The goal of this review is to provide an overview of the advancements made in the field of bipolar electrochemistry over the past 2 years, with an emphasis on analysis. Bipolar electrodes (BPEs) are versatile, and in electroanalysis, they have been used extensively to screen electrocatalysts(1−4) and to sense biomarkers.(5−10) Their ability to modulate local electric fields lends them to the manipulation of cells and to the enrichment and separation of analytes.(11−17) Finally, by virtue of the polar and often graded profile of the interfacial potential across BPEs, they provide a platform for synthesis of Janus particles, useful as sensors and as microswimmers(18−22) and other materials with compositional gradients.(23,24) BPEs are particularly well-suited to analytical challenges that demand multiplexing or amenability to point-of-need (PON) application because even large arrays of BPEs can be controlled with simple equipment yet yield quantitative information about a system. In this review, we discuss recent progress in reactions that transduce current to a visible signal, sensing mechanisms, bipolar electrochemical cell design, integration of bipolar electrochemistry with spectroscopic techniques, BPEs at the nanoscale, and the application of BPEs to electrokinetics and materials preparation. Throughout the discussion, we identify promising trends, innovative directions, and remaining challenges in the field. Disciplines Disciplines Analytical Chemistry Comments Comments

A headspace single drop microextraction (HS-SDME) method and a dispersive liquid-liquid microextraction (DLLME) method were developed using two tetrachloromanganate ([MnCl])-based magnetic ionic liquids (MIL) as extraction solvents for the determination of twelve aromatic compounds, including four polyaromatic hydrocarbons, by reversed phase high-performance liquid chromatography (HPLC). The analytical performance of the developed HS-SDME method was compared to the DLLME approach employing the same MILs. In the HS-SDME approach, the magnetic field generated by the magnet was exploited to suspend the MIL solvent from the tip of a rod magnet. The utilization of MILs in HS-SDME resulted in a highly stable microdroplet under elevated temperatures and long extraction times, overcoming a common challenge encountered in traditional SDME approaches of droplet instability. The low UV absorbance of the [MnCl]-based MILs permitted direct analysis of the analyte enriched extraction solvent by HPLC. In HS-SDME, the effects of ionic strength of the sample solution, temperature of the extraction system, extraction time, stir rate, and headspace volume on extraction efficiencies were examined. Coefficients of determination (R) ranged from 0.994 to 0.999 and limits of detection (LODs) varied from 0.04 to 1.0μgL with relative recoveries from lake water ranging from 70.2% to 109.6%. For the DLLME method, parameters including disperser solvent type and volume, ionic strength of the sample solution, mass of extraction solvent, and extraction time were studied and optimized. Coefficients of determination for the DLLME method varied from 0.997 to 0.999 with LODs ranging from 0.05 to 1.0μgL. Relative recoveries from lake water samples ranged from 68.7% to 104.5%. Overall, the DLLME approach permitted faster extraction times and higher enrichment factors for analytes with low vapor pressure whereas the HS-SDME approach exhibited better extraction efficiencies for analytes with relatively higher vapor pressure.

Bipolar electrochemistry allows for facile arraying of tens to thousands of electrochemical sensors that can be controlled by a single pair of driving electrodes. While bipolar electrodes (BPEs) have been applied to many sensing motifs, their sensitivity and specificity are limited by the lack of diversity in voltammetric methods that have been developed for these wireless electrodes. In this study, electrochemiluminescence (ECL) from the co‐oxidation of Ru(bpy)32+ and tripropylamine (TPA) is evaluated as a reporting reaction for alternating current voltammetry (ACV) on a BPE at frequencies of 1.0 Hz and 5.0 Hz. We observe sinusoidal alternating luminescence that follows a similar trend to that of the simultaneously monitored current – a plot of the amplitude versus potential approximates a bell‐shaped curve. Notably, the luminescent response to the current is detected with a smartphone, which underscores the portability of this method. The fidelity of the transduced signal is determined both in a traditional 3‐electrode configuration and at a BPE. These experimental results indicate that the alternating luminescence follows the current sufficiently for quantitative sensing but is diminished at higher frequencies and peaks at a shifted potential. These results are significant because they demonstrate the potential for application of ACV at BPE arrays for multiplexed point‐of‐need sensors and provide guidance for the selection of reporting reactions in this context.

Label-free electrochemical biosensing leverages the advantages of label-free techniques, low cost, and fewer user steps, with the sensitivity and portability of electrochemical analysis. In this review, we identify four label-free electrochemical biosensing mechanisms: ( a) blocking the electrode surface, ( b) allowing greater access to the electrode surface, ( c) changing the intercalation or electrostatic affinity of a redox probe to a biorecognition unit, and ( d) modulating ion or electron transport properties due to conformational and surface charge changes. Each mechanism is described, recent advancements are summarized, and relative advantages and disadvantages of the techniques are discussed. Furthermore, two avenues for gaining further diagnostic information from label-free electrochemical biosensors, through multiplex analysis and incorporating machine learning, are examined. Expected final online publication date for the Annual Review of Analytical Chemistry, Volume 16 is June 2023. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.