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

Stochastic Impact Electrochemistry of Alkanethiolate-Functionalized Silver Nanoparticles

This study uses single-impact experiments to explore how the nanoparticles' surface chemistry influences their redox activity. 20 and 40 nm-sized silver nanoparticles are functionalized with alkanethiol ligands of various chain lengths (n = 3, 6, 8, and 11) and moieties (carboxyl ─COOH / hydroxyl ─OH), and the critical role of the particle shell is systematically examined. Short COOH-terminated ligands enable efficient charge transfer, resulting in higher impact rates and fast, high-amplitude transients. Even elevated potentials fail to overcome tunneling barriers for ligand lengths of n ≥ 6 and risk oxidizing the electrode, forming an insulating layer. Electrostatic interactions play a key role in governing reaction dynamics. In general, particles with a COOH-group exhibit higher impact rates and current amplitudes in KCl than those with an OH-group. This effect is more pronounced for 40 nm-sized particles; although, they rarely oxidize completely. The influence of electrolyte composition-concentration, pH, and a biologically relevant electrolyte-reveals that its impact on the redox activity can be as critical as that of the particle shell, with both determining particle adsorption and electron tunneling. These findings provide insights into the complex interdependencies at the electrode-particle-electrolyte interface, aiding the design of custom redox-active (silver) nanoparticles for ultrasensitive electrochemical sensing.

Stability and SERS signal strength of laser-generated gold, silver, and bimetallic nanoparticles at different KCl concentrations

Noble metal nanoparticles, specifically gold and silver, are extensively utilized in sensors, catalysts, surface-enhanced Raman scattering (SERS), and optical-electronic components due to their unique localized surface plasmon resonance (LSPR) properties. The production of these nanoparticles involves various methods, but among the environmentally friendly approaches, laser ablation stands out as it eliminates the need for toxic chemicals during purification. However, nanoparticle aggregation poses a challenge in laser ablation, necessitating the addition of extra materials that contaminate the otherwise clean process. In this study, we investigate the effectiveness of a biocompatible material, potassium chloride (KCl), in preventing particle aggregation. Although salt is known to trigger aggregation, we observed that certain concentrations of KCl can slow down this process. Over an eight-week period, we examined the aggregation rate, extinction behavior, and stability of gold, silver, and hybrid nanoparticles generated in different KCl concentrations. Extinction spectra, SEM images, SERS signal strength, and zeta potential were analyzed. Our results demonstrate that laser ablation in water and salt solutions yields nanoparticles with a spherical shape and a negative zeta potential. Importantly, we identified the optimal concentration of potassium chloride salt that maintains solution stability and SERS signal strength. Adsorbed chloride ions on silver nanoparticles were evidenced by low-frequency SERS band near 242 cm-1. A better understanding of the effect of KCl concentration on the properties of noble metal nanoparticles can lead to improved generation protocols and the development of tailored nanoparticle systems with enhanced stability and SERS activity.

Single Entity Electrocatalysis

Making a measurement over millions of nanoparticles or exposed crystal facets seldom reports on reactivity of a single nanoparticle or facet, which may depart drastically from ensemble measurements. Within the past 30 years, science has moved toward studying the reactivity of single atoms, molecules, and nanoparticles, one at a time. This shift has been fueled by the realization that everything changes at the nanoscale, especially important industrially relevant properties like those important to electrocatalysis. Studying single nanoscale entities, however, is not trivial and has required the development of new measurement tools. This review explores a tale of the clever use of old and new measurement tools to study electrocatalysis at the single entity level. We explore in detail the complex interrelationship between measurement method, electrocatalytic material, and reaction of interest (e.g., carbon dioxide reduction, oxygen reduction, hydrazine oxidation, etc.). We end with our perspective on the future of single entity electrocatalysis with a key focus on what types of measurements present the greatest opportunity for fundamental discovery.

Impact electrochemistry for biosensing: advances and future directions

Impact electrochemistry allows for the investigation of the properties of single entities, ranging from nanoparticles (NPs) to soft bio-particles. It has introduced a novel dimension in the field of biological analysis, enhancing researchers' ability to comprehend biological heterogeneity and offering a new avenue for developing novel diagnostic devices for quantifying biological analytes. This review aims to summarize the recent advancements in impact electrochemistry-based biosensing over the past two to three years and provide insights into the future directions of this field.

Homogeneous and Label-Free Detection and Monitoring of Protein Kinase Activity Using the Impact Electrochemistry of Silver Nanoparticles

Protein kinase activity correlates closely with that of many human diseases. However, the existing methods for quantifying protein kinase activity often suffer from limitations such as low sensitivity, harmful radioactive labels, high cost, and sophisticated detection procedures, underscoring the urgent need for sensitive and rapid detection methods. Herein, we present a simple and sensitive approach for the homogeneous detection of protein kinase activity based on nanoimpact electrochemistry to probe the degree of aggregation of silver nanoparticles (AgNPs) before and after phosphorylation. Phosphorylation, catalyzed by protein kinases, introduces two negative charges into the substrate peptide, leading to alterations in electrostatic interactions between the phosphorylated peptide and the negatively charged AgNPs, which, in turn, affects the aggregation status of AgNPs. Via direct electro-oxidation of AgNPs in nanoimpact electrochemistry experiments, protein kinase activity can be quantified by assessing the impact frequency. The present sensor demonstrates a broad detection range and a low detection limit for protein kinase A (PKA), along with remarkable selectivity. Additionally, it enables monitoring of PKA-catalyzed phosphorylation processes. In contrast to conventional electrochemical sensing methods, this approach avoids the requirement of complex labeling and washing procedures.

Mass Transport and Electron Transfer at the Electrochemical-Confined Interface

Single entity measurements based on the stochastic collision electrochemistry provide a promising and versatile means to study single molecules, single particles, single droplets, etc. Conceptually, mass transport and electron transfer are the two main processes at the electrochemically confined interface that underpin the most transient electrochemical responses resulting from the stochastic and discrete behaviors of single entities at the microscopic scale. This perspective demonstrates how to achieve controllable stochastic collision electrochemistry by effectively altering the two processes. Future challenges and opportunities for stochastic collision electrochemistry are also highlighted.

Influence of Charged Self-Assembled Monolayers on Single Nanoparticle Collision

Studying nanoparticle (NP)-electrode interactions in single nanoparticle collision events is critical to understanding dynamic processes such as nanoparticle motion, adsorption, oxidation, and catalytic activity, which are abundant on electrode surfaces. Herein, NP-electrode electrostatic interactions are studied by tracking the oxidation of AgNPs at Au microelectrodes functionalized with charged self-assembled monolayers (SAMs). Tuning the charge of short alkanethiol-based monolayers and selecting AgNPs that can be partially or fully oxidized upon impact enabled probing the influence of attractive and repulsive NP-electrode electrostatic interactions on collision frequency, electron transfer, and nanoparticle sizing. We find that repulsive electrostatic interactions lead to a significant decrease in collision frequency and erroneous nanoparticle sizing. In stark difference, attractive electrostatic interactions dramatically increase the collision frequency and extend the sizing capability to larger nanoparticle sizes. Thus, these findings demonstrate how NP-monolayer interactions can be studied and manipulated by combining nanoimpact electrochemistry and functionalized SAMs.

Seeing Is Not Believing: Filtering Effects on Random Nature in Electrochemical Measurements of Single-Entity Collision

To clarify the discrete nature of electrochemistry, single-entity electrochemistry of collision (SEEC) utilizes a confinement space in a nanoscale local electric field at a microscale electrode interface for characterizing single freely diffusing entities. This promising method provides new insights at the single entity level. However, the precise measurement is challenging because of the short residence time and wide current fluctuations caused by the dynamic and stochastic motion of a single entity at the interface of the electrode. Moreover, the enormous noise in the electrochemical system would submerge these weak transient electrochemical signals. To increase the signal-to-noise ratio, the low-pass filter (LPF) is often used but at the cost of lower temporal resolution. Therefore, a deeper understanding of the filtering effects on the electrochemical signal is required in SEEC. Here, we build a random walk model to simulate the dynamic electrochemical oxidation of individual silver nanoparticles (AgNPs) in the local electric field near the electrode. This model considers the effect of the effective potential during the interaction between NP and electrode. Results reveal that the shape of the signal is seriously distorted as the cutoff frequency (f _c) of LPF is set at <20 kHz. Due to the filtering effects, hundreds of subpeaks originating from the dynamic motion of NP are merged in a simple peak, which muddies our "believing" from the "seeing" signals. However, the entire interaction time of single NPs with the electrodes can be acquired at f _c ≥ 10 kHz. Moreover, an integral charge of the signal is conserved at any LPF, which enables quantitative analysis of SEEC. Our understanding of the filtering effect on single AgNPs oxidation is generally applicable to nano-electrochemical techniques (e.g., nanopore electrochemistry and nanopipette sensing) that generate transient current signals.

On-Chip Electrokinetic Micropumping for Nanoparticle Impact Electrochemistry

Single-entity electrochemistry is a powerful technique to study the interactions of nanoparticles at the liquid-solid interface. In this work, we exploit Faradaic (background) processes in electrolytes of moderate ionic strength to evoke electrokinetic transport and study its influence on nanoparticle impacts. We implemented an electrode array comprising a macroscopic electrode that surrounds a set of 62 spatially distributed microelectrodes. This configuration allowed us to alter the global electrokinetic transport characteristics by adjusting the potential at the macroscopic electrode, while we concomitantly recorded silver nanoparticle impacts at the microscopic detection electrodes. By focusing on temporal changes of the impact rates, we were able to reveal alterations in the macroscopic particle transport. Our findings indicate a potential-dependent micropumping effect. The highest impact rates were obtained for strongly negative macroelectrode potentials and alkaline solutions, albeit also positive potentials lead to an increase in particle impacts. We explain this finding by reversal of the pumping direction. Variations in the electrolyte composition were shown to play a critical role as the macroelectrode processes can lead to depletion of ions, which influences both the particle oxidation and the reactions that drive the transport. Our study highlights that controlled on-chip micropumping is possible, yet its optimization is not straightforward. Nevertheless, the utilization of electro- and diffusiokinetic transport phenomena might be an appealing strategy to enhance the performance in future impact-based sensing applications.

Prototype Digital Lateral Flow Sensor Using Impact Electrochemistry in a Competitive Binding Assay

This work demonstrates a lateral flow assay concept on the basis of stochastic-impact electrochemistry. To this end, we first elucidate requirements to employ silver nanoparticles as redox-active labels. Then, we present a prototype that utilizes nanoimpacts from biotinylated silver nanoparticles as readouts to detect free biotin in solution based on competitive binding. The detection is performed in a membrane-based microfluidic system, where free biotin and biotinylated particles compete for streptavidin immobilized on embedded latex beads. Excess nanoparticles are then registered downstream at an array of detection electrodes. In this way, we establish a proof of concept that serves as a blueprint for future "digital" lateral flow sensors.

Single-Impact Electrochemistry in Paper-Based Microfluidics

Microfluidic paper-based analytical devices (μPADs) have experienced an unprecedented story of success. In particular, as of today, most people have likely come into contact with one of their two most famous examples─the pregnancy or the SARS-CoV-2 antigen test. However, their sensing performance is constrained by the optical readout of nanoparticle agglomeration, which typically allows only qualitative measurements. In contrast, single-impact electrochemistry offers the possibility to quantify species concentrations beyond the pM range by resolving collisions of individual species on a microelectrode. Within this work, we investigate the integration of stochastic sensing into a μPAD design by combining a wax-patterned microchannel with a microelectrode array to detect silver nanoparticles (AgNPs) by their oxidative dissolution. In doing so, we demonstrate the possibility to resolve individual nanoparticle collisions in a reference-on-chip configuration. To simulate a lateral flow architecture, we flush previously dried AgNPs along a microchannel toward the electrode array, where we are able to record nanoparticle impacts. Consequently, single-impact electrochemistry poses a promising candidate to extend the limits of lateral flow-based sensors beyond current applications toward a fast and reliable detection of very dilute species on site.

Collision, Adhesion, and Oxidation of Single Ag Nanoparticles on a Polysulfide-Modified Microelectrode

We report the collision, adhesion, and oxidation behavior of single silver nanoparticles (Ag NPs) on a polysulfide-modified gold microelectrode. Despite its remarkable success in volume analysis for smaller Ag NPs, the method of NP-collision electrochemistry has failed to analyze particles greater than 50 nm due to uncontrollable collision behavior and incomplete NP oxidation. Herein, we describe the unique capability of an ultrathin polysulfide layer in controlling the collision behavior of Ag NPs by drastically improving their sticking probability on the electrode. The ultrathin sulfurous layer is formed on gold by sodium thiosulfate electro-oxidation and serves both as an adhesive interface for colliding NPs and as a preconcentrated reactive medium to chemically oxidize Ag to form Ag₂S. Rapid particle dissolution is further promoted by the presence of bulk sodium thiosulfate serving as a Lewis base, which drastically improves the solubility of generated Ag₂S by a factor of 10¹³. The combined use of polysulfide and sodium thiosulfate allows us to observe a 25× increase in NP detection frequency, a 3× increase in peak amplitude, and more complete oxidation for larger Ag NPs. By recognizing how volumetric analysis using transmission electron microscopy (TEM) may overestimate quasi-spherical NPs, we believe we can have full NP oxidation for particles up to 100 nm. By focusing on the electrode/solution interface for more effective NP-electrode contact, we expect that the knowledge learned from this study will greatly benefit future NP collision systems for mechanistic studies in single-entity electrochemistry as well as designing ultrasensitive biochemical sensors.

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.

Quantification of Tumor Protein Biomarkers from Lung Patient Serum Using Nanoimpact Electrochemistry

Protein quantification with high throughput and high sensitivity is essential in the early diagnosis and elucidation of molecular mechanisms for many diseases. Conventional approaches for protein assay often suffer from high costs, long analysis time, and insufficient sensitivity. The recently emerged nanoimpact electrochemistry (NIE), as a contrast, allows in situ detection of analytes one at a time with simplicity, fast response, high throughput, and the potential of reducing the detection limits down to the single entity level. Herein, we propose a NIE-enabled electrochemical immunoassay using silver nanoparticles (AgNPs) as labels for the detection of CYFRA21-1, a typical protein marker for lung carcinoma. This strategy is based on the measurement of the impact frequency and the charge intensity of the electrochemical oxidation of individual AgNPs before and after they are modified with anti-CYFRA21-1 and in turn immunocomplexed with CYFRA21-1. Both the frequency and intensity modes of single-nanoparticle electrochemistry correlate well with each other, resulting in a self-validated immunoassay that provides linear ranges of two orders of magnitude and a limit of detection of 0.1 ng/mL for CYFRA21-1 analysis. The proposed immunoassay also exhibits excellent specificity when challenged with other possible interfering proteins. In addition, the CYFRA21-1 content is validated by a conventional, well-known enzyme-linked immunosorbent assay and successfully quantified in a diluted healthy serum with a satisfactory recovery. Moreover, CYFRA21-1 detection in serum samples of lung cancer patients is successfully demonstrated, suggesting the feasibility of the NIE-based immunoassay in clinically relevant diagnosis. To the best of our knowledge, this is the first report to construct NIE-based electrochemical immunoassays for the specific detection of tumor protein biomarkers.

Detection, size characterization and quantification of silver nanoparticles in consumer products by particle collision coulometry

Silver nanoparticles (AgNPs) are widely used in industrial and consumer products owing to its antimicrobial nature and multiple applications. Consequently, their release into the environment is becoming a big concern because of their negative impacts on living organisms. In this work, AgNPs were detected at a potential of + 0.70 V vs. Ag/AgCl reference electrode, characterized, and quantified in consumer products by particle collision coulometry (PCC). The electrochemical results were compared with those measured with electron microscopy and single-particle inductively coupled plasma mass spectrometry. The theoretical and practical peculiarities of the application of PCC technique in the characterization of AgNPs were studied. Reproducible size distributions of the AgNPs were measured in a range 10-100 nm diameters. A power allometric function model was found between the frequency of the AgNPs collisions onto the electrode surface and the number concentration of nanoparticles up to a silver concentration of 10¹⁰ L^-1 (ca. 25 ng L^-1 for 10 nm AgNPs). A linear relationship between the number of collisions and the number concentration of silver nanoparticles was observed up to 5 × 10⁷ L^-1. The PCC method was applied to the quantification and size determination of the AgNPs in three-silver containing consumer products (a natural antibiotic and two food supplements). The mean of the size distributions (of the order 10-20 nm diameters) agrees with those measured by electron microscopy. The areas of current spikes from the chronoamperogram allow the rapid calculation of size distributions of AgNPs that impact onto the working electrode.

Exploring dynamic interactions of single nanoparticles at interfaces for surface-confined electrochemical behavior and size measurement

With the development of new instruments and methodologies, the highly dynamic behaviors of nanoparticle at the liquid-solid interface have been studied. However, the dynamic nature of the electrochemical behavior of individual nanoparticles on the electrode interface is still poorly understood. Here, we generalize scaling relations to predict nanoparticle-electrode interactions by examining the adsorption energy of nanoparticles at an ultramicroelectrode interface. Based on the theoretical predictions, we investigate the interaction-modulated dynamic electrochemical behaviors for the oxidation of individual Ag nanoparticles. Typically, significantly distinct current traces are observed owing to the adsorption-mediated motion of Ag nanoparticles. Inspired by restraining the stochastic paths of particles in the vicinity of the electrode interface to produce surface-confined current traces, we successfully realize high-resolution size measurements of Ag nanoparticles in mixed-sample systems. This work offers a better understanding of dynamic interactions of nanoparticles at the electrochemical interface and displays highly valuable applications of single-entity electrochemistry.

Single-entity electrochemistry has been proposed for studying properties of single nanoparticles (NPs). Here, the authors make use of adsorption-mediated motion of Ag NPs to take individual NP size measurements using electrochemical impacts with excellent agreement to standard imaging techniques.

Single Particle Electrochemistry of Collision

A novel electrochemistry method using stochastic collision of particles at microelectrode to study their performance in single-particle scale has obtained remarkable development in recent years. This convenient and swift analytical method, which can be called "nanoimpact," is focused on the electrochemical process of the single particle rather than in complex ensemble systems. Many researchers have applied this nanoimpact method to investigate various kinds of materials in many research fields, including sensing, electrochemical catalysis, and energy storage. However, the ways how they utilize the method are quite different and the key points can be classified into four sorts: sensing particles at ultralow concentration, theory optimization, kinetics of mediated catalytic reaction, and redox electrochemistry of the particles. This review gives a brief overview of the development of the nanoimpact method from the four aspects in a new perspective.

Single Nanoparticle Electrochemistry

Experimental techniques to monitor and visualize the behaviors of single nanoparticles have not only revealed the significant spatial and temporal heterogeneity of those individuals, which are hidden in ensemble methods, but more importantly, they have also enabled researchers to elucidate the origin of such heterogeneity. In pursuing the intrinsic structure-function relations of single nanoparticles, the recently developed stochastic collision approach demonstrated some early promise. However, it was later realized that the appropriate sizing of a single nanoparticle by an electrochemical method could be far more challenging than initially expected owing to the dynamic motion of nanoparticles in electrolytes and complex charge-transfer characteristics at electrode surfaces. This clearly indicates a strong necessity to integrate single nanoparticle electrochemistry with high-resolution optical microscopy. Hence, this review aims to give a timely update of the latest progress for both electrochemically sensing and seeing single nanoparticles. A major focus is on collision-based measurements, where nanoparticles or single entities in solution impact on a collector electrode and the electrochemical response is recorded. These measurements are further enhanced with optical measurements in parallel. For completeness, advances in other related methods for single nanoparticle electrochemistry are also included.

Silver Nanoparticle Detection in Real-World Environments via Particle Impact Electrochemistry

Silver nanoparticles (AgNPs) suspended in bottled mineral water and in tap water were successfully detected via the nanoimpact method without the deliberate addition of electrolyte. The recorded spike charge was used to indicate the stability of the AgNPs in their suspensions. It is found that the AgNPs largely agglomerated in potable water within the first 20 min. Addition of high concentrations of citrate (>2 mM) improved the stability of the AgNPs and enabled the detection and sizing of the AgNPs monomers in these media. Aging of the potable water suspensions was independently confirmed via UV-vis spectroscopy, validating the electrochemical method for detecting nanoparticles in real-world media.

Size-Selective Electrophoretic Deposition of Gold Nanoparticles Mediated by Hydroquinone Oxidation

Here we describe the size-selective, hydroquinone (HQ)-mediated electrophoretic deposition of 4 and 15 nm diameter citrate-stabilized Au nanoparticles (NPs) onto a glass/indium-tin-oxide (ITO) electrode. Protons liberated from HQ during electrochemical oxidation at the Au NP surface during collisions with the glass/ITO electrode lead to Au NP deposition through neutralization of the citrate stabilizer surrounding the Au NPs, protonation of the glass/ITO electrode, or some combination of the two. Interestingly, the 4 nm Au NPs deposit at about 300-400 mV more negative potential than that of 15 nm diameter Au NPs because of faster HQ oxidation kinetics at the 4 nm NPs, leading to lower overpotentials. This allows for selective deposition of the 4 nm Au NPs over 15 nm Au NPs in a solution containing a mixture of the two by controlling the electrode potential. Controlled pH experiments indicate that significant NP deposition occurs on glass/ITO at a pH of ∼3, giving insight into the local pH needed from HQ oxidation in order to deposit the Au NPs. Experiments performed at different ionic strengths confirm that migration is a major mode of mass transport of the NPs to the glass/ITO. Long deposition times lead to films of densely packed Au NPs that are mostly free from NP-NP contact, indicating that some electrostatic repulsion between the NPs remains during the deposition. This simple method of Au NP deposition may find use for separation of Au NPs and electrode device preparation for a variety of sensor and electrocatalysis applications.

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.

Size Determination of Metal Nanoparticles Based on Electrochemically Measured Surface-Area-to-Volume Ratios

Here we report the electrochemical determination of the surface-area-to-volume ratio (SA/ V) of Au nanospheres (NSs) attached to electrode surfaces for size analysis. The SA is determined by electrochemically measuring the number of coulombs of charge passed during the reduction of surface Au₂O₃ following Au NS oxidation in HClO₄, whereas V is determined by electrochemically measuring the coulombs of charge passed during the complete oxidative dissolution of all of the Au in the Au NSs in the presence of Br^- to form aqueous soluble AuBr₄^-. Assuming a spherical geometry and taking into account the total number of Au NSs on the electrode surface, the SA/ V is theoretically equal to 3/radius. A plot of the electrochemically measured SA/ V versus 1/radius for five different-sized Au NSs is linear with a slope of 1.8 instead of the expected value of 3. Following attachment of the Au NSs to the electrode and ozone treatment, the plot of SA/ V versus 1/radius is linear with a slope of 3.5, and the size based on electrochemistry matches very closely with those measured by scanning electron microscopy. We believe the ozone cleans the Au NS surface, allowing a more accurate measurement of the SA.

A quantitative methodology for the study of particle-electrode impacts

Herein we provide a generic framework for use in the acquisition and analysis of the electrochemical responses of individual nanoparticles, summarising aspects that must be considered to avoid mis-interpretation of data. Specifically, we threefold highlight the importance of the nanoparticle shape, the effect of the nanoparticle diffusion coefficient on the probability of it being observed and the influence of the used measurement bandwidth. Using the oxidation of silver nanoparticles as a model system, it is evidenced that when all of the above have been accounted for, the experimental data is consistent with being associated with the complete oxidation of the nanoparticles (50 nm diameter). The duration of many single nanoparticle events are found to be ca. milliseconds in duration over a range of experiments. Consequently, the insight that the use of lower frequency filtered data yields a more accurate description of the charge passed during a nano-event is likely widely applicable to this class of experiment; thus we report a generic methodology. Conversely, information regarding the dynamics of the nano redox event is obscured when using such lower frequency measurements; hence, both data sets are complementary and are required to provide full insight into the behaviour of the reactions at the nanoscale.

A thermostated cell for electrochemistry: minimising natural convection and investigating the role of evaporation and radiation

An optimised thermostated electrochemical cell is designed and implemented. This is informed by experimental and computational studies characterizing the extent to which the thermostating of an electrochemical cell via a heated bath can be realised, both with the cell closed and open to the environment. The heat transfer in the system is simulated and probed experimentally; special emphasis is put on heat loss due to radiation and evaporation. Experiments and simulations demonstrate that these two mechanisms of heat transfer lead to a steady temperature in the cell that differs from that of the thermostat by ∼0.1 K. Simulations indicate that spatial inhomogeneities in the stationary temperature drive natural convective flows with a significant velocity. These new physical insights inform the optimization of a new electrochemical cell and its application in measurements of the impact frequency of silver nanoparticles as a function of temperature.

Coupled Optical and Electrochemical Probing of Silver Nanoparticle Destruction in a Reaction Layer

The oxidation of silver nanoparticles is induced to occur near to, but not at, an electrode surface. This reaction at a distance from the electrode is studied through the use of dark-field microscopy, allowing individual nanoparticles and their reaction with the electrode product to be visualized. The oxidation product diffuses away from the electrode and oxidizes the nanoparticles in a reaction layer, resulting in their destruction. The kinetics of the silver nanoparticle solution-phase reaction is shown to control the length scale over which the nanoparticles react. In general, the new methodology offers a route by which nanoparticle reactivity can be studied close to an electrode surface.

The fate of silver nanoparticles in authentic human saliva

The physicochemical properties of silver nanoparticles (AgNPs) in human whole saliva are investigated herein. In authentic saliva samples, AgNPs exhibit a great stability with over 70% of the nanomaterial remaining intact after a 24-h incubation in the presence of ∼0.3 mM dissolved oxygen. The small loss of AgNPs from the saliva sample has been demonstrated to be a result of two processes: agglomeration/aggregation (not involving oxygen) and oxidative dissolution of AgNPs (assisted by oxygen). In authentic saliva, AgNPs are also shown to be more inert both chemically (silver oxidative dissolution) and electrochemically (electron transfer at an electrode) than in synthetic saliva or aqueous electrolytes. The results thus predict based on the chemical persistence (over a 24-h study) of AgNPs in saliva and hence the minimal release of hazardous Ag⁺ and reactive oxygen species that the AgNPs are less likely to cause serious harm to the oral cavity but this persistence may enable their transport to other environments.

On-Chip Stochastic Detection of Silver Nanoparticles without a Reference Electrode

We report the electrochemical detection of 20 nm silver nanoparticles at a chip-based microelectrode array (MEA) without the need for a conventional reference electrode. This is possible due to the system's open-circuit potential allowing the oxidation of silver nanoparticles in the presence of phosphate-buffered saline (PBS). The hypothesis is confirmed by modulating the open-circuit potential via addition of ascorbic acid in solution, effectively inhibiting the detection of silver nanoparticle events. Employing the reference-free detection concept, we observe a linear relationship between the nanoparticle impact frequency at the microelectrodes and the nanoparticle concentration. This allows for viable quantification of silver nanoparticle concentrations in situ. The presented concept is ideal for the development of simple lab-on-a-chip or point-of-use systems enabling fast and low-cost screening of nanoparticles.

Single Ag nanoparticle collisions within a dual-electrode micro-gap cell

An adjustable width (between 600 nm and 20 μm) gap between two Au microelectrodes is used to probe the electrodissolution dynamics of single Ag nanoparticles. One Au microelectrode is used to drive the oxidation and subsequent dissolution of a single Ag nanoparticle, which displays a multi-peak oxidation behavior, while a second Au microelectrode is used to collect the Ag+ that is produced. Careful analysis of the high temporal resolution current-time traces reveals capacitive coupling between electrodes due to the sudden injection of Ag+ ions into the gap between the electrodes. The current-time traces allow measurement of the effect of citrate concentration on the electrodissolution dynamics of a single Ag nanoparticle, which reveals that the presence of 2 mM citrate significantly slows down the release of Ag+. Intriguingly, these experiments also reveal that only a portion (ca. 50%) of the oxidized Ag nanoparticle is released as free Ag+ regardless of citrate concentration.