Nitric oxide (NO) has been implicated in a variety of physiological and pathological processes. Monitoring cellular levels of NO requires a sensor to feature adequate sensitivity, transient recording ability and biocompatibility. Herein we report a single-atom catalysts (SACs)-based electrochemical sensor for the detection of NO in live cellular environment. The system employs nickel single atoms anchored on N-doped hollow carbon spheres (Ni SACs/N-C) that act as an excellent catalyst for electrochemical oxidation of NO. Notably, Ni SACs/N-C shows superior electrocatalytic performance to the commonly used Ni based nanomaterials, attributing from the greatly reduced Gibbs free energy that are required for Ni SACs/N-C in activating NO oxidation. Moreover, Ni SACs-based flexible and stretchable sensor shows high biocompatibility and low nanomolar sensitivity, enabling the real-time monitoring of NO release from cells upon drug and stretch stimulation. Our results demonstrate a promising means of using SACs for electrochemical sensing applications.

We describe a method for the electrodeposition of an isolated single Pt atom or small cluster, up to 9 atoms, on a bismuth ultramicroelectrode (UME). This deposition was immediately followed by electrochemical characterization via the hydrogen evolution reaction (HER) that occurs readily on the electrodeposited Pt but not on Bi. The observed voltammetric current plateau, even for a single atom, which behaves as an electrode, allows the estimation of deposit size. Pt was plated from solutions of femtomolar PtCl, which allowed precise control of the arrival of ions and thus the plating rate on the Bi UME, to one ion every few seconds. This allowed the atom-by-atom fabrication of isolated platinum deposits, ranging from single atoms to 9-atom clusters. The limiting currents in voltammetry gave the size and number of atoms of the clusters. Given the stochasticity of the plating process, we show that the number of atoms plated over a given time (10 and 20 s) follows a Poisson distribution. Taking the potential at a certain current density as a measure of the relative rate of the HER, we found that the potential shifted positively as the size increased, with single atoms showing the largest overpotentials compared to bulk Pt.

Electrochemistry at metal nanoparticles (NPs) is of significant current interest because of its applications in catalysis, energy conversion and storage, and sensors. The electrocatalytic activity of NPs depends strongly on their size, shape, and surface attachment. The use of a large number of particles in most reported kinetic experiments obscured the effects of these factors because of polydispersity and different NP orientations. Recent efforts to probe electrochemistry at single NPs included recording of the catalytically amplified current produced by random collisions of particles with the electrode surface, immobilizing an NP on the surface of a small electrode, and delivering individual NPs to electrode surfaces. Although the signals recorded in such experiments were produced by single NPs, the characterization issues and problems with separating an individual particle from other NPs present in the system made it difficult to obtain spatially and/or temporally resolved information about heterogeneous processes occurring at a specific NP. To carry out electrochemical experiments involving only one NP and characterize such an NP in situ, one needs nanoelectrochemical tools with the characteristic dimension smaller than or comparable to those of the particle of interest. This Account presents fundamentals of two complementary approaches to studying NP electrochemistry, i.e., probing single immobilized NPs with the tip of a scanning electrochemical microscope (SECM) and monitoring the collisions between one catalytic NP and a carbon nanopipette. The former technique can provide spatially resolved information about NP geometry and measure its electron transfer properties and catalytic activity under steady-state conditions. The emphasis here is on the extraction of quantitative physicochemical information from nanoelectrochemical data. By employing a polished disk-type nanoelectrode as an SECM tip, one can characterize a specific nanoparticle in situ and then use the same NP for kinetic experiments. A new mode of SECM operation based on tunneling between the tip and nanoparticle can be used to image the NP topography with a lateral resolution of ∼1 nm. An alternative approach employs carbon nanoprobes produced by chemical vapor deposition of carbon into quartz nanopipettes. One metal NP is captured inside the carbon nanocavity to probe the dynamics of its interactions with the electrode surface on the microsecond time scale. The use of high-resolution transmission electron microscopy is essential for interpreting the results of single-NP collision experiments. A brief discussion of the nanoelectrochemical methodology, recent advances, and future directions is included.

We report the catalytic activity of a single, isolated Pt deposit on Bi and Pb supports to probe the size and substrate effects on the electrochemical hydrogen evolution reaction (HER). Deposits were made electrolytically by an atom-by-atom method in a controlled plating; we prepared an individual Pt deposit on Bi and Pb ultramicroelectrodes (UMEs) such as a single isolated atom, clusters containing one to five Pt atoms, and nanoparticles to about 10 nm radius. A steady-state voltammogram on the single Pt deposits is observed by electrocatalytic amplification of the HER, with a negligible contribution by the HER at the substrate UME. A single Pt atom can act as an electrode for the HER, showing a diffusion-limiting current plateau in the voltammogram that can be used to estimate the radius of a single deposit. We simulated the voltammograms of the individual deposits, assuming the Volmer step of the HER is appropriate for a Pt cluster deposit, to obtain kinetic parameters for each deposit. The HER kinetics increases as the particle radius increases from ∼0.2 to ∼4 nm for Bi and Pb substrates and then reaches a limiting plateau. The limiting kinetics on the Bi substrate approaches that of bulk Pt while that on the Pb substrate is much smaller.

The electrocatalytic reduction of CO has been studied extensively and produces a number of products. The initial reaction in the CO reduction is often taken to be the 1e formation of the radical anion, CO. However, the electrochemical detection and characterization of CO is challenging because of the short lifetime of CO, which can dimerize and react with proton donors and even mild oxidants. Here, we report the generation and quantitative determination of CO in N,N-dimethylformamide (DMF) with the tip generation/substrate collection (TG/SC) mode of scanning electrochemical microscopy (SECM). CO was reduced at a hemisphere-shaped Hg/Pt ultramicroelectrode (UME) or a Hg/Au film UME, which were utilized as the SECM tips. The CO produced can either dimerize to form oxalate within the nanogap between SECM tip and substrate or collected at SECM substrate (e.g., an Au UME). The collection efficiency (CE) for CO depends on the distance (d) between the tip and substrate. The dimerization rate (6.0 × 10 M s) and half-life (10 ns) of CO can be evaluated by fitting the collection efficiency vs distance curve. The dimerized species of CO, oxalate, can also be determined quantitatively. Furthermore, the formal potential (E') and heterogeneous rate constant (k) for CO reduction were determined with different quaternary ammonium electrolytes. The significant difference in k is due to a tunneling effect caused by the adsorption of the electrolytes on the electrode surface at negative potentials.

for electron and charge transfer. Therefore, the OV-T n QDs@ PCN/S cathode delivers a superb long-term cycling stability (88% capacity retention over 1000 cycles at 2C) under a S-mass loading of 2.2 mg cm −1 and an E/S ratio of 10 µL mg −1 . In addition, the cathode exhibits good Li + storage at high S-mass loading (4.8 mg cm −1 ) and lean electrolyte (E/S ratio: 4.5 µL mg −1 ), demonstrating its great potential for practical implementation. Our strategy may be extended to other MXenes (e.g., Ti 3 CNT x , Nb 2 CT x , and V 2 CT x ) and pave the way to realize the facile synthesis of QDs with rich OVs for advanced Li-S batteries.

Development of better catalysts for the oxygen reduction reaction (ORR) and other electrocatalytic processes requires detailed knowledge of reaction pathways and intermediate species. Here we report a new methodology for detecting charged reactive intermediates and its application to the mechanistic analysis of ORR. A nanopipette filled with an organic phase that is immiscible with the external aqueous solution was used as a tip in the scanning electrochemical microscope to detect and identify a short-lived superoxide (O2(●-)) intermediate and to determine the rate of its generation at the catalytic Pt substrate and its lifetime in neutral aqueous solution. The voltammogram of the O2(●-) anion transfer to the organic phase provides a unique signature for unambiguous identification of superoxide. The extremely short attainable separation distance between the pipette tip and substrate surface (∼1 nm) makes this technique suitable for detecting and identifying charged intermediates of catalytic processes with a lifetime of a few nanoseconds.

Investigating the collisions of individual metal nanoparticles (NPs) with electrodes can provide new insights into their electrocatalytic behavior, mass transport, and interactions with surfaces. Here we report a new experimental setup for studying NP collisions based on the use of carbon nanopipettes to enable monitoring multiple collision events involving the same NP captured inside the pipet cavity. A patch clamp amplifier capable of measuring pA-range currents on the microsecond time scale with a very low noise and stable background was used to record the collision transients. The analysis of current transients produced by oxidation of hydrogen peroxide at one IrO NP provided information about the origins of deactivation of catalytic NPs and the effects of various experimental conditions on the collision dynamics. High-resolution TEM of carbon pipettes was used to attain better understanding of the NP capture and collisions.