Redox cycling (RC) is often used to amplify the faradaic current associated with the electrochemical transformation of redox-active species. For a given concentration, this allows a considerable increase in sensitivity and an improvement in the detection limit of analytical devices. However, all previous reports have focused only on the detection of achiral molecules, e.g., ferrocene derivatives. In this contribution, we report an original setup composed of two metal electrodes encoded with chiral features, facing each other to form a microchannel. The presence of the same chiral feature on the two parallel-oriented electrodes enables the preferential cyclic transformation of a specific enantiomer. It will be more easily oxidized at the anode with the right chirality and can subsequently diffuse across the microchannel to be reduced back into the starting enantiomer at a cathode with an identical chiral feature. As a function of the width of the microchannel, this cycle is repeated multiple times, significantly enhancing the overall current in terms of chiral recognition. Herein, we illustrate this fundamental concept with mesoporous Pt-Ir alloy electrodes, imprinted with either L-DOPA or D-DOPA as transducers constituting the microchannel. The latter allows an amplification of the analytical signal of DOPA as a model compound by more than one order of magnitude and with an enantiodiscrimination efficiency reaching 90%. Thus, the proposed strategy opens up very interesting perspectives for chiral-sensing applications.

Tip-enhanced Raman spectroscopy (TERS) is a powerful technique for nanoscale chemical imaging. However, its worldwide expansion is still limited by the challenging fabrication of cheap, robust and efficient TERS tips as optical nanosources to amplify the Raman signal. An original method based on bipolar electrodeposition is described here to prepare gold-coated AFM cantilevers used as TERS tips. This wireless method is simple to implement, cost-effective, and allows for the parallel fabrication of several TERS tips with good reproducibility of the metal thickness and a relatively long lifetime. The TERS activity was confirmed by imaging graphene oxide flakes with high spatial resolution (below 10 nm). A promising yield of 64% was achieved for the fabrication of active TERS tips. Therefore, this method could pave the way for the development of new chemical routes for the preparation of TERS tips and other plasmonic nanostructures.

Externally driven chemo-electromagnets were designed by coupling bipolar electrochemistry and the geometry of a solenoid. The chemically induced magnetic dipole allows control of the dynamic displacement, without the use of ferromagnetic materials.

Water splitting has become a sustainable and clean alternative for hydrogen production. Commonly, the efficiency of such reactions is intimately related to the physico-chemical properties of the catalysts that constitute the electrolyzer. Thus, the development of simple and fast methods to evaluate the electrocatalytic efficiency of an electrolyzer is highly required. In this work, we present an unconventional method based on the combination of bipolar electrochemistry and light-emitting diodes, which allows the evaluation of the electrocatalytic performance of the two types of catalysts, composing an electrolyzer, namely for oxygen and hydrogen evolution reactions, respectively. The integrated light emission of the diode acts as an optical readout of the electrocatalytic information, which simultaneously depends on the composition of the anode and the cathode. The electrocatalytic activity of Au, Pt, and Ni electrodes, connected to the LED in multiple anode/cathode configurations, towards the water splitting reactions has been evaluated. The efficiency of the electrolyzer can be represented in terms of the onset electric field (ϵonset) for light emission, obtaining variations that are in agreement with data reported with conventional electrochemistry. This work introduces a straightforward method for evaluating electrocatalysts and underscores the importance of material characterization in developing efficient electrolyzers for hydrogen production.

An electromagnet is a particular device that takes advantage of electrical currents to produce concentrated magnetic fields. The most well-known example is a conventional solenoid, having the form of an elongated coil and creating a strong magnetic field through its center when it is connected to a current source. Spontaneous redox reactions located at opposite ends of an anisotropic Janus swimmer can effectively mimic a standard power source, due to their ability to wirelessly generate a local electric current. Herein, we propose the coupling of thermodynamically spontaneous redox reactions occurring at the extremities of a hybrid Mg/Pt Janus swimmer with a solenoidal geometry to generate significant magnetic fields. These chemically driven electromagnets spontaneously transform the redox-induced electric current into a magnetic field with a strength in the range of μT upon contact with an acidic medium. Such on-board magnetization allows them to perform compass-like rotational motion and magnetotactic displacement in the presence of external magnetic field gradients, without the need of using ferromagnetic materials for the swimmer design. The torque force experienced by the swimmer is proportional to the internal redox current, and by varying the composition of the solution, it is possible to fine-tune its angular velocity.

Electrochemical water-splitting is one of the key reactions that lay the basis for the storage of energy produced by sustainable sources. The development of fast methods to evaluate the electrocatalytic activity of electrode materials with respect to the oxidation and reduction of water are therefore required to make further progress in this exciting field of research. Here, we present a novel method based on bipolar electrochemistry, which allows evaluating the electrocatalytic activity of a material towards hydrogen and oxygen evolution (HER and OER, respectively) with a single device. This method is based on the capability of a conducting polymer layer to behave as a variable-resistance switch as a function of its electrochemical doping. This approach was used to evaluate the electrocatalytic activity of Au, Pt and Ni surfaces towards HER and OER in terms of the half-wave electric field, ε1/2, obtained during the insulating/conducting transition of poly-3,4-ortho-xylen-dioxythiophene (PXDOT). The trends of ε1/2 variations are in agreement with data reported with classical electrochemical methods. In this work, we developed an easy and straightforward method to evaluate the electrocatalytic activity of electrode materials for water-splitting.

Redox cycling (RC) is a powerful tool capable of amplifying faradaic currents in electroanalytical measurements, thus allowing an enhancement of sensitivity through fast multiple sequential oxidation and reduction reactions of a redox-active analyte. Present state-of-the-art RC devices are mostly based on planar electrode geometries either in 2D or 3D configurations, requiring cleanroom facilities and expensive microfabrication techniques. Here, the electrochemical elaboration and characterization of a 3D coaxial macroporous twin-electrode is reported, obtained by following a low-cost bottom-up approach. A nanoengineered highly organized porous material is the basis for the design of two threaded cylindrical porous gold microelectrodes with a gap in the micrometer range that can be fine-tuned. The potentials of the outer and inner electrodes are biased at values above and below the redox potential of the analyte so that a given molecule can participate several times in the electron exchange reaction by shuttling between both electrodes. The resulting signal amplification, combined with a straightforward synthesis strategy of the electrode architecture, allows envisioning numerous (bio)electroanalytical applications.

A novel, cost‐effective strategy for sensing performance enhancement of cantilever‐shaped, mass‐sensitive sensors is presented. The developed strategy relies on the introduction of macroporosity in the recognition unit. The developed sensors are successfully applied in air humidity monitoring as well as in chemical sensing. As mass‐sensitive transducers, polymer microelectromechanical systems (MEMS) in the form of poly(vinylidene fluoride‐trifluoroethylene) [P(VDF‐TrFE)] cantilever resonators are specifically developed. These resonators are modified with a hierarchically structured macroporous poly(2,3′‐bithiophene) film, acting as a sensing layer. The design of the recognition layer takes advantage of the synergistic combination of inverse opal structuring, surface imprinting, and semi‐covalent imprinting of proteins. The resulting cantilever resonators are first successfully tested for air humidity monitoring in the whole relative humidity range (ca.10 – 90%). When the recognition layer is composed of human serum albumin‐imprinted polymer (HSA‐MIP) with a hierarchical structure, it can also be used as a selective chemosensor in a concentration range from 10 pM to 20 µM. The obtained results demonstrate that the macroporosity of the receptor film significantly enhances the sensor performance.

Potentiostatic electrodeposition was used to manufacture gold-coated atomic force microscopy (AFM) tips for applications in tip-enhanced Raman spectroscopy (TERS). The growth of the gold film was found to be initiated by the instantaneous nucleation mechanism on the doped-silicon AFM tip taken as substrate. This cost-effective method allowed robust AFM-TERS probes with localized surface plasmon resonances in the visible range to be produced with 53 % yield. These probes enable TERS mapping of a graphene oxide flake and a carbon nanotube to be performed with lateral spatial resolution below 20 nm, which was uncorrelated with the curvature radius of the metallized tip apex.

Actuators controlled by external stimuli have received a lot of attention in recent years. Herein a polymer based dual stimuli actuator is reported, triggered by light and an electric field. This allows better control of actuation, enlarging the field of potential applications, like, for example, in the frame of soft robotics. The actuator is composed of polypyrrole and TiO2 modified with methylene blue. In an aqueous solution, the resulting freestanding hybrid film shows reversible actuation due to the synergy of light and an applied electric field. Illumination with light produces electron-hole pairs in the TiO2 layer, which are shuttled to the opposite ends of the actuator by the potential gradient present in the solution. This results in electrochemical oxidation and reduction reactions at the two extremities and consequently in site selective swelling of the polymer, which finally leads to a controlled motion of the actuator, following the principles of logic gate operations. Such synergistically induced switching allows developing original actuation schemes for performing complex mechanical tasks triggered by more than one stimulus.

The immobilization of bilirubin oxidase (BOD) on macroporous gold electrodes for the optimization of bioelectrocatalytic activity is described. A bilirubin oxidase mutant S362C (cys-BOD) engineered with a cysteine residue located on purpose at the enzyme surface close to the T1 active center was used. It allows the attachment in one-step of a self-assembled monolayer of the enzyme to gold through a reaction between the thiol group of the cysteine residue and the metal surface. BOD immobilization of wild type and S362C mutant in macroporous gold electrodes allowed high retention of activity and perfect control of the overall BOD loading due to the fine-tuning of the macroporous structure. The macroporous arrangement together with the use of cys-BOD makes these rationally designed enzyme-modified electrodes very promising candidates for high-performance bioelectrocatalytic devices with improved activity and stability.

The EFSA Panel on Food Additives and Nutrient Sources added to Food (ANS) was asked to deliver a scientific opinion re-evaluating sulfur dioxide (E 220), sodium sulfite (E 221), sodium bisulfite (E 222), sodium metabisulfite (E 223), potassium metabisulfite (E 224), calcium sulfite (E 226), calcium bisulfite (E 227) and potassium bisulfite (E 228) when used as food additives. The Panel noted that sulfur dioxide, bisulfite and sulfite ions existed in a series of equilibria and that these would favour bisulfite ions at the pH of the stomach and sulfite ions at physiological pHs. Therefore, it was considered that once ingested, based on their capacity to form sulfite ions, read across between the different sulfite sources is possible; however, the Panel noted the uncertainties about the reactivity of sulfites in different foods and the resulting reaction products. The overall limited database did not indicate any concern for genotoxicity and did not report any effect in the available chronic, carcinogenicity and reprotoxicity studies after oral exposure in the diet, by gavage, or in the drinking water. A no observed adverse effect level (NOAEL) of 70 mg SO2 equivalent/kg body weight (bw) per day was identified from a long-term toxicity study in rats. However, the Panel noted several uncertainties and limitations in the database and concluded that the current group acceptable daily intake (ADI) of 0.7 mg SO2 equivalent/kg bw per day (derived using a default uncertainty factor) would remain adequate but should be considered temporary while the database was improved. The Panel recommended that the database and the temporary group ADI should be re-evaluated and noted that the recommended studies could require 5 years for completion. The Panel further concluded that exposure estimates to sulfur dioxide and sulfites were higher than the group ADI of 0.7 mg SO2 equivalent/kg bw per day for all population groups.

Electrochemical water splitting is a crucially important process for energy conversion and storage. In this context, we report the synthesis of hierarchical multiporous nickel nanosheets obtained by a templated two‐step electrodeposition of nickel in the presence of silica beads and an assembly of nonionic surfactant in order to generate a combined macro‐ and mesoporous structure. Interestingly, the synergistic effect of these highly ordered meso‐ and macroporous structures promotes the catalytic performance for oxygen evolution reaction with one order of magnitude higher current densities and a better stability of the electrodes compared to flat nickel or electrodes having a single type of porosity. This example illustrates a promising strategy for the rational design of high‐performance porous metals, not only in the frame of water electrolysis, but also for other electrocatalytic applications.

This article reports on a procedure to predict the optimal thickness of cylindrical porous electrodes operating a single redox reaction. This is obtained from a macroscopic model for the coupled diffusion-reaction process that is first validated with voltammetry experiments of the H2O2/H2O reduction reaction carried out with a series of porous electrodes elaborated in this work. An analytical solution to this model is developed in the steady regime and for electrodes featuring a thickness to mean radius ratio small enough compared to unity. An analytical expression of the optimal electrode thickness is derived corresponding to the crossover value of two asymptotic regimes characterizing the dependence of the volume current density produced by the electrode upon its thickness. The predictive tool of the optimal thickness is general, regardless of the porous microstructure. The case of the electrodes used in the reported experiments illustrates that the optimal thickness is not intrinsic to the microsctructure characterized by the size of the representative volume, its specific area and effective diffusion coefficient. It also depends on the operating conditions reflected in the kinetic number, Ki, and the thickness of the diffusion layer surrounding the electrode. The dependence of the optimal thickness on these two parameters is quite significant in a range of very small values of Ki but remains quasi constant beyond a threshold value.

The Cover Feature illustrates the electrochemical conversion of oxygen on a highly ordered macroporous gold wire. The internal pore space being modified with a genetically engineered enzyme, it allows a controlled tuning of the bioelectrocatalytic current and its stability. More information can be found in the Article by L. Zhang et al. on page 4980 in Issue 19, 2019 (DOI: 10.1002/celc.201901022).