived peptide with both human and murine VEGF was probed using a functionalized microelectrode array.

3-D Printed Adjustable Microelectrode Arrays for Electrochemical Sensing and Biosensing

Printed Electronics has emerged as an important fabrication technique that overcomes several shortcomings of conventional lithography and provides custom rapid prototyping for various sensor applications. In this work, silver microelectrode arrays (MEA) with three different electrode spacing were fabricated using 3-D printing by the aerosol jet technology. The microelectrodes were printed at a length scale of about 15 μm, with the space between the electrodes accurately controlled to about 2 times (30 μm, MEA30), 6.6 times (100 μm, MEA100) and 12 times (180 μm, MEA180) the trace width, respectively. Hydrogen peroxide and glucose were chosen as model analytes to demonstrate the performance of the MEA for sensor applications. The electrodes are shown to reduce hydrogen peroxide with a reduction current proportional to the concentration of hydrogen peroxide for certain concentration ranges. Further, the sensitivity of the current for the three electrode configurations was shown to decrease with an increase in the microelectrode spacing (sensitivity of MEA30: MEA100: MEA180 was in the ratio of 3.7: 2.8: 1), demonstrating optimal MEA geometry for such applications. The noise of the different electrode configurations is also characterized and shows a dramatic reduction from MEA30 to MEA100 and MEA180 electrodes. Further, it is shown that the response current is proportional to MEA100 and MEA180 electrode areas, but not for the area of MEA30 electrode (the current density of MEA30 : MEA100 : MEA180 is 0.25 : 1 : 1), indicating that the MEA30 electrodes suffer from diffusion overlap from neighboring electrodes. The work thus establishes the lower limit of microelectrode spacing for our geometry. The lowest detection limit of the MEAs was calculated (with S/N = 3) to be 0.45 μM. Glucose oxidase was immobilized on MEA100 microelectrodes to demonstrate a glucose biosensor application. The sensitivity of glucose biosensor was 1.73 μAmM^-1 and the calculated value of detection limit (S/N = 3) was 1.7 μM. The electrochemical response characteristics of the MEAs were in agreement with the predictions of existing models. The current work opens up the possibility of additive manufacturing as a fabrication technique for low cost custom-shaped MEA structures that can be used as electrochemical platforms for a wide range of sensor applications.