Electrogeneration of single nanobubbles at sub-50-nm-radius platinum nanodisk electrodes

Exploring single-entity electrochemistry beyond conventional potential windows: mechanistic insights into hydrazine/hydrazinium ion oxidation

Single-entity electrochemistry (SEE) enables research into the electrochemical properties of nanoparticles (NPs) at the individual NP level. Recent studies on active particle-active electrode systems have expanded the scope of SEE measurements, moving beyond the limitations of inert electrode-based methods that rely on distinct NP-electrode catalytic differences, thereby enhancing mechanistic understanding of catalytic reactions. In this study, we investigated SEE signals from Pt NPs colliding with Au ultramicroelectrodes (UME) at elevated potentials where both Pt and Au UME exhibit electrocatalytic activity. Under conditions where Au UME is activated for hydrazine oxidation, distinctive combined spike and staircase current responses were observed. SEE signals exhibited varied shapes depending on pH and hydrazine concentration. Analyzing these variations provided insights into changes in reaction mechanisms according to pH and hydrazine concentration.

Gas Evolution in Water Electrolysis

Gas bubbles generated by the hydrogen evolution reaction and oxygen evolution reaction during water electrolysis influence the energy conversion efficiency of hydrogen production. Here, we survey what is known about the interaction of gas bubbles and electrode surfaces and the influence of gas evolution on practicable devices used for water electrolysis. We outline the physical processes occurring during the life cycle of a bubble, summarize techniques used to characterize gas evolution phenomena in situ and in practical device environments, and discuss ways that electrodes can be tailored to facilitate gas removal at high current densities. Lastly, we review efforts to model the behavior of individual gas bubbles and multiphase flows produced at gas-evolving electrodes. We conclude our review with a short summary of outstanding questions that could be answered by future efforts to characterize gas evolution in electrochemical device environments or by improved simulations of multiphase flows.

The smallest electrochemical bubbles

Many of the relevant electrochemical processes in the context of catalysis or energy conversion and storage, entail the production of gases. This often implicates the nucleation of bubbles at the interface, with the concomitant blockage of the electroactive area leading to overpotentials and Ohmic drop. Nanoelectrodes have been envisioned as assets to revert this effect, by inhibiting bubble formation. Experiments show, however, that nanobubbles nucleate and attach to nanoscale electrodes, imposing a limit to the current, which turns out to be independent of size and applied potential in a wide range from 3 nm to tenths of microns. Here we investigate the potential-current response for disk electrodes of diameters down to a single-atom, employing molecular simulations including electrochemical generation of gas. Our analysis reveals that nanoelectrodes of 1 nm can offer twice as much current as that delivered by electrodes with areas four orders of magnitude larger at the same bias. This boost in the extracted current is a consequence of the destabilization of the gas phase. The grand potential of surface nanobubbles shows they can not reach a thermodynamically stable state on supports below 2 nm. As a result, the electroactive area becomes accessible to the solution and the current turns out to be sensitive to the electrode radius. In this way, our simulations establish that there is an optimal size for the nanoelectrodes, in between the single-atom and ∼3 nm, that optimizes the gas production.

Bridging Innovations of Phase Change Heat Transfer to Electrochemical Gas Evolution Reactions

Bubbles play a ubiquitous role in electrochemical gas evolution reactions. However, a mechanistic understanding of how bubbles affect the energy efficiency of electrochemical processes remains limited to date, impeding effective approaches to further boost the performance of gas evolution systems. From a perspective of the analogy between heat and mass transfer, bubbles in electrochemical gas evolution reactions exhibit highly similar dynamic behaviors to them in the liquid-vapor phase change. Recent developments of liquid-vapor phase change systems have substantially advanced the fundamental knowledge of bubbles, leading to unprecedented enhancement of heat transfer performance. In this Review, we aim to elucidate a promising opportunity of understanding bubble dynamics in electrochemical gas evolution reactions through a lens of phase change heat transfer. We first provide a background about key parallels between electrochemical gas evolution reactions and phase change heat transfer. Then, we discuss bubble dynamics in gas evolution systems across multiple length scales, with an emphasis on exciting research problems inspired by new insights gained from liquid-vapor phase change systems. Lastly, we review advances in engineered surfaces for manipulating bubbles to enhance heat and mass transfer, providing an outlook on the design of high-performance gas evolving electrodes.

Electrochemical Imaging of Precisely-Defined Redox and Reactive Interfaces

Understanding the diverse electrochemical reactions occurring at electrode-electrolyte interfaces (EEIs) is a critical challenge to developing more efficient energy conversion and storage technologies. Establishing a predictive molecular-level understanding of solid electrolyte interphases (SEIs) is challenging due to the presence of multiple intertwined chemical and electrochemical processes occurring at battery electrodes. Similarly, chemical conversions in reactive electrochemical systems are often influenced by the heterogeneous distribution of active sites, surface defects, and catalyst particle sizes. In this mini review, we highlight an emerging field of interfacial science that isolates the impact of specific chemical species by preparing precisely-defined EEIs and visualizing the reactivity of their individual components using single-entity characterization techniques. We highlight the broad applicability and versatility of these methods, along with current state-of-the-art instrumentation and future opportunities for these approaches to address key scientific challenges related to batteries, chemical separations, and fuel cells. We establish that controlled preparation of well-defined electrodes combined with single entity characterization will be crucial to filling key knowledge gaps and advancing the theories used to describe and predict chemical and physical processes occurring at EEIs and accelerating new materials discovery for energy applications.

Dual-Mode Imaging of Dynamic Interaction between Bubbles and Single Nanoplates during the Electrocatalytic Hydrogen Evolution Process

Gas bubble formation at electrochemical interfaces can significantly affect the efficiency and durability of electrocatalysts. However, obtaining comprehensive details on bubble evolution dynamics, particularly their dynamic interaction with high-performance structured electrocatalysts, poses a considerable challenge. Herein, dual-mode interference/total internal reflection fluorescence microscopy is introduced, which allows for the simultaneous capture of the evolution pathway of bubbles and the 3D motion of nanoplate electrocatalysts, providing high-resolution and accurate spatiotemporal information. During the hydrogen evolution reaction, the dynamics of hydrogen bubble generation and their interactions with single nanoplate electrocatalysts at the electrochemical interface are observed. The results unveiled that, under constant potential, bubbles initially manifest as fast-moving nanobubbles, transforming into stationary microbubbles subsequently. The morphology of stationary nanoplates regulates the trajectories of these moving nanobubbles while the pinned microbubbles induce the motion of the electrocatalysts. The dual-mode microscopy can be employed to scrutinize numerous multiphase electrochemical interactions with high spatiotemporal resolution, which can facilitate the rational design of high-performance electrocatalysts.

A Systematic Study on Long-acting Nanobubbles: Current Advancement and Prospects on Theranostic Properties

Delivery of diagnostic drugs via nanobubbles (NBs) has shown to be an emerging field of study. Due to their small size, NBs may more easily travel through constricted blood vessels and precisely target certain bodily parts. NB is considered the major treatment for cancer treatment and other diseases which are difficult to diagnose. The field of NBs is dynamic and continues to grow as researchers discover new properties and seek practical applications in various fields. The predominant usage of NBs in novel drug delivery is to enhance the bioavailability, and controlled drug release along with imaging properties NBs are important because they may change interfacial characteristics including surface force, lubrication, and absorption. The quick diffusion of gas into the water was caused by a hypothetical film that was stimulated and punctured by a strong acting force at the gas/water contact of the bubble. In this article, various prominent aspects of NBs have been discussed, along with the long-acting nature, and the theranostical aspect which elucidates the potential marketed drugs along with clinical trial products. The article also covers quality by design aspects, different production techniques that enable method-specific therapeutic applications, increasing the floating time of the bubble, and refining its properties to enhance the prepared NB's quality. NB containing both analysis and curing properties makes it special from other nano-carriers. This work includes all the possible methods of preparing NB, its application, all marketed drugs, and products in clinical trials.

Scanning Bubble Electrochemical Microscopy: Mapping of Electrocatalytic Activity with Low-Solubility Reactants

Determining electrocatalytic activity for reactions that involve reactants with limited solubility presents a significant challenge due to the reduced mass-transport to the electrocatalyst surface. This limitation is seen in important reactions such as the oxygen reduction reaction, nitrogen reduction reaction, and carbon dioxide reduction reaction. We introduce a new approach, which we call scanning bubble electrochemical microscopy, to enable the detection and high-resolution mapping of electrocatalytic activity with low-solubility reactants at high mass-transport rates. Using a scanning probe approach, a dual-barreled nanopipette is used to precisely deliver the gas-phase reactant within micrometers of an electrocatalyst surface, which results in high mass-transport rates at the electrocatalyst surface directly under the probe. We demonstrate the scanning bubble electrochemical microscopy approach by mapping the oxygen reduction reaction on model platinum microelectrode surfaces. We anticipate that scanning bubble electrochemical microscopy will provide an effective tool for measuring the electrocatalytic activity of reactants that have limited solubility.

Threshold current density for diffusion-controlled stability of electrolytic surface nanobubbles

Understanding the stability mechanism of surface micro/nanobubbles adhered to gas-evolving electrodes is essential for improving the efficiency of water electrolysis, which is known to be hindered by the bubble coverage on electrodes. Using molecular simulations, the diffusion-controlled evolution of single electrolytic nanobubbles on wettability-patterned nanoelectrodes is investigated. These nanoelectrodes feature hydrophobic islands as preferential nucleation sites and allow the growth of nanobubbles in the pinning mode. In these simulations, a threshold current density distinguishing stable nanobubbles from unstable nanobubbles is found. When the current density remains below the threshold value, nucleated nanobubbles grow to their equilibrium states, maintaining their nanoscopic size. However, for the current density above the threshold value, nanobubbles undergo unlimited growth and can eventually detach due to buoyancy. Increasing the pinning length of nanobubbles increases the degree of nanobubble instability. By connecting the current density with the local gas oversaturation, an extension of the stability theory for surface nanobubbles [Lohse and Zhang, Phys. Rev. E 91, 031003(R) (2015)] accurately predicts the nanobubble behavior found in molecular simulations, including equilibrium contact angles and the threshold current density. For larger systems that are not accessible to molecular simulations, continuum numerical simulations with the finite difference method combined with the immersed boundary method are performed, again demonstrating good agreement between numerics and theories.

Solutal Marangoni force controls lateral motion of electrolytic gas bubbles

Electrochemical gas-evolving reactions have been widely used for industrial energy conversion and storage processes. Gas bubbles form frequently at the electrode surface due to a small gas solubility, thereby reducing the effective reaction area and increasing the over-potential and ohmic resistance. However, the growth and motion mechanisms for tiny gas bubbles on the electrode remains elusive. Combining molecular dynamics (MD) and fluid dynamics simulations (CFD), we show that there exists a lateral solutal Marangoni force originating from an asymmetric distribution of dissolved gas near the bubble. Both MD and CFD simulations deliver a similar magnitude of the Marangoni force of ∼0.01 nN acting on the bubble. We demonstrate that this force may lead to lateral bubble oscillations and analyze the phenomenon of dynamic self-pinning of bubbles at the electrode boundary.

Controlling the Collision Type and Frequency of Single Pt Nanoparticles at Chemically Modified Gold Electrodes

Studying the electrochemical response of single nanoparticles at an electrode surface gives insight into the dynamic and stochastic processes that occur at the electrode interface. Herein, we investigated single platinum nanoparticle collision dynamics and type (elastic vs inelastic) at gold electrode surfaces modified with self-assembled monolayers (SAMs) of varying terminal chemistries. Collision events are measured via the faradaic current from catalytic reactions at the Pt surface. By changing the terminal, solution-facing group of a thiolate monolayer, we observed the effect of hydrophobicity at the solution-electrode interface on single-particle collisions by employing either a hydrophobic -CH3 terminal group (1-hexanethiol), a hydrophilic -OH terminal group (6-mercaptohexanol), or an equimolar mixture of the two. Changes in the terminal group lead to alterations in collision-induced current magnitude, collisional frequency, and the distinct shape of the collision event current transient. The effects of the terminal group of the SAM were probed by measuring quantitative differences in the events monitored through both the hydrogen evolution reaction (HER) and hydrazine oxidation. In both cases, a platinum nanoparticle (PtNP) favors adsorption to bare and hydrophilic surfaces but demonstrates elastic collision behavior when it collides with a hydrophobic surface. In the case of a mixed monolayer, distinct characteristics of hydrophobic and hydrophilic surfaces are observed. We report how single nanoparticle collisions can reveal nanoscale surface heterogeneity and can be used to manipulate the nature of single-particle interactions on an electrode surface by functionalized self-assembled monolayers.

Superaerophilic/Superaerophobic NiFe-LDHs Electrode for Enhancing Overall Water Splitting in Alkaline Media

Overall water splitting, as a critical approach to producing green hydrogen, is greatly impeded by the mass transfer of gaseous bubbles and dissolved gas molecules. Herein, a bifunctional superaerophilic/superaerophobic (SAL/SAB) NiFe layered-double-hydroxides (LDHs) electrode has been developed, which can drive H2 and O2 bubbles out of the reaction system by asymmetric Laplace pressure and accelerate dissolved gases diffusion through reducing their diffusion distance. Consequently, the SAL/SAB NiFe-LDHs electrode exhibits excellent HER activity with an overpotential of -76 mV at -10 mA cm-2 and outstanding oxygen evolution reaction activity with an overpotential of 253 mV at 100 mA cm-2. The bifunctional SAL/SAB NiFe-LDHs electrode is further utilized in overall water splitting, which can achieve 10 mA cm-2 with a cell voltage of 1.54 V. This work provides an efficient strategy to improve the efficiency of overall water splitting and can stimulate new electrode design in various gas-involved processes.

Molecular Mechanisms of the Generation and Accumulation of Gas at the Interface

Gas-evolving reactions are widespread in chemical and energy fields. However, the generated gas will accumulate at the interface, which reduces the rate of gas generation. Understanding the microscopic processes of the generation and accumulation of gas at the interface is crucial for improving the efficiency of gas generation. Here, we develop an algorithm to reproduce the process of catalytic gas generation at the molecular scale based on the all-atom molecular dynamics simulations and obtain the quantitative evolution of the gas generation, which agrees well with the experimental results. In addition, we demonstrate that under an external electric field, the generated gas molecules do not accumulate at the electrode surface, which implies that the electric field can significantly increase the rate of the gas generation. The results suggest that the external electric field changes the structure of the water molecules near the electrode surface, making it difficult for gas molecules to accumulate on the electrode surface. Furthermore, it is found that gas desorption from the electrode surface is an entropy-driven process, and its accumulation at the electrode surface depends mainly on the competition between the entropy and the enthalpy of the water molecules under the influence of the electric field. These results provide deep insight into gas generation and inhibition of gas accumulation.

Nucleation of surface nanobubbles in electrochemistry: Analysis with nucleation theorem

The formation of single bubbles at nanoelectrodes during electrochemical reactions allows to accurately identify the critical nucleus for bubble formation. As demonstrated before, combining nanoelectrode experiments and an analysis approach based on classical nucleation theory (CNT) delivers useful insight into bubble nucleation. In this work we propose an alternative approach to analyze the critical nuclei by applying the nucleation theorem (NT), which is able to overcome the inherent shortcomings of CNT. The size of the critical nucleus can be calculated more accurately by fitting experimental data in a simple form of the NT. Simulating the local gas concentration using a finite element approach, and considering the effect of gas oversaturation on the interfacial tension and the real gas compressibility, we obtain a more realistic estimation of the critical nuclei morphology. With the NT-based analysis presented, we re-analyze the nucleation data reported before. The properties of the critical nuclei obtained here are roughly consistent with those obtained from the CNT-based approach. In addition, we confirm that the critical nucleus for bubble formation in high gas oversaturation is featured with a contact angle much larger than Young's contact angle.

Bubble Engineering on Micro-/Nanostructured Electrodes for Water Splitting

Bubble behaviors play crucial roles in mass transfer and energy efficiency in gas evolution reactions. Combining multiscale structures and surface chemical compositions, micro-/nanostructured electrodes have drawn increasing attention. With the aim to identify the exciting opportunities and rationalize the electrode designs, in this review, we present our current comprehension of bubble engineering on micro-/nanostructured electrodes, focusing on water splitting. We first provide a brief introduction of gas wettability on micro-/nanostructured electrodes. Then we discuss the advantages of micro-/nanostructured electrodes for mass transfer (detailing the lowered overpotential, promoted supply of electrolyte, and faster bubble growth kinetics), localized electric field intensity, and electrode stability. Following that, we outline strategies for promoting bubble detachment and directional transportation. Finally, we offer our perspectives on this emerging field for future research directions.

Solutal Marangoni effect determines bubble dynamics during electrocatalytic hydrogen evolution

Understanding and manipulating gas bubble evolution during electrochemical water splitting is a crucial strategy for optimizing the electrode/electrolyte/gas bubble interface. Here gas bubble dynamics are investigated during the hydrogen evolution reaction on a well-defined platinum microelectrode by varying the electrolyte composition. We find that the microbubble coalescence efficiency follows the Hofmeister series of anions in the electrolyte. This dependency yields very different types of H2 gas bubble evolution in different electrolytes, ranging from periodic detachment of a single H2 gas bubble in sulfuric acid to aperiodic detachment of small H2 gas bubbles in perchloric acid. Our results indicate that the solutal Marangoni convection, induced by the anion concentration gradient developing during the reaction, plays a critical role at practical current density conditions. The resulting Marangoni force on the H2 gas bubble and the bubble departure diameter therefore depend on how surface tension varies with concentration for different electrolytes. This insight provides new avenues for controlling bubble dynamics during electrochemical gas bubble formation.

Nanoelectrochemistry in electrochemical phase transition reactions

Electrochemical phase transition is important in a range of processes, including gas generation in fuel cells and electrolyzers, as well as in electrodeposition in battery and metal production. Nucleation is the first step in these phase transition reactions. A deep understanding of the kinetics, and mechanism of the nucleation and the structure of the nuclei and nucleation sites is fundamentally important. In this perspective, theories and methods for studying electrochemical nucleation are briefly reviewed, with an emphasis on nanoelectrochemistry and single-entity electrochemistry approaches. Perspectives on open questions and potential future approaches are also discussed.

Toward Precision Deposition of Conductive Charge-Transfer Complex Crystals Using Nanoelectrochemistry

The lack of understanding for precise synthesis and assembly of nano-entities remains a major challenge for nanofabrication. Electrocrystallization of a charge-transfer complex (CTC), tetrathiafulvalene bromide (TTF)Br, is studied on micro/nanoelectrodes for precision deposition of functional materials. The study reveals new insights into the entire CTC electrocrystallization process from the initial nanocluster nucleation to the final elongated crystals with hollow ends grown from the working electrode to the neighboring receiving electrode. On microelectrodes, the number of nucleation sites is reduced to one by lowering the applied overpotential or precursor concentration. Certain current-time transients exhibit significant induction periods prior to stable nucleus growth. The induction regime contains small fluctuating current spikes consistent with stochastic formation of precritical nanoclusters with lifetimes of 0.1-30 s and sizes of 20-160 nm. Electrochemical analyses further reveal rate, size distribution, and formation/dissipation dynamics of the nanoclusters. Crystal growth of (TTF)Br is further studied on triangular nanoelectrode patterns with thickness of 5-500 nm, which shows a mass-transfer-controlled process applicable for precision deposition of functional (TTF)Br crystals. This study, for the first time, establishes CTC nanoelectrochemistry as a platform technology for precise deposition of conductive crystal assemblies spanning the source and drain electrode for sensing applications.

Electrochemical Redox Cycling Behavior of Gold Nanoring Electrodes Microfabricated on a Silicon Micropillar

We report the microfabrication and characterization of concentric gold nanoring electrodes (Au NREs), which were fabricated by patterning two gold nanoelectrodes on the same silicon (Si) micropillar tip. Au NREs of 165 ± 10 nm in width were micropatterned on a 6.5 ± 0.2 µm diameter 80 ± 0.5 µm height Si micropillar with an intervening ~ 100 nm thick hafnium oxide insulating layer between the two nanoelectrodes. Excellent cylindricality of the micropillar with vertical sidewalls as well as a completely intact layer of a concentric Au NRE including the entire micropillar perimeter has been achieved as observed via scanning electron microscopy and energy dispersive spectroscopy data. The electrochemical behavior of the Au NREs was characterized by steady-state cyclic voltammetry and electrochemical impedance spectroscopy. The applicability of Au NREs to electrochemical sensing was demonstrated by redox cycling with the ferro/ferricyanide redox couple. The redox cycling amplified the currents by 1.63-fold with a collection efficiency of > 90% on a single collection cycle. The proposed micro-nanofabrication approach with further optimization studies shows great promise for the creation and expansion of concentric 3D NRE arrays with controllable width and nanometer spacing for electroanalytical research and applications such as single-cell analysis and advanced biological and neurochemical sensing.

Current oscillations from bipolar nanopores for statistical monitoring of hydrogen evolution on a confined electrochemical catalyst

Taking advantage of bipolar electrochemistry and a glass nanopipette, continuous single bubbles can be controlled which are generated and detached from a nanometer-sized area of confined electrochemical catalysts. The observed current oscillations offer opportunities to rapidly collect data for the statistical analysis of single-bubble generation on and departure from the catalysts.

Confined Electrochemical Behaviors of Single Platinum Nanoparticles Revealing Ultrahigh Density of Gas Molecules inside a Nanobubble

Understanding the basic physicochemical properties of gas molecules confined within nanobubbles is of fundamental importance for chemical and biological processes. Here, we successfully monitored the nanobubble-confined electrochemical behaviors of single platinum nanoparticles (PtNPs) at a carbon fiber ultramicroelectrode in HClO₄ and H₂O₂ solution. Due to the catalytic decomposition of H₂O₂, a single oxygen nanobubble was formed on individual PtNPs to block the active surface of particles for proton reduction and to suppress their stochastic motion, resulting in significantly distinguished current traces. Furthermore, the combination of theoretical calculations and high-resolution electrochemical measurements allowed the nanobubble size and the oxygen gas density inside a single nanobubble to be quantified. Moreover, the ultrahigh oxygen density inside (1046 kg/m³) was revealed, indicating that gas molecules in a nanosized space existed with a high state of aggregation. Our approach sheds light on the gas aggregation behaviors of nanoscale bubbles using single-entity electrochemical measurements.

Monitoring Bipolar Electrochemistry and Hydrogen Evolution Reaction of a Single Gold Microparticle under Sub-Micropipette Confinement

Herein, an approach to track the process of autorepeating bipolar reactions and hydrogen evolution reaction (HER) on a micro gold bipolar electrode (BPE) is established. Once blocking the channel of the sub-micropipette tip, the formed gold microparticle is polarized into the wireless BPE, which induces the dissolution of the gold at the anode and the HER at the cathode. The current response shows a periodic behavior with three regions: the bubble generation region (I), the bubble rupture/generation region (II), and the channel opening region (III). After a stable low baseline current of region I, a series of positive spike signals caused by single H2 nanobubbles rupture/generation are recorded standing for the beginning of region II. Meanwhile, the dissolution of the gold blocking at the orifice will create a new channel, increasing the baseline current for region III, where the synthesis of gold occurs again, resulting in another periodic response. Finite element simulations are applied to unveil the mechanism thermodynamically. In addition, the integral charge of the H2 nanobubbles in region II corresponds to the consumption of the anode gold. It simultaneously monitors autorepeating bipolar reactions of a single gold microparticle and HER of a single H2 nanobubble electrochemically, which reveals an insightful physicochemical mechanism in nanoscale confinement and makes the glass nanopore an ideal candidate to further reveal the heterogeneity of catalytic capability at the single particle level.

A nanoelectrode-based study of water splitting electrocatalysts

The development of low-cost and efficient catalytic materials for key reactions like water splitting, CO₂ reduction and N₂ reduction is crucial for fulfilling the growing energy consumption demands and the pursuit of renewable and sustainable energy. Conventional electrochemical measurements at the macroscale lack the potential to characterize single catalytic entities and nanoscale surface features on the surface of a catalytic material. Recently, promising results have been obtained using nanoelectrodes as ultra-small platforms for the study of the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) on innovative catalytic materials at the nanoscale. In this minireview, we summarize the recent progress in the nanoelectrode-based studies on the HER and OER on various nanostructured catalytic materials. These electrocatalysts can be generally categorized into two groups: 0-dimensional (0D) single atom/molecule/cluster/nanoparticles and 2-dimensional (2D) nanomaterials. Controlled growth as well as the electrochemical characterization of single isolated atoms, molecules, clusters and nanoparticles has been achieved on nanoelectrodes. Moreover, nanoelectrodes greatly enhanced the spatial resolution of scanning probe techniques, which enable studies at the surface features of 2D nanomaterials, including surface defects, edges and nanofacets at the boundary of a phase. Nanoelectrode-based studies on the catalytic materials can provide new insights into the reaction mechanisms and catalytic properties, which will facilitate the pursuit of sustainable energy and help to solve CO₂ release issues.

Xenon and Krypton Dissolved in Water Form Nanoblobs: No Evidence for Nanobubbles

We demonstrate, experimentally and by molecular dynamics simulations, that krypton and xenon form nanostructured water-gas domains. High pressure was applied to force the inert gases to dissolve in water following Henry's law, then the liquid was depressurized, centrifuged, and inspected by dynamic light scattering. The observed objects have similar sizes and electrical properties to nanobubbles, but we found that they have fairly neutral buoyancy even at high gravitational fields. We posit that the formed nano objects are not bubbles but blobs, unique structures conceived as clathrate-hydrate precursors, thus resolving the so-called Laplace pressure bubble catastrophe.

Single-Entity Electrochemistry of Nano- and Microbubbles in Electrolytic Gas Evolution

Gas bubbles are found in diverse electrochemical processes, ranging from electrolytic water splitting to chlor-alkali electrolysis, as well as photoelectrochemical processes. Understanding the intricate influence of bubble evolution on the electrode processes and mass transport is key to the rational design of efficient devices for electrolytic energy conversion and thus requires precise measurement and analysis of individual gas bubbles. In this Perspective, we review the latest advances in single-entity measurement of gas bubbles on electrodes, covering the approaches of voltammetric and galvanostatic studies based on nanoelectrodes, probing bubble evolution using scanning probe electrochemistry with spatial information, and monitoring the transient nature of nanobubble formation and dynamics with opto-electrochemical imaging. We emphasize the intrinsic and quantitative physicochemical interpretation of single gas bubbles from electrochemical data, highlighting the fundamental understanding of the heterogeneous nucleation, dynamic state of the three-phase boundary, and the correlation between electrolytic bubble dynamics and nanocatalyst activities. In addition, a brief discussion of future perspectives is presented.

Electroprecipitation of Nanometer-Thick Films of Ln(OH)3 [Ln = La, Ce, and Lu] at Pt Microelectrodes and Their Effect on Electron-Transfer Reactions

We report investigations of the deposition of nanometer-thick Ln(OH)₃ films (Ln = La, Ce, and Lu) and their effect on outer-sphere and inner-sphere electron-transfer reactions. Insoluble Ln(OH)₃ films are deposited from aqueous solutions of LaCl₃ onto the surface of 12.5 μm radius Pt microdisk electrodes during water or oxygen reduction. Both reactions produce interfacial OH^-, which complexes with Ln³⁺, resulting in the precipitation of Ln(OH)₃. Surface analyses by scanning electron microscopy (SEM), SEM-energy-dispersive X-ray spectroscopy, and atomic force microscopy indicate the formation of a 1-2 nm thick uniform film. Outer-sphere electron-transfer reactions (Ru(NH₃)₆³⁺ reduction, FcMeOH oxidation, and Fe(CN)₆^4-/3- oxidation/reduction) were investigated at Ln(OH)₃-modified electrodes of different film thicknesses. The results demonstrate that the steady-state transport-limited current for these reactions decreases with an increase in the film thickness. Moreover, the degree of blockage depends upon the redox species, suggesting that the Ln(OH)₃ films are free from pinholes greater than the size of the redox molecules. This suggests that the films are either ionically conducting or that electron tunneling occurs across these thin layers. A similar blocking effect was observed for the inner-sphere reductions of H₂O and O₂. We further demonstrate that the thickness of La(OH)₃ films can be controlled by anodic dissolution. Additionally, we show that La³⁺ lowers the supersaturation of dissolved H₂ required to nucleate a stable nanobubble.

Unraveling the effects of gas species and surface wettability on the morphology of interfacial nanobubbles

The morphology of interfacial nanobubbles (INBs) is a crucial but controversial topic in nanobubble research. We carried out atomistic molecular dynamics (MD) simulations to comprehensively study the morphology of INBs controlled by several determinant factors, including gas species, surface wettability, and bubble size. The simulations show that H₂, O₂ and N₂ can all form stable INBs, with the contact angles (CAs, on the liquid side) following the order CA(H₂) < CA(N₂) < CA(O₂), while CO₂ prefers to form a gas film (pancake) structure on the substrate. The CA of INBs demonstrates a linear relation with the strength of interfacial interaction; however, a limited bubble CA of ∼25° is found on superhydrophilic surfaces. The high gas density and high internal pressure of the INBs are further confirmed, accompanied by strong interfacial gas enrichment (IGE) behavior. The morphology study of differently sized INBs shows that the internal density of the gas is drastically decreased with the bubble size at the initial stage of bubble nucleation, while the CA remains almost constant. Based on the simulation results, a modified Young's equation is presented for describing the extraordinary morphology of INBs.

Generation methods, stability, detection techniques, and applications of bulk nanobubbles in agro-food industries: a review and future perspective

Nanobubble (NB) technologies have received considerable attention for various applications due to their low cost, eco-friendliness, scale-up potential, process control, and unique physical characteristics. NB stands for nanoscopic gaseous cavities, typically <1 μm in diameter. NBs can exist on surfaces (surface or interfacial NBs) and be dispersed in a bulk liquid phase (bulk NBs). Compared to the microbubbles, NBs exhibit high specific surface area, negative surface charge, and better adsorption. Bulk NBs can be generated by hydrodynamic/acoustic cavitation, electrolysis, water-solvent mixing, nano-membrane filtration, and so on. NBs exhibit extraordinary longevity compared to microbubbles, prompting the interest of the scientific community aiming for potential applications including medicine, agriculture, food, wastewater treatment, surface cleaning, and so on. Based on the limited amount of research work available regarding the influence of NBs on food matrices, further research, however, needs to be done to provide more insights into its applications in food industries. This review provides an overview of the generation methods for NBs, techniques to evaluate them, and a discussion of their stability and several applications in various fields of science were discussed. However, recent studies have revealed that, despite the many benefits of NB technologies, several NB generating approaches are still limited in their application in specific agro-food industries. Further study should focus on process optimization, integrating various NB generation techniques/combining with other emerging technologies in order to achieve rapid technical progress and industrialization of NB-based technologies.HighlightsNanobubbles (NBs) are stable spherical entities of gas within liquid and are operationally defined as having diameters less than 1 µm.Currently, various reported theories still lack the ability to explain the evidence and stability of NBs in water, numerous NB applications have emerged due to the unique properties of NBs.NB technologies can be applied to various food and dairy products (e.g. yogurt and ice cream) and other potential applications, including agriculture (e.g. seed germination and plant growth), wastewater treatment, surface cleaning, and so on.

Nanopore-Based Single-Entity Electrochemistry for the Label-Free Monitoring of Single-Molecule Glycoprotein-Boronate Affinity Interaction and Its Sensing Application

Nanopipettes provide a promising confined space that enables advances in single-molecule analysis, and their unique conical tubular structure is also suitable for single-cell analysis. In this work, functionalized-nanopore-based single-entity electrochemistry (SEE) analysis tools were developed for the label-free monitoring of single-molecule glycoprotein-boronate affinity interaction for the first time, and immunoglobulin G (IgG, one of the important biomarkers for many diseases such as COVID-19 and cancers) was employed as the model glycoprotein. The principle of this method is based on a single glycoprotein molecule passing through 4-mercaptophenylboronic acid (4-MPBA)-modified nanopipettes under a bias voltage and in the meantime interacting with the boronate group from modified 4-MPBA. This translocation and affinity interaction process can generate distinguishable current blockade signals. Based on the statistical analysis of these signals, the equilibrium association constant (κ_a) of single-molecule glycoprotein-boronate affinity interaction was obtained. The results show that the κ_a of IgG in the confined nanopore at the single-molecule level is much larger than that measured in the open system at the ensemble level, which is possibly due to the enhanced multivalent synergistic binding in the restricted space. Moreover, the functionalized-nanopore-based SEE analysis tools were further applied for the label-free detection of IgG, and the results indicate that our method has potential application value for the detection of glycoproteins in real samples, which also paves way for the single-cell analysis of glycoproteins.

On the growth regimes of hydrogen bubbles at microelectrodes

Beside classical growth (regime I), depending on potential and concentration, new growth regimes of hydrogen bubbles were found. These differ with respect to the existence of a carpet of microbubbles underneath and of current oscillations.

Nanobubble Labeling and Imaging with a Solvatochromic Fluorophore Nile Red

Herein, we report the use of a polarity-sensitive, solvatochromic fluorophore Nile red to label and probe individual hydrogen nanobubbles on the surface of an indium-tin oxide (ITO) electrode. Nanobubbles are generated from the reduction of water on ITO and fluorescently imaged from the transient adsorption and desorption process of single Nile red molecules at the nanobubble surface. The ability to label and fluorescently image individual nanobubbles with Nile red suggests that the gas/solution interface is hydrophobic in nature. Compared to the short labeling events using rhodamine fluorophores, Nile red-labeled events appear to be longer in duration, suggesting that Nile red has a higher affinity to the bubble surface. The stronger fluorophore-bubble interaction also leads to certain nanobubbles being co-labeled by multiple Nile red molecules, resulting in the observation of super-bright and long-lasting labeling events. Based on these interesting observations, we hypothesize that Nile red molecules may start clustering and form some kind of molecular aggregates when they are co-adsorbed on the same nanobubble surface. The ability to observe super-bright and long-lasting multifluorophore labeling events also allows us to verify the high stability and long lifetime of electrochemically generated surface nanobubbles.

Stimuli-responsive nanobubbles for biomedical applications

Stimuli-responsive nanobubbles have received increased attention for their application in spatial and temporal resolution of diagnostic techniques and therapies, particularly in multiple imaging methods, and they thus have significant potential for applications in the field of biomedicine. This review presents an overview of the recent advances in the development of stimuli-responsive nanobubbles and their novel applications. Properties of both internal- and external-stimuli responsive nanobubbles are highlighted and discussed considering the potential features required for biomedical applications. Furthermore, the methods used for synthesis and characterization of nanobubbles are outlined. Finally, novel biomedical applications are proposed alongside the advantages and shortcomings inherent to stimuli-responsive nanobubbles.

Integrated Bundle Electrode with Wettability-Gradient Copper Cones Inducing Continuous Generation, Directional Transport, and Efficient Collection of H2 Bubbles

The hydrogen evolution reaction (HER), as an efficient process of converting various energies into high-purity hydrogen, has attracted much attention from both scientific research studies and industrial productions. However, its wide applications still confront considerable difficulties, for example, bubble coverage on the electrode and bubble dispersion in the electrolyte, which will disturb current distribution and isolate active sites from reaction ions resulting in a high reaction overpotential and large Ohmic voltage drop. Consequently, timely removing the generated gas bubbles from the electrode as well as avoiding their direct release into the electrolyte can be an effective approach to address these issues. In this work, we have developed an elegant electrode, that is, the integrated bundle electrode with wettability-gradient copper cones, which is endowed with the multifunctions of continuous generation, direct transport, and efficient collection of hydrogen bubbles. All processes are proceeding on the electrode, which not only remove the generated hydrogen bubbles efficiently but also prevent the hydrogen bubbles from releasing into the electrolyte, which should greatly advance the development of water electrolysis and offer inspirations for people to fabricate more efficient HER devices.

Single-Entity Electrochemistry for Digital Biosensing at Ultralow Concentrations

Quantifying ultralow analyte concentrations is a continuing challenge in the analytical sciences in general and in electrochemistry in particular. Typical hurdles for affinity sensors at low concentrations include achieving sufficiently efficient mass transport of the analyte, dealing with slow reaction kinetics, and detecting a small transducer signal against a background signal that itself fluctuates slowly in time. Recent decades have seen the advent of methods capable of detecting single analytes ranging from the nanoscale to individual molecules, representing the ultimate mass sensitivity to these analytes. However, single-entity detection does not automatically translate into a superior concentration sensitivity. This is largely because electrochemical transducers capable of such detection are themselves miniaturized, exacerbating mass transport and binding kinetic limitations. In this Perspective, we discuss how these challenges can be tackled through so-called digital sensing: large arrays of separately addressable single-entity detectors that provide real-time information on individual binding events. We discuss the advantages of this approach and the barriers to its implementation.

The effects of nanobubbles on the assembly of glucagon amyloid fibrils

Some recent studies have shown that the surface and interface play an important role in the assembly and aggregation of amyloid proteins. However, it is unclear how the gas-liquid interface affects the protein assembly at the nanometer scale although the presence of gas-liquid interfaces is very common in in vitro experiments. Nanobubbles have a large specific surface area, which provides a stage for interactions with various proteins and peptides on the nanometer scale. In this work, nanobubbles produced in solution were employed for studying the effects of the gas-liquid interface on the assembly of glucagon proteins. Atomic force microscopy (AFM) studies showed that nanobubble-treated glucagon solution formed fibrils with an apparent height of 4.02 ± 0.71 nm, in contrast to the fibrils formed with a height of 2.14 ± 0.53 nm in the control. Transmission electron microscopy (TEM) results also showed that nanobubbles promoted the assembly of glucagon to form more fibrils. Thioflavin T (ThT) fluorescence and Fourier transform infrared (FTIR) analyses indicated that the nanobubbles induced the change of the glucagon conformation to a β-sheet structure. A mechanism that explains how nanobubbles affect the assembly of glucagon amyloid fibrils was proposed based on the above-mentioned experimental results. Given the fact that there are a considerable amount of nanobubbles existing in protein solutions, our results indicate that nanobubbles should be considered for fully understanding the protein aggregation events in vitro.

Dynamic Equilibrium Model for Surface Nanobubbles in Electrochemistry

Gas bubbles are ubiquitous in electrochemical processes, particularly in water electrolysis. Due to the development of gas-evolving electrocatalysis and energy conversion technology, a deep understanding of gas bubble behaviors at the electrode surface is highly desirable. In this work, by combining theoretical analysis and molecular simulations, we study the behaviors of a single nanobubble electrogenerated at a nanoelectrode. With the dynamic equilibrium model, the stability criteria for stationary surface nanobubbles are established. We show theoretically that a slight change in either the gas solubility or solute concentration results in various nanobubble dynamic states at a nanoelectrode: contact line pinning in aqueous and ethylene glycol solutions, oscillation of pinning states in dimethyl sulfoxide, and mobile nanobubbles in methanol. The above complex nanobubble behavior at the electrode/electrolyte interface is explained by the competition between gas influx into the nanobubble and outflux from the nanobubble.

Visualization and Quantification of Electrochemical H2 Bubble Nucleation at Pt, Au, and MoS2 Substrates

Electrolytic gas evolution is a significant phenomenon in many electrochemical technologies from water splitting, chloralkali process to fuel cells. Although it is known that gas evolution may substantially affect the ohmic resistance and mass transfer, studies focusing on the electrochemistry of individual bubbles are critical but also challenging. Here, we report an approach using scanning electrochemical cell microscopy (SECCM) with a single channel pipet to quantitatively study individual gas bubble nucleation on different electrode substrates, including conventional polycrystalline Pt and Au films, as well as the most interesting two-dimensional semiconductor MoS₂. Due to the confinement effect of the pipet, well-defined peak-shaped voltammetric features associated with single bubble nucleation and growth are consistently observed. From stochastic bubble nucleation measurement and finite element simulation, the surface H₂ concentration corresponding to bubble nucleation is estimated to be ∼218, 137, and 157 mM, with critical nuclei contact angles of ∼156°, ∼161°, and ∼160° at polycrystalline Pt, Au, and MoS₂ substrates, respectively. We further demonstrated the surface faceting at polycrystalline Pt is not specifically correlated with the bubble nucleation behavior.

Stochasticity in Single-Entity Electrochemistry

Most electrochemical processes are stochastic and discrete in nature. Yet experimental observables, e.g., i vs E, are typically smooth and deterministic, due to many events/processes, e.g., electron transfers, being averaged together. However, when the number of entities measured approaches a few or even one, stochasticity frequently emerges. Yet all is not lost! Probabilistic and statistical interpretation can generate insights matching or superseding those from macroscale/ensemble measurements, revealing phenomena that were hitherto averaged over. Herein, we review recent literature examples of stochastic processes in single-entity electrochemistry, highlighting strategies for interpreting stochasticity, contrasting them with macroscale measurements, and describing the insights generated.

Harnessing bubble behaviors for developing new analytical strategies

Gas bubbles are easily accessible and offer many unique characteristic properties of a gas/liquid two-phase system for developing new analytical methods. In this minireview, we discuss the newly developed analytical strategies that harness the behaviors of bubbles. Recent advancements include the utilization of the gas/liquid interfacial activity of bubbles for detection and preconcentration of surface-active compounds; the employment of the gas phase properties of bubbles for acoustic imaging and detection, microfluidic analysis, electrochemical sensing, and emission spectroscopy; and the application of the mass transport behaviors at the gas/liquid interface in gas sensing, biosensing, and nanofluidics. These studies have demonstrated the versatility of gas bubbles as a platform for developing new analytical strategies.

Transport-Based Modeling of Bubble Nucleation on Gas Evolving Electrodes

Bubble nucleation is ubiquitous in gas evolving reactions that are instrumental for a variety of electrochemical systems. Fundamental understanding of the nucleation process, which is critical to system optimization, remains limited as prior works generally focused on the thermodynamics and have not considered the coupling between surface geometries and different forms of transport in the electrolytes. Here, we establish a comprehensive transport-based model framework to identify the underlying mechanism for bubble nucleation on gas evolving electrodes. We account for the complex effects on the electrical field, ion migration, ion diffusion, and gas diffusion arising from surface heterogeneities and gas pockets initiated from surface crevices. As a result, we show that neglecting these effects leads to significant underprediction of the energy needed for nucleation. Our model provides a non-monotonic relationship between the surface cavity size and the overpotential required for nucleation, which is physically more consistent than the monotonic relationship suggested by a traditional thermodynamics-based model. We also identify the significance of the gas diffuse layer thickness, a parameter controlled by external flow fields and overall electrode geometries, which has been largely overlooked in previous models. Our model framework offers guidelines for practical electrochemical systems whereby, without changing the surface chemistry, nucleation on electrodes can be tuned by engineering the cavity size and the gas diffuse layer thickness.

Acid-base chemistry at the single ion limit

We present the results of acid–base experiments performed at the single ion (H⁺ or OH⁻) limit in ∼6 aL volume nanopores incorporating electrochemical zero-mode waveguides (E-ZMWs). At pH 3 each E-ZMW nanopore contains ca. 3600H⁺ ions, and application of a negative electrochemical potential to the gold working electrode/optical cladding layer reduces H⁺ to H₂, thereby depleting H⁺ and increasing the local pH within the nanopore. The change in pH was quantified by tracking the intensity of fluorescein, a pH-responsive fluorophore whose intensity increases with pH. This behavior was translated to the single ion limit by changing the initial pH of the electrolyte solution to pH 6, at which the average pore occupancy 〈n〉_pore ∼3.6H⁺/nanopore. Application of an electrochemical potential sufficiently negative to change the local pH to pH 7 reduces the proton nanopore occupancy to 〈n〉_pore ∼0.36H⁺/nanopore, demonstrating that the approach is sensitive to single H⁺ manipulations, as evidenced by clear potential-dependent changes in fluorescein emission intensity. In addition, at high overpotential, the observed fluorescence intensity exceeded the value predicted from the fluorescence intensity-pH calibration, an observation attributed to the nucleation of H₂ nanobubbles as confirmed both by calculations and the behavior of non-pH responsive Alexa 488 fluorophore. Apart from enhancing fundamental understanding, the approach described here opens the door to applications requiring ultrasensitive ion sensing, based on the optical detection of H⁺ population at the single ion limit.

Visualizing dynamic change in the number of protons during electroreduction of protons in attoliter volume zero-mode waveguides.

Electrochemically Generated Nanobubbles: Invariance of the Current with Respect to Electrode Size and Potential

Gas-producing electrochemical reactions are key to energy conversion and generation technologies. Bubble formation dramatically decreases gas-production rates on nanoelectrodes, by confining the reaction to the electrode boundary. This results in the collapse of the current to a stationary value independent of the potential. Startlingly, these residual currents also appear to be insensitive to the nanoelectrode diameter in the 5 to 500 nm range. These results are counterintuitive, as it may be expected that the current be proportional to the circumference of the electrode, i.e., the length of the three-phase line where the reaction occurs. Here, we use molecular simulations and a kinetic model to elucidate the origin of current insensitivity with respect to the potential and establish its relationship to the size of nanoelectrodes. We provide critical insights for the design and operation of nanoscale electrochemical devices and demonstrate that nanoelectrode arrays maximize conversion rates compared to macroscopic electrodes with same total area.

Real-Time Visualization of the Single-Nanoparticle Electrocatalytic Hydrogen Generation Process and Activity under Dark Field Microscopy

Visualizing a chemical reaction process is critical for understanding the mechanism of the reaction. For example, information on chemical reactions involving single nanocatalysts has significant implications for mechanism research and is vital for guiding the selection of the most active nanocatalysts. In this work, dark field microscopy (DFM) is utilized to observe the electrocatalytic reaction process of Au-Pt core-shell nanoparticles (AuNPs@Pt) as an example. Hydrogen ions were reduced to hydrogen (H₂) on the surface of AuNPs@Pt under a certain potential, forming H₂ nanobubbles covering the surface of AuNPs@Pt. As a result, the scattering intensity of the nanomaterial was observed to significantly increase under DFM. Therefore, the electrocatalytic reaction process could be monitored in real time by simply observing the scattering intensity change via DFM. Our investigation reveals a different nanobubble evolution process with an average nucleation time and lifetime of 0.69 and 32.34 s, respectively. Moreover, the catalytic activity between different nanomaterials was studied. The relationship between the Pt shell thickness and the average scattering intensity change reveals that the electrocatalytic activity is closely related to the Pt content. Finally, from the brightness of the scattering spot observed by DFM, the temporal and spatial distribution information on the catalytic activity could also be obtained, which is more abundant than the information obtained using the traditional electrochemical method.