Electrochemical Collision of Single Silver Nanoparticles in Carbon Nanopipettes

An Ultramicroelectrode Electrochemistry and Surface Plasmon Resonance Coupling Method for Cell Exocytosis Study

Exocytosis of a single cell has been extensively researched in recent years due to its close association with numerous diseases. However, current methods only investigate exocytosis at either the single-cell or multiple-cell level, and a method for simultaneously studying exocytosis at both levels has yet to be established. In this study, a combined device incorporating ultramicroelectrode (UME) electrochemistry and surface plasmon resonance (SPR) was developed, enabling the simultaneous monitoring of single-cell and multiple-cell exocytosis. PC12 cells were cultured directly on the SPR sensing Au film, with a carboxylated carbon nanopipette (c-CNP) electrode employed for electrochemical detection in the SPR reaction cell. Upon exocytosis, the released dopamine diffuses onto the inner wall of c-CNP, undergoing an electrochemical reaction to generate a current peak. Concurrently, exocytosis can also induce changes in the refractive index of the Au film surface, leading to the SPR signal. Consequently, the device enables real-time monitoring of exocytosis from both single and multiple cells with a high spatiotemporal resolution. The c-CNP electrode exhibited excellent resistance to protein contamination, high sensitivity for dopamine detection, and the capability to continuously monitor dopamine exocytosis over an extended period. Analysis of both SPR and electrochemical signals revealed a positive correlation between changes in the SPR signal and the frequency of exocytosis. This study introduces a novel method and platform for the simultaneous investigation of single-cell and multiple-cell exocytosis.

Collision Oxidation Behavior of Silver Nanoparticles in Alkaline Solution

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

Molecular Electrocatalytic Processes in Carbon Nanopipettes

Conductive nanopipettes have been recognized as powerful multifunctional platforms for electrochemical sensing applications in confined spaces. However, the electron-transfer processes of many biological analytes (i.e., enzymes or proteins) are slow and coupled with chemical reactions, which have not been well elucidated in conductive nanopipettes. In this Letter, both experimental and simulation methods are used to study electron-transfer processes coupled to chemical reactions (EC mechanism) in carbon nanopipettes (CNPs). It is demonstrated that the electroactive species can serve as redox mediator to help oxidize and reduce the nonelectroactive analytes of interest in the solution and produce noticeable catalytic current signals. Besides, glutathione was directly measured by using ferrocenemethanol as the redox mediator in the CNPs. The elucidated EC processes in CNPs would offer a new opportunity to measure nonelectroactive analytes in biological fields.

Current oscillations from bipolar nanopores for statistical monitoring of hydrogen evolution on a confined electrochemical catalyst

Taking advantage of bipolar electrochemistry and a glass nanopipette, continuous single bubbles can be controlled which are generated and detached from a nanometer-sized area of confined electrochemical catalysts. The observed current oscillations offer opportunities to rapidly collect data for the statistical analysis of single-bubble generation on and departure from the catalysts.

Electrodeposition of Metal Nanoparticles inside Carbon Nanopipettes for Sensing Applications

Conductive nanopipettes offer promising confined spaces to enable advanced electrochemical sensing applications in small spaces. Herein, a series of metal-decorated carbon nanopipettes (CNPs) were developed, in which Au, Ag, and Pt are modified at the inner walls of CNPs by a simple electrodeposition method. The fabricated tips show good sensing performances for a variety of important analytes, such as glucose, hydrogen peroxide, and chloride and hydrogen ions in biological and catalytic systems. This simple and effective approach can be further extended to prepare other functionalized nanopipette electrodes toward more versatile and powerful measurements in electrochemical sensing and imaging applications.

Scanning Electrochemical Microscopy Featuring Transient Current Signals in Carbon Nanopipets with Dilute or No Redox Mediator

Herein, we report a sensitive scanning electrochemical microscopy (SECM) method based on the high transient current signals in carbon nanopipets (CNPs) under step potential waveforms. Taking advantage of the transient peak current, the approach curve can be conducted with very dilute (1 μM) or even no redox mediator and fitted by the scanning ion conductance microscopy (SICM) theory. In addition, a trace amount of electroactive species generated at the substrate can also be directly revealed from the transient current at the CNP tips. With the established feedback and generation/collection methods, we present the constant-height topography and electroactivity imaging of the substrates with only 1 μM K₄Fe(CN)₆. The developed new SECM method would allow the usage of CNPs to achieve both high sensitivity and spatial resolution with dilute or no redox mediator and thus find great potential applications in biological and electrocatalytic studies.

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

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

Pressure-Regulated Single-Entity Electrochemistry Inside Carbon Nanopipettes

Conductive nanopipettes have been widely used as a multifunctional platform for emerging sensing applications in small spaces, although the electrochemical processes involved are not well controlled and fully quantified. Herein, we use an external pressure to precisely control the solution volume and regulate the electrochemical signals in carbon nanopipettes. In addition to polarizing the redox concentration profile, the pressure is found to generate a convective flow to control the transport processes of redox molecules and nanoparticles as well, and their quantitative correlation is established by a numerical simulation. The elucidated pressure-regulated electrochemistry in conductive nanopipettes would reveal the fundamental charge transport processes at the nanoscale and promote better usage of conductive nanopipettes for delivery and sensing applications in single-cell analysis.

Quantification of the charge transport processes inside carbon nanopipettes

Conductive nanopipettes have been extensively used as powerful multifunctional probes for electrochemical and ion transport measurements, while the involved charge transfer processes have not been fully explored. In this paper, we use both experimental and simulation methods to de-convolute and quantify the respective electron transfer (ET) and ion transport (IT) contributions to the resulting current signals in carbon nanopipettes (CNPs). The results present that the current signals in CNPs are determined by ET in the case of low solution depth and long timescales, while IT becomes dominant at short timescales or high solution depth. In addition, the electrochemically and chemically irreversible ET processes in CNPs were also quantified. The elucidated and quantified charge transport processes inside CNPs will help control and optimize the IT and ET processes at the nanoscale, promoting better and broad usage of conductive nanopipettes in single-entity sensing and imaging applications.

The coupled electron transfer (ET) and ion transport (IT) processes in conductive nanopipettes, at both steady and transient states, are elucidated and quantified by experiments and simulation.