Electrochemical impedance spectroscopy

Insights into the Importance of Native Passivation Layer and Interface Reactivity of Metallic Lithium by Electrochemical Impedance Spectroscopy

Lithium-metal batteries offer substantial advantages over lithium-ion batteries in terms of gravimetric and volumetric energy densities. However, their widespread practical use is hindered by safety concerns, often attributed to the poor stability of the metallic lithium interface, where electrochemical impedance spectroscopy (EIS) can provide crucial information. The EIS spectra of metallic lithium electrodes proved to be more complex than expected, especially when studying thin lithium metal foils. Here, it is identified that charge-transfer impedance becomes one of the main components of the EIS spectra, the magnitude of which is found to be strongly dependent on the native passivation layer of metallic lithium and on the nature of electrolyte. "Asymmetricity" of the EIS spectra in symmetric cells when separated the working and counter electrode contributions to the total impedance using three-electrode cells is also identified.

Nanoscale electrochemical charge transfer kinetics investigated by electrochemical scanning microwave microscopy

We show how microwave microscopy can be used to probe local charge transfer reactions with unprecedented sensitivity, visualizing surface reactions with only a few hundred molecules involved. While microwaves are too fast under classical conditions to interact and sense electrochemical processes, this is different at the nanoscale, where our heterodyne microwave sensing method allows for highly sensitive local cyclic voltammetry (LCV) and local electrochemical impedance spectroscopy (LEIS). LCV and LEIS allow for precise measurement of the localized charge transfer kinetics, as illustrated in this study for a ferrocene self-assembled monolayer immersed in an electrolyte. The theoretical analysis presented here enables a consistent mapping of the faradaic kinetics and the parasitic contributions (nonfaradaic) to be spectrally resolved and subtracted. In particular, this methodology reveals an undistorted assessment of accessible redox site density of states associated with faradaic capacitance, fractional surface coverage and electron transfer kinetics at the nanoscale. The developed methodology opens a new perspective on comprehending electrochemical reactivity at the nanoscale.

Recent Developments in DNA-Nanotechnology-Powered Biosensors for Zika/Dengue Virus Molecular Diagnostics

Zika virus (ZIKV) and dengue virus (DENV) are highly contagious and lethal mosquito-borne viruses. Global warming is steadily increasing the probability of ZIKV and DENV infection, and accurate diagnosis is required to control viral infections worldwide. Recently, research on biosensors for the accurate diagnosis of ZIKV and DENV has been actively conducted. Moreover, biosensor research using DNA nanotechnology is also increasing, and has many advantages compared to the existing diagnostic methods, such as polymerase chain reaction (PCR) and enzyme-linked immunosorbent assay (ELISA). As a bioreceptor, DNA can easily introduce a functional group at the 5' or 3' end, and can also be used as a folded structure, such as a DNA aptamer and DNAzyme. Instead of using ZIKV and DENV antibodies, a bioreceptor that specifically binds to viral proteins or nucleic acids has been fabricated and introduced using DNA nanotechnology. Technologies for detecting ZIKV and DENV can be broadly divided into electrochemical, electrical, and optical. In this review, advances in DNA-nanotechnology-based ZIKV and DENV detection biosensors are discussed.

Toward Understanding of the Li-Ion Migration Pathways in the Lithium Aluminum Sulfides Li3AlS3 and Li4.3AlS3.3Cl0.7 via 6,7Li Solid-State Nuclear Magnetic Resonance Spectroscopy

Li-containing materials providing fast ion transport pathways are fundamental in Li solid electrolytes and the future of all-solid-state batteries. Understanding these pathways, which usually benefit from structural disorder and cation/anion substitution, is paramount for further developments in next-generation Li solid electrolytes. Here, we exploit a range of variable temperature ⁶Li and ⁷Li nuclear magnetic resonance approaches to determine Li-ion mobility pathways, quantify Li-ion jump rates, and subsequently identify the limiting factors for Li-ion diffusion in Li₃AlS₃ and chlorine-doped analogue Li_4.3AlS_3.3Cl_0.7. Static ⁷Li NMR line narrowing spectra of Li₃AlS₃ show the existence of both mobile and immobile Li ions, with the latter limiting long-range translational ion diffusion, while in Li_4.3AlS_3.3Cl_0.7, a single type of fast-moving ion is present and responsible for the higher conductivity of this phase. ⁶Li-⁶Li exchange spectroscopy spectra of Li₃AlS₃ reveal that the slower moving ions hop between non-equivalent Li positions in different structural layers. The absence of the immobile ions in Li_4.3AlS_3.3Cl_0.7, as revealed from ⁷Li line narrowing experiments, suggests an increased rate of ion exchange between the layers in this phase compared with Li₃AlS₃. Detailed analysis of spin-lattice relaxation data allows extraction of Li-ion jump rates that are significantly increased for the doped material and identify Li mobility pathways in both materials to be three-dimensional. The identification of factors limiting long-range translational Li diffusion and understanding the effects of structural modification (such as anion substitution) on Li-ion mobility provide a framework for the further development of more highly conductive Li solid electrolytes.

Highly sensitive pork meat detection using copper(ii) tetraaza complex by electrochemical biosensor

Three copper(ii) tetraaza complexes [Cu(ii)LBr]Br (1a), [Cu(ii)L(CIO4)](CIO4) (2a) and [Cu(ii)L](CIO4)2 (2b), where L = 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-7,14-diene were prepared and confirmed by FTIR, 1HNMR and 13CNMR. The binding interaction of complex (1a, 2a, 2b) with calf thymus DNA (CT-DNA) was investigated using UV-vis absorption, luminescence titrations, viscosity measurements and molecular docking. The findings suggested that complex 1a, 2a and 2b bind to DNA by electrostatic interaction, and the strengths of the interaction were arranged according to 2b > 1a > 2a. The differences in binding strengths were certainly caused by the complexes' dissimilar charges and counter anions. Complex 2b, with the biggest binding strength towards the DNA, was further applied in developing the porcine sensor. The developed sensor exhibits a broad linear dynamic range, low detection limit, good selectivity, and reproducibility. Analysis of real samples showed that the biosensor had excellent selectivity towards the pork meat compared to chicken and beef meat.

Versatile electrical stimulator for cardiac tissue engineering-Investigation of charge-balanced monophasic and biphasic electrical stimulations

The application of biomimetic physical stimuli replicating the in vivo dynamic microenvironment is crucial for the in vitro development of functional cardiac tissues. In particular, pulsed electrical stimulation (ES) has been shown to improve the functional properties of in vitro cultured cardiomyocytes. However, commercially available electrical stimulators are expensive and cumbersome devices while customized solutions often allow limited parameter tunability, constraining the investigation of different ES protocols. The goal of this study was to develop a versatile compact electrical stimulator (ELETTRA) for biomimetic cardiac tissue engineering approaches, designed for delivering controlled parallelizable ES at a competitive cost. ELETTRA is based on an open-source micro-controller running custom software and is combinable with different cell/tissue culture set-ups, allowing simultaneously testing different ES patterns on multiple samples. In particular, customized culture chambers were appositely designed and manufactured for investigating the influence of monophasic and biphasic pulsed ES on cardiac cell monolayers. Finite element analysis was performed for characterizing the spatial distributions of the electrical field and the current density within the culture chamber. Performance tests confirmed the accuracy, compliance, and reliability of the ES parameters delivered by ELETTRA. Biological tests were performed on neonatal rat cardiac cells, electrically stimulated for 4 days, by comparing, for the first time, the monophasic waveform (electric field = 5 V/cm) to biphasic waveforms by matching either the absolute value of the electric field variation (biphasic ES at ±2.5 V/cm) or the total delivered charge (biphasic ES at ±5 V/cm). Findings suggested that monophasic ES at 5 V/cm and, particularly, charge-balanced biphasic ES at ±5 V/cm were effective in enhancing electrical functionality of stimulated cardiac cells and in promoting synchronous contraction.

Synthesis and photocatalytic performance of g-C3N4/MeTMC-COP composite photocatalyst

Covalent organic polymers have excellent application prospects in photocatalysis due to their excellent visible light absorption and structural designability. However, their fast recombination efficiency and complex preparation process limit their applications. Because of the above problems, this paper used urea to prepare g-C₃N₄ by high-temperature thermal polymerization and prepared g-C₃N₄ composite photocatalyst loaded with MeTMC-COP (g-C₃N₄/MeTMC-COP) by hydrothermal method. The photocatalytic hydrogen generation and photocatalytic degradation capabilities of composite photocatalysts with various mass ratios were investigated by characterizing the catalyst and using the organic dye Rhodamine B (RhB) as the pollutant. According to the research, the specific surface area of the g-C₃N₄/MeTMC-COP composite may reach 40.95 m² g^-1 when the mass ratio of g-C₃N₄ and MeTMC-COP is 3:1 (25.22 m² g^-1). It can offer more active sites for the photocatalytic process, and because the fluorescence peak intensity is the lowest, it has the lowest photogenerated electron-hole recombination efficiency. In comparison to g-C₃N₄, 3:1 g-C₃N₄/MeTMC-COP can breakdown rhodamine B up to 100% after 75 min of light irradiation; its photocatalytic hydrogen generation efficiency is 1.62 times that of g-C₃N₄, and the hydrogen evolution rate is 11.8 μmol g^-1 h^-1.

Recent Developments in Electrochemical-Impedimetric Biosensors for Virus Detection

Viruses, including influenza viruses, MERS-CoV (Middle East respiratory syndrome coronavirus), SARS-CoV (severe acute respiratory syndrome coronavirus), HAV (Hepatitis A virus), HBV (Hepatitis B virus), HCV (Hepatitis C virus), HIV (human immunodeficiency virus), EBOV (Ebola virus), ZIKV (Zika virus), and most recently SARS-CoV-2 (severe acute respiratory syndrome coronavirus 2), are responsible for many diseases that result in hundreds of thousands of deaths yearly. The ongoing outbreak of the COVID-19 disease has raised a global concern and intensified research on the detection of viruses and virus-related diseases. Novel methods for the sensitive, rapid, and on-site detection of pathogens, such as the recent SARS-CoV-2, are critical for diagnosing and treating infectious diseases before they spread and affect human health worldwide. In this sense, electrochemical impedimetric biosensors could be applied for virus detection on a large scale. This review focuses on the recent developments in electrochemical-impedimetric biosensors for the detection of viruses.

Spatial-Division Multiplexing Approach for Simultaneous Detection of Fiber-Optic Ball Resonator Sensors: Applications for Refractometers and Biosensors

Fiber-optic ball resonators are an attractive technology for refractive index (RI) sensing and optical biosensing, as they have good sensitivity and allow for a rapid and repeatable manufacturing process. An important feature for modern biosensing devices is the multiplexing capacity, which allows for interrogating multiple sensors (potentially, with different functionalization methods) simultaneously, by a single analyzer. In this work, we report a multiplexing method for ball resonators, which is based on a spatial-division multiplexing approach. The method is validated on four ball resonator devices, experimentally evaluating both the cross-talk and the spectral shape influence of one sensor on another. We show that the multiplexing approach is highly efficient and that a sensing network with an arbitrary number of ball resonators can be designed with reasonable penalties for the sensing capabilities. Furthermore, we validate this concept in a four-sensor multiplexing configuration, for the simultaneous detection of two different cancer biomarkers across a widespread range of concentrations.

Study on the Electrical Conduction Mechanism of Unipolar Resistive Switching Prussian White Thin Films

The electrical conduction mechanism of resistive switching Prussian white (PW) thin films obtained by the electrodeposition method was examined by AC impedance spectroscopy and DC current-voltage measurements. Using an electrode tip to contact PW grown over Au, robust unipolar resistive switching was observed with a current change of up to three orders of magnitude, high repeatability, and reproducibility. Moreover, electrical impedance spectroscopy showed that the resistive switching comes from small conductive filaments formed by potassium ions before the establishment of larger conductive channels. Both voltammetry and EIS measurements suggest that the electrical properties and conductive filament formation are influenced by defects and ions present in the grain boundaries. Thus, PW is a potential material for the next generation of ReRAM devices.

Impedimetric Detection Based on Label-Free Immunoassay Developed for Targeting Spike S1 Protein of SARS-CoV-2

After the COVID-19 pandemic started all over the world, great importance was placed on the development of sensitive and selective bioanalytical assays for the rapid detection of the highly pathogenic SARS-CoV-2 virus causing COVID-19 disease. In this present work, an impedimetric immunosensor was developed and applied for rapid, reliable, sensitive and selective detection of the SARS-CoV-2 S1 protein. To detect the SARS-CoV-2 virus, targeting of the spike S1 protein was achieved herein by using S1 protein-specific capture antibody (Cab-S1) immobilized screen-printed electrode (SPE) in combination with the electrochemical impedance spectroscopy (EIS) technique. With the impedimetric immunosensor, the detection limit for S1 protein in buffer medium was found to be 0.23 ng/mL (equal to 23.92 amol in 8 µL sample) in the linear concentration range of S1 protein from 0.5 to 10 ng/mL. In the artificial saliva medium, it was found to be 0.09 ng/mL (equals to 9.36 amol in 8 µL sample) in the linear concentration range of S1 protein between 0.1 and 1 ng/mL. The selectivity of the impedimetric immunosensor toward S1 protein was tested against influenza hemagglutinin antigen (HA) in the buffer medium as well as in artificial saliva.

Lithium-Ion Battery Solid Electrolytes Based on Poly(vinylidene Fluoride)-Metal Thiocyanate Ionic Liquid Blends

Solid polymer electrolytes (SPEs) are required to improve battery safety through the elimination of the liquid electrolyte solution in current batteries. This work is focused on the development of a hybrid SPE based on poly(vinylidene fluoride), PVDF, and 1-butyl-3-methylimidazolium cobalt(II) isothiocyanate, [BMIM]2[(SCN)4Co] magnetic ionic liquid (MIL), and its battery cycling behavior at room temperature. The addition of MIL in filler contents up to 40 wt % to the PVDF matrix does not influence the compact morphology of the samples obtained by solvent casting. The polar β-phase of PVDF increases with increasing MIL content, whereas the degree of crystallinity, thermal degradation temperature, and mechanical properties of the MIL/PVDF blends decrease with increasing MIL content. The ionic conductivity of the MIL/PVDF blends increases both with temperature and MIL content, showing the highest ionic conductivity of 7 × 10-4 mS cm-1 at room temperature for the MIL/PVDF blend with 40 wt % of MIL. The cathodic half-cells prepared with this blend as SPE show good reversibility and excellent cycling behavior at different C-rates, with a discharge capacity of 80 mAh g-1 at a C/10-rate with a Coulombic efficiency of 99%. The developed magnetic SPE, with excellent performance at room temperature, shows potential for the implementation of sustainable lithium-ion batteries, which can be further tuned by the application of an external magnetic field.

Structure and Evolution of Quasi-Solid-State Hybrid Electrolytes Formed Inside Electrochemical Cells

Solid-state electrolytes (SSEs) formed inside an electrochemical cell by polymerization of a liquid precursor provide a promising strategy for overcoming problems with electrolyte wetting in solid-state batteries. Hybrid solid-state polymer electrolytes (HSPEs) created by in situ polymerization of a conventional liquid precursor containing electrochemically inert nanostructures are of particular interest because they offer a mechanism for selectively reinforcing or adding new functionalities to the electrolyte-removing the need for high degrees of polymerization. The synthesis, structure, chemical kinetics, ion-transport properties and electrochemical characteristics of HSPEs created by Al(OTf)₃ -initiated polymerization of 1,3-dioxolane (DOL) containing hairy, nano-sized SiO₂  particles are reported. Small-angle X-ray scattering reveals the particles are well-dispersed in liquid DOL. Strong interaction between poly(ethylene glycol) molecules tethered to the SiO₂  particles and poly(DOL) lead to co-crystallization-anchoring the nanoparticles in their host It also enables polymerization-depolymerization processes in DOL to be studied and controlled. The utility of the in-situ-formed HSPE, is demonstrated first in Li|HSPE|Cu half cells, which manifest Coulombic efficiencies (CE) values approaching 99%. HSPEs are also demonstrated in solid-state lithium-sulfur-polyacrylonitrile (SPAN) composite full-cell batteries. The in-situ-formed Li|HSPE|SPAN cells show good cycling stability and thus provide a promising path toward all-solid-state batteries.

Electrochemical impedance spectroscopy applied to microbial fuel cells: A review

Electrochemical impedance spectroscopy (EIS) is an efficient and non-destructive test for analyzing the bioelectrochemical processes of microbial fuel cells (MFCs). The key factors limiting the output performance of an MFC can be identified by quantifying the contribution of its various internal parts to the total impedance. However, little attention has been paid to the measurement conditions and diagrammatic processes of the EIS for MFC. This review, starting with the analysis of admittance of bioelectrode, introduces conditions for the EIS measurement and summarizes the representative equivalent circuit plots for MFC. Despite the impedance from electron transfer and diffusion process, the effect of unnoticeable capacitance obtained from the Nyquist plot on MFCs performance is evaluated. Furthermore, given that distribution of relaxation times (DRT) is an emerging method for deconvoluting EIS data in the field of fuel cell, the application of DRT-analysis to MFC is reviewed here to get insight into bioelectrode reactions and monitor the biofilm formation. Generally, EIS measurement is expected to optimize the construction and compositions of MFCs to overcome the low power generation.

Copper–Ruthenium Composite as Perspective Material for Bioelectrodes: Laser-Assisted Synthesis, Biocompatibility Study, and an Impedance-Based Cellular Biosensor as Proof of Concept

Copper is an inexpensive material that has found wide application in electronics due to its remarkable electric properties. However, the high toxicity of both copper and copper oxide imposes restrictions on the application of this metal as a material for bioelectronics. One way to increase the biocompatibility of pure copper while keeping its remarkable properties is to use copper-based composites. In the present study, we explored a new copper–ruthenium composite as a potential biocompatible material for bioelectrodes. Sample electrodes were obtained by subsequent laser deposition of copper and ruthenium on glass plates from a solution containing salts of these metals. The fabricated Cu–Ru electrodes exhibit high effective area and their impedance properties can be described by simple R-CPE equivalent circuits that make them perspective for sensing applications. Finally, we designed a simple impedance cell-based biosensor using this material that allows us to distinguish between dead and alive HeLa cells.

Iron (Fe)-doped mesoporous 45S5 bioactive glasses: Implications for cancer therapy

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Fe-doped mesoporous 45S5 BGs were successfully synthesized using the sol-gel route.

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Fe-doped MBGs exhibited a particles size of 12 nm with a high surface area of 306 m²/g.

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Fe-doped MBGs could generate H₂O₂ in a cathodic potential higher than −0.2 V.

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Fe-doped MBGs increased the standard rate constant of Electro-Fenton's (EF) reaction up to 38.44 times as compared with the Fe-free glasses.

The utilization of bioactive glasses (BGs) in cancer therapy has recently become quite promising; herein, a series of Fe-doped mesoporous 45S5-based BGs (MBGs) were synthesized via the sol-gel method in the presence of Pluronic P123 as a soft template. The physico-chemical and biological properties of the prepared glasses were well-characterized through structural assessments, thermal analyses, and electron microscopic studies. Electrochemical analyses, including cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), were also performed to investigate the actual potential of the Fe₂O₃-containing MBGs in modulating the Fenton's reaction. The XRD results confirmed the glassy state of the Fe-doped samples before immersion in simulated body fluid (SBF). The prepared Fe-doped MBGs exhibited a particle size in the range of 11–86 nm, surface charge of 27–30 mV, S_BET of 95–306 m²/g, and M_s of 0.08 to 0.2 emu/g. The incorporation of Fe₂O₃ led to a negligible decrease in the bioactivity of the glasses. The CV analysis indicated that the Fe-doped MBGs could generate H₂O₂ in a cathodic potential higher than -0.2 V (vs. Ag/AgCl) in the O₂-saturated Na₂SO₄ solution. Additionally, the data of the EIS test revealed that the Fe₂O₃-doped MBGs could increase the standard rate constant of Electro-Fenton's (EF) reaction up to 38.44 times as compared with the Fe-free glasses. In conclusion, Fe-doped 45S5-derived glasses may be useful in cancer therapy strategies due to their capability of activating Fenton's reaction and subsequent production of reactive oxygen species (ROS) such as ^•OH free radicals.

Development of an Online Monitoring Device for the Mixing Ratio of Two-Part Epoxy Adhesives Using an Electrical Impedance Spectroscopy Technique and Machine Learning

Two-part epoxy adhesives are widely used in a range of industries. Two-part epoxy adhesive is composed of a resin and a hardener. Both materials remain stable in the general environment but curing begins when mixed in the specified mixing ratio. However, it has the disadvantage of requiring a specific mixing device. In addition, if the mixing ratio is different from the specified ratio due to the error of the mixing system, it has a fatal effect on the adhesion performance. The dielectric constant is a characteristic constant of a material. Therefore, it represents the mixing ratio of mixed two-part epoxy adhesives. With the electrical impedance spectroscopy technique, it can be measured indirectly by measuring impedance according to frequency and temperature. In this study, a sensor and embedded device for an online monitoring of its integrity using a regression method among machine learning are developed, which can acquire impedance data with frequency and temperature data according to the change in the mixing ratio of a two-part epoxy adhesive. The experimentally collected data were used as training data for the machine learning algorithm. It was found that the learned machine learning algorithm effectively estimates the mixing ratio of the two-part epoxy with an arbitrary value.

Nano-enabled sensing of per-/poly-fluoroalkyl substances (PFAS) from aqueous systems - A review

Per-/poly-fluoroalkyl substances (PFAS) are an emerging class of environmental contaminants used as an additive across various commodity and fire-retardant products, for their unique thermo-chemical stability, and to alter their surface properties towards selective liquid repellence. These properties also make PFAS highly persistent and mobile across various environmental compartments, leading to bioaccumulation, and causing acute ecotoxicity at all trophic levels particularly to human populations, thus increasing the need for monitoring at their repositories or usage sites. In this review, current nano-enabled methods towards PFAS sensing and its monitoring in wastewater are critically discussed and benchmarked against conventional detection methods. The discussion correlates the materials' properties to the sensitivity, responsiveness, and reproducibility of the sensing performance for nano-enabled sensors in currently explored electrochemical, spectrophotometric, colorimetric, optical, fluorometric, and biochemical with limits of detection of 1.02 × 10^-6 μg/L, 2.8 μg/L, 1 μg/L, 0.13 μg/L, 6.0 × 10^-5 μg/L, and 4.141 × 10^-7 μg/L respectively. The cost-effectiveness of sensing platforms plays an important role in the on-site analysis success and upscalability of nano-enabled sensors. Environmental monitoring of PFAS is a step closer to PFAS remediation. Electrochemical and biosensing methods have proven to be the most reliable tools for future PFAS sensing endeavors with very promising detection limits in an aqueous matrix, short detection times, and ease of fabrication.

One-step electrochemical deposition of antifouling polymers with pyrogallol for biosensing applications

Electrochemical techniques are highly sensitive and label-free sensing methods for the detection of various biomarkers, toxins, or pathogens. An ideal sensing element should be electroconductive, nonfouling, and readily available for conjugation of ligands. In this work, we have developed a facile, one-step electrodeposition method based on pyrogallol polymerization for preparation of a nonfouling and biotinylated surface on indium tin oxide (ITO). A copolymer of sulfobetaine methacrylate and aminoethyl methacrylate (pSBAE) was synthesized and deposited on ITO in the presence of pyrogallol via cyclic voltammetry. The deposition took less than 15 minutes to sufficiently inhibit cell adhesion. Using biotinylated pSBAE, the modified surface resisted nonspecific protein adsorption from the fetal bovine serum solution and detected added avidin concentrations. The results show an efficient platform to fabricate an electrochemical biosensor for the detection of biomarkers. We expect that this facile one-step technology could be applied to conjugate various biosensing elements for nonfouling biosensors.

Highly Deformable, Conductive Double-Network Hydrogel Electrolytes for Durable and Flexible Supercapacitors

Developing flexible energy storage devices with the ability to retain capacitance under extreme deformation is promising but remains challenging. Here, we report the development of a durable supercapacitor with remarkable capacitance retention under mechanical deformation by utilizing a physical double-network (DN) hydrogel as an electrolyte. The first network is hydrophobically associating polyacrylamide cross-linked by nanoparticles, and the second network is Zn²⁺ cross-linked alginate. Through soaking such a DN hydrogel into a high concentration of ZnSO₄ solution, a highly deformable electrolyte with good conductivity is fabricated, which also shows adhesion to diverse surfaces. Directly attaching the hydrogel electrolyte to two pieces of an active carbon cloth facilely produces a flexible supercapacitor with a high specific capacitance and theoretical energy density. Remarkable capacitance retention under tension, compression, and bending is observed for the supercapacitor, which can also maintain above 87% of the initial capacitance after 4000 charge-discharge cycles. This study provides a simple way to fabricate hydrogel electrolytes for deformable yet durable supercapacitors, which is expected to inspire the development of next-generation flexible energy storage devices.

Solid Polymer Electrolytes Based on Gellan Gum and Ionic Liquid for Sustainable Electrochromic Devices

Materials sustainability is becoming increasingly relevant in every developed technology and, consequently, environmentally friendly solid polymer electrolytes (SPEs) based on gellan gum and different quantities of ionic liquid (IL) 1-ethyl-3-methyl-imidazolium-thiocyanate [Emim][SCN] have been prepared and applied in electrochromic devices (ECDs). The addition of the IL does not affect the crystalline phase of gellan gum, and the samples show a compact morphology, surface uniformity, no phase separation, and good distribution of the IL within the carrageenan matrix. The developed SPE are thermally stable up to ∼100 °C and show suitable mechanical properties. The most concentrated sample (39 wt % IL content) reaches a maximum ionic conductivity value of 6.0 × 10^-3 S cm^-1 and 1.8 × 10^-2 S cm^-1 at 30 and 90 °C, respectively. The electrochromic device (ECD) was fabricated with poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS) as working electrode and the developed SPE was compared with an aqueous 0.1 M KNO₃ solution. The electrochromic performance of the electrolyte was assessed in terms of spectroelectrochemistry, demonstrating a fully flexible ECD operating at voltages below 1.0 V. This novel electrolyte opens the door to the preparation of high performance sustainable ECD.

The role of the double layer for the pseudocapacitance of the hydrogen adsorption on platinum

Pseudocapacitances such as the hydrogen adsorption on platinum (HAoPt) are associated with faradaic chemical processes that appear as capacitive in their potentiodynamic response, which was reported to result from the kinetics of adsorption processes. This study discusses an alternative interpretation of the partly capacitive response of the HAoPt that is based on the proton transport of ad- or desorbed hydrogen in the double layer. Potentiodynamic perturbations of equilibrated surface states of the HAoPt lead to typical double layer responses with the characteristic resistive–capacitive relaxations that overshadow the fast adsorption kinetics. A potential-dependent double layer representation by a dynamic transmission line model incorporates the HAoPt in terms of capacitive contributions and can computationally reconstruct the charge exchanged in full range cyclic voltammetry data. The coupling of charge transfer with double layer dynamics displays a novel physicochemical theory to explain the phenomenon of pseudocapacitance and the mechanisms in thereon based supercapacitors.

Selective Passivation of Three-Dimensional Carbon Microelectrodes by Polydopamine Electrodeposition and Local Laser Ablation

In this article, a novel approach for selective passivation of three-dimensional pyrolytic carbon microelectrodes via a facile electrochemical polymerization of a non-conductive polymer (polydopamine, PDA) onto the surface of carbon electrodes, followed by a selective laser ablation is elaborated. The 3D carbon electrodes consisting of 284 micropillars on a circular 2D carbon base layer were fabricated by pyrolysis of lithographically patterned negative photoresist SU-8. As a second step, dopamine was electropolymerized onto the electrode by cyclic voltammetry (CV) to provide an insulating layer at its surface. The CV parameters, such as the scan rate and the number of cycles, were investigated and optimized to achieve a reliable and uniform non-conductive coating on the surface of the 3D pyrolytic carbon electrode. Finally, the polydopamine was selectively removed only from the tips of the pillars, by using localized laser ablation. The selectively passivated electrodes were characterized by scanning electron microscopy, cyclic voltammetry and electrochemical impedance spectroscopy methods. Due to the surface being composed of highly biocompatible materials, such as pyrolytic carbon and polydopamine, these 3D electrodes are particularly suited for biological application, such as electrochemical monitoring of cells or retinal implants, where highly localized electrical stimulation of nerve cells is beneficial.

Nickel-Based Electrocatalysts for Water Electrolysis

Currently, hydrogen production is based on the reforming process, leading to the emission of pollutants; therefore, a substitute production method is imminently required. Water electrolysis is an ideal alternative for large-scale hydrogen production, as it does not produce any carbon-based pollutant byproducts. The production of green hydrogen from water electrolysis using intermittent sources (e.g., solar and eolic sources) would facilitate clean energy storage. However, the electrocatalysts currently required for water electrolysis are noble metals, making this potential option expensive and inaccessible for industrial applications. Therefore, there is a need to develop electrocatalysts based on earth-abundant and low-cost metals. Nickel-based electrocatalysts are a fitting alternative because they are economically accessible. Extensive research has focused on developing nickel-based electrocatalysts for hydrogen and oxygen evolution. Theoretical and experimental work have addressed the elucidation of these electrochemical processes and the role of heteroatoms, structure, and morphology. Even though some works tend to be contradictory, they have lit up the path for the development of efficient nickel-based electrocatalysts. For these reasons, a review of recent progress is presented herein.

Continuous monitoring of molecular biomarkers in microfluidic devices

The ability to monitor molecular targets is crucial in fields ranging from healthcare to industrial processing to environmental protection. Devices employing biomolecules to achieve this goal are called biosensors. Over the last half century researchers have developed dozens of different biosensor approaches. In this chapter we analyze recent advances in the biosensing field aiming at adapting these to the problem of continuous molecular monitoring in complex sample streams, and how the merging of these sensors with lab-on-a-chip technologies would be beneficial to both. To do so we discuss (1) the components that comprise a biosensor, (2) the challenges associated with continuous molecular monitoring in complex sample streams, (3) how different sensing strategies deal with (or fail to deal with) these challenges, and (4) the implementation of these technologies into lab-on-a-chip architectures.

Performance of graphene-zinc oxide nanocomposite coated-glassy carbon electrode in the sensitive determination of para-nitrophenol

Graphene: zinc oxide nanocomposite (GN:ZnO NC) platform was tried for the sensitive determination of para-nitrophenol (p-NP) through the electrochemical method. ZnO nanoparticles (NPs) were synthesized by the modified wet-chemical method where in potassium hydroxide and zinc nitrate were used as precursors and starch as a stabilizing agent. A green and facile approach was applied to synthesize GN:ZnO NC in which glucose was employed as a reductant to reduce graphene-oxide to graphene in the presence of ZnO NPs. The synthesized NC was characterized using scanning and high-resolution transmission electron microscopy, energy dispersive x-ray analysis, X-ray diffraction and Raman spectroscopic techniques to examine the crystal phase, crystallinity, morphology, chemical composition and phase structure. GN:ZnO NC layer deposited over the glassy carbon electrode (GCE) was initially probed for its electrochemical performance using the standard 1 mM K3[Fe(CN)6] model complex. GN:ZnO NC modified GCE was monitored based on p-NP concentration. An enhanced current response was observed in 0.1 M phosphate buffer of pH 6.8 for the determination of p-NP in a linear working range of 0.09 × 10-6 to 21.80 × 10-6 M with a lower detection limit of 8.8 × 10-9 M employing square wave adsorptive stripping voltammetric technique at a deposition-potential and deposition-time of - 1.0 V and 300 s, respectively. This electrochemical sensor displayed very high specificity for p-NP with no observed interference from some other possible interfering substances such as 2, 4-di-NP, ortho-NP, and meta-NP. The developed strategy was useful for sensitive detection of p-NP quantity in canals/rivers and ground H2O samples with good recoveries.

Free-base porphyrin polymer for bifunctional electrochemical water splitting

Water splitting is considered a promising approach for renewable and sustainable energy conversion. The development of water splitting electrocatalysts that have low-cost, long-lifetime, and high-performance is an important area of research for the sustainable generation of hydrogen and oxygen gas. Here, we report a metal-free porphyrin-based two-dimensional crystalline covalent organic polymer obtained from the condensation of terephthaloyl chloride and 5,10,15,20-tetrakis(4-aminophenyl) porphyrin which is stabilized by an extensive hydrogen bonding network. This material exhibits bifunctional electrocatalytic performance towards water splitting with onset overpotentials, η, of 36 mV and 110 mV for HER (in 0.5 M H₂SO₄) and OER (in 1.0 M KOH), respectively. The as-synthesized material is also able to perform water splitting in neutral phosphate buffer saline solution, with 294 mV for HER and 520 mV for OER, respectively. Characterized by electrochemical impedance spectroscopy (EIS) and chronoamperometry, the as-synthesized material also shows enhanced conductivity and stability compared to its molecular counterpart. Inserting a non-redox active zinc metal center in the porphyrin unit leads to a decrease in electrochemical activity towards both HER and OER, suggesting the four-nitrogen porphyrin core is the active site. The high performance of this metal-free material towards water splitting provides a sustainable alternative to the use of scarce and expensive metal electrocatalysts in energy conversion for industrial applications.

Water splitting is considered a promising approach for renewable and sustainable energy conversion.

Development of a POCT type insulin sensor employing anti-insulin single chain variable fragment based on faradaic electrochemical impedance spectroscopy under single frequency measurement

To improve glycemic control managed through insulin administration, recent studies have focused on developing hand-held point-of-care testing (POCT) electrochemical biosensors for insulin measurement. Amongst them, anti-insulin IgG-based sensors show promise in detecting insulin with high specificity and sensitivity. However, fabrication of electrochemical sensors with IgG antibodies can prove challenging because of their larger molecular size. To overcome these limitations, this study focuses on utilizing the anti-insulin single chain variable fragment (scFv) as a biosensing molecule with single-frequency faradaic electrochemical impedance spectroscopy (EIS). By comparing two different immobilization methods, covalent conjugation via succinimidyl ester and non-covalent poly-histidine chelation, we demonstrated effective modification of the electrode surface with anti-insulin scFv, while retaining its specific recognition toward insulin. Sensor performance was confirmed via the concentration-dependent faradaic electrochemical impedance change using potassium ferricyanide as a redox probe. The optimal frequency for measurement was determined to be the peak slope of the calculated impedance correlation with respect to frequency. Based on the identified optimized frequency, we performed single-frequency measurement of insulin within a concentration range of 10 pM-100 nM. This study can aid in developing a future point-of-care sensor which rapidly and sensitively measures insulin across a dynamic range of physiological concentrations, with label-free detection.

Anomalous Enhancement of Electrochemical Charge Transfer by a Ru Complex Ion Intercalator

We have found that the DNA intercalator [Ru(bpy)₂DPPZ]²⁺ (bpy = 2,2'-bipyridine; DPPZ = dipyrido[3,2-a:2',3'-c]phenazine) causes an anomalous increase in charge transfer in electrochemical impedance spectroscopy (EIS). With a carbonaceous electrode and a 1 mM hexacyanoferrate (1 mM [Fe(CN)₆]^3- and 1 mM [Fe(CN)₆]^4-) mediator, we found that adding only 1 μM [Ru(bpy)₂DPPZ]²⁺ greatly enhanced the charge transfer between the electrode and hexacyanoferrate mediator, independently of other electrolytes or buffer components. The effect started with a one millionth amount of hexacyanoferrate. Since [Ru(bpy)₂DPPZ]²⁺ can intercalate with dsDNA, the effect is highly applicable for dsDNA detection or PCR monitoring. With further developments of this method, EIS sensors not requiring specific electrode modifications should be possible.

Feasibility of Applying Fourier Transform Electrochemical Impedance Spectroscopy in Fast Cyclic Square Wave Voltammetry for the In Vivo Measurement of Neurotransmitters

We previously reported on the use of fast cyclic square wave voltammetry (FCSWV) as a new voltammetric technique. Fourier transform electrochemical impedance spectroscopy (FTEIS) has recently been utilized to provide information that enables a detailed analytical description of an electrified interface. In this study, we report on attempts to combine FTEIS with FCSWV (FTEIS-FCSWV) and demonstrate the feasibility of FTEIS-FCSWV in the in vivo detection of neurotransmitters, thus giving a new type of electrochemical impedance information such as biofouling on the electrode surface. From FTEIS-FCSWV, three new equivalent circuit element voltammograms, consisting of charge-transfer resistance (R_ct), solution-resistance (R_s), and double-layer capacitance (C_dl) voltammograms were constructed and investigated in the phasic changes in dopamine (DA) concentrations. As a result, all R_ct, R_s, and C_dl voltammograms showed different DA redox patterns and linear trends for the DA concentration (R² > 0.99). Furthermore, the R_ct voltammogram in FTEIS-FCSWV showed lower limit of detection (21.6 ± 15.8 nM) than FSCV (35.8 ± 17.4 nM). FTEIS-FCSWV also showed significantly lower prediction errors than FSCV in selectivity evaluations of unknown mixtures of catecholamines. Finally, C_dl from FTEIS-FCSWV showed a significant relationship with fouling effect on the electrode surface by showing decreased DA sensitivity in both flow injection analysis experiment (r = 0.986) and in vivo experiments. Overall, this study demonstrates the feasibility of FTEIS-FCSWV, which could offer a new type of neurochemical spectroscopic information concerning electrochemical monitoring of neurotransmitters in the brain, and the ability to estimate the degree of sensitivity loss caused by biofouling on the electrode surface.

MoS2 quantum dots and titanium carbide co-modified carbon nanotube heterostructure as electrode for highly sensitive detection of zearalenone

A novel electrochemical sensor has been fabricated for sensitive determination of zearalenone (ZEA) in food samples based on molybdenum disulfide quantum dots (MoS₂ QDs) and two-dimensional titanium carbide (2D-Ti₃C₂T_x MXene) co-modified multi-walled carbon nanotube (MWCNT) heterostructure. Physical and electrochemical characterizations reveal that 2D-Ti₃C₂T_x and MoS₂ QDs co-modified MWCNTs yields synergistic signal amplification effect, together with large specific surface area and excellent conductivity for the heterostructure, endowing the developed sensor with high detection performance to ZEA. Under optimized conditions, the sensor shows a wide linear range from 3.00 to 300 ng mL^-1 and a low limit of detection (LOD) of 0.32 ng mL^-1_, which is far lower than the maximum residue limits (MRLs) settled by the European Commission. In addition, it exhibits excellent selectivity, high reproducibility with a relative standard deviation (RSD) of 1.1%, and good repeatability (RSD 1.1%). In real sample analysis recoveries ranged from 94.8 to 105% showing the proposed electrochemical sensor has high potential in practical applications. This work presents an effective and valuable pathway for the use of novel heterostructure in the biosensing field.

Extensively Reducing Interfacial Resistance by the Ultrathin Pt Layer between the Garnet-Type Solid-State Electrolyte and Li-Metal Anode

All-solid-state Li-ion batteries (ASSLIBs), also known as next-generation batteries, have attracted much attention due to their high energy density and safety. The best advantage of ASSLIBs is the Li-metal anodes that could be used without safety issues. In this study, a highly conductive garnet solid electrolyte (Li_6.75La₃Zr_1.75Ta_0.25O₁₂, LLZTO) was used in the ASSLIB, and a Pt film was used to modify the surface of LLZTO to prove the solution of the Li-metal anode for LLZTO. Li-Pt alloy was synthesized to improve the wettability and contact of the interface. The interfacial resistance was reduced by 21 times, at only 9 Ω cm². The symmetric cell could stably cycle over 3500 h at a current density of 0.1 mA cm^-2. The full cell of Li|Li-Pt|LLZTO|LiFePO₄ and Li|Li-Pt|LLZTO|LiMn_0.8Fe_0.2PO₄ achieved high stability in terms of battery performance. Point-to-point contact transformed into homogeneous surface contact made the Li-ion flux faster and more stable. This surface modification method could provide researchers with a new choice for fixing interface issues and promoting the application of high-performance ASSLIBs in the future.

Continuous and Polarization-Tuned Redox Capacitive Anion Sensing at Electroactive Interfaces

Continuous, real-time ion sensing is of great value across various environmental and medical scenarios but remains underdeveloped. Herein, we demonstrate the potential of redox capacitance spectroscopy as a sensitive and highly adaptable ion sensing methodology, exemplified by the continuous flow sensing of anions at redox-active halogen bonding ferrocenylisophthalamide self-assembled monolayers. Upon anion binding, the redox distribution of the electroactive interface, and its associated redox capacitance, are reversibly modulated, providing a simple and direct sensory readout. Importantly, the redox capacitance can be monitored at a freely chosen, constant electrode polarization, providing a facile means of tuning both the sensor analytical performance and the anion binding affinity, by up to 1 order of magnitude. In surpassing standard voltammetric methods in terms of analytical performance and adaptability, these findings pave the way for the development of highly sensitive and uniquely tunable ion sensors. More generally, this methodology also serves as a powerful and unprecedented means of simultaneously modulating and monitoring the thermodynamics and kinetics of host-guest interactions at redox-active interfaces.

Probing polymer brushes with electrochemical impedance spectroscopy: a mini review

Polymer brushes are frequently used as surface-tethered antifouling layers in biosensors to improve sensor surface-analyte recognition in the presence of abundant non-target molecules in complex biological samples by suppressing nonspecific interactions. However, because brushes are complex systems highly responsive to changes in their surrounding environment, studying their properties remains a challenge. Electrochemical impedance spectroscopy (EIS) is an emerging method in this context. In this mini review, we aim to elucidate the potential of EIS for investigating the physicochemical properties and structural aspects of polymer brushes. The application of EIS in brush-based biosensors is also discussed. Most common principles employed in these biosensors are presented, as well as interpretation of EIS data obtained in such setups. Overall, we demonstrate that the EIS-polymer brush pairing has a considerable potential for providing new insights into brush functionalities and designing highly sensitive and specific biosensors.

A label-free graphene-based impedimetric biosensor for real-time tracing of the cytokine storm in blood serum; suitable for screening COVID-19 patients

Concurrent with the pandemic announcement of SARS-CoV-2 infection by the WHO, a variety of reports were published confirming the cytokine storm as the most mortal effect of the virus on the infected patients. Hence, cytokine storm as an evidenced consequence in most of the COVID-19 patients could offer a promising opportunity to use blood as a disease progression marker. Here, we have developed a rapid electrochemical impedance spectroscopy (EIS) sensor for quantifying the overall immune activity of the patients. Since during the cytokine storm many types of cytokines are elevated in the blood, there is no need for specific detection of a single type of cytokine and the collective behavior is just measured without any electrode functionalization. The sensor includes a monolayer graphene on a copper substrate as the working electrode (WE) which is able to distinguish between the early and severe stage of the infected patients. The charge transfer resistance (R CT) in the moderate and severe cases varies about 65% and 138% compared to the normal groups, respectively and a specificity of 77% and sensitivity of 100% based on ELISA results were achieved. The outcomes demonstrate a significant correlation between the total mass of the three main hypercytokinemia associated cytokines including IL-6, TNF-α and IFN-γ in patients and the R CT values. As an extra application, the biosensor's capability for diagnosis of COVID-19 patients was tested and a sensitivity of 92% and specificity of 50% were obtained compared to the RT-PCR results.

High-Performance Room Temperature Lithium-Ion Battery Solid Polymer Electrolytes Based on Poly(vinylidene fluoride-co-hexafluoropropylene) Combining Ionic Liquid and Zeolite

The demand for more efficient energy storage devices has led to the exponential growth of lithium-ion batteries. To overcome the limitations of these systems in terms of safety and to reduce environmental impact, solid-state technology emerges as a suitable approach. This work reports on a three-component solid polymer electrolyte system based on poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), the ionic liquid 1-butyl-3-methylimidazolium thiocyanate ([BMIM][SCN]), and clinoptilolite zeolite (CPT). The influences of the preparation method and of the dopants on the electrolyte stability, ionic conductivity, and battery performance were studied. The developed electrolytes show an improved room temperature ionic conductivity (1.9 × 10^-4 S cm^-1), thermal stability (up to 300 °C), and mechanical stability. The corresponding batteries exhibit an outstanding room temperature performance of 160.3 mAh g^-1 at a C/15-rate, with a capacity retention of 76% after 50 cycles. These results represent a step forward in a promising technology aiming the widespread implementation of solid-state batteries.

Detection and Characterization of Single Particles by Electrochemical Impedance Spectroscopy

We present an electrochemical impedance spectroscopy (EIS) technique that can detect and characterize single particles as they collide with an electrode in solution. This extension of single-particle electrochemistry offers more information than typical amperometric single-entity measurements, as EIS can isolate concurrent capacitive, resistive, and diffusional processes on the basis of their time scales. Using a simple model system, we show that time-resolved EIS can detect individual polystyrene particles that stochastically collide with an electrode. Discrete changes are observed in various equivalent circuit elements, corresponding to the physical properties of the single particles. The advantages of EIS are leveraged to separate kinetic and diffusional processes, enabling enhanced precision in measurements of the size of the particles. In a broader context, the frequency analysis and single-object resolution afforded by this technique can provide valuable insights into single pseudocapacitive microparticles, electrocatalysts, and other energy-relevant materials.

Se4P4nanoparticles confined within porous carbon as a lithium-ion battery anode with superior electrochemical performance

The exploration of advanced anode materials through rational structure/phase design is the key to developing high-performance rechargeable batteries. Herein, tetraphosphorus tetraselenide (Se₄P₄) nanoparticles confined within porous carbon (named SeP@C) are developed for lithium-ion batteries. The designed SeP@C shows a set of structural/compositional advantages as lithium-ion battery anodes including high electrical conductivity, low ion diffusion barrier, and relieved lithiation stress. Consequently, the SeP@C electrode displays superior comprehensive lithium storage performance, e.g., high reversible capacity (640.8 mA h g^-1at 0.1 A g^-1), excellent cycling stability (500 cycles with respective capacity retention of over or nearly 100%), and good rate capability, representing a comparable lithium storage performance in reported phosphide-based anodes. More significantly, it shows excellent energy storage properties in lithium-ion full cells which can light up 85 red LEDs for over 3.2 h. This work offers an advanced electrode construction guidance of phosphorous-based anodes for the development of high-performance energy storage devices.

Insect odorant receptor-based biosensors: Current status and prospects

Whilst the senses of vision and hearing have been successfully automated and miniaturized in portable formats (e.g. smart phone), this is yet to be achieved with the sense of smell. This is because the sensing challenge is not trivial as it involves navigating a chemosensory space comprising thousands of volatile organic compounds. Distinct aroma recognition is based on detecting unique combinations of volatile organic compounds. In natural olfactory systems this is accomplished by employing odorant receptors (ORs) with varying specificities, together with combinatorial neural coding mechanisms. Attempts to mimic the remarkable sensitivity and accuracy of natural olfactory systems has therefore been challenging. Current portable chemical sensors for odorant detection are neither sensitive nor selective, prompting research exploring artificial olfactory devices that use natural OR proteins for sensing. Much research activity to develop OR based biosensors has concentrated on mammalian ORs, however, insect ORs have not been explored as extensively. Insects possess an extraordinary sense of smell due to a repertoire of odorant receptors evolved to interpret olfactory cues vital to the insects' survival. The potential of insect ORs as sensing elements is only now being unlocked through recent research efforts to understand their structure, ligand binding mechanisms and development of odorant biosensors. Like their mammalian counterparts, there are many challenges with working with insect ORs. These include expression, purification and presentation of the insect OR in a stable display format compatible with an effective transduction methodology while maintaining OR structure and function. Despite these challenges, significant progress has been demonstrated in developing OR-based biosensors which exploit insect ORs in cells, lipid bilayers, liposomes and nanodisc formats. Ultrasensitive and highly selective detection of volatile organic compounds has been validated by coupling these insect OR display formats with transduction methodologies spanning optical (fluorescence) and electrical (field effect transistors, electrochemical impedance spectroscopy) techniques. This review summarizes the current status of insect OR based biosensors and their future outlook.