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.

The Effect of the Capping Agents of Nanoparticles on Their Redox Potential

Engineered metallic nanoparticles, which are found in numerous applications, are usually stabilized by organic ligands influencing their interfacial properties. We found that the ligands affect tremendously the electrochemical peak oxidation potentials of the nanoparticles. In this work, identical gold nanoparticles were ligand-exchanged and carefully analyzed to enable a precise and highly reproducible comparison. The peak potential difference between gold nanoparticles stabilized by various ligands, such as 2- and 4-mercaptobenzoic acid, can be as high as 71 mV, which is substantial in energetic terms. A detailed study supported by density functional theory (DFT) calculations aimed to determine the source of this interesting effect. The DFT simulations of the ligand adsorption modes on Au surfaces were used to calculate the redox potentials through the thermodynamic cycle method. The DFT results of the peak potential shift were in good agreement with the experimental results for a few ligands, but showed some discrepancy, which was attributed to kinetic effects. The kinetic rate constant of the oxidation of Au nanoparticles stabilized by 4-mercaptobenzoic acid was found to be twice as large as that of the Au nanoparticles stabilized by citrate, as calculated from Laviron's theory and the Tafel equation. Finally, these findings could be applied to some novel applications such as determining the distribution of nanoparticle population in a dispersion as well as monitoring the ligand exchange between nanoparticles.

Collision Oxidation Behavior of Silver Nanoparticles in Alkaline Solution

In recent years, silver nanoparticles (Ag NPs) have been used as positive electrode material for zinc/silver batteries, and the silver oxides formed during the charging process determine the discharge performance of batteries. Therefore, it is important to study the oxidation behavior of Ag NPs in alkaline solution. Single-nanoparticle collision is an important tool for analyzing oxidation behavior of individual nanoparticles. Based on thermodynamic information from collision events, it is known that oxidation products are potential-dependent and size-dependent. Based on dynamic information, including collisional peak shapes and duration time, it was observed that the Ag NP collision oxidation process changed from stepwise oxidation to direct oxidation as the potential increased or size decreased. This work provides guidance for application of Ag NPs in zinc/silver batteries and proposed a strategy for oxidation behavior of individual NP that could be tracked in situ through an all-encompassing view of thermodynamic and dynamic information.

Electrochemical Characterization of Neurotransmitters in a Single Submicron Droplet

Single-entity electrochemistry, which employs electrolysis during the collision of single particles on ultramicroelectrodes, has witnessed significant advancements in recent years, enabling the observation and characterization of individual particles. Information on a single aqueous droplet (e.g., size) can also be studied based on the redox species contained therein. Dopamine, a redox-active neurotransmitter, is usually present in intracellular vesicles. Similarly, in the current study, the electrochemical properties of neurotransmitters in submicron droplets were investigated. Because dopamine oxidation is accompanied by proton transfer, unique electrochemical properties of dopamine were observed in the droplet. We also investigated the electrochemical properties of the adsorbed droplets containing DA and the detection of oxidized dopamine by the recollision phenomenon.

Electrochemical Metal Recycling: Recovery of Palladium from Solution and In Situ Fabrication of Palladium-Carbon Catalysts via Impact Electrochemistry

Recycling of critical materials, regeneration of waste, and responsible catalyst manufacture have been repeatedly documented as essential for a sustainable future with respect to the environment and energy production. Electrochemical methods have become increasingly recognized as capable of achieving these goals, and “impact” electrochemistry, with the advantages associated with dynamic nanoelectrodes, has recently emerged as a prime candidate for the recovery of metals from solution. In this report, the nanoimpact technique is used to generate carbon-supported palladium catalysts from low-concentration palladium(II) chloride solutions (i.e., a waste stream mimic) as a proof of concept. Subsequently, the catalytic properties of this material in both synthesis (Suzuki coupling reaction) and electrocatalysis (hydrogen evolution) are demonstrated. Transient reductive impact signals are shown and analyzed at potentials negative of +0.4 V (vs SCE) corresponding to the onset of palladium deposition in traditional voltammetry. Direct evidence of Pd modification was obtained through characterization by environmental scanning electron microscopy/energy-dispersive X-ray spectroscopy, inductively coupled plasma mass spectrometry, X-ray photoelectron spectroscopy, transmission electron microscopy, and thermogravimetric analysis of impacted particles. This showed the formation of deposits of Pd0 partially covering the 50 nm carbon black particles with approximately 14% Pd (wt %) under the conditions used. This material was then used to demonstrate the conversion of iodobenzene into its biphenyl product (confirmed through nuclear magnetic resonance) and the successful production of hydrogen as an electrocatalyst under acidic conditions (under cyclic voltammetry).

Selecting an Optimal Faraday Cage To Minimize Noise in Electrochemical Experiments

The ubiquitous Faraday cage, an experimental component particularly essential for nanoelectrochemical measurements, is responsible for neutralizing noise introduced by electromagnetic interference (EMI). Faraday cage designs abound in the literature, often exhibiting varying thicknesses, mesh sizes, and base materials. The fact that the Faraday cage composition most often goes unreported underscores the fact that many electrochemical researchers assume a 100% EMI reduction for any given design. In this work, this assumption is challenged from a theoretical and empirical perspective by highlighting the physical principles producing the Faraday effect. A brief history of the Faraday cage and a simplified theoretical approach introduce fundamental considerations regarding optimal design properties. In practice, time-domain noise profiles and corresponding Fourier transform frequency domain information for custom-built Faraday cages reveal that maximally conductive cages provide more optimal EMI exclusion.

Electrochemical Detection and Analysis of Various Current Responses of a Single Ag Nanoparticle Collision in an Alkaline Electrolyte Solution

A single silver (Ag) nanoparticle (NP) collision was observed and analyzed in an alkaline solution using the electrocatalytic amplification (EA) method. Previously, the observation of a single Ag NP collision was only possible through limited methods based on a self-oxidation of Ag NPs or a blocking strategy. However, it is difficult to characterize the electrocatalytic activity of Ag NPs at a single NP level using a method based on the self-oxidation of Ag NPs. When using a blocking strategy, size analysis is difficult owing to the edge effect in the current signal. The fast oxidative dissolution of Ag NPs has been a problem for observing the staircase response of a single Ag NP collision signal using the EA method. In alkaline electrolyte conditions, Ag oxides are stable, and the oxidative dissolution of Ag NPs is sluggish. Therefore, in this study, the enhanced magnitude and frequency of the current response for single Ag NP collisions were obtained using the EA method in an alkaline electrolyte solution. The peak height and frequency of single Ag NP collisions were analyzed and compared with the theoretical estimation.

The discrete single-entity electrochemistry of Pickering emulsions

Single-entity analysis is an important research topic in electrochemistry. To date, electrode collisions and subsequent electrode-particle interactions have been studied for many types of nano-objects, including metals, polymers, and micelles. Here we extend this nano-object electrochemistry analysis to Pickering emulsions for the first time. The electrochemistry of Pickering emulsions is important because the internal space of a Pickering emulsion can serve as a reactor or template; this leads to myriad possible applications, all the while maintaining mechanical stability far superior to what is exhibited by conventional emulsions. This work showed that Pickering emulsions exhibit similar hydrodynamic behavior to other nano-objects, despite the complex structure involving hard nanoparticle surfactants, and the electron-transport mechanism into the internal volume of Pickering emulsions was elucidated. The Pickering emulsion electrochemistry platform developed here can be applied to electrochemical nanomaterial synthesis, surmounting the challenges faced by conventional synthetic strategies involving normal emulsions.

Current Lifetime of Single-Nanoparticle Collision for Sizing Nanoparticles

Accurate size analysis of nanoparticles (NPs) is vital for nanotechnology. However, this cannot be realized based on conventional single-nanoparticle collision (SNC) because the current intensity, a thermodynamic parameter of SNC for sizing NPs, is always smaller than the theoretical value due to the effect of NP movements on the electrode surface. Herein, a size-dependent dynamic parameter of SNC, current lifetime, which refers to the time that the current intensity decays to 1/e of the original value, was originally utilized to distinguish differently sized NPs. Results showed that the current lifetime increased with NP size. After taking the current lifetime into account rather than the current intensity, the overlap rates for the peak-type current transients of differently sized Pt NPs (10 and 15 nm) and Au NPs (18 and 35 nm) reduced from 73 and 7% to 45 and 0%, respectively, which were closer to the theoretical values (29 and 0%). Hence, the proposed SNC dynamics-based method holds great potential for developing reliable electrochemical approaches to evaluate NP sizes accurately.

Particle Impact Electrochemistry

Experiments involving collisions between a single entity and the electrode surface have become an active area of research. The electrochemical contribution of individual nanoparticles (NPs), enzymes, and other entities, such as aggregates or agglomerates, can be determined using particle impact experiments. Destructive nanoimpact experiments of materials, such as Ag, and the electrocatalytic amplification (ECA) are used to detect the NP/electrode interactions. This review covers the seminal work, critical theoretical studies, and some recent applications. The applications to electrocatalysis include measurements of electron transfer rate constants on individual nanoparticles. Applications in analytical chemistry have allowed the detection of nonelectroactive species by detecting the collisions of soft materials, e.g. micellar suspensions and proteins have increased the technique's analytical possibilities. With ECA, NPs can be used as tags for the electrochemical detection of bioanalytes such as DNA, proteins, and liposomes. The theory of ECA collisions, including frequency of collision and the size of the electrochemical current transients, are also covered. For nanoimpacts, the charge measured during a NP electrolysis, such as Ag NP, is used to detect the NP. Measurements of NP diameter are possible, but limitations to this analysis are covered. The electron transfer studies to the electrolysis of Ag and of metal oxides are discussed. Finally, key experimental instrumentations are discussed, including instrumentation techniques for the small currents inherent to single NP measurement. The effect of filtering, instrumentations rise time, and sampling frequency are also covered.

Rapid and Accurate Data Processing for Silver Nanoparticle Oxidation in Nano-Impact Electrochemistry

In recent years, nano-impact electrochemistry (NIE) has attracted widespread attention as a new electroanalytical approach for the analysis and characterization of single nanoparticles in solution. The accurate analysis of the large volume of the experimental data is of great significance in improving the reliability of this method. Unfortunately, the commonly used data analysis approaches, mainly based on manual processing, are often time-consuming and subjective. Herein, we propose a spike detection algorithm for automatically processing the data from the direct oxidation of sliver nanoparticles (AgNPs) in NIE experiments, including baseline extraction, spike identification and spike area integration. The resulting size distribution of AgNPs is found to agree very well with that from transmission electron microscopy (TEM), showing that the current algorithm is promising for automated analysis of NIE data with high efficiency and accuracy.

Time-Resolved Selective Electrochemical Sensing of Carbon Particles

This work demonstrated a new method for electrochemical detection of carbon black particles based on impact electrochemistry that was capable of selective detection of carbon black from the insulating oxide particles. We systematically studied the electrochemical collision events with carbon black particle suspension solution (pH 7.0 phosphate buffer) at varying carbon black concentrations using a convective condition and a gold microelectrode. We evaluated the effect of bias potential on the number and magnitude of collision spikes by changing the applied potential in chronoamperometry experiments. Our results showed that the biased potential of +0.4 V was the most suitable potential among the tested potential biases. Current blips were observed in the amperometric i-t response, and the spike numbers scaled linearly with the concentration of carbon black particles in the range of 2.5-20 μM (i.e., mass/volume concentration of 0.03 to 0.24 mg L^-1) with the lowest detection limit of 0.396 μM (i.e., mass/volume concentration of 0.00475 mg L^-1). The selective detection of carbon particles in the presence of representative poorly conductive oxide particles in our experimental conditions was achieved. The sensing mechanism of the sensitive and selective detection of carbon black particles is proposed. This work provides the basis for the development of powerful electroanalytical methods and technologies for the detection and classification of carbon particles in varying environmental conditions such as coalmines, engineered carbon particle factories, and coal power plants.

In Situ Monitoring of Collision and Recollision Events of Single Attoliter Droplets via Single-Entity Electrochemistry

We describe a simple method for real-time observation of collision and recollision behavior of a single aqueous attoliter droplet in an organic solvent through single-entity electrochemistry. The dynamics and morphology of the droplet after the collision event at the Au ultramicroelectrode (Au-UME) were monitored by consecutive cyclic voltammetry and amperometric current-time measurements. By sequentially applying oxidative potential and reductive potential at the Au-UME in the presence of attoliter droplets containing reversible redox species (e.g., ferrocyanide), we successfully detected the oxidative collision spike and its reductive recollision spike successively owing to the reversible redox reactions inside the droplet. Because the redox species was dissolved in a reduced form, the reductive collision spikes observed are the direct evidence that the water droplets colliding at the electrode surface are detached after the oxidation reaction. The collided droplet properties, such as size change and contact area, are also investigated and discussed.

Stochastic Particle Approach Electrochemistry (SPAE): Estimating Size, Drift Velocity, and Electric Force of Insulating Particles

Stochastic particle impact electrochemistry (SPIE) is considered one of the most important electro-analytical methods to understand the physicochemical properties of single entities. SPIE of individual insulating particles (IPs) has been particularly crucial for analyses of bioparticles. In this article, we introduce stochastic particle approach electrochemistry (SPAE) for electrochemical analyses of IPs, which is the advanced version of SPIE; SPAE is analogous to SPIE but focuses on deciphering a sudden current drop (SCD) by an IP-approach toward the edge of an ultramicroelectrode (UME). Polystyrene particles (PSPs) with and without different surface functionalities (-COOH and - NH₃) as well as fixed human platelets (F-HPs) were used as model IPs. From theory based on finite element analysis, a sudden current drop (SCD) induced by an IP during electro-oxidation (or reduction) of a redox mediator on a UME can represent the rapid approach of an IP toward an edge of a UME, where a strong electric field is generated. It is also found that the amount of current drop, i_drop, of an SCD depends strongly on both the size of an IP and the concentration of redox electrolyte. From simulations based on the SPAE model that fit the experimentally obtained SCDs of three types of PSPs or F-HP dispersed in solutions with two redox electrolytes, their size distribution histograms are estimated, from which their average radii determined by SPAE are compared to those from scanning electron microscopic images. In addition, the drift velocity and corresponding electric force of the PSPs and F-HPs during their approach toward an edge of a Pt UME are estimated, which cannot be addressed currently with SPIE. We further learned that the estimated drift velocity and the corresponding electric force could provide a relative order of the number of excess surface charges on the IPs.

Stochastic Electrochemical Cytometry of Human Platelets via a Particle Collision Approach

The quantitative analysis of human platelets is important for the diagnosis of various hematologic and cardiovascular diseases. In this article, we present a stochastic particle impact electrochemical (SPIE) approach for human platelets with fixation (F-HPs). Carboxylate-functionalized polystyrene particles (PSPs) are studied as well as a standard platform of SPIE-F-HPs. For SPIE-PSPs (or F-HPs), [Fe(CN)₆]^4- was used as the redox mediator, and electro-oxidation of [Fe(CN)₆]^4- to [Fe(CN)₆]^3- was conducted on a Pt ultramicroelectrode (UME) by applying a constant potential, where the corresponding oxidation current is mass-transfer-controlled. When PSPs (or F-HPs) are introduced into aqueous solution with [Fe(CN)₆]^4-, sudden current drops (SCDs) were observed, which resulted from the partial blockage of a Pt UME by collision of an individual PSP (or F-HP). For SPIE-PSPs (or F-HPs), we found that it is essential to enhance the migration of PSPs (F-HPs) toward a Pt UME by maximizing the steady state current associated with electro-oxidation of [Fe(CN)₆]^4-. This was accomplished by increasing its concentration to the solubility limit. We successfully measured the concentration of F-HPs dispersed in aqueous solution containing [Fe(CN)₆]^4- with a minimum detectable concentration of 0.1 fM, and the size distribution of F-HPs was also estimated from the obtained i_drop distribution based on the SPIE analysis, where i_drop stands for the magnitude of the current drop of each SCD. Lastly, we revealed that HPs without the fixation process (WF-HPs) are difficult to quantitatively analyze by SPIE because of their transient activation process, which results in changes from their spherical shape. The observed difficulty was also confirmed by finite element analysis, which shows that i_drop can be significantly increased, as an elongated WF-HP is adsorbed on the edge of an UME.

Viologen-Bromide Dual-Redox Ionic Solid Complexes: Understanding Their Electrochemical Formation and Proton-Accompanied Redox Chemistry

The inhibition of self-discharge in a redox-enhanced electrochemical capacitor (Redox-EC) is crucial for excellent energy retention. Heptyl viologen dibromide (HVBr₂) was chosen as a strong candidate of a dual-redox species in Redox-EC due to its solid complexations during the charging process, at which HV²⁺ is electrochemically reduced to HV^+• and form a solid complex, [HV^+•·Br^-], on an anode while Br^- is electro-oxidized to Br₃^- and renders [HV²⁺·2Br₃^-] on a cathode. The solid complexes could not transfer across the separator, resulting in significant diminution of the self-discharge. In this Article, we present detailed electrochemical studies of formation of [HV²⁺·2Br₃^-] and [HV^+•·Br^-], their redox features, and galvanic exchange reactions between the two types of dual-redox ionic solids on a Pt ultra-microelectrode (UME) in neutral (0.33 M Na₂SO₄) and acidic (1 M H₂SO₄) solutions. Most importantly, through voltammetric and particle-impact electrochemical analyses, we found that the redox and galvanic exchange reactions of the two dual-redox ionic solid complexes involve H⁺ transfer, which is the key process to limit the overall kinetics of the electrochemical reactions. We also rationalize the proton-accompanied galvanic exchange reaction based on computational simulation.

Single-Particle Investigation of Environmental Redox Processes of Arsenic on Cerium Oxide Nanoparticles by Collision Electrochemistry

Quantification of chemical reactions of nanoparticles (NPs) and their interaction with contaminants is a fundamental need to the understanding of chemical reactivity and surface chemistry of NPs released into the environment. Herein, we propose a novel strategy employing single-particle electrochemistry showing that it is possible to measure reactivity, speciation, and loading of As³⁺ on individual NPs, using cerium oxide (CeO₂) as a model system. We demonstrate that redox reactions and adsorption processes can be electrochemically quantified with high sensitivity via the oxidation of As³⁺ to As⁵⁺ at 0.8 V versus Ag/AgCl or the reduction of As³⁺ to As⁰ at -0.3 V (vs Ag/AgCl) generated by collisions of single particles at an ultramicroelectrode. Using collision electrochemistry, As³⁺ concentrations were determined in basic conditions showing a maximum adsorption capacity at pH 8. In acidic environments (pH < 4), a small fraction of As³⁺ was oxidized to As⁵⁺ by surface Ce⁴⁺ and further adsorbed onto the CeO₂ surface as a As⁵⁺ bidentate complex. The frequency of current spikes (oxidative or reductive) was proportional to the concentration of As³⁺ accumulated onto the NPs and was found to be representative of the As³⁺ concentration in solution. Given its sensitivity and speciation capability, the method can find many applications in the analytical, materials, and environmental chemistry fields where there is a need to quantify the reactivity and surface interactions of NPs. This is the first study demonstrating the capability of single-particle collision electrochemistry to monitor the interaction of heavy metal ions with metal oxide NPs. This knowledge is critical to the fundamental understanding of the risks associated with the release of NPs into the environment for their safe implementation and practical use.

Electrochemical impacts complement light scattering techniques for in situ nanoparticle sizing

We show that the electrochemical particle-impact technique (or 'nano-impacts') complements light scattering techniques for sizing both mono- and poly-disperse nanoparticles. It is found that established techniques - Dynamic Light Scattering (DLS) and Nanoparticle Tracking Analysis (NTA) - can accurately measure the diameters of '30 nm' silver particles assuming spherical shapes, but are unable to accurately size a smaller '20 nm' sample. In contrast, nano-impacts have a high accuracy (<5% error in effective diameters) and are able to size both individual '20 nm' and '30 nm' silver NPs in terms of the number of constituent atoms. Further study of a '20 nm and 30 nm' bimodal sample shows that the electrochemical technique resolves the two very similar sizes well, demonstrating accurate sizing regardless of particle size polydispersity, whereas due to inherent limitations of light scattering measurements this is not possible for DLS and NTA. Electrochemical sizing is concluded to offer significant attractions over light scattering methods.

Chronoamperometric Observation and Analysis of Electrocatalytic Ability of Single Pd Nanoparticle for Hydrogen Peroxide Reduction Reaction

The current generated by the collision of a single nanoparticle (NP) of palladium (Pd) on a gold (Au) ultramicroelectrode (UME) surface was observed using an electrocatalytic amplification method. The hydrogen peroxide reduction reaction was used for the electrocatalytic reaction because the hydrogen peroxide reduction reaction has no gas-phase product, which would induce rapid signal decay. The electrocatalytic current resulting from a single Pd nanoparticle on the Au UME shows a staircase response with accompanying slow current decay. The applying potential and concentration of hydrogen peroxide were optimized for clear distinction of signal. The height of the current step and signal frequency were analyzed and compared with the theoretical expectation. The analysis of the electrocatalytic activity of single Pd NPs provides insight toward their future application.

Electrochemical Behavior of Single CuO Nanoparticles: Implications for the Assessment of their Environmental Fate

The electrochemical behavior of copper oxide nanoparticles is investigated at both the single particle and at the ensemble level in neutral aqueous solutions through the electrode-particle collision method and cyclic voltammetry, respectively. The influence of Cl^- and NO₃^- anions on the electrochemical processes occurring at the nanoparticles is further evaluated. The electroactivity of CuO nanoparticles is found to differ between the two types of experiments. At the single-particle scale, the reduction of the CuO nanoparticles proceeds to a higher extent in the presence of chloride ion than of nitrate ion containing solutions. However, at the multiparticle scale the CuO reduction proceeds to the same extent regardless of the type of anions present in solution. The implications for assessing realistically the environmental fate and therefore the toxicity of metal-based nanoparticles in general, and copper-based nanoparticles in particular, are discussed.

Particle-impact analysis of the degree of cluster formation of rutile nanoparticles in aqueous solution

Cluster formation can profoundly influence the bioavailability and (bio)geochemical activity of nanoparticles in natural aquatic systems. While colloidal properties of nanoparticles are commonly investigated using light-scattering techniques, the requirement to dilute samples can affect the fundamental nature and extent of the cluster size. Hence, an alternative in situ approach that can cover a much higher and wider concentration range of particles is desirable. In this study, particle impact chronoamperometry is employed to probe the degree of cluster formation of Alizarin Red S modified rutile nanoparticles of diameter ca. 167 nm in conditions approximating those existing in the environment. Random collisions of individual clusters of the modified rutile particles with a stationary electrode result in transient current signals during a chronoamperometric measurement, indicative of the reduction of the adsorbed Alizarin Red S dye molecules. The results from the particle-impact analysis reveal that the nanoparticles are heavily clustered with an average 91 monomeric particles per cluster. As the spherical equivalent size of the clusters (ca. 754 nm in diameter) is considerably larger than that from nanoparticle tracking analysis (ca. 117 nm), the present work highlights the impact of the dilution on the fundamental nature of the colloidal suspension and introduces the electrochemical determination of the size distribution of inert mineral nanoparticles in highly concentrated media.

Electrode-particle impacts: a users guide

We present a comprehensive guide to nano-impact experiments, in which we introduce newcomers to this rapidly-developing field of research. Central questions are answered regarding required experimental set-ups, categories of materials that can be detected, and the theoretical frameworks enabling the analysis of experimental data. Commonly-encountered issues are considered and presented alongside methods for their solutions.

Single Oxidative Collision Events of Silver Nanoparticles: Understanding the Rate-Determining Chemistry

The oxidative dissolution of citrate-capped silver nanoparticles (AgNPs, ∼50 nm diameter) is investigated herein by two electrochemical techniques: nano-impacts and anodic stripping voltammetry. Nano-impacts or single nanoparticle-electrode collisions allow the detection of individual nanoparticles. The technique offers an advantage over surface-immobilized methods such as anodic stripping voltammetry as it eliminates the effects of particle agglomeration/aggregation. The electrochemical studies are performed in different electrolytes (KNO₃ , KCl, KBr and KI) at varied concentrations (≤20 mm). In nano-impact measurements, the AgNP undergoes complete oxidation upon impact at a suitably potentiostated electrode. The frequency of the nanoparticle-electrode collisions observed as current-transient spikes depends on the electrolyte identity, its concentration and the potential applied at the working electrode. The frequencies of the spikes are significantly higher in the presence of halide ions and increase with increasing potentials. From the frequency, the rate of AgNP oxidation as compared with the timescale the AgNP is in electrical contact with the electrode can be inferred, and hence is indicative of the relative kinetics of the oxidation process. Primarily based on these results, we propose the initial formation of the silver (I) nucleus (Ag⁺ , AgCl, AgBr or AgI) as the rate-determining process of silver oxidation on the nanoparticle.

Real-time plasmon spectroscopy study of the solid-state oxidation and Kirkendall void formation in copper nanoparticles

Oxidation and corrosion reactions have a major effect on the application of non-noble metals. Kinetic information and simple theoretical models are often insufficient for describing such processes in metals at the nanoscale, particularly in cases involving formation of internal voids (nano Kirkendall effect, NKE) during oxidation. Here we study the kinetics of solid-state oxidation of chemically-grown copper nanoparticles (NPs) by in situ localized surface plasmon resonance (LSPR) spectroscopy during isothermal annealing in the range 110-170 °C. We show that LSPR spectroscopy is highly effective in kinetic studies of such systems, enabling convenient in situ real-time measurements during oxidation. Change of the LSPR spectra throughout the oxidation follows a common pattern, observed for different temperatures, NP sizes and substrates. The well-defined initial Cu NP surface plasmon (SP) band red-shifts continuously with oxidation, while the extinction intensity initially increases to reach a maximum value at a characteristic oxidation time τ, after which the SP intensity continuously drops. The characteristic time τ is used as a scaling parameter for the kinetic analysis. Evolution of the SP wavelength and extinction intensity during oxidation at different temperatures follows the same kinetics when the oxidation time is normalized to τ, thus pointing to a general oxidation mechanism. The characteristic time τ is used to estimate the activation energy of the process, determined to be 144 ± 6 kJ mol^-1, similar to previously reported values for high-temperature Cu thermal oxidation. The central role of the NKE in the solid-state oxidation process is revealed by electron microscopy, while formation of Cu₂O as the major oxidation product is established by X-ray diffraction, XPS, and electrochemical measurements. The results indicate a transition of the oxidation mechanism from a Valensi-Carter (VC) to NKE mechanism with the degree of oxidation. To interpret the optical evolution during oxidation, Mie scattering solutions for metal core-oxide shell spherical particles are computed, considering formation of Kirkendall voids. The model calculations are in agreement with the experimental results, showing that the large red-shift of the LSPR band during oxidation is the result of Kirkendall voiding, thus establishing the major role of the NKE in determining the optical behavior of such systems.

Impact and oxidation of single silver nanoparticles at electrode surfaces: one shot versus multiple events

Single nanoparticle (NP) electrochemical impacts is a rapidly expanding field of fundamental electrochemistry, with applications from electrocatalysis to electroanalysis. These studies, which involve monitoring the electrochemical (usually current-time, I-t) response when a NP from solution impacts with a collector electrode, have the scope to provide considerable information on the properties of individual NPs. Taking the widely studied oxidative dissolution of individual silver nanoparticles (Ag NPs) as an important example, we present measurements with unprecedented noise (< 5 pA) and time resolution (time constant 100 μs) that are highly revealing of Ag NP dissolution dynamics. Whereas Ag NPs of diameter, d = 10 nm are mostly dissolved in a single event (on the timescale of the measurements), a wide variety of complex processes operate for NPs of larger diameter (d ≥ 20 nm). Detailed quantitative analysis of the I-t features, consumed charge, event duration and impact frequency leads to a major conclusion: Ag NPs undergo sequential partial stripping (oxidative dissolution) events, where a fraction of a NP is electrochemically oxidized, followed by the NP drifting away and back to the tunnelling region before the next partial stripping event. As a consequence, analysis of the charge consumed by single events (so-called "impact coulometry") cannot be used as a general method to determine the size of colloidal NPs. However, a proper analysis of the I-t responses provides highly valuable information on the transient physicochemical interactions between NPs and polarized surfaces.

DNA capping agent control of electron transfer from silver nanoparticles

DNA capping of silver nanoparticles gates electron transfer from the nanoparticle and is controlled by the potentials at which the electroactive base pairs undergo oxidation.

Combined Blip and Staircase Response of Ascorbic Acid-Stabilized Copper Single Nanoparticle Collision by Electrocatalytic Glucose Oxidation

The current response of the collision of ascorbic acid-stabilized copper (Cu) single nanoparticles (NPs) on a gold (Au) ultramicroelectrode (UME) surface was observed by using an electrocatalytic amplification method. Here, the glucose oxidation electrocatalyzed by oxidized Cu NPs was used as the indicating reaction. In this system, the NP collision signals were obtained simultaneously by both direct particle electrolysis and electrocatalytic amplification. For example, when the applied potential was high enough for Cu NP oxidation, a blip response combined with a staircase response was observed as a current signal. The blip part in the single Cu NP collision signal indicates the self-oxidation of a Cu NP, and the staircase part indicates the steady-state electrocatalytic reaction by oxidized Cu NP.

Stochastic detection and characterisation of individual ferrocene derivative tagged graphene nanoplatelets

Graphene nanoplatelets (GNPs) are ‘tagged’ with 1-(biphen-4-yl)ferrocene, which has been studied via nano-impacts to derive the corresponding surface coverage.

Nanoparticle electrochemistry

This perspective article provides a survey of recent advances in nanoscale electrochemistry, with a brief theoretical background and a detailed discussion of experimental results of nanoparticle based electrodes, including the rapidly expanding field of “impact electrochemistry”.

Exploring the mineral-water interface: reduction and reaction kinetics of single hematite (α-Fe2O3) nanoparticles

Here we show that particle impact chronoamperometry allows the quantitative electrochemical characterization of individual mineral nanoparticles with adequate proton concentrations. Through this approach, we extract the kinetics and thermodynamics of the reductive dissolution of single hematite (α-Fe₂O₃) nanoparticles.

In spite of their natural and technological importance, the intrinsic electrochemical properties of hematite (α-Fe₂O₃) nanoparticles are not well understood. In particular, particle agglomeration, the presence of surface impurities, and/or inadequate proton concentrations are major obstacles to uncover the fundamental redox activities of minerals in solution. These are particularly problematic when samples are characterized in common electrochemical analyses such as cyclic voltammetry in which nanoparticles are immobilized on a stationary electrode. In this work, the intrinsic reaction kinetics and thermodynamics of individual hematite nanoparticles are investigated by particle impact chronoamperometry. The particle radius derived from the integrated area of spikes recorded in a chronoamperogram is in excellent agreement with electron microscopy results, indicating that the method provides a quantitative analysis of the reduction of the nanoparticles to the ferrous ion. A key finding is that the suspended individual nanoparticles undergo electrochemical reduction at potentials much more positive than those immobilized on a stationary electrode. The critical importance of the solid/water interface on nanoparticle activity is further illustrated by a kinetic model. It is found that the first electron transfer process is the rate determining step of the reductive dissolution of hematite nanoparticles, while the overall process is strongly affected by the interfacial proton concentration. This article highlights the effects of the interfacial proton and ferrous ion concentrations on the reductive dissolution of hematite nanoparticles and provides a highly effective method that can be readily applied to study a wide range of other mineral nanoparticles.

Analyzing Benzene and Cyclohexane Emulsion Droplet Collisions on Ultramicroelectrodes

We report the collisions of single emulsion oil droplets with extremely low dielectric constants (e.g., benzene, ε of 2.27, or cyclohexane, ε of 2.02) as studied via emulsion droplet reactor (EDR) on an ultramicroelectrode (UME). By applying appropriate potentials to the UME, we observed the electrochemical effects of single-collision signals from the bulk electrolysis of single emulsion droplets. Different hydrophobic redox species (ferrocene, decamethyl-ferrocene, or metalloporphyrin) were trapped in a mixed benzene (or cyclohexane) oil-in-water emulsion using an ionic liquid as the supporting electrolyte and emulsifier. The emulsions were prepared using ultrasonic processing. Spike-like responses were observed in each i-t response due to the complete electrolysis of all of the above-mentioned redox species within the droplet. On the basis of these single-particle collision results, the collision frequency, size distribution, i-t decay behavior of the emulsion droplets, and possible mechanisms are analyzed and discussed. This work demonstrated that bulk electrolysis can be achieved in a few seconds in these attoliter reactors, suggesting many applications, such as analysis and electrosynthesis in low dielectric constant solvents, which have a much broader potential window.

Electrochemical Nanoparticle Sizing Via Nano-Impacts: How Large a Nanoparticle Can be Measured?

The field of nanoparticle (NP) sizing encompasses a wide array of techniques, with electron microscopy and dynamic light scattering (DLS) having become the established methods for NP quantification; however, these techniques are not always applicable. A new and rapidly developing method that addresses the limitations of these techniques is the electrochemical detection of NPs in solution. The 'nano-impacts' technique is an excellent and qualitative in situ method for nanoparticle characterization. Two complementary studies on silver and silver bromide nanoparticles (NPs) were used to assess the large radius limit of the nano-impact method for NP sizing. Noting that by definition a NP cannot be larger than 100 nm in diameter, we have shown that the method quantitatively sizes at the largest limit, the lower limit having been previously reported as ∼6 nm.1.