Electrocatalytic Amplification of Single Nanoparticle Collisions Using DNA-Modified Surfaces

Artificial Intelligence-Assisted Multiparameter Size Discrimination of Silver Nanoparticles through Electrochemical Collision

Single particle collision is an important tool for size analysis at the individual particle level; however, due to complex dynamic behaviors of nanoparticles on the surface of an electrode, the accuracy of size discrimination is limited. A silver (Ag) nanoparticle (NP) was chosen as the research target, and the dynamic behavior of Ag NPs was simplified by enhancing adsorption between Ag NP and Au ultramicroelectrode (UME) in alkaline media. Immediately after, accurate dynamic and thermodynamic information on single Ag NP was accurately extracted from collision events, including current intensity, transferred charge, and duration time. On the basis that there were differences between parameters of different-sized Ag NPs, multiparameter size discrimination was proposed, which improved the accuracy compared to single-parameter discrimination. More intriguingly, multiparameter analysis was combined with artificial intelligence, a tool adept at processing multidimensional data, for the first time. Finally, artificial intelligence-assisted multiparameter size discrimination was successfully used to intelligently distinguish mixed Ag NPs, with an optimal accuracy of more than 95%. To sum up, the artificial intelligence-assisted multiparameter method showed an excellent ability to quickly achieve the most accurate size discrimination of nanoparticles at the level of individual particle and provide an effective guidance for the application of nanoparticles.

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.

Electrocatalytic Activities of a Platinum Nanostructured Electrode Modified by Gold Adatom toward Methanol and Glycerol Electrooxidation in Acid and Alkaline Media

For practical applications such as fuel cells, it is important to exploit electrocatalysis with high activity for methanol and glycerol oxidation. A platinum nanostructured electrode (PtNPs) is modified by gold adatoms and is created by application of a square wave potential regime to a tantalum surface electrode. In nanostructured platinum, the structure and the surface properties are characterized by scanning electron microscopy (SEM), X-ray powder diffraction (XRD) and cyclic voltammetry (CV). In acid and alkaline media, the CV and Chronoamperometric (CA) are studied to investigate the catalytic activity of the PtNPs nanoparticles for the electrooxidation of methanol and glycerol. The prepared nanostructured platinum on a tantalum electrode was allowed to balance an open circuit with a 1.0×10^-3 M solution containing an Au ion. Consequently, the proximity of the irreversibly adsorbed Au-adatoms on the already described Pt-nanostructured electrode. In acidic and alkaline solutions, the electrocatalytically activities toward methanol and glycerol oxidation were evaluated and is found to strongly on the surface of the gold-modified PtNPs. The PtNPs modified by Au electrode system used direct methanol fuel cell (DMFC) and direct glycerol fuel cell (DGFC). The DMFC and DGFC are much higher than in acid output in alkaline. Comparison of the i-E curves of nanostructure platinum electrode with that of a platinum nanostructure electrode modified by Au under similar conditions for the letter, the charge under the peak (i-E curve) in the oxidation region was higher. Furthermore, rough chronoamperometric measurements confirmed the results. The results of showed that the electrocatalytic properties of the nanostructured prepared surface were enhanced by the inclusion of gold adatoms with a variable extent of advancement. The current peak (I_p) and the current chronoamperometric (I_CA) of glycerol oxidation on the PtNPs electrode modified by Au in acid media (130 mA/cm², 47 µA/cm²) were higher than those of the bare PtNPs electrode and in alkaline media (171 mA/cm², 66 µA/cm²). The stronger catalytic behavior in alkaline media of the Au-PtNP electrode indicates its promising use in alkaline direct alcohol cells.

Stochastic Collision Electrochemistry from Single Pt Nanoparticles: Electrocatalytic Amplification and MicroRNA Sensing

Single-particle collisions have made many achievements in basic research, but challenges still exist due to their low collision frequency and selectivity in complex samples. In this work, we developed an "on-off-on" strategy based on Pt nanoparticles (PtNPs) that catalyze N₂H₄ collision signals on the surface of carbon ultramicroelectrodes and established a new method for the detection of miRNA21 with high selectivity and sensitivity. PtNPs catalyze the reduction of N₂H₄ on the surface of carbon ultramicroelectrodes to generate a stepped collision signal, which is in the "on" state. The single-stranded DNA paired with miRNA21 is coupled with PtNPs to form the complex DNA/PtNPs. Because PtNPs are covered by DNA, the electrocatalytic collision of N₂H₄ oxidation is inhibited. At this time, the signal is in the "off" state. When miRNA21 is added, the strong complementary pairing between miRNA21 and DNA destroys the electrostatic adsorption of DNA/PtNP conjugates and restores the electrocatalytic performance of PtNPs, and the signal is in the "on" state again. Based on this, a new method for detecting miRNA21 was established. It provides a new way for small-molecule sensing and has a wide range of applications in electroanalysis, electrocatalysis, and biosensing.

Faradaic Counter for Liposomes Loaded with Potassium, Sodium Ions, or Protonated Dopamine

Collisional electrochemistry between single particles and a biomimetic polarized micro-liquid/liquid interface has emerged as a novel and powerful analytical method for measurements of single particles. Using this platform, rapid detection of liposomes at the single particle level is reported herein. Individual potassium, sodium, or protonated dopamine-encapsulated (pristine or protein-decorated) liposomes collide and fuse with the polarized micro-liquid/liquid interface accompanying the release of ions, which are recorded as spike-like current transients of stochastic nature. The sizing and concentration of the liposomes can be readily estimated by quantifying the amount of encapsulated ions in individual liposomes via integrating each current spike versus time and the spike frequency, respectively. We call this type of nanosensing technology "Faradaic counter". The estimated liposome size distribution by this method is in line with the dynamic light scattering (DLS) measurements, implying that the quantized current spikes are indeed caused by the collisions of individual liposomes. The reported electrochemical sensing technology may become a viable alternative to DLS and other commercial nanoparticle analysis systems, for example, nanoparticle tracking analysis.

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.

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.

Advanced Nanoscale Approaches to Single-(Bio)entity Sensing and Imaging

Individual (bio)chemical entities could show a very heterogeneous behaviour under the same conditions that could be relevant in many biological processes of significance in the life sciences. Conventional detection approaches are only able to detect the average response of an ensemble of entities and assume that all entities are identical. From this perspective, important information about the heterogeneities or rare (stochastic) events happening in individual entities would remain unseen. Some nanoscale tools present interesting physicochemical properties that enable the possibility to detect systems at the single-entity level, acquiring richer information than conventional methods. In this review, we introduce the foundations and the latest advances of several nanoscale approaches to sensing and imaging individual (bio)entities using nanoprobes, nanopores, nanoimpacts, nanoplasmonics and nanomachines. Several (bio)entities such as cells, proteins, nucleic acids, vesicles and viruses are specifically considered. These nanoscale approaches provide a wide and complete toolbox for the study of many biological systems at the single-entity level.

Lithium-Ion-Transfer Kinetics of Single LiFePO4 Particles

The Li-ion desertion process of a single LiFePO₄ particle with a diameter of 400 nm in aqueous media at high potential is investigated by stochastic collision of the particle at a microelectrode. No extra additive, such as polymeric binder or conductive carbon, is involved in this stochastic measurement. The ion diffusion inside the particle is proved to be the rate-determining step that limits the discharge rate of batteries using LiFePO₄ in an aqueous environment. This result offers guidance for exploring the most effective enhancement of Li-ion batteries. Moreover, this general method can be applied to study the electrochemical behavior of other electrode materials as an alternative and complementary route.

Programmable DNA switches and their applications

DNA switches are ideally suited for numerous nanotechnological applications, and increasing efforts are being directed toward their engineering. In this review, we discuss how to engineer these switches starting from the selection of a specific DNA-based recognition element, to its adaptation and optimisation into a switch, with applications ranging from sensing to drug delivery, smart materials, molecular transporters, logic gates and others. We provide many examples showcasing their high programmability and recent advances towards their real life applications. We conclude with a short perspective on this exciting emerging field.

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.

Fluorescence Electrochemical Microscopy: Capping Agent Effects with Ethidium Bromide/DNA Capped Silver Nanoparticles

Fluorescence microscopy and electrochemistry were employed to examine capping agent dynamics in silver nanoparticles capped with DNA intercalated with ethidium bromide, a fluorescent molecule. The capped NPs were studied first electrochemically, demonstrating that the intercalation of the capping agent promotes oxidation of the silver core, occurring at 0.50 V (vs. Ag, compared with 1.15 V for Ag NPs capped in DNA alone). Second, fluorescence electrochemical microscopy revealed that the electron transfer from the nanoparticles is gated by the capping agent, allowing dynamic insights unobservable using electrochemistry alone.

Imaging Dynamic Collision and Oxidation of Single Silver Nanoparticles at the Electrode/Solution Interface

The electrochemical interface is an ultrathin interfacial region between the electrode surface and the electrolyte solution and is often characterized by numerous dynamic processes, such as solvation and desolvation, heterogeneous electron transfer, molecular adsorption and desorption, diffusion, and surface rearrangement. Many of these processes are driven and modulated by the presence of a large interfacial potential gradient. The study and better understanding of the electrochemical interface is important for designing better electrochemical systems where their applications may include batteries, fuel cells, electrocatalytic water splitting, corrosion protection, and electroplating. This, however, has proved to be a challenging analytical task due to the ultracompact and dynamic evolving nature of the electrochemical interface. Here, we describe the use of an electrochemical nanocell to image the dynamic collision and oxidation process of single silver nanoparticles at the surface of a platinum nanoelectrode. A nanocell is prepared by depositing a platinum nanoparticle at the tip of a quartz nanopipette forming a bipolar nanoelectrode. The compact size of the nanocell confines the motion of the silver nanoparticle in a 1-D space. The highly dynamic process of nanoparticle collision and oxidation is imaged by single-particle fluorescence microscopy. Our results demonstrate that silver nanoparticle collision and oxidation is highly dynamic and likely controlled by a strong electrostatic effect at the electrode/solution interface. We believe that the use of a platinum nanocell and single molecule/nanoparticle fluorescence microscopy can be extended to other systems to yield highly dynamic information about the electrochemical interface.

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.

Observation of Multipeak Collision Behavior during the Electro-Oxidation of Single Ag Nanoparticles

The dynamic collision behavior of the electro-oxidation of single Ag nanoparticles is observed at Au microelectrodes using stochastic single-nanoparticle collision amperometry. Results show that an Ag nanoparticle collision/oxidation event typically consists of a series of 1 to ∼10 discrete "sub-events" over an ∼20 ms interval. Results also show that the Ag nanoparticles typically undergo only partial oxidation prior to diffusing away from the Au electrode into the bulk solution. Both behaviors are characterized and shown to exist under a variety of experimental conditions. These previously unreported behaviors suggest that nanoparticle collision and electro-dissolution is a highly dynamic process driven by fast particle-electrode interactions and nanoparticle diffusion.

Single-Nanoparticle Electrochemistry through Immobilization and Collision

Metal nanoparticles are key electrode materials in a variety of electrochemical applications including basic electron-transfer study, electrochemical sensing, and electrochemical surface enhanced Raman spectroscopy (SERS). Metal nanoparticles have also been extensively applied to electrocatalytic processes in recent years due to their high catalytic activity and large surface areas. Because the catalytic activity of metal nanoparticle is often highly dependent on their size, shape, surface ligands, and so forth, methods for examining and better understanding the correlation between particle structure and function are of great utility in the development of more efficient catalytic systems. Despite considerable progress in this field, the understanding of the structure-activity relationships remains challenging in nanoparticle-based electrochemistry and electrocatalysis due to limitations associated with traditional ensemble measurements. One of the major issues is the ensemble averaging of the electrocatalytic response which occurs over a very large number of nanoparticles of various sizes and shapes. Additionally, the electrochemical response can also be greatly affected by properties of the ensemble itself, such as the particle spacing. The ability to directly measure kinetics of electrochemical reactions at structurally well-characterized single nanoparticles opens up new possibilities in many important areas including nanoscale electrochemistry, electrochemical sensing, and nanoparticle electrocatalysis. When a macroscopic electrode is placed in a solution containing redox molecules and metal nanoparticles, random collision and adsorption of nanoparticles occurs at the electrode surface in addition to redox reactions when a suitable potential is present on the electrode. In a special case where particles are catalytically more active than the substrate, the faradaic signals can be greatly amplified on particle surfaces and a steady shift in the baseline current would be expected due to many particles adsorbing on the electrode. Single particle events can be temporally resolved when an ultramicroelectrode (UME) is used as the recording electrode. The use of an UME not only reduces the collision frequency, but also greatly decreases baseline noise, thereby resulting in clear resolution of single collision events. Single particle collision has quickly grown into a popular electroanalytical technique in recent years. Alternatively, one can use nanoelectrodes to immobilize single nanoparticles so that they can be individually studied in electrochemistry and electrocatalysis. Nanoparticle immobilization also allows one to obtain detailed structural information on the same particles and offers enormous potential for developing more comprehensive understanding of the structure-function relationship in nanoparticle-based electrocatalysts. This Account summarizes recent electrochemical experiments of single metal nanoparticles which have been performed by our group using both of these schemes.

Enzyme-free electrochemical immunosensor based on methylene blue and the electro-oxidation of hydrazine on Pt nanoparticles

Enzyme-free electrochemical sensors enable rapid, high sensitivity measurements without the limitations associated with enzyme reporters. However, the performance of non-enzymatic electrochemical sensors tends to suffer from slow electrode kinetics and poor signal stability. We report a new enzyme-free electrochemical immunosensor based on a unique competitive detection scheme using methylene blue (MB), hydrazine and platinum nanoparticles (Pt NPs). This scheme is coupled with a robust immunosandwich format employing a MB-labelled detection antibody as a non-enzymatic reporter. In the presence of the target antigen, surface-immobilized MB consumes interfacial hydrazine thereby diminishing the electro-oxidation of hydrazine on Pt NPs. Thus, the concentration of the antigen is directly proportional to the reduction in the electrochemical signal. For proof-of-concept, this sensor was used to detect Plasmodium falciparum histidine-rich protein 2 (PfHRP2), an important malaria biomarker, in unadulterated human saliva samples. Chronocoulometric measurements showed that this platform exhibits pM-range sensitivity, high specificity and good reproducibility, making it well suited for many biosensing applications including noninvasive diagnostic testing.

Electrocatalytic amplification of DNA-modified nanoparticle collisions via enzymatic digestion

We report a new and general approach that will be useful for adapting the method of electrocatalytic amplification (ECA) to biosensing applications. In ECA, individual collisions of catalytic nanoparticles with a noncatalytic electrode surface lead to bursts of current. In the work described here, the current arises from catalytic electrooxidation of N2H4 at the surface of platinum nanoparticles (PtNPs). The problem with using ECA for biosensing applications heretofore, is that it is necessary to immobilize a receptor, such as DNA (as in the case here) or an antibody on the PtNP surface. This inactivates the colliding NP, however, and leads to very small collision signatures. In the present article, we show that single-stranded DNA (ssDNA) present on the PtNP surface can be detected by selectively removing a fraction of the ssDNA using the enzyme Exonuclease I (Exo I). About half of the current associated with collisions of naked PtNPs can be recovered from fully passivated PtNPs after exposure to Exo I. Experiments carried out using both Au and Hg ultramicroelectrodes reveal some mechanistic aspects of the collision process before and after treatment of the ssDNA-modified PtNPs with Exo I.

Biomolecular detection at ssDNA-conjugated nanoparticles by nano-impact electrochemistry

We describe the use of ssDNA functionalized silver nanoparticle (AgNP) probes for quantitative investigation of biorecognition and real time detection of biomolecular targets using nano-impact electrochemistry. The method is based on measurements of the individual collision events between ssDNA aptamer-functionalized AgNPs and a carbon fiber miroelectrode (CFME). Specific binding events of target analyte induced collision frequency changes enabling ultrasensitive detection of the aptamer target in a single step. These changes are assigned to the surface coverage of the NP by the ssDNA aptamers and subsequent conformational changes of the aptamer probe which affect the electron transfer between the NP and the electrode surface. The method enables sensitive and selective detection of ochratoxin A (OTA), chosen here as a model target, with a limit of detection of 0.05nM and a relative standard deviation of 4.9%. The study provides a means of characterizing bioconjugation of AgNPs with aptamers and assessing biomolecular recognition events with high sensitivity and without the use of exogenous reagents or enzyme amplification steps. This methodology can be broadly applicable to other bioconjugated systems, biosensing and related bioanalytical applications.

Addressing Colloidal Stability for Unambiguous Electroanalysis of Single Nanoparticle Impacts

Herein the problem of colloidal instability on electrochemically detected nanoparticle (NP) collisions with a Hg ultramicroelectrode (UME) by electrocatalytic amplification is addressed. NP tracking analysis (NTA) shows that rapid aggregation occurs in solution after diluting citrate-stabilized Pt NPs with hydrazine/phosphate buffers of net ionic strength greater than 70 mM. Colloidal stability improves by lowering the ionic strength, indicating that aggregation processes were strongly affected by charge screening of the NP double layer interactions at high cation concentrations. For the system of lowest ionic strength, the overwhelming majority of observed electrocatalytic current signals represent single NP/electrode impacts, as confirmed by NTA kinetic monitoring. NP diffusion coefficients determined by NTA and NP impact electroanalysis are in excellent agreement for the stable colloids, which signifies that the sticking probability of Pt NPs interacting with Hg is unity and that the observed NP impact rate agrees with the expected steady-state diffusive flux expression for the spherical cap Hg UME.

Mechanistic aspects of hydrazine-induced Pt colloid instability and monitoring aggregation kinetics with nanoparticle impact electroanalysis

Here we investigate the mechanistic aspects of Pt nanoparticle (NP) aggregation in solutions typically used for detecting NP/electrode impacts by electrocatalytic amplification (ECA). We previously proposed a general mechanism for Pt colloid destabilization that involved the participation of both the hydrazine redox probe and the pH buffer species as coagulants. Herein the Pt NP coagulation and aggregation mechanisms were further investigated with microscopic kinetic NP concentration monitoring and zeta potential measurements using nanoparticle tracking analysis (NTA), as well as open circuit potential experiments with a citrate-treated polycrystalline Pt surface to assess electrical double layer potential. After considering the combined results of these experiments we propose that the colloidal stability of citrate-capped platinum nanoparticles involves much more than the typical physicochemical interactions predicted by DLVO theory. A structure based on intermolecular H-bonding in the citrate capping layer is the most plausible explanation for the exceptional stability of large Pt NPs in high ionic strength buffers. Thus, the mechanism of Pt NP aggregation includes specific reactive contributions from hydrazine. The catalytic decomposition of hydrazine, in particular, is thought to occur to some extent at the citrate-coated Pt surface while the citrate remains adsorbed. Evolved gases such as ammonia and possible surface bound intermediates from Pt-catalyzed decomposition of hydrazine may disrupt the stability of the citrate layer, causing colloidal instability and thus promoting Pt NP coagulation. In the closing section, we demonstrate nanoparticle impact electroanalysis by ECA detection as a method to quantify Pt NP concentration with adequate time resolution for monitoring the kinetics of Pt NP coagulation.

Concluding remarks: single entity electrochemistry one step at a time

This article provides a summary of the Faraday Discussion on single entity electrochemistry held in York, U.K., in early September, 2016. The introduction provides some context for thinking about electrochemical studies of single entities. The next four sections follow the themes of the meeting as they relate to single-entity electrochemistry: (1) nanoparticles, nanotubes, and nanowires; (2) nanopores and nanofluidics; (3) complex surfaces and reactions at the nanoscale; and (4) molecular electroanalysis. Each paper presented at the Discussion is summarized, and some personal thoughts as to the significance of the findings, the technical advances that made the experiments possible, and common themes between articles are interspersed. Finally, at the end, I round-up my impressions and provide them in a succinct list.