Electrochemistry at carbon nanotube forests: sidewalls and closed ends allow fast electron transfer

State-of-the-art colloidal particles and unique interfaces-based SARS-CoV-2 detection methods and COVID-19 diagnosis

In March 2020, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)-based infections were declared 'COVID-19 pandemic' by the World Health Organization. Pandemic raised the necessity to design and develop genuine and sensitive tests for precise specific SARS-CoV-2 infections detection. Nanotechnological methods offer new ways to fight COVID-19. Nanomaterials are ideal for unique sensor platforms because of their chemically versatile properties and they are easy to manufacture. In this context, selected examples for integrating nanomaterials and distinct biosensor platforms are given to detect SARS-CoV-2 biological materials and COVID-19 biomarkers, giving researchers and scientists more goals and a better forecast to design more relevant and novel sensor arrays for COVID-19 diagnosis.

Local probe investigation of electrocatalytic activity

As the world energy crisis remains a long-term challenge, development and access to renewable energy sources are crucial for a sustainable modern society. Electrochemical energy conversion devices are a promising option for green energy supply, although the challenge associated with electrocatalysis have caused increasing complexity in the materials and systems, demanding further research and insights. In this field, scanning probe microscopy (SPM) represents a specific source of knowledge and understanding. Thus, our aim is to present recent findings on electrocatalysts for electrolysers and fuel cells, acquired mainly through scanning electrochemical microscopy (SECM) and other related scanning probe techniques. This review begins with an introduction to the principles of several SPM techniques and then proceeds to the research done on various energy-related reactions, by emphasizing the progress on non-noble electrocatalytic materials.

Porous carbon derived from metal–organic framework@graphene quantum dots as electrode materials for supercapacitors and lithium-ion batteries

The C@GQD composite was prepared by the combination of metal–organic framework (ZIF-8)-derived porous carbon and graphene quantum dots (GQDs) by a simple method. The resulting composite has a high specific surface area of 668 m² g⁻¹ and involves numerous micro- and mesopores. As a supercapacitor electrode, the material showed an excellent double-layer capacitance and a high capacity retention of 130 F g⁻¹ at 2 A g⁻¹. The excellent long-term stability was observed even after ∼10 000 charge–discharge cycles. Moreover, the composite as an anode material for a lithium-ion battery exhibited a good reversible capacity and outstanding cycle stability (493 mA h g⁻¹ at 100 mA g⁻¹ after 200 cycles). The synergistic effect of a MOF-derived porous carbon and GQDs was responsible for the improvement of electrochemical properties.

The C@GQD composite was prepared by the combination of metal–organic framework (ZIF-8)-derived porous carbon and graphene quantum dots (GQDs) by a simple method.

Single gold nanowire electrodes and single Pt@Au nanowire electrodes: electrochemistry and applications

Single Au nanowire electrodes and single Pt@Au nanowire electrodes have been prepared and used to investigate electrochemical properties, fabricate an E-DNA sensor and study the oxygen reduction reaction.

Nanoscale Electrochemistry of sp(2) Carbon Materials: From Graphite and Graphene to Carbon Nanotubes

Carbon materials have a long history of use as electrodes in electrochemistry, from (bio)electroanalysis to applications in energy technologies, such as batteries and fuel cells. With the advent of new forms of nanocarbon, particularly, carbon nanotubes and graphene, carbon electrode materials have taken on even greater significance for electrochemical studies, both in their own right and as components and supports in an array of functional composites. With the increasing prominence of carbon nanomaterials in electrochemistry comes a need to critically evaluate the experimental framework from which a microscopic understanding of electrochemical processes is best developed. This Account advocates the use of emerging electrochemical imaging techniques and confined electrochemical cell formats that have considerable potential to reveal major new perspectives on the intrinsic electrochemical activity of carbon materials, with unprecedented detail and spatial resolution. These techniques allow particular features on a surface to be targeted and models of structure-activity to be developed and tested on a wide range of length scales and time scales. When high resolution electrochemical imaging data are combined with information from other microscopy and spectroscopy techniques applied to the same area of an electrode surface, in a correlative-electrochemical microscopy approach, highly resolved and unambiguous pictures of electrode activity are revealed that provide new views of the electrochemical properties of carbon materials. With a focus on major sp(2) carbon materials, graphite, graphene, and single walled carbon nanotubes (SWNTs), this Account summarizes recent advances that have changed understanding of interfacial electrochemistry at carbon electrodes including: (i) Unequivocal evidence for the high activity of the basal surface of highly oriented pyrolytic graphite (HOPG), which is at least as active as noble metal electrodes (e.g., platinum) for outer-sphere redox processes. (ii) Demonstration of the high activity of basal plane HOPG toward other reactions, with no requirement for catalysis by step edges or defects, as exemplified by studies of proton-coupled electron transfer, redox transformations of adsorbed molecules, surface functionalization via diazonium electrochemistry, and metal electrodeposition. (iii) Rationalization of the complex interplay of different factors that determine electrochemistry at graphene, including the source (mechanical exfoliation from graphite vs chemical vapor deposition), number of graphene layers, edges, electronic structure, redox couple, and electrode history effects. (iv) New methodologies that allow nanoscale electrochemistry of 1D materials (SWNTs) to be related to their electronic characteristics (metallic vs semiconductor SWNTs), size, and quality, with high resolution imaging revealing the high activity of SWNT sidewalls and the importance of defects for some electrocatalytic reactions (e.g., the oxygen reduction reaction). The experimental approaches highlighted for carbon electrodes are generally applicable to other electrode materials and set a new framework and course for the study of electrochemical and interfacial processes.

Effects of carbon surface topography on the electrode/electrolyte interface structure and relevance to Li–air batteries

Carbon surface topography influences the solvent structure at the interface, concentration distribution of reactants (Li⁺, O₂), and their absorption kinetics.

Multiscale electrochemistry of hydrogels embedding conductive nanotubes

The local functionalities of biocompatible objects can be characterized under conditions similar to the operating ones, using scanning electrochemical microscopy (SECM). In the case of alginate beads entrapping carbon nanotubes (CNTs), SECM allows evidencing of the local conductivity, organization, and communication between the CNTs. It shows that the CNT network is active enough to allow long range charge evacuation, enabling the use of alginate/CNT beads as soft 3D electrodes. Direct connection or local interrogation by a microelectrode allows visualization of their communication as a network and eventually the study of them individually at the nanoscale.

Electrochemical activation of pristine single walled carbon nanotubes: impact on oxygen reduction and other surface sensitive redox processes

The effect of systematic anodic pre-treatments of pristine single walled carbon nanotube (SWNT) forests on the electrochemical response towards a variety of redox processes is investigated. An experimental arrangement is adopted whereby a microcapillary containing the solution of interest and a quasi reference-counter electrode is brought into contact with a small portion of the forest to enable measurements on the surface before and after controlled anodic polarisation (AP). AP of the surface is found to both improve the voltammetric response (faster apparent heterogeneous electron transfer kinetics) of surface sensitive redox processes, such as Fe(2+/3+), and enhance the electrocatalytic response of the SWNTs towards oxygen reduction; the extent of which can be carefully controlled via the applied anodic potential. AP is expected to remove any trace organic (atmospheric) contaminants that may accumulate on the forest over extended periods as well as allowing the controlled introduction of defects, as confirmed by micro-Raman spectroscopy.

Controlled functionalisation of single-walled carbon nanotube network electrodes for the enhanced voltammetric detection of dopamine

Acid functionalised SWNT network electrodes enhance the voltammetric detection of dopamine and minimise surface fouling.

Positionable vertical microfluidic cell based on electromigration in a theta pipet

A microscale vertical fluidic cell system has been implemented, based on a simple theta pipet pulled to a sharp point (ca. 10-20 μm diameter for the studies herein) and positioned with a high degree of control on a surface. The dual channel arrangement allows an electric field to be generated between an electrode in each compartment of the pipet that can be used to control the electromigration of charged species between the two compartments, across a thin liquid meniscus in contact with the substrate of interest. By visualizing the interfacial region using laser scanning confocal microscopy, the adsorption of fluorescently-labeled materials on surfaces is monitored quantitatively in real time, exemplified through studies of the adsorption of anionic microparticles (1.1 μm diameter) on positively and negatively charged substrate surfaces of poly-L-lysine (PLL) and poly-L-glutamic acid (PGA), respectively, on glass. These studies highlight significant electrostatic effects on adsorption rates and also that the adsorption of these particles is dominated by the three phase meniscus/solid/air boundary. The technique is easily modified to the case of a submerged substrate, resulting in a much larger deposition area. Finite element method modeling is used to calculate local electric field strengths that are used to understand surface deposition patterns. To demonstrate the applicability of the technique to live biological substrates, the delivery of fluorescent particles directly to the surface of a single root hair cell of Zea mays is demonstrated. The mobile pipet allows deposition to be directed to specific regions of the cell, allowing discrete sites to be labeled with particles. Finally, the technique is used to study the uptake of fluorescent polymer molecules to single root hair cells, with quantitative analysis of the adsorption rates of vinyl-sulfonic acid copolymers, with varying rhodamine B content.

The edge- and basal-plane-specific electrochemistry of a single-layer graphene sheet

Graphene has a unique atom-thick two-dimensional structure and excellent properties, making it attractive for a variety of electrochemical applications, including electrosynthesis, electrochemical sensors or electrocatalysis, and energy conversion and storage. However, the electrochemistry of single-layer graphene has not yet been well understood, possibly due to the technical difficulties in handling individual graphene sheet. Here, we report the electrochemical behavior at single-layer graphene-based electrodes, comparing the basal plane of graphene to its edge. The graphene edge showed 4 orders of magnitude higher specific capacitance, much faster electron transfer rate and stronger electrocatalytic activity than those of graphene basal plane. A convergent diffusion effect was observed at the sub-nanometer thick graphene edge-electrode to accelerate the electrochemical reactions. Coupling with the high conductivity of a high-quality graphene basal plane, graphene edge is an ideal electrode for electrocatalysis and for the storage of capacitive charges.

Selection, characterisation and mapping of complex electrochemical processes at individual single-walled carbon nanotubes: the case of serotonin oxidation

The electrochemical (EC) oxidation of the neurotransmitter, serotonin, at individual single-walled carbon nanotubes (SWNTs) is investigated at high resolution using a novel platform that combines flow-aligned SWNTs with atomic force microscopy, Raman microscopy, electronic conductance measurements, individual SWNT electrochemistry and high-resolution scanning electrochemical cell microscopy (SECCM). SECCM has been used to visualise the EC activity along side-wall sections of metallic SWNTs to assess the extent to which side-walls promote the electrochemistry of this complex multi-step process. Uniform and high EC activity is observed that is consistent with significant reaction at the side-wall, rather than electrochemistry being driven by defects alone. By scanning forward and reverse (trace and retrace) over the same region of a SWNT, it is also possible to assess any blocking of EC activity by serotonin oxidation reaction products. At a physiologically relevant concentration (5 μM), there is no detectable blocking of SWNTs, which can be attributed, at least in part, to the high diffusion rate to an individual, isolated SWNT in the SECCM format. At higher serotonin concentration (2 mM), oligomer formation from oxidation products is much more significant and major blocking of the EC process is observed from line profiles recorded as the SECCM meniscus moves over an SWNT. The SECCM line profile morphology is shown to be highly diagnostic of whether blocking occurs during EC processes. The studies herein add to a growing body of evidence that various EC processes at SWNTs, from simple outer sphere redox reactions to complex multi-step processes, occur readily at pristine SWNTs. The platform described is of general applicability to various types of nanostructures and nanowires.

Concluding remarks: there's nowt so queer as carbon electrodes

This contribution provides a personal overview and summary of Faraday Discussion 172 on “Carbon in Electrochemistry”, covering some of the key points made at the meeting within the broader context of other recent developments on carbon materials for electrochemical applications. Although carbon electrodes have a long history of use in electrochemistry, methods and techniques are only just becoming available that can test long-established models and identify key features for further exploration. This Discussion has highlighted the need for a better understanding of the impact of surface structure, defects, local density of electronic states, and surface functionality and contamination, in order to advance fundamental knowledge of various electrochemical processes and phenomena at carbon electrodes. These developments cut across important materials such as graphene, carbon nanotubes, conducting diamond and high surface area carbon materials. With more detailed pictures of structural and electronic controls of electrochemistry at carbon electrodes (and electrodes generally), will come rational advances in various technological applications, from sensors to energy technology (particularly batteries, supercapacitors and fuel cells), that have been well-illustrated at this Discussion.

Electrochemical characterisation of graphene nanoflakes with functionalised edges

Graphene nanoflakes (GNF) of diameter ca. 30 nm and edge-terminated with carboxylic acid (COOH) or amide functionalities were characterised electrochemically after drop-coating onto a boron-doped diamond (BDD) electrode. In the presence of the outer-sphere redox probe ferrocenemethanol there was no discernible difference in electrochemical response between the clean BDD and GNF-modified electrodes. When ferricyanide or hydroquinone were used as redox probes there was a marked difference in response at the electrode modified with COOH-terminated GNF in comparison to the unmodified BDD and amide-terminated GNF electrode. The response of the COOH-terminated GNF electrode was highly pH dependent, with the most dramatic differences in response noted at pH < 8. This pH range coincides with partial protonation of the carboxylic acid groups as determined by titration. The acid edge groups occupy a range of bonding environments and are observed to undergo deprotonation over a pH range ca. 3.7 to 8.3. The protonation state of the GNF influences the oxidation mechanism of hydroquinone and in particular the number of solution protons involved in the reaction mechanism. The voltammetric response of ferricyanide is very inhibited by the presence of COOH-terminated GNF at pH < 8, especially in low ionic strength solution. While the protonation state of the GNF is clearly a major factor in the observed response, the exact role of the acid group in the redox process has not been firmly established. It may be that the ferricyanide species is unstable in the solution environment surrounding the GNF, where dynamic protonation equilibria are at play, perhaps through disruption to ion pairing.

Electrochemistry of well-defined graphene samples: role of contaminants

We report the electrochemical characterisation of well-defined graphene samples, prepared by mechanical exfoliation. Mechanical exfoliation is the method of choice for high purity graphene samples, despite the inherent complexity of the approach and the small scale of the resultant flakes. However, one important, yet presently unclear area, is the role of adsorbates such as processing residue, on the properties of the graphene layer. We report high resolution microscopic and electrochemical characterisation of a variety of poly(methyl methacrylate) (PMMA) transferred graphene samples, with the explicit aim of investigating the relationship between electrochemical activity and sample purity.

Pseudo-single-crystal electrochemistry on polycrystalline electrodes: visualizing activity at grains and grain boundaries on platinum for the Fe2+/Fe3+ redox reaction

The influence of electrode surface structure on electrochemical reaction rates and mechanisms is a major theme in electrochemical research, especially as electrodes with inherent structural heterogeneities are used ubiquitously. Yet, probing local electrochemistry and surface structure at complex surfaces is challenging. In this paper, high spatial resolution scanning electrochemical cell microscopy (SECCM) complemented with electron backscatter diffraction (EBSD) is demonstrated as a means of performing 'pseudo-single-crystal' electrochemical measurements at individual grains of a polycrystalline platinum electrode, while also allowing grain boundaries to be probed. Using the Fe(2+/3+) couple as an illustrative case, a strong correlation is found between local surface structure and electrochemical activity. Variations in electrochemical activity for individual high index grains, visualized in a weakly adsorbing perchlorate medium, show that there is higher activity on grains with a significant (101) orientation contribution, compared to those with (001) and (111) contribution, consistent with findings on single-crystal electrodes. Interestingly, for Fe(2+) oxidation in a sulfate medium a different pattern of activity emerges. Here, SECCM reveals only minor variations in activity between individual grains, again consistent with single-crystal studies, with a greatly enhanced activity at grain boundaries. This suggests that these sites may contribute significantly to the overall electrochemical behavior measured on the macroscale.

Scanning electrochemical cell microscopy: a versatile technique for nanoscale electrochemistry and functional imaging

Scanning electrochemical cell microscopy (SECCM) is a new pipette-based imaging technique purposely designed to allow simultaneous electrochemical, conductance, and topographical visualization of surfaces and interfaces. SECCM uses a tiny meniscus or droplet, at the end of a double-barreled (theta) pipette, for high-resolution functional imaging and nanoscale electrochemical measurements. Here we introduce this technique and provide an overview of its principles, instrumentation, and theory. We discuss the power of SECCM in resolving complex structure-activity problems and provide considerable new information on electrode processes by referring to key example systems, including graphene, graphite, carbon nanotubes, nanoparticles, and conducting diamond. The many longstanding questions that SECCM has been able to answer during its short existence demonstrate its potential to become a major technique in electrochemistry and interfacial science.