Focused-Ion-Beam-Milled Carbon Nanoelectrodes for Scanning Electrochemical Microscopy

Nanoelectrochemistry at liquid/liquid interfaces for analytical, biological, and material applications

Herein, we feature our recent efforts toward the development and application of nanoelectrochemistry at liquid/liquid interfaces, which are also known as interfaces between two immiscible electrolyte solutions (ITIES). Nanopipets, nanopores, and nanoemulsions are developed to create the nanoscale ITIES for the quantitative electrochemical measurement of ion transfer, electron transfer, and molecular transport across the interface. The nanoscale ITIES serves as an electrochemical nanosensor to enable the selective detection of various ions and molecules as well as high-resolution chemical imaging based on scanning electrochemical microscopy. The powerful nanoelectroanalytical methods will be useful for biological and material applications as illustrated by in situ studies of solid-state nanopores, nuclear pore complexes, living bacteria, and advanced nanoemulsions. These studies provide unprecedented insights into the chemical reactivity of important biological and material systems even at the single nanostructure level.

A troubleshooting guide for laser pulling platinum nanoelectrodes

While there are numerous publications on laser-assisted fabrication and characterization of Pt nanoelectrodes, the exact replication of those procedures is not as straightforward as following a single recipe across laboratories. Often, the working procedures vary by day, by laser puller, or by person. Only a handful of nanoelectrode fabrication papers record their parameters, and even fewer offer troubleshooting advice. Here, we provide a step-by-step guide for laser-assisted Pt nanoelectrode fabrication using low-cost equipment including a laser puller, voltammetry, and simple microscope images captured via cell phone. We also offer solutions for common failures experienced throughout the process to guide beginners as they troubleshoot their own fabrication procedures.

A nanoelectrode-based study of water splitting electrocatalysts

The development of low-cost and efficient catalytic materials for key reactions like water splitting, CO₂ reduction and N₂ reduction is crucial for fulfilling the growing energy consumption demands and the pursuit of renewable and sustainable energy. Conventional electrochemical measurements at the macroscale lack the potential to characterize single catalytic entities and nanoscale surface features on the surface of a catalytic material. Recently, promising results have been obtained using nanoelectrodes as ultra-small platforms for the study of the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) on innovative catalytic materials at the nanoscale. In this minireview, we summarize the recent progress in the nanoelectrode-based studies on the HER and OER on various nanostructured catalytic materials. These electrocatalysts can be generally categorized into two groups: 0-dimensional (0D) single atom/molecule/cluster/nanoparticles and 2-dimensional (2D) nanomaterials. Controlled growth as well as the electrochemical characterization of single isolated atoms, molecules, clusters and nanoparticles has been achieved on nanoelectrodes. Moreover, nanoelectrodes greatly enhanced the spatial resolution of scanning probe techniques, which enable studies at the surface features of 2D nanomaterials, including surface defects, edges and nanofacets at the boundary of a phase. Nanoelectrode-based studies on the catalytic materials can provide new insights into the reaction mechanisms and catalytic properties, which will facilitate the pursuit of sustainable energy and help to solve CO₂ release issues.

Precise Polishing and Electrochemical Applications of Quartz Nanopipette-Based Carbon Nanoelectrodes

Quartz nanopipette-based carbon nanoelectrodes (CNEs) have attracted extensive attention in nanoscale electrochemistry due to their simple and efficient fabrication, chemically inert materials, flexible size (down to a few nanometers), and ultrathin insulating encapsulation. However, these pristine CNEs usually have significantly irregular morphology on the surface, which greatly limits the applications where inlaid nanodisks are urgently needed. To address this critical issue, we have developed a new precise polishing strategy using paraffin coating protection (i.e., avoiding breakage of quartz materials) and real-time monitoring with a high impedance meter (i.e., indicating electrode exposure) to produce flat carbon nanodisk electrodes. The surface flatness of polished CNEs has been confirmed by a combination of scanning electron microscopy, fast-scan cyclic voltammetry, and scanning electrochemical microscopy. As compared to the expensive focused ion beam processing, this strategy is competitive in terms of the low cost and availability of the equipment and enables the preparation of polished CNEs with sufficiently small size. The flattened CNEs have been exemplified for grafting molecular catalysts to achieve the durable catalysis of reactive molecules or for immobilizing single-particle electrocatalysts to measure the intrinsic activity under sufficient mass-transfer rates.

Potentiometric pH Nanosensor for Intracellular Measurements: Real-Time and Continuous Assessment of Local Gradients

We present a pH nanosensor conceived for single intracellular measurements. The sensing architecture consisted of a two-electrode system evaluated in the potentiometric mode. We used solid-contact carbon nanopipette electrodes tailored to produce both the indicator (pH nanosensor) and reference electrodes. The indicator electrode was a membrane-based ion-selective electrode containing a receptor for hydrogen ions that provided a favorable selectivity for intracellular measurements. The analytical features of the pH nanosensor revealed a Nernstian response (slope of −59.5 mV/pH unit) with appropriate repeatability and reproducibility (variation coefficients of <2% for the calibration parameters), a fast response time (<5 s), adequate medium-term drift (0.7 mV h^–1), and a linear range of response including physiological and abnormal cell pH levels (6.0–8.5). In addition, the position and configuration of the reference electrode were investigated in cell-based experiments to provide unbiased pH measurements, in which both the indicator and reference electrodes were located inside the same cell, each of them inside two neighboring cells, or the indicator electrode inside the cell and the reference electrode outside of (but nearby) the studied cell. Finally, the pH nanosensor was applied to two cases: (i) the tracing of the pH gradient from extra-to intracellular media over insertion into a single PC12 cell and (ii) the monitoring of variations in intracellular pH in response to exogenous administration of pharmaceuticals. It is anticipated that the developed pH nanosensor, which is a label-free analytical tool, has high potential to aid in the investigation of pathological states that manifest in cell pH misregulation, with no restriction in the type of targeted cells.

Enhancement of heavy ion track-etching in polyimide membranes with organic solvents

The effect of organic solvents on the ion track-etching of polyimide (PI) membranes is studied to enhance the nanopore fabrication process and the control over pore diameter growth. To this end, two approaches are employed to investigate the influence of organic solvents on the nanopore fabrication in PI membranes. In the first approach, the heavy ion irradiated PI samples are pretreated with organic solvents and then chemically etched with sodium hypochlorite (NaOCl) solution, resulting up to ∼4.4 times larger pore size compared to untreated ones. The second approach is based on a single-step track-etching process where the etchant (NaOCl) solution contains varying amounts of organic solvent (by vol%). The experimental data shows that a significant increase in both the bulk-etch and track-etch rates is observed by using the etchant mixture, which leads to ∼47% decrease in the nanopore fabrication time. This enhancement of nanopore fabrication process in PI membranes would open up new opportunities for their implementation in various potential applications.

3D-Printed Carbon Nanoelectrodes for In Vivo Neurotransmitter Sensing

Direct laser writing, a nano 3D-printing approach, has enabled fabrication of customized carbon microelectrode sensors for neurochemical detection. However, to detect neurotransmitters in tiny biological organisms or synapses, submicrometer nanoelectrodes are required. In this work, we used 3D printing to fabricate carbon nanoelectrode sensors. Customized structures were 3D printed and then pyrolyzed, resulting in free-standing carbon electrodes with nanotips. The nanoelectrodes were insulated with atomic layer deposition of Al₂O₃ and the nanotips were polished by a focused ion beam to form 600 nm disks. Using fast-scan cyclic voltammetry, the electrodes successfully detected stimulated dopamine in the adult fly brain, demonstrating that they are robust and sensitive enough to use in tiny biological systems. This work is the first demonstration of 3D printing to fabricate free-standing carbon nanoelectrode sensors and will enable batch fabrication of customized nanoelectrode sensors with precise control and excellent reproducibility.

Evaluation of the electroanalytical performance of carbon-on-gold films prepared by electron-beam evaporation

Carbon film electrodes can often be used without pretreatment, and their fabrication allows for flexibility in size and shape and for mass production. In this work, we are exploring layered structures comprised of thin films of carbon on gold (eC/Au) prepared by electron-beam evaporation. These extremely flat films are not pyrolyzed and are comprised of mainly amorphous carbon but still exhibit reasonable conductivity due to the underlying gold layer. eC/Au electrodes, without any pretreatment, yield similar heterogeneous electron-transfer rates for benchmark redox systems and significantly lower background current in comparison with polished glassy carbon. Interestingly, they show insignificant adsorption of quinones, which is uncommon for most carbon electrode materials. However, eC/Au is still prone to adsorption of airborne hydrocarbons when exposed to ambient air like most graphitic materials. With reproducibly fast electron transfer kinetics, low background current, negligible adsorption, and ultraflat surface, eC/Au films are a promising candidate for electrochemical and sensor applications.

Review-Recent Advances in Nanosensors Built with Pre-Pulled Glass Nanopipettes and Their Applications in Chemical and Biological Sensing

Nanosensors built with pre-pulled glass nanopipettes, including bare or chemically modified nanopipettes and fully or partially filled solid nanoelectrodes, have found applications in chemical and biological sensing via resistive-pulse, current rectification, and electrochemical sensing. These nanosensors are easily fabricated and provide advantages through their needle-like geometry with nanometer-sized tips, making them highly sensitive and suitable for local measurements in extremely small samples. The variety in the geometry and layout have extended sensing capabilities. In this review, we will outline the fundamentals in fabrication, modification, and characterization of those pre-pulled glass nanopipette based nanosensors and highlight the most recent progress in their development and applications in real-time monitoring of biological processes, chemical ion sensing, and single entity analysis.

Nanotip-assisted photoreduction of silver nanostructures on chemically patterned ferroelectric crystals for surface enhanced Raman scattering

Nanotips made of metal and semiconductor have been widely utilized in versatile applications to strengthen the electric field through lightning rod effect and localized surface plasmon resonance (LSPR) effect. Here, we present the utilization of ferroelectric nanotips to assist photoreduction of silver nanostructures for surface enhanced Raman scattering (SERS). Ferroelectric nanotips with spontaneous polarization posses the unique feature of producing the permanent electrostatic field without requiring external excitation, which differs from the present nanotips requiring electrical and optical excitation. The enhanced electrostatic field promotes the formation of silver nanoparticles by reducing the effect of Stern layer and accelerating the movement of photoelectrons and silver ions to the template surface. Experimental results show that sharp ferroelectric nanotips facilitate the formation of large-diameter nanoparticles with strong LSPR action. Compared to the conventional ferroelectric templates, the SERS substrates using nanotip-equipped ferroelectric templates produce 5.51 times larger Raman intensity, which can be further increased by >10.76 times by increasing the reaction time. The proposed SERS substrate owns the limit of detection <10-8 M and the enhancement factor of 2.3 × 109. The presented ferroelectric nanotips with permanent electrostatic field would open promising applications in the versatile areas, such as nanomaterial fabrication and optoelectronic devices.

Electrochemistry at the Synapse

Electrochemical measurements of neurotransmitters provide insight into the dynamics of neurotransmission. In this review, we describe the development of electrochemical measurements of neurotransmitters and how they started with extrasynaptic measurements but now are pushing toward synaptic measurements. Traditionally, biosensors or fast-scan cyclic voltammetry have monitored extrasynaptic levels of neurotransmitters, such as dopamine, serotonin, adenosine, glutamate, and acetylcholine. Amperometry and electrochemical cytometry techniques have revealed mechanisms of exocytosis, suggesting partial release. Advances in nanoelectrodes now allow spatially resolved, electrochemical measurements in a synapse, which is only 20-100 nm wide. Synaptic measurements of dopamine and acetylcholine have been made. In this article, electrochemical measurements are also compared to optical imaging and mass spectrometry measurements, and while these other techniques provide enhanced spatial or chemical information, electrochemistry is best at monitoring real-time neurotransmission. Future challenges include combining electrochemistry with these other techniques in order to facilitate multisite and multianalyte monitoring.

Probing High Permeability of Nuclear Pore Complexes by Scanning Electrochemical Microscopy: Ca2+ Effects on Transport Barriers

The nuclear pore complex (NPC) solely mediates molecular transport between the nucleus and cytoplasm of a eukaryotic cell to play important biological and biomedical roles. However, it is not well-understood chemically how this biological nanopore selectively and efficiently transports various substances, including small molecules, proteins, and RNAs by using transport barriers that are rich in highly disordered repeats of hydrophobic phenylalanine and glycine intermingled with charged amino acids. Herein, we employ scanning electrochemical microscopy to image and measure the high permeability of NPCs to small redox molecules. The effective medium theory demonstrates that the measured permeability is controlled by diffusional translocation of probe molecules through water-filled nanopores without steric or electrostatic hindrance from hydrophobic or charged regions of transport barriers, respectively. However, the permeability of NPCs is reduced by a low millimolar concentration of Ca2+, which can interact with anionic regions of transport barriers to alter their spatial distributions within the nanopore. We employ atomic force microscopy to confirm that transport barriers of NPCs are dominantly recessed (∼80%) or entangled (∼20%) at the high Ca2+ level in contrast to authentic populations of entangled (∼50%), recessed (∼25%), and "plugged" (∼25%) conformations at a physiological Ca2+ level of submicromolar. We propose a model for synchronized Ca2+ effects on the conformation and permeability of NPCs, where transport barriers are viscosified to lower permeability. Significantly, this result supports a hypothesis that the functional structure of transport barriers is maintained not only by their hydrophobic regions, but also by charged regions.

Real-time characterization of uptake kinetics of glioblastoma vs. astrocytes in 2D cell culture using microelectrode array

Extracellular measurement of uptake/release kinetics and associated concentration dependencies provides mechanistic insight into the underlying biochemical processes. Due to the recognized importance of preserving the natural diffusion processes within the local microenvironment, measurement approaches which provide uptake rate and local surface concentration of adherent cells in static media are needed. This paper reports a microelectrode array device and a methodology to measure uptake kinetics as a function of cell surface concentration in adherent 2D cell cultures in static fluids. The microelectrode array simultaneously measures local concentrations at five positions near the cell surface in order to map the time-dependent concentration profile which in turn enables determination of surface concentrations and uptake rates, via extrapolation to the cell plane. Hydrogen peroxide uptake by human astrocytes (normal) and glioblastoma multiforme (GBM43, cancer) was quantified for initial concentrations of 20 to 500 μM over time intervals of 4000 s. For both cell types, the overall uptake rate versus surface concentration relationships exhibited non-linear kinetics, well-described by a combination of linear and Michaelis-Menten mechanisms and in agreement with the literature. The GBM43 cells showed a higher uptake rate over the full range of concentrations, primarily due to a larger linear component. Diffusion-reaction models using the non-linear parameters and standard first-order relationships are compared. In comparison to results from typical volumetric measurements, the ability to extract both uptake rate and surface concentration in static media provides kinetic parameters that are better suited for developing reaction-diffusion models to adequately describe behavior in more complex culture/tissue geometries. The results also highlight the need for characterization of the uptake rate over a wider range of cell surface concentrations in order to evaluate the potential therapeutic role of hydrogen peroxide in cancerous cells.

Electrochemistry at a single nanoparticle: from bipolar regime to tunnelling

This paper is concerned with long-distance interactions between an unbiased metal nanoparticle (NP) and a nanoelectrode employed as a tip in the scanning electrochemical microscope (SECM).

Ultraflat, Pristine, and Robust Carbon Electrode for Fast Electron-Transfer Kinetics

Electron-beam (e-beam) deposition of carbon on a gold substrate yields a very flat (0.43 nm of root-mean-square roughness), amorphous carbon film consisting of a mixture of sp²- and sp³-hybridized carbon with sufficient conductivity to avoid ohmic potential error. E-beam carbon (eC) has attractive properties for conventional electrochemistry, including low background current and sufficient transparency for optical spectroscopy. A layer of KCl deposited by e-beam to the eC surface without breaking vacuum protects the surface from the environment after fabrication until dissolved by an ultrapure electrolyte solution. Nanogap voltammetry using scanning electrochemical microscopy (SECM) permits measurement of heterogeneous standard electron-transfer rate constants (k°) in a clean environment without exposure of the electrode surface to ambient air. The ultraflat eC surface permitted nanogap voltammetry with very thin electrode-to-substrate gaps, thus increasing the diffusion limit for k° measurement to >14 cm/s for a gap of 44 nm. Ferrocene trimethylammonium as the redox mediator exhibited a diffusion-limited k° for the previously KCl-protected eC surface, while k° was 1.45 cm/s for unprotected eC. The k° for Ru(NH₃)₆^3+/2+ increased from 1.7 cm/s for unprotected eC to above the measurable limit of 6.9 cm/s for a KCl-protected eC electrode.

Characterization of Nanopipet-Supported ITIES Tips for Scanning Electrochemical Microscopy of Single Solid-State Nanopores

Nanoscale scanning electrochemical microscopy (SECM) is a powerful scanning probe technique that enables high-resolution imaging of chemical processes at single nanometer-sized objects. However, it has been a challenging task to quantitatively understand nanoscale SECM images, which requires accurate characterization of the size and geometry of nanoelectrode tips. Herein, we address this challenge through transmission electron microscopy (TEM) of quartz nanopipets for SECM imaging of single solid-state nanopores by using nanopipet-supported interfaces between two immiscible electrolyte solutions (ITIES) as tips. We take advantage of the high resolution of TEM to demonstrate that laser-pulled quartz nanopipets reproducibly yield not only an extremely small tip diameter of ∼30 nm, but also a substantial tip roughness of ∼5 nm. The size and roughness of a nanopipet can be reliably determined by optimizing the intensity of the electron beam not to melt or deform the quartz nanotip without a metal coating. Electrochemically, the nanoscale ITIES supported by a rough nanotip gives higher amperometric responses to tetrabutylammonium than expected for a 30 nm diameter disc tip. The finite element simulation of sphere-cap ITIES tips accounts for the high current responses and also reveals that the SECM images of 100 nm diameter Si3N4 nanopores are enlarged along the direction of the tip scan. Nevertheless, spatial resolution is not significantly compromised by a sphere-cap tip, which can be scanned in closer proximity to the substrate. This finding augments the utility of a protruded tip, which can be fabricated and miniaturized more readily to facilitate nanoscale SECM imaging.

3D Printed and Microcontrolled: The One Hundred Dollars Scanning Electrochemical Microscope

The design and fabrication of a versatile and low-cost electrochemical-scanning probe microscope (EC-SPM) is presented. The proposed equipment relies on the use of modern prototyping tools such as 3D printers and microcontroller boards and only a few "off-the-shelf" parts to deliver a simple yet powerful EC-SPM equipment capable of performing simple space-resolved electrochemical measurements. The equipment was able to perform space-resolved electrochemical measurements using a platinum ultramicroelectrode (UME) as the working electrode on a scanning electrochemical microscopy (SECM) configuration and was used to record approach curves, line scans, and array scans over an insulating substrate. The performance of the proposed equipment was found to be adequate for simple SECM measurements under hindered diffusion conditions. Because of its flexible design (software and hardware), more complex array scan patterns, only found on high-end EC-SPM setups such as hopping mode scan, were easily implemented on the built equipment. Despite its simplicity, the versatility and low cost of the proposed design make it an attractive alternative as a teaching platform as well as a platform for developing more elaborate EC-SPM setups.

Scanning Electrochemical Microscopy of Carbon Nanomaterials and Graphite

Carbon materials are tremendously important as electrode materials in both fundamental and applied electrochemistry. Recently, significant attention has been given not only to traditional carbon materials, but also to carbon nanomaterials for various electrochemical applications in energy conversion and storage as well as sensing. Importantly, many of these applications require fast electron-transfer (ET) reactions between a carbon surface and a redox-active molecule in solution. It, however, has not been well understood how heterogeneous ET kinetics at a carbon/solution interface is affected by the electronic structure, defect, and contamination of the carbon surface. Problematically, it is highly challenging to measure the intrinsic electrochemical reactivity of a carbon surface, which is readily passivated by adventitious organic contaminants. This Account summarizes our recent studies of carbon nanomaterials and graphite by scanning electrochemical microscopy (SECM) not only to reveal the fast ET kinetics of simple ferrocene derivatives at their graphitic surfaces, but also to obtain mechanistic insights into their extraordinary electrochemical reactivity. Specifically, we implemented new principles and technologies to reliably and reproducibly enable nanoscale SECM measurements. We took advantage of a new SECM imaging principle to resolve the high reactivity of the sidewall of individual single walled carbon nanotubes. In addition, we developed SECM-based nanogap voltammetry to find that monolayer graphene grown by chemical vapor deposition yields an unprecedentedly high standard ET rate constant, k(0), of ≥25 cm/s, which was >1000 times higher than that reported in the literature. Remarkably, the nonideal asymmetry of paired nanogap voltammograms revealed that the high reactivity of graphitic surfaces is compromised by their contamination with airborne hydrocarbons. Most recently, we protected the clean surface of highly oriented pyrolytic graphite from the airborne contaminants during its exfoliation and handling by forming a water adlayer to obtain a reliable k(0) value of ≥12 cm/s from symmetric pairs of nanogap voltammograms. We envision that SECM of clean graphitic surfaces will enable us to reliably address not only effects of their electronic structures on their electrochemical reactivity, but also the activity of carbon-based or carbon-supported electrocatalysts for fuel cells and batteries.

Recent advances in the development and application of nanoelectrodes

Nanoelectrodes have key advantages compared to electrodes of conventional size and are the tool of choice for numerous applications in both fundamental electrochemistry research and bioelectrochemical analysis. This Minireview summarizes recent advances in the development, characterization, and use of nanoelectrodes in nanoscale electroanalytical chemistry. Methods of nanoelectrode preparation include laser-pulled glass-sealed metal nanoelectrodes, mass-produced nanoelectrodes, carbon nanotube based and carbon-filled nanopipettes, and tunneling nanoelectrodes. Several new topics of their recent application are covered, which include the use of nanoelectrodes for electrochemical imaging at ultrahigh spatial resolution, imaging with nanoelectrodes and nanopipettes, electrochemical analysis of single cells, single enzymes, and single nanoparticles, and the use of nanoelectrodes to understand single nanobubbles.