Quantification of the charge transport processes inside carbon nanopipettes

Controlled Chemical Vapor Deposition and Modification of Carbon Layers inside Quartz Nanopipettes

Carbon nanopipettes (CNPs) have attracted much attention in nanoscale electrochemical applications recently, while the carbon structure and surface oxygen-containing groups limit its applications. Herein, we grow the carbon nanotubes (CNTs) inside the quartz nanopipet via the chemical vapor deposition method, and the fabricated carbon nanotube nanopipettes (CNT-NPs) exhibit better electrochemical responses toward biomolecules such as glutathione and ascorbic acid, compared to the conventional CNPs. In addition, the carbon nanopipette can also be easily doped by a chemical reaction with urea, to display positive surface charges and high electrochemical activity for H2O2 oxidation/reduction. This work provides an easy way to tailor the surface structure and charges of the deposited carbon inside the pipettes and thus would further promote its broader usage in electrochemical sensing applications in biological fields.

Bridging Ionic Current Rectification and Resistive-Pulse Sensing for Reliable Wide-Linearity Detection

As two mainstream ionic detection techniques, ionic current rectification (ICR) suffers from large fluctuations in trace level detection, while resistive-pulse sensing (RPS) encounters easy clogs in high-concentration detection. By rationally matching the nanopore size with the DNA tetrahedron (TDN), this work bridges the two techniques to achieve reliable detection with wide linearity. As a representative analyte, miRNA-10b could specifically combine with and release TDN from the interior wall, which thus induced the simultaneous generation of distinct ICR and RPS signals. The ICR signals could be attributed to the balance between the effective orifice and surface charge density of the inner wall, while the RPS signals were induced by the complex of miRNA-10b and TDN passing through the nanopore. Such an operation contributed to a wide detection range of 1 fM-1 nM with a good linearity. The feasibility of this method is also validated in single-cell and real plasma detection.

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.

Elucidating the Shape of Current Transients in Electrochemical Resistive-Pulse Sensing of Single Liposomes

Electrochemical resistive-pulse (ERP) sensing with conductive carbon nanopipettes (CNPs) has recently been developed and employed for the detection of single liposomes and biological vesicles, and for the analysis of redox molecules contained in such vesicles. However, the origins of different shapes of current transients produced by the translocation of single vesicles through the CNP remain poorly understood. Herein, we report extensive finite-element simulations of both portions of an ERP transient, the current blockage by a vesicle approaching and passing through the pipet orifice and the faradaic current spike due to oxidation/reduction of redox species released from a vesicle on the carbon surface, for different values of parameters defining the geometry and dynamics of the vesicle/CNP system. The effects of the pipet geometry, surface charge, transport, vesicle trajectory, and collision location on the shape of current transients are investigated. The possibility of quantitative analysis of experimental ERP transients produced by translocations of liposomes and extracellular vesicles by fitting them to simulated curves is demonstrated. The developed theory can enable a more reliable interpretation of complicated ERP signals and characterization of the size and contents of single biological and artificial vesicles.

Multivalent Ion-Modulated Electron Transfer Processes in Carbon Nanopipettes

Conductive nanopipettes with both an electroactive interface and a pipet geometry have been recognized as powerful multifunctional probes in various electrochemical sensing and imaging applications. As confined inside the nanopipette, the excess surface charges at the solid/solution interface would then play a dominant role in the resulting charge transport processes. Herein, the effects of a multivalent ion on the resulting electron transfer (ET) processes in the carbon nanopipettes are investigated with both experimental and simulation methods. The multivalent cations (i.e., Ca²⁺, Mg²⁺, Co²⁺, and Ni²⁺) are shown to strongly adsorb at the negatively charged carbon surface and attract more Fe(CN)₆^4- ions inside the cavity, as indicated by the increasing ET current responses. In addition to elucidating the fundamental charge transport processes in conductive nanopipettes to afford better usage as electrochemical probes, these results could also help in the development of new sensing methods for measuring the non-electroactive ions in biological or environmental systems.

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.

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.