Flexible Three‐Dimensional Graphene Hydrogels with Superior Conductivity and Excellent Electrochemical Performance for Supercapacitor Electrodes

Three‐dimensional porous graphene hydrogels have been prepared by a green and facile but very efficient approach using glucose as an assistant. Based on a one‐step hydrothermal reaction with optimal experimental conditions such as the reaction time and temperature, the graphene hydrogels exhibit a superior electrical conductivity (95.3 S/m) and can be used as supercapacitor electrode without any binder or conducting additives but showing a high specific capacitance of 384.6 F/g at a current density of 1 A/g. The results show that addition of glucose can not only greatly decrease the reaction temperature but also shorten the reaction time. The superior performance of the three‐dimensional porous graphene hydrogels as electrode for supercapacitor suggests its promising potentials in the field of energy storage devices.     

Recent Progresses on the High Performance Organic Electrochemical Transistors

Organic electrochemical transistor(OECT) with bulk current modulation capability based on the ion penetration into the organic semiconducting channel exhibits unique features, including high transconductance, low voltage and large capacitance. The high current at a low voltage, together with the compatibility with aqueous environment, makes OECT particularly suitable for bioelectronic applications, such as biological interfacing, printed logic circuitry and neuromorphic devices. However, the operation mechanism and structure-performance relationship of OECT are rather complicated and remain unclear to date. One of the critical issues is the ion penetration and transportation process. This review focuses on the research progresses of how to improve the OECT performance specifically through materials design, interfacing and morphology modulation. Different strategies of promoting the ion doping process are compared and discussed in order to optimize the device performance so that a deep understanding of the OECT operation principle could be gained.     

Suitability of Liquid Crystal Polymer substrates for high frequency acoustic devices

Polymer substrates are widely seen as a low-cost route to flexible circuits for systems incorporating displays, sensing functions and transistors. To date most polymer-based devices have involved passive components such as humidity sensors, constructed using metals and additional organic layers. The starting point of the study described here, is whether more advanced components incorporating functional materials can be integrated into devices on polymer substrates. An understanding will be required of the material factors that limit performance and, if possible, techniques developed to circumvent them so that their performance can be compared to that of standard silicon-based components.     

Recent advances in large-scale assembly of semiconducting inorganic nanowires and nanofibers for electronics, sensors and photovoltaics

Semiconducting inorganic nanowires (NWs), nanotubes and nanofibers have been extensively explored in recent years as potential building blocks for nanoscale electronics, optoelectronics, chemical/biological/optical sensing, and energy harvesting, storage and conversion, etc. Besides the top-down approaches such as conventional lithography technologies, nanowires are commonly grown by the bottom-up approaches such as solution growth, template-guided synthesis, and vapor-liquid-solid process at a relatively low cost. Superior performance has been demonstrated using nanowires devices. However, most of the nanowire devices are limited to the demonstration of single devices, an initial step toward nanoelectronic circuits, not adequate for production on a large scale at low cost. Controlled and uniform assembly of nanowires with high scalability is still one of the major bottleneck challenges towards the materials and device integration for electronics. In this review, we aim to present recent progress toward nanowire device assembly technologies, including flow-assisted alignment, Langmuir-Blodgett assembly, bubble-blown technique, electric/magnetic- field-directed assembly, contact/roll printing, planar growth, bridging method, and electrospinning, etc. And their applications in high-performance, flexible electronics, sensors, photovoltaics, bioelectronic interfaces and nano-resonators are also presented.     

Ultra Flexible Paper Based Electrochemical Sensors: Effect of Mechanical Contortion upon Electrochemical Performance

The influence of mechanical contortion upon the electrochemical performance of screen‐printed graphite paper‐based electroanalytical sensing platforms is evaluated and contrasted with traditionally employed polymeric based screen‐printed graphite sensors. Such a situation of implementation can be envisaged for the potential sensing of analytes on the skin where such sensors are based, for example in clothing where mechanical contortion, viz, bending will occur, and as such, its effect upon electrochemical sensors is of both fundamental and applied importance. The effect of mechanical contortion or stress upon electrochemical behaviour and performance is of screen printed sensors is explored. Comparisons are made between both paper‐ and polymeric‐ based sensing platforms that are evaluated towards the sensing of the well characterised electrochemical probes potassium ferrocyanide(II), hexaammine‐ruthenium(III) chloride and nicotinamide adenine dinucleotide (NADH). It is determined that the paper‐based sensors offer greater resilience in terms of electrochemical performance after mechanical stress. We gain insights into the role played by both the effect of the time of mechanical contortion and additionally the potentially detrimental effects of repeated contortion are explored. These unique paper‐based sensors hold promise for widespread applications where flexible and ultra‐low cost sensors are required such as applications into medical devices were ultra‐low cost sensors are a pre‐requisite, but also for utilisation within applications which require the implementation of ultra‐flexible electroanalytical sensing platforms such as in the case of wearable sensors, whilst maintaining useful electrochemical performances.     

Functional one-dimensional lipid bilayers on carbon nanotube templates

Use of biological machines and environments in novel bioinorganic nanostructures is critical for development of new types of biosensors, bio-NEMS devices, and functional materials. Lipid bilayers that mimic a cell membrane have already played an important role in such applications. We present supported lipid bilayers that spontaneously assemble in a continuous nanoshell around a template of a carbon nanotube wrapped with hydrophilic polymer cushion layers. We demonstrate that such 1-D lipid membranes are fluid and can heal defects, even over repeated damage-recovery cycles. A simple diffusion model can describe mobility of lipid molecules in these 1-D nanoshells. These structures could lead to the development of new classes of biosensors and bioelectronic devices.     

Nickel electrodes as a cheap and versatile platform for studying structure and function of immobilized redox proteins

Practical use of many bioelectronic and bioanalytical devices is limited by the need of expensive materials and time consuming fabrication. Here we demonstrate the use of nickel electrodes as a simple and cheap solid support material for bioelectronic applications. The naturally nanostructured electrodes showed a surprisingly high electromagnetic surface enhancement upon light illumination such that immobilization and electron transfer reactions of the model redox proteins cytochrome b₅ (Cyt b₅) and cytochrome c (Cyt c) could be followed via surface enhanced resonance Raman spectroscopy. It could be shown that the nickel surface, when used as received, promotes a very efficient binding of the proteins upon preservation of their native structure. The immobilized redox proteins could efficiently exchange electrons with the electrode and could even act as an electron relay between the electrode and solubilized myoglobin. Our results open up new possibility for nickel electrodes as an exceptional good support for bioelectronic devices and biosensors on the one hand and for surface enhanced spectroscopic investigations on the other hand.     

Evaluation of the performance of sensors based on optical imaging of a chemically sensitive layer

Interest in the use of the optical properties of chemical indicators is growing steadily. Among the optical methods that can be used to capture changes in sensing layers, those producing images of large-area devices are particularly interesting for chemical sensor array development. Until now, few studies addressed the characterization of image sensors from the point of view of their chemical sensor application. In this paper, a method to evaluate such performance is proposed. It is based on the simultaneous measurement of absorption events in a metalloporphyrin layer with an image sensor and a quartz microbalance (QMB). Exploiting the well-known behaviour of QMB, comparison of signals enables estimation of the minimum amount of absorbed molecules that the image sensor can detect. Results indicate that at the single pixel level a standard image sensor (for example a webcam) can easily detect femtomoles of absorbed molecules. It should therefore be possible to design sensor arrays in which the pixels of images of large-area sensing layers are regarded as individual chemical sensors providing a ready and simple method for large sensor array development.     

石墨烯在自供能传感系统中的应用

随着现代社会智能化的加速发展,传感系统中传感器的数量、密度和分布范围不断增加,传统的供能方式难以满足如此复杂多变的传感器供能需求,从周围环境中收集能量并转化为电能的自供能传感器件是解决这一难题的有效途径。石墨烯不仅具有优异的传感性能,而且在各种能源器件中有广泛的应用,这为基于石墨烯的自供能传感器件设计提供了便利。近年来,人们已经研究和发展了多种多样的石墨烯自供能传感器件。本文基于自供能器件的基本能量供给原理,包括电化学供能、光伏供能、摩擦电供能、水伏供能以及热电、压电、热释电等其它供能,分别介绍了石墨烯在自供能传感器件中的应用,并展望了基于石墨烯的自供能传感器件的未来发展、挑战和前景。     

Biological Components and Bioelectronic Interfaces of Water Splitting Photoelectrodes for Solar Hydrogen Production

Artificial photosynthesis (AP) is inspired by photosynthesis in nature. In AP, solar hydrogen can be produced by water splitting in photoelectrochemical cells (PEC). The necessary photoelectrodes are inorganic semiconductors. Light‐harvesting proteins and biocatalysts can be coupled with these photoelectrodes and thus form bioelectronic interfaces. We expand this concept toward PEC devices with vital bio‐organic components and interfaces, and their integration into the built environment.     

Measuring Sheet Resistances of Dielectrics Using Co‐Planar Hydrogel Electrochemical Cells with Practical Applications to Characterize the Protective Quality of Paints on Sculptures

Electrical properties of thin dielectric films at the solid‐liquid phase boundary are an important performance characteristic of many devices, coatings and sensors. In this paper, co‐polymeric hydrogels of polyacrylic acid co‐sulfonic acid, swollen with a salt solution to act as the solid electrolyte, were used to assess interfaces using electrochemical impedance spectroscopy (EIS) in a co‐planar geometry. Silane‐modified glasses were characterized by the co‐planar hydrogel EIS cell and found to be distinguishable based on their surface monolayer chemistry. EIS measurements were also made on primed and painted metal substrates, using both test panels and an outdoor sculpture, Tony Smith's Stinger . The panels were then exposed to accelerated and outdoor weathering and showed degradation on the surface paint layers, which was observable electrochemically using EIS and confirmed visually by SEM. Electrochemical spectral features were compared between data from a standard paint‐test cell versus this co‐planar hydrogel cell; both cell types were able to measure coating capacitance, providing useful information about the condition of the bulk coating. Yet, sheet resistance (R_s) was a spectral feature seen only by the co‐planar hydrogel cell. Thus, it can be concluded that the use of such co‐planar hydrogel cells can provide an earlier warning sign to coating degradation and such cells provide a new type of spectral information that is not assessable by the standard geometry.     

胶体离子超级电容器的比容量评价

胶体离子超级电容器作为一种新型的超级电容器,其同时具有能量密度和功率密度高的独特优势。 目前已经发展了包括多种过渡金属阳离子和稀土阳离子,例如Mn²⁺、Fe²⁺、Co²⁺、Ni²⁺、Cu²⁺、Sn²⁺、Sn⁴⁺、La³⁺、Ce³⁺、Er³⁺和Yb³⁺的胶体离子超级电容器体系。 在电化学反应中,识别出电活性物质的存在形式对研究电极反应机理和提高比容量具有重要价值。 本文主要通过对电活性物质比容量的探讨,理解这种新型胶体离子超级电容器的电化学储能机理。 评述了胶体离子超级电容器的比容量核算方式,提出了以阳离子为标准核算比容量的原因,并与传统超级电容器的核算方式进行了比较,表明胶体离子超级电容器在提高能量密度方面具有潜在优势,有望突破现有电化学储能设备的技术瓶颈,实现下一代高能量储能器件的开发。     

Carbon‐Nanotube Based Electrochemical Biosensors: A Review

This review addresses recent advances in carbon‐nanotubes (CNT) based electrochemical biosensors. The unique chemical and physical properties of CNT have paved the way to new and improved sensing devices, in general, and electrochemical biosensors, in particular. CNT‐based electrochemical transducers offer substantial improvements in the performance of amperometric enzyme electrodes, immunosensors and nucleic‐acid sensing devices. The greatly enhanced electrochemical reactivity of hydrogen peroxide and NADH at CNT‐modified electrodes makes these nanomaterials extremely attractive for numerous oxidase‐ and dehydrogenase‐based amperometric biosensors. Aligned CNT “forests” can act as molecular wires to allow efficient electron transfer between the underlying electrode and the redox centers of enzymes. Bioaffinity devices utilizing enzyme tags can greatly benefit from the enhanced response of the biocatalytic‐reaction product at the CNT transducer and from CNT amplification platforms carrying multiple tags. Common designs of CNT‐based biosensors are discussed, along with practical examples of such devices. The successful realization of CNT‐based biosensors requires proper control of their chemical and physical properties, as well as their functionalization and surface immobilization.     

Integration of Layered Redox Proteins and Conductive Supports for Bioelectronic Applications

Integration of redox enzymes with an electrode support and formation of an electrical contact between the biocatalysts and the electrode is the fundamental subject of bioelectronics and optobioelectronics. This review addresses the recent advances and the scientific progress in electrically contacted, layered enzyme electrodes, and discusses the future applications of the systems in various bioelectronic devices, for example, amperometric biosensors, sensoric arrays, logic gates, and optical memories. This review presents the methods for the immobilization of redox enzymes on electrodes and discusses the covalent linkage of proteins, the use of supramolecular affinity complexes, and the reconstitution of apo-redox enzymes for the nanoengineering of electrodes with protein monolayers of electrodes with protein monolayers and multilayers. Electrical contact in the layered enzyme electrode is achieved by the application of diffusional electron mediators, such as ferrocene derivatives, ferricyanide, quinones, and bipyridinium salts. Covalent tethering of electron relay units to layered enzyme electrodes, the cross-linking of affinity complexes formed between redox proteins and electrodes functionalized with relay-cofactor units, or surface reconstitution of apo-enzymes on relay-cofactor-functionalized electrodes yield bioelectrocatalytic electrodes. The application of the functionalized electrodes as biosensor devices is addressed and further application of electrically "wired" enzymes as catalytic interfaces in biofuel cells is discussed. The organization of sensor arrays, self-calibrated biosensors, or gated bioelectronic devices requires the microstructuring of biomaterials on solid supports in the form of ordered micro-patterns. For example, light-sensitive layers composed of azides, benzophenone, or diazine derivatives associated with solid supports can be irradiated through masks to enable the patterned covalent linkage of biomaterials to surfaces. Alternatively, patterning of biomaterials can be accomplished by noncovalent interactions (such as in affinity complexes between avidin and a photolabeled biotin, or between an antibody and a photoisomerizable antigen layer) to provide a means of organizing protein microstructures on surfaces. The organization of patterned hydrophilic/hydrophobic domains on surfaces, by using photolithography, stamping, or micromachining methods, allows the selective patterning of surfaces by hydrophobic, noncovalent interactions. Photoactivated layered enzyme electrodes act as light-switchable optobioelectronic systems for the amperometric transduction of recorded photonic information. These systems can act as optical memories, biomolecular amplifiers, or logic gates. The photoswitchable enzyme electrodes are generated by the tethering of photoisomerizable groups to the protein, the reconstitution of apo-enzymes with semisynthetic photoisomerizable cofactor units, or the coupling of photoisomerizable electron relay units.     

Electrochemical Performance of Transition Metal‐Coordinated Polypyrrole: A Mini Review

The conductive polymer of polypyrrole can be acted as electroactive electrode material of supercapacitor due to reversible redox behavior and high capacitance. It usually suffers from low electrochemical stability due to the breakdown of polymer molecule chain in the long‐term charge and discharge process. The monometallic or bimetallic‐coordinated polypyrrole usually exhibits the improved electrochemical performance. The transition metal ions such as ruthenium, iron, copper and cobalt are adopted for the coordination modification. The transition metal‐coordinated polypyrrole includes the intrachain and interchain coordination structure between transition metal ion and nitrogen atom of pyrrole ring. It is able to reinforce the polymer molecule chain strength to overcome excessive volumetric swelling and shrinking during charge‐discharge process, improving the cycling stability and rate capability of polypyrrole. Accordingly, the transition metal‐coordinated polypyrrole keeps simultaneously high capacitance performance and electrochemical stability, acting as the promising conductive polymer‐based supercapacitor electrode material for effective energy storage.     

An Intrinsically Stretchable and Compressible Supercapacitor Containing a Polyacrylamide Hydrogel Electrolyte

Stretchability and compressibility of supercapacitors is an essential element of modern electronics, such as flexible, wearable devices. Widely used polyvinyl alcohol‐based electrolytes are neither very stretchable nor compressible, which fundamentally limits the realization of supercapacitors with high stretchability and compressibility. A new electrolyte that is intrinsically super‐stretchable and compressible is presented. Vinyl hybrid silica nanoparticle cross‐linkers were introduced into polyacrylamide hydrogel backbones to promote dynamic cross‐linking of the polymer networks. These cross‐linkers serve as stress buffers to dissipate energy when strain is applied, providing a solution to the intrinsically low stretchability and compressibility shortcomings of conventional supercapacitors. The newly developed supercapacitor and electrolyte can be stretched up to an unprecedented 1000 % strain with enhanced performance, and compressed to 50 % strain with good retention of the initial performance.     

Crack‐Free Periodic Porous Thin Films Assisted by Plasma Irradiation at Low Temperature and Their Enhanced Gas‐Sensing Performance

Homogenous thin films are preferable for high‐performance gas sensors because of their remarkable reproducibility and long‐term stability. In this work, a low‐temperature fabrication route is presented to prepare crack‐free and homogenous metal oxide periodic porous thin films by oxygen plasma irradiation instead of high temperature annealing by using a sacrificial colloidal template. Rutile SnO₂ is taken as an example to demonstrate the validity of this route. The crack‐free and homogenous porous thin films are successfully synthesized on the substrates in situ with electrodes. The SnO₂ porous thin film obtained by plasma irradiation is rich in surface OH groups and hence superhydrophilic. It exhibits a more homogenous structure and lower resistance than porous films generated by annealing. More importantly, such thin films display higher sensitivity, a lower detection threshold (100 ppb to acetone) and better durability than those that have been directly annealed, resulting in enhanced gas‐sensing performance. The presented method could be applied to synthesize other metal oxide homogenous thin films and to fabricate gas‐sensing devices with high performances.     

Graphene as a spacer to layer-by-layer assemble electrochemically functionalized nanostructures for molecular bioelectronic devices

This study demonstrates the capability of graphene as a spacer to form electrochemically functionalized multilayered nanostructures onto electrodes in a controllable manner through layer-by-layer (LBL) chemistry. Methylene green (MG) and positively charged methylimidazolium-functionalized multiwalled carbon nanotubes (MWNTs) were used as examples of electroactive species and electrochemically useful components for the assembly, respectively. By using graphene as the spacer, the multilayered nanostructures of graphene/MG and graphene/MWNT could be readily formed onto electrodes with the LBL method on the basis of the electrostatic and/or π-π interaction(s) between graphene and the electrochemically useful components. Scanning electron microscopy (SEM), ultraviolet-visible spectroscopy (UV-vis), and cyclic voltammetry (CV) were used to characterize the assembly processes, and the results revealed that nanostructure assembly was uniform and effective with graphene as the spacer. Electrochemical studies demonstrate that the assembled nanostructures possess excellent electrochemical properties and electrocatalytic activity toward the oxidation of NADH and could thus be used as electronic transducers for bioelectronic devices. This potential was further demonstrated by using an alcohol dehydrogenase-based electrochemical biosensor and glucose dehydrogenase-based glucose/O(2) biofuel cell as typical examples. This study offers a simple route to the controllable formation of graphene-based electrochemically functionalized nanostructures that can be used for the development of molecular bioelectronic devices such as biosensors and biofuel cells.     

Semisynthetic DNA–Protein Conjugates for Biosensing and Nanofabrication

Conjugation with artificial nucleic acids allows proteins to be modified with a synthetically accessible, robust tag. This attachment is addressable in a highly specific manner by means of molecular recognition events, such as Watson–Crick hybridization. Such DNA–protein conjugates, with their combined properties, have a broad range of applications, such as in high‐performance biomedical diagnostic assays, fundamental research on molecular recognition, and the synthesis of DNA nanostructures. This Review surveys current approaches to generate DNA–protein conjugates as well as recent advances in their applications. For example, DNA–protein conjugates have been assembled into model systems for the investigation of catalytic cascade reactions and light‐harvesting devices. Such hybrid conjugates are also used for the biofunctionalization of planar surfaces for micro‐ and nanoarrays, and for decorating inorganic nanoparticles to enable applications in sensing, materials science, and catalysis.     

Unusual Inherent Electrochemistry of Graphene Oxides Prepared Using Permanganate Oxidants

Graphene and graphene oxides are materials of significant interest in electrochemical devices such as supercapacitors, batteries, fuel cells, and sensors. Graphene oxides and reduced graphenes are typically prepared by oxidizing graphite in strong mineral acid mixtures with chlorate (Staudenmaier, Hofmann) or permanganate (Hummers, Tour) oxidants. Herein, we reveal that graphene oxides pose inherent electrochemistry, that is, they can be oxidized or reduced at relatively mild potentials (within the range ±1 V) that are lower than typical battery potentials. This inherent electrochemistry of graphene differs dramatically from that of the used oxidants. Graphene oxides prepared using chlorate exhibit chemically irreversible reductions, whereas graphene oxides prepared through permanganate‐based methods exhibit very unusual inherent chemically reversible electrochemistry of oxygen‐containing groups. Insight into the electrochemical behaviour was obtained through cyclic voltammetry, chronoamperometry, and X‐ray photoelectron spectroscopy experiments. Our findings are of extreme importance for the electrochemistry community as they reveal that electrode materials undergo cyclic changes in charge/discharge cycles, which has strong implications for energy‐storage and sensing devices.