Ultrasensitive Electroanalysis: Femtomolar Determination of Lead, Cobalt, and Nickel

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.

Electrochemical-Shock Synthesis of Nanoparticles from Sub-femtoliter Nanodroplets

ConspectusNanoparticles have witnessed immense development in the past several decades due to their intriguing physicochemical properties. The modern chemist is interested not only in methods of synthesizing nanoparticles with tunable properties but also in the chemistry that nanoparticles can drive. While several methods exist to synthesize nanoparticles, it is often advantageous to put nanoparticles on a variety of conductive substrates for multiple applications (such as energy storage and conversion). Despite enjoying over 200 years of development, electrodeposition of nanoparticles suffers from a lack of control over nanoparticle size and morphology. There have been heroic efforts to address these issues over time. With an understanding that structure-function studies are imperative to understand the chemistry of nanoparticles, new methods are necessary to electrodeposit a variety of nanoparticles with control over macromorphology and also microstructure.This Account details our group's efforts in overcoming challenges of classical nanoparticle electrodeposition by electrodepositing nanoparticles from water nanodroplets. When a nanodroplet full of metal salt precursor is incident on the electrode biased sufficiently negative to drive electroplating, nanoparticles form at a fast rate (on the order of microseconds to milliseconds). We start with the general nuts-and-bolts of the experiment (nanodroplet formation and methods for electrodeposition). The deposition of new nanomaterials often requires one to develop new methods of measurement, and we detail new measurement tools for quantifying nanoparticle porosity and nanopore tortuosity within single nanoparticles. We achieve nanopore characterization by using Focused Ion Beam milling and Scanning Electron Microscopy. Owing to the small size of the nanodroplets and fast mass transfer (the contents of a femtoliter droplet can be electrolyzed in only a few milliseconds), the use of nanodroplets also allows the electrodeposition of high entropy alloy nanoparticles at room temperature.We detail how a deep understanding of ion transfer mechanisms can be used to expand the library of possible metals that can be deposited. Furthermore, simple ion changes in the dispersed droplet phase can decrease the cost per experiment by orders of magnitude. Finally, electrodeposition in aqueous nanodroplets can also be combined with stochastic electrochemistry for a variety of interesting studies. We detail the quantification of the growth kinetics of single nanoparticles in single aqueous nanodroplets. Nanodroplets can also be used as tiny reactors to trap only a few molecules of a metal salt precursor. Upon reduction to the zerovalent metal, electrocatalysis at very small metal clusters can be probed and evaluated with time using steady-state electrochemical measurements. Overall, this burgeoning synthetic tool is providing unexpected avenues of tunability of metal nanoparticles on conductive substrates.

High-Spatiotemporal-Resolution Electrochemical Measurements of Electrocatalytic Reactivity

The real-time measurement of the individual or local electrocatalytic reactivity of catalyst particles instead of ensemble behavior is considerably challenging but very critical to uncover fundamental insights into catalytic mechanisms. Recent remarkable efforts have been made to the development of high-spatiotemporal-resolution electrochemical techniques, which allow the imaging of the topography and reactivity of fast electron-transfer processes at the nanoscale. This Perspective summarizes emerging powerful electrochemical measurement techniques for studying various electrocatalytic reactions on different types of catalysts. Principles of scanning electrochemical microscopy, scanning electrochemical cell microscopy, single-entity measurement, and molecular probing technique have been discussed for the purpose of measuring important parameters in electrocatalysis. We further demonstrate recent advances in these techniques that reveal quantitative information about the thermodynamic and kinetic properties of catalysts for various electrocatalytic reactions associated with our perspectives. Future research on the next-generation electrochemical techniques is anticipated to be focused on the development of instrumentation, correlative multimodal techniques, and new applications, thus enabling new opportunities for elucidating structure-reactivity relationships and dynamic information at the single active-site level.

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.

Tracking the Electrocatalytic Activity of a Single Palladium Nanoparticle for the Hydrogen Evolution Reaction

The nanoparticle-based electrocatalysts' performance is directly related to their working conditions. In general, a number of nanoparticles are uncontrollably fixed on a millimetre-sized electrode for electrochemical measurements. However, it is hard to reveal the maximum electrocatalytic activity owing to the aggregation and detachment of nanoparticles on the electrode surface. To solve this problem, here, we take the hydrogen evolution reaction (HER) catalyzed by palladium nanoparticles (Pd NPs) as a model system to track the electrocatalytic activity of single Pd NPs by stochastic collision electrochemistry and ensemble electrochemistry, respectively. Compared with the nanoparticle fixed working condition, Pd NPs in the nanoparticle diffused working condition results in a 2-5 orders magnitude enhancement of electrocatalytic activity for HER at various bias potential. Stochastic collision electrochemistry with high temporal resolution gives further insights into the accurate study of NPs' electrocatalytic performance, enabling to dramatically enhance electrocatalytic efficiency.

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.

Nanoscale electrochemical kinetics & dynamics: the challenges and opportunities of single-entity measurements

The development of nanoscale electrochemistry since the mid-1980s has been predominately coupled with steady-state voltammetric (i-E) methods. This research has been driven by the desire to understand the mechanisms of very fast electrochemical reactions, by electroanalytical measurements in small volumes and unusual media, including in vivo measurements, and by research on correlating electrocatalytic activity, e.g., O2 reduction reaction, with nanoparticle size and structure. Exploration of the behavior of nanoelectrochemical structures (nanoelectrodes, nanoparticles, nanogap cells, etc.) of a characteristic dimension λ using steady-state i-E methods generally relies on the well-known relationship, λ2 ∼ Dt, which relates diffusional lengths to time, t, through the coefficient, D. Decreasing λ, by performing measurements at a nanometric length scales, results in a decrease in the effective timescale of the measurement, and provides a direct means to probe the kinetics of steps associated with very rapid electrochemical reactions. For instance, steady-state voltammetry using a nanogap twin-electrode cell of characteristic width, λ ∼ 10 nm, allows investigations of events occurring at timescales on the order of ∼100 ns. Among many other advantages, decreasing λ also increases spatial resolution in electrochemical imaging, e.g., in scanning electrochemical microscopy, and allows probing of the electric double layer. This Introductory Lecture traces the evolution and driving forces behind the "λ2 ∼ Dt" steady-state approach to nanoscale electrochemistry, beginning in the late 1950s with the introduction of the rotating ring-disk electrode and twin-electrode thin-layer cells, and evolving to current-day investigations using nanoelectrodes, scanning nanocells for imaging, nanopores, and nanoparticles. The recent focus on so-called "single-entity" electrochemistry, in which individual and very short redox events are probed, is a significant departure from the steady-state approach, but provides new opportunities to probe reaction dynamics. The stochastic nature of very fast single-entity events challenges current electrochemical methods and modern electronics, as illustrated using recent experiments from the authors' laboratory.

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.

Collision of Aptamer/Pt Nanoparticles Enables Label-Free Amperometric Detection of Protein in Rat Brain

Single particle collision is emerging as a powerful and sensitive technique for analyzing small molecules, however, its application in biomacromolecules detection, for example, protein, in complex biological environments is still challenging. Here, we present the first demonstration on the single particle collision that can be developed for the detection of platelet-derived growth factor (PDGF), an important protein involved in the central nervous system in living rat brain. The system features Pt nanoparticles (PtNPs) conjugated with the PDGF recognition aptamer, suppressing the electrocatalytic collision of PtNPs toward the oxidation of hydrazine. In the presence of PDGF, the stronger binding between targeted protein and the aptamer disrupts the aptamer/PtNPs conjugates, recovering the electrocatalytic performance of PtNPs, and allowing quantitative, selective, and highly sensitive detection of PDGF in cerebrospinal fluid of rat brain.