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Advances in Electroanalytical Chemistry
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electrochemistry instrumentation utilizing screen-printed electrodes for luminescence assays. This device is one of the first plug-and-play spectroelectrochemistry instruments for the novice in the field. Tao and co-workers have developed a plasmonics-based electrochemical current microscopy technique for studying surface redox reactions at individual gold nanowires, which allows for the study of the local distribution of redox activity across a single nanowire. These techniques are not commonly used for assaying individual analytes, but they will be critical in future research to understand complex electrodes, photoelectrochemistry fundamentals, and even the double layer. Scanning probe electroanalytical techniques have also made significant advances over the past decade with the enhanced abilities of scanning electrochemical microscopy (SECM),2 scanning ion conductance microscopy (SICM),3 and scanning electrochemical cell microscopy (SECCM).3 Rodriǵ uezLópez’s group has utilized mercury-based ultramicroelectrodes to eliminate positive feedback mechanisms and unwanted side reactions, as well as changing traditional potentiostatic approach curves to cyclic voltammetric approach curves. Yu and colleagues have developed new materials for modifying pipets, including polyimidazolium brushes that allow for micrometer-scale ion rectification. Many of these scanning probe techniques help researchers study the heterogeneity of an electrode. Zhang, Bond, and co-workers have transitioned from traditional amperometry and cyclic voltammetry to Fouriertransformed alternating current voltammetry to better investigate the heterogeneous nanostructured electrode surfaces. These techniques will be important for studying the new materials that are being designed for electrochemical applications, such as biosensors, electrocatalysts for fuel cells and electrolyzers, and batteries. Over the past couple of decades, there has been increasing interest in in vivo electrochemical measurements of biological systems,4 including single cell measurements and living organisms. Researchers led by Mirkin and Amatore have developed a platinized carbon nanoelectrode for studying the production of reactive oxygen species and reactive nitrogen species in cancer cells. They utilize scanning electrochemical microscopy to image the production of these reactive species inside non-transformed and metastatic human breast cells. Zhu, Shi, and co-workers have rationally designed polymer-modified electrodes for the in vivo analysis of sialic acid to study the role of sialic acid in the progression of Alzheimer’s disease in mouse brains. Kelley and co-workers have also developed DNA hybridization electrodes for sensing three secreted proteins from stem cell culture. Dempsey and colleagues have developed cellular electrochemical nanotoxicity assays to study the effect of CdTe quantum dots on mammalian cells. This method studies the secretion of the cellular enzyme acid phosphatase, whose product 2-naphthol can be detected via chronocoulometry. Although many techniques have been
lectroanalytical chemistry is a subfield of electrochemistry focused on the development of new techniques, methods, and modified electrodes for quantitative analytical investigations. In some instances, those quantitative analytical investigations are used for sensing and detecting an analyte (i.e., glucose concentrations in blood), but in recent years, there has been considerable technique development for studying electrocatalysis in fuel cells and electrolyzers, investigating intercalation in batteries, interrogating the electrode/electrolyte interface, examining corrosion and plating, and even studying nanomaterial properties (i.e., size, toxicity, etc.). These techniques have also included materials and methods innovations for in vivo analysis, combinations of techniques for in situ or operando analysis, new chemically and biologically modified electrodes for analysis of non-redox-active molecules, and improvements in voltammetric techniques. This ACS Select Virtual Issue highlights publications in the field of electroanalytical chemistry from the Journal of the American Chemical Society, Analytical Chemistry, and ACS Sensors over the past year. Although there are many outstanding contributions, these selections showcase the diversity of applications of electroanalytical chemistry from fundamental understanding of the electrode double layer to futuristic applications in fitness monitoring. Specifically, this ACS Select Virtual Issue looks at recent advancements in single-molecule electrochemistry, spectroelectrochemistry, in vivo electroanalysis, sensors, and new in situ and operando techniques. Analytical chemists have been improving single-molecule and single-entity detection methods for several decades. Singlemolecule detection is difficult in electrochemistry, but several researchers have improved the signal from a single redox event by potentiostatic redox cycling in the gap between a tip and macroelectrodes in scanning electrochemical microscopy or in a nanogap in a microfabricated device.1 However, Zhang’s group has developed an interesting hybrid spectroelectrochemical technique combining total internal reflection fluorescence with electrochemistry, utilizing a fluorogenic redox species. This technique allows for single-molecule detection of resorufin molecules, but it could be applied to a wide variety of electrochemistry applications. Although amperometric measurements are commonly used for single-molecule electrochemistry, other electroanalytical techniques can be explored. For instance, potentiometry can be used for single-entity detection. Recently, Percival and Bard have shown that opencircuit potentials are very sensitive to the electrode environment and may be able to detect single-electron-transfer events with very small electrode sizes. Researchers are developing enhanced spectroelectrochemical techniques for indirectly detecting redox events, studying photoelectrochemical reactions, heterogeneity of electrode surfaces, and modification of electrode surfaces. Compton’s group has developed a fluorescence electrochemical microscopy technique for evaluating reaction layers via “indirect” detection of the occurrence of an electron-transfer reaction. FanjulBolado and co-workers have developed time-resolved spectro© XXXX American Chemical Society
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DOI: 10.1021/jacs.8b00986 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Journal of the American Chemical Society
Editorial
developed to study redox-active biomolecules in vivo, and a few techniques have been developed to study redox-inactive molecules in vivo, there are a wealth of redox-inactive molecules in biological systems, so there is still a large research effort to develop in vivo electrochemical measurements for redox-inactive molecules. Electrochemical sensors typically utilize potentiometric, impedimetric, voltammetric, or amperometric methods for detecting an analyte at a conductive transducer. Recent research has focused on improving the chemically selective layer, studying new strategies for detecting non-redox-active molecules at an electrochemical transducer, and improving reference electrodes. For instance, nanopore-based biosensors have been popular in recent years for detecting DNA, DNA damage, and peptides,5,6 but this strategy can be applied to small molecules as well. Li and co-workers have used an α-hemolysin nanopore to detect cocaine with a cocaine aptamer that loses a short complementary single-stranded DNA during binding of cocaine. The released short complementary single-stranded DNA is then detected by the nanopore. This method allows for a limit of detection of cocaine down to 50 nM in a variety of biological fluids, including saliva and serum. This technique could be applied to a wide variety of non-redox-active molecules with the use of selective predesigned aptamers. Aptamers have also been employed in other electrochemical sensing platforms, but similar to any DNA- or protein-modified electrode, there have been problems with sensor-to-sensor fabrication variation and drift over time. To overcome these problems, Plaxco’s group has recently developed a calibrationfree square wave voltammetry method that uses the ratio of currents collected at responsive and non-responsive frequencies. Although most research has focused on liquid samples, there have been a number of works exploring electrochemical gas sensors. Ishihara and co-workers have developed a formaldehyde sensor using hydroxylamine hydrochloride that will react with formaldehyde and inject an electron into a semiconducting single-walled carbon nanotube. The device operates at common atmospheric conditions, with low limit of detection, suitable for so-called “detect-to-warn” sensors due to the low cost, portability, and ability to perform real-time monitoring. It is important to note that the previously discussed work focuses on the development of better working/sensing electrodes, but reference electrodes also need improvement. Lodge, Hillmyer, Bühlmann, and colleagues have utilized ionic liquids for improving the resistance, stability, and reproducibility of Ag/AgCl reference electrodes. This method allows for miniaturization and the ability to operate in aqueous solutions from deionized to 100 mM electrolytes. It is important to note that researchers are also considering deep eutectic solvents rather than ionic liquids for electrochemical applications, due to their low cost and environmental sustainability, but much research is still needed to understand the fundamental electrochemistry of these highly viscous and limited conductivity solvents. Benedetti and co-workers have developed an efficient tool using semi-integrative voltammetry to determine fundamental electrochemical parameters of the glyceline−FeCl3 system in deep eutectic solvents. All of these developments are making portable and even wearable sensors possible. Wang and co-workers have recently transitioned electrochemical lab-on-achip devices to wearable devices. They have designed a flexible, skin-mounted sensor system for continuous, real-time electro-
chemical monitoring of lactate and glucose metabolites in sweat, which delivers the analytical information via Bluetooth to the signal processing and readout device. Since many devices for energy conversion and storage are electrochemical, analytical chemists have also been focused on developing in situ and operando techniques for studying the electrodes and electrochemical cells during electrochemical perturbation.7−9 Researchers led by Schuhmann and Conzuelo have developed new operando techniques for studying photobioelectrochemical cells in operando utilizing bipolar electrochemistry, which is a technique that has become quite popular for wirelessly addressing redox reactions. This advance has allowed them to study both photosystem I and photosystem II under photo- and electrochemical perturbation. Zare and co-workers have developed new methods for integrating electrochemistry with in situ mass spectroscopy for analyzing reaction reactants, products, and intermediates. In summary, I hope this ACS Select Virtual Issue has shown the reader the diversity of different applications for new and improved electroanalytical techniques ranging from in vivo electroanalysis to the study of new materials for energy conversion and storage devices. It is important to note that these new techniques and methods are transitioning from requiring large quantities of sample/analyte to studying single molecules, single entities, and the very thin electrode/ electrolyte interface. Currently, researchers are trying to improve the sensitivity, selectivity, reliability, stability, reproducibility, portability, and analysis time of these techniques, so I am looking forward to the future advances in electroanalytical chemistry.
Shelley D. Minteer
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JACS Associate Editor and USTAR Professor of Chemistry, University of Utah
FEATURED ARTICLES Lu, J.; Fan, Y.; Howard, M. D.; Vaughan, J. C.; Zhang, B. J. Am. Chem. Soc. 2017, 139, 2964−2971. Percival, S. J.; Bard, A. J. Anal. Chem. 2017, 89, 9843− 9849. Yang, M.; Batchelor-McAuley, C.; Kätelhön, E.; Compton, R. G. Anal. Chem. 2017, 89, 6870−6877. ́ Martin-Yerga, D.; Pérez-Junquera, A.; Hernández-Santos, D.; Fanjul-Bolado, P. Anal. Chem. 2017, 89, 10649− 10654. Wang, Y.; Shan, X.; Wang, H.; Wang, S.; Tao, N. J. Am. Chem. Soc. 2017, 139, 1376−1379. ́ Barton, Z. J.; Rodriguez-Ló pez, J. Anal. Chem. 2017, 89, 2708−2715. He, X.; Zhang, K.; Li, T.; Jiang, Y.; Yu, P.; Mao, L. J. Am. Chem. Soc. 2017, 139, 1396−1399. Tan, S.-y.; Unwin, P. R.; Macpherson, J. V.; Zhang, J.; Bond, A. M. Anal. Chem. 2017, 89, 2830−2837. Li, Y.; Hu, K.; Yu, Y.; Rotenberg, S. A.; Amatore, C.; Mirkin, M. V. J. Am. Chem. Soc. 2017, 139, 13055− 13062. Ding, S.; Cao, S.; Liu, Y.; Lian, Y.; Zhu, A.; Shi, G. ACS Sensors 2017, 2, 394−400. Zhou, W.; Mahshid, S. S.; Wang, W.; Vallée-Bélisle, A.; Zandstra, P. W.; Sargent, E. H.; Kelley, S. O. ACS Sensors 2017, 2, 495−500. DOI: 10.1021/jacs.8b00986 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Journal of the American Chemical Society
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Editorial
O’Hara, T.; Seddon, B.; O’Connor, A.; McClean, S.; Singh, B.; Iwuoha, E.; Fuku, X.; Dempsey, E. ACS Sensors 2017, 2, 165−171. Rauf, S.; Zhang, L.; Ali, A.; Liu, Y.; Li, J. ACS Sensors 2017, 2, 227−234. Li, H.; Dauphin-Ducharme, P.; Ortega, G.; Plaxco, K. W. J. Am. Chem. Soc. 2017, 139, 11207−11213. Ishihara, S.; Labuta, J.; Nakanishi, T.; Tanaka, T.; Kataura, H. ACS Sensors 2017, 2, 1405−1409. Chopade, S. A.; Anderson, E. L.; Schmidt, P. W.; Lodge, T. P.; Hillmyer, M. A.; Bühlmann, P. ACS Sensors 2017, 2, 1498−1504. Perdizio Sakita, A. M.; Della Noce, R.; Fugivara, C. S.; Benedetti, A. V. Anal. Chem. 2017, 89, 8296−8303. ́ A.; Kim, J.; Kurniawan, J. F.; Sempionatto, J. R.; Martin, Moreto, J. R.; Tang, G.; Campbell, A. S.; Shin, A.; Lee, M. Y.; Liu, X.; Wang, J. ACS Sensors 2017, 2, 1860−1868. Eßmann, V.; Zhao, F.; Hartmann, V.; Nowaczyk, M. M.; Schuhmann, W.; Conzuelo, F. Anal. Chem. 2017, 89, 7160−7165. Cheng, H.; Yan, X.; Zare, R. N. Anal. Chem. 2017, 89, 3191−3198.
AUTHOR INFORMATION
ORCID
Shelley D. Minteer: 0000-0002-5788-2249 Notes
Views expressed in this editorial are those of the author and not necessarily the views of the ACS.
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REFERENCES
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DOI: 10.1021/jacs.8b00986 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
