Carbon Fiber Ultramicroelectrodes Research Articles

Impact of Surface Chemistry on Emulsion-Electrode Interactions and Electron-Transfer Kinetics in the Single-Entity Electrochemistry.

Single-entity electrochemistry (SEE) provides powerful means to track a single particle, single cell, and even single molecule from the nano to microscale. The electrode serves as not only the detector of collision but also the surface supplier in SEE, and the fundamental understanding of the electrode surface chemistry on the dynamic particle-electrode interactions and electrochemical responses of a single particle still remains unexplored, particularly for soft particles. Herein, dynamic interactions of microemulsions and the interaction-controlled electron-transfer (ET) kinetics are studied employing SEE and fluorescence spectroscopy. The o/w-type nitrobenzene emulsions were prepared with the surfactant-type room temperature ionic liquids (RTILs). Biased the electrode potential for the reduction of 7,7,8,8-tetracyanoquinodimethane within emulsions, it is surprising to see the distinct collision current signals on the carbon fiber ultramicroelectrode (C UME) and Au ultramicroelectrode (Au UME) in the late stage of chronoamperometric measurements. Theoretical understanding was made to determine the ET kinetics behind the disparate current signals. It is believed that the electrode surface chemistry, i.e., the surface energy, has a great influence on the dynamic emulsion-electrode interactions and ET kinetics. On the hydrophilic surface of Au UME, emulsions tend to decompose/detach from the electrode surface immediately after colliding. In contrast, on the lipophilic surface of C UME with lower surface energy, a layer of oil phase accumulated by the coalescence of emulsions and the migration of the precedent colliding emulsions, which would serve as a barrier to block ET via tunneling as manifested by the gradual slowdown of ET rate and the reduced collision frequency in the late stage of measurement. The impacts of the emulsion size and amphiphilicity of RTILs on the C UME-emulsion interactions and ET kinetics were also investigated.

Utilizing Microelectrodes to Understand Indirect Redox Targeting Reactions for High Energy Density Flow Batteries

One of the major problems with widespread implementation of renewable energy is the lack of cost-effective long-duration grid-scale energy storage filling in for the intermittent nature of solar and wind energy. Redox flow batteries (RFBs) are considered a promising solution to this problem due to their decoupled energy and power capabilities, allowing them to be scaled up for long-duration more efficiently and cost-effectively than other electrochemical energy storage devices [1]. The energy capacity of RFBs is primarily determined by the quantity of active species in the electrolyte, which is a product of the volume and the concentration. Thus, the limiting factor for energy density in RFBs is typically the solubility of the active species [2]. One probable path to enhancing the energy density without increasing the concentration is by utilizing the concept of indirect redox targeting reactions [3]. These redox-targeting flow batteries (RTFB) use solid charge storage materials in the electrolyte tanks to boost capacity, allowing the active species to act as mediators which transfer their charge by reversibly reducing or oxidizing the solid material [4].In this study, cobalt hexacyanoferrate (CoHCF) was used as a solid capacity booster in an electrolyte containing vanadium(iv/v)bis-hydroxyiminodiacetate (VBH) as mediators in acetonitrile. Constant current cycling was performed in a symmetric cell configuration with vanadium ions shuttling between vanadium(iv) and vanadium(v) oxidation states. To monitor the concentration of redox active species accurately and in real-time, a carbon fiber ultramicroelectrode (UME) was installed on the capacity limiting half-cell. This in-line UME set-up provides a better understanding of how CoHCF interacts with mediators and insights on active species crossover between half-cells. In addition, this study aims to examine the kinetics of the indirect redox-targeting reactions. As a rarely studied charge transfer process, very few studies have been reported on the kinetics of indirect redox targeting reactions [5]. Scanning electrochemical microscopy is used to understand how counter cations affect the kinetics of the reaction between the solid booster material and the mediator. Various counter cations (e.g., Ca2+, Na+, Li+) are used with the previously mentioned VBH-based mediators in order to determine the effective rate constants (keff) as well as the reversibility of the charge transfer reactions.

Confined Electrochemical Behaviors of Single Platinum Nanoparticles Revealing Ultrahigh Density of Gas Molecules inside a Nanobubble.

Understanding the basic physicochemical properties of gas molecules confined within nanobubbles is of fundamental importance for chemical and biological processes. Here, we successfully monitored the nanobubble-confined electrochemical behaviors of single platinum nanoparticles (PtNPs) at a carbon fiber ultramicroelectrode in HClO4 and H2O2 solution. Due to the catalytic decomposition of H2O2, a single oxygen nanobubble was formed on individual PtNPs to block the active surface of particles for proton reduction and to suppress their stochastic motion, resulting in significantly distinguished current traces. Furthermore, the combination of theoretical calculations and high-resolution electrochemical measurements allowed the nanobubble size and the oxygen gas density inside a single nanobubble to be quantified. Moreover, the ultrahigh oxygen density inside (1046 kg/m3) was revealed, indicating that gas molecules in a nanosized space existed with a high state of aggregation. Our approach sheds light on the gas aggregation behaviors of nanoscale bubbles using single-entity electrochemical measurements.

(General Student Poster Award Winner - 1st Place) Nitrogen-Doped and Phosphorus-Doped Carbon Fiber Ultramicroelectrodes As Electrochemical Sensors for Detection of Hydrogen Peroxide

Ultramicroelectrodes (UMEs) are generally defined as electrodes with sizes ≤25 µm. UMEs can be prepared by several methods including by sealing a conductive filament like thin metal wire or a single carbon fiber in a glass capillary. The small size of UMEs makes them useful as probes for measuring electroactive species in confined spaces (for example, inside living cells, etc.), and also enables very effective mass transport, resulting in rapid achievement of steady-state response and facilitating measurement of fast electrochemical reactions. Application of UMEs often requires modification of the electrode surface to improve the selectivity and the sensitivity towards the target analyte. Surface modification methods are time-consuming and may require expensive equipment. Previous research in our group demonstrated that a simple soft nitriding method could be employed to introduce surface nitrogen on carbon fiber (CF). The technique improved electrochemical response of CF-UMEs towards hydrogen peroxide (a reactive oxygen species that has been related to various malignancies and disorders) and, in separate experiments, also enabled deposition of electroactive metal nanoparticles on the UME surface. Since the presence of phosphorus heteroatoms on carbon electrodes has been shown to impart similar benefits, here we investigate a simple phosphorus doping strategy to make P-doped CF-UMEs. We compare their properties towards the electrocatalytic reduction of H2O2 to both N-doped CF-UMEs and unmodified CF-UMEs.

Non-enzymatic amperometric glucose sensor based on bimetal-oxide modified carbon fiber ultra-microelectrode

In this study, an efficient non-enzymatic glucose sensor is proposed based on a bimetal-oxide-modified carbon fiber microelectrode (CFME). The NiO-CuO/CFME nanocomposite was synthesized by consecutive electro-deposition of CuO and NiO nanoparticles on the CFME. The morphology and structure of NiCuCFME were studied with scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX). NiCuCFME provides both a large surface area and electrocatalytic properties to improve electrochemical reactivity toward glucose oxidation. The electrocatalytic behavior of the sensing microelectrode is analyzed with cyclic voltammetry (CV) and amperometric techniques in an alkaline medium. The fabricated glucose microsensor exhibits a linear range from 1.32 μM to 570 μM, a sensitivity of 0.07 mA μM−1 cm−2, and a low limit of detection of 0.40 μM. The NiCuCFME shows long-term stability, good reproducibility, favorable repeatability, excellent selectivity, and satisfactory applicability for glucose detection which is a promising candidate for the development of non-enzymatic glucose sensors.

Spatial Transcriptomics as a Novel Approach to Redefine Electrical Stimulation Safety.

Current standards for safe delivery of electrical stimulation to the central nervous system are based on foundational studies which examined post-mortem tissue for histological signs of damage. This set of observations and the subsequently proposed limits to safe stimulation, termed the “Shannon limits,” allow for a simple calculation (using charge per phase and charge density) to determine the intensity of electrical stimulation that can be delivered safely to brain tissue. In the three decades since the Shannon limits were reported, advances in molecular biology have allowed for more nuanced and detailed approaches to be used to expand current understanding of the physiological effects of stimulation. Here, we demonstrate the use of spatial transcriptomics (ST) in an exploratory investigation to assess the biological response to electrical stimulation in the brain. Electrical stimulation was delivered to the rat visual cortex with either acute or chronic electrode implantation procedures. To explore the influence of device type and stimulation parameters, we used carbon fiber ultramicroelectrode arrays (7 μm diameter) and microwire electrode arrays (50 μm diameter) delivering charge and charge density levels selected above and below reported tissue damage thresholds (range: 2–20 nC, 0.1–1 mC/cm2). Spatial transcriptomics was performed using Visium Spatial Gene Expression Slides (10x Genomics, Pleasanton, CA, United States), which enabled simultaneous immunohistochemistry and ST to directly compare traditional histological metrics to transcriptional profiles within each tissue sample. Our data give a first look at unique spatial patterns of gene expression that are related to cellular processes including inflammation, cell cycle progression, and neuronal plasticity. At the acute timepoint, an increase in inflammatory and plasticity related genes was observed surrounding a stimulating electrode compared to a craniotomy control. At the chronic timepoint, an increase in inflammatory and cell cycle progression related genes was observed both in the stimulating vs. non-stimulating microwire electrode comparison and in the stimulating microwire vs. carbon fiber comparison. Using the spatial aspect of this method as well as the within-sample link to traditional metrics of tissue damage, we demonstrate how these data may be analyzed and used to generate new hypotheses and inform safety standards for stimulation in cortex.

Front Cover: Single‐Entity Electrochemical Detection of As‐Prepared Metallic and Dielectric Nanoparticle Stochastic Impacts in a Phosphonium Ionic Liquid (ChemElectroChem 13/2022)

The Front Cover shows a platinum nanoparticle stochastic impact at a carbon fiber ultramicroelectrode along with the associated, recorded signal in an ionic liquid medium. More information can be found in the Research Article by N. Ahmadinasab and T. J. Stockmann.

Single Entity Electrochemical Detection of As‐prepared Metallic and Dielectric Nanoparticle Stochastic Impacts in a Phosphonium Ionic Liquid

Invited for this issue's Front Cover is the group of Dr. Talia Jane Stockmann from Memorial University of Newfoundland (Canada). The cover picture shows a platinum nanoparticle stochastic impact at a carbon fiber ultramicroelectrode along with the associated, recorded signal in an ionic liquid medium. Read the full text of the Research Article at 10.1002/celc.202200162.

An Antifouling Photoelectrochemical Ultramicrosensor for Unbiased Single‐Cell Analysis

Comprehensive SummaryThe nanoelectrode‐based electrochemical method has been widely used for single‐cell analysis because of the advantages of high spatiotemporal resolution and in‐situ detection capability. As an evolutional generation of the electrochemical analysis, photoelectrochemical (PEC) sensors not only inherit the above‐mentioned merits, but also possess a lower background and higher sensitivity, because they can operate under unbiased conditions. Moreover, electrodes are often impaired by nonspecific binding in complex biological environments. Thus, an antifouling PEC ultramicrosensor is developed for unbiased single‐cell analysis in this report. The ultramicrosensor is fabricated by self‐assembling of photoactive material ZnCaInS nanosheets onto the carbon fiber ultramicroelectrode surface and further dip‐coating of a porous antifouling layer of cross‐linked bovine serum albumin supported by carbon nanotubes. Benefiting from the minimal influence of the ultrathin nanosheet on the ultramicroelectrode morphology and the smooth protective interface formed by the antifouling film, the obtained photoelectrode can be readily inserted into a single cell with negligible impact on cell viability and shows excellent antifouling performance. The PEC ultramicrosensor achieves selective detection of dopamine (DA) with a low detection limit (3.97 nmol/L) and exhibits good sensing capability for cytosolic DA in single PC12 cells. This exploration provides new opportunities for single‐cell analysis.

Theoretical study of the chemical interactions between carbon fiber ultramicroelectrodes and the dihydroxybenzene isomers for electrochemical sensor understanding

Herein we present a study using the density functional theory in periodic systems to describe the interaction of carbon fiber ultramicroelectrodes during the detection of the dihydroxybenzene isomers: catechol (CT), resorcinol (RS), and hydroquinone (HQ). We used graphene as the surface model in the conformations: carbon basal plane, zig-zag, and armchair edges. We modeled the surface models functionalized with oxygenated groups. We predicted chemisorption for the oxidized form of RS when the COOH group is present in the armchair edge. Also, the susceptibility of the RS to form dimers and trimers bonded to the electrode surface. Experimentally, the electrooxidation of RS showed inactivation of the ultramicroelectrode, which can be explained considering the chemisorptions mentioned above and the formation of no-electroactive polymers. In the case of the other two molecules (CT and HQ), the model describes favorable interaction energies with the electrode; none of them as strong as RS, thus, experimentally, we did not see affections to the electrochemical responses, and we could measure a well-defined steady-state current at different concentrations of these molecules. Furthermore, the characteristic half-wave potential (E1/2) of the isomers ensured their detection and quantification in single or mixed solutions in concentrations from 40 μM to 1 mM.

Scanning Ion Conductance Microscopy: Surface Charge Effects on Electroosmotic Flow Delivery from a Nanopipette.

Scanning ion conductance microscopy (SICM) is a powerful and versatile technique that allows an increasingly wide range of interfacial properties and processes to be studied. SICM employs a nanopipette tip that contains electrolyte solution and a quasi-reference counter electrode (QRCE), to which a potential is applied with respect to a QRCE in a bathing solution, in which the tip is placed. The work herein considers the potential-controlled delivery of uncharged electroactive molecules (solute) from an SICM tip to a working electrode substrate to determine the effect of the substrate on electroosmotic flow (EOF). Specifically, the local delivery of hydroquinone from the tip to a carbon fiber ultramicroelectrode (CF UME) provides a means of quantifying the rate of mass transport from the nanopipette and mapping electroactivity via the CF UME current response for hydroquinone oxidation to benzoquinone. EOF, and therefore species delivery, has a particularly strong dependence on the charge of the substrate surface at close nanopipette-substrate surface separations, with implications for retaining neutral solute within the tip predelivery and for the delivery process itself, both controlled via the applied tip potential. Finite element method (FEM) simulations of mass transport and reactivity are used to explain the experimental observations and identify the nature of EOF, including unusual flow patterns under certain conditions. The combination of experimental results with FEM simulations provides new insights on mass transport in SICM that will enhance quantitative applications and enable new possibilities for the use of nanopipettes for local delivery.

Artificial Synapse: Spatiotemporal Heterogeneities in Dopamine Electrochemistry at a Carbon Fiber Ultramicroelectrode.

An artificial synapse is developed that mimics ultramicroelectrode (UME) amperometric detection of single cell exocytosis. It comprises the nanopipette of a scanning ion conductance microscope (SICM), which delivers rapid pulses of neurotransmitter (dopamine) locally and on demand at >1000 defined locations of a carbon fiber (CF) UME in each experiment. Analysis of the resulting UME current-space-time data reveals spatiotemporal heterogeneous electrode activity on the nanoscale and submillisecond time scale for dopamine electrooxidation at typical UME detection potentials. Through complementary surface charge mapping and finite element method (FEM) simulations, these previously unseen variations in electrochemical activity are related to heterogeneities in the surface chemistry of the CF UME.

Sensitive pH measurement using EGFET pH-microsensor based on ZnO nanowire functionalized carbon-fibers

Herein, we report the fabrication of zinc oxide nanowire (ZnO NW) coated carbon fiber (CF) ultra-microelectrodes (UME). ZnO NWs were grown on commercial multifilament CFs through hydrothermal process in a teflon-lined autoclave at 90 °C for 4 h. X-ray diffraction (XRD), Raman and scanning electron microscopy characterizations showed that crystalline and well oriented NW structures were successfully obtained. The fabrication of the pH sensitive UME was carried out by a novel approach which allowed controlling the protruding length of the modified CF surface. The UME was then integrated with a metal-oxide-semiconductor field effect transistor (MOSFET) for the construction of an EGFET pH-microsensor. The present pH microsensor is expected to be useful for localized pH measurement in small volumes such single cell analysis.

Amperometric Biosensor Based on Coimmobilization of Multiwalled Carbon Nanotubes and Horseradish Peroxidase-Gold Nanocluster Bioconjugates for Detecting H2O2

An enzyme-based amperometric biosensor was fabricated for detecting hydrogen peroxide (H2O2). Horseradish peroxidase (HRP) was modified using functionalized fluorescent gold nanoclusters (AuNCs) via biomineralization. HRP-AuNCs were successfully immobilized on multiwalled carbon nanotube- (MWCNT-) coated carbon fiber ultramicroelectrodes (CFUMEs). The AuNCs, which act as molecular electric wires, effectively promote the electron transfer between the enzyme active center and the electrode. Additionally, the HRP conjugated with the AuNCs retains its biological activity, which enables the catalytic reaction of H2O2. The HRP-AuNCs/MWCNTs/CFUMEs have been proven as excellent amperometric sensors for H2O2. The sensitivity of the H2O2 biosensor is 3.0×10−4 A/M, and the detection limit is estimated to be 443 nM. Furthermore, the biosensor exhibited long-term stability and good reproducibility. Moreover, the biosensor has been tested by determining the H2O2 concentration in calf serum samples.

A carbon fiber ultramicroelectrode as a simple tool to direct antioxidant estimation based on caffeic acid oxidation.

This work describes the construction and evaluation of carbon fiber ultramicroelectrodes (CF-UMEs) in the voltammetric estimation of the antioxidant capacity of wine and grape samples based on caffeic acid (HCAF) oxidation. For this, lab-made CF-UMEs were constructed using an arrangement of six carbon fibers (7 μm diameters individual) assembled in a glass capillary, and caffeic acid (HCAF) was used as a standard solution. By using the most straightforward 2-electrode cell arrangement (the CF-UME as a working electrode and Ag/AgCl as a reference/auxiliary electrode), voltammetric measurements of a 1.0 mmol L-1 HCAF solution were done in the absence of a supporting electrolyte. A sigmoidal voltammetric profile was observed in CF-UMEs caused by a more effective mass transport by radial diffusion, which leads to a rapid formation of the diffusion layer. Reproducibility studies for different 6-fiber electrodes manually constructed in different batches showed an RSD of less than 5%. For the same electrode surface, a variation of 2.7% was observed. Under optimized conditions, a linear relationship between anodic peak current and HCAF concentration from 3.0 to 500 μmol L-1 with a sensitivity of 12 μA L mol-1 was reached. The limits of detection (LOD) and quantification (LOQ) were calculated to be 0.41 and 1.26 μmol L-1, respectively. The proposed electrochemical method was applied in the estimation of the antioxidant capacity in three different wine samples as well as in green and red grapes. Concordant and satisfactory results by comparison with a proper method were obtained, which suggests that the proposed sensor can be successfully applied for direct analysis of wine and grape samples by estimation of HCAF content.

Electrodeposition of Co and Pd Onto Carbon Fiber and Platinum Ultramicroelectrodes

The synthesis of Co and Pd structures with tuned magnetic properties have scarcely been investigated from the electrochemical point of view [1-7]. Probably, this is due that, these properties are highly dependent on their shape, size and composition. In this sense, the use of ultramicroelectrodes (UME) allow to form a limited number of nuclei onto its surface, which is useful to analyse the electrochemical variables that modify the shape, size and specific composition of these particles [8-11]. In order to analyze the influence of the electrochemical variables during the synthesis of Co and Pd materials employing ultramicroelectrodes; in this work we conducted voltamperometric and chronoamperometric studies from two aqueous solutions. The first one containing PdCl2 0.01 M + NH4Cl 0.1 M and the second one CoCl2 0.01 M + NH4Cl 0.1 M. The working electrodes were a platinum UME (Pt-UME) of 10 μm of diameter and a carbon fiber UME (CUME) of 11 μm of diameter. A graphite bar was used as a counterelectrode and all potentials are reported against an Ag/AgCl electrode. The electrochemical experiments were controlled by mean a bipotentiostat UNISCAN M370. Fig. 1a shows a set of transients obtained in the potential range [-0.8 – -0.95] V applying a potential pulse during 32 s from the system Pt-UME/CoCl2 0.01 M + NH4Cl 0.1 M, while in Fig. 1b are depicted the transients obtained from the system Pt-UME /PdCl2 0.01 M + NH4Cl 0.1 M in the potential range [-0.4 – -0.7] V. Figure 1c and 1d, shows the transients obtained onto CUME for the case of Co and Pd, respectively. All the transients reported in Fig. 1 were compared with the theoretical ones predicted by the mathematical model proposed by Correia et al [12], by means a nonlinear fitting employing the Levenberg-Marquardt algorithm. In all cases, the kinetic parameters such as the nucleation rate (A) and the number of active nucleation sites (N0) were potential dependent. Also, the N0 values in each potential allowed us to simulate the AFM image associated to each potential considering a random distribution on the UME surface. [1] E.O. Berlanga-Ramírez et al., Phys. Rev. B, 70 (2004) 014410. [2] T. Sondon and J. Guevara, J. Mag. Mag. Mat. 272-276 (2004) e1247. [3] S. R. Brankovic, X. Yang, T. J. Klemmer, M. Seigler, IEEE T. Magn., 42(2) (2006) 132. [4] S. Ouazi, S. Vlaic S. Rusponi, G. Moulas, P. Buluschek, K. Halleux, S. Bornemann, S. Mankovsky, J. Minar, J.B. Staunton, H. Ebert, H. Brune. Nature Communications, 3 (2012) 1313 [5] M. Rezaei, S. Hadi, D. Fatmehsari, Journal of Materials Chemistry A, 1 (2014) [6] C. Xu, Y. Liu, J. Wang, H. Geng, H. Qiu, ACS Appl. Mater. Interfaces, 12 (2011) 4626. [7] F. Nasirpouri, S. M. Peighambari, A. S. Samardak, A. V. Ognev, E. V. Sukovatitsina, E. B. Modin, L. A. Chebotkevich, S. V. Komogortsev, S. J. Bending. Journal of Applied Physics 117, (2015) 17E715 [8] M. Peña, R. Celdran, R. Duo, Journal of Electroanalytical Chemistry, 367, (1994) 85. [9] C. Barin, A. Correia, L. Avaca, S. Machado, Journal of the Brazilian Chemical Society, 11 (2000) 175. [10] A. Kandory, H. Cattey, L. Saviot, T. Gharbi, J. Vigneron, M. Fregnaux, A. Etcheberry, G. Herlem, The Journal of Physical Chemistry C, 121 (2017) 1129. [11] A. Correia, S. Machado y L. Avaca, Journal of the Brazilian Chemical Society, 5 (1994) 173. [12] A. N. Correia, S. A. Machado, J. C. Sampaio, L. A. Avaca, Journal of Electroanalytical Chemistry, 407, (1996), 37-43. [13] C. T. Rueden, J. Schindelin, M. C. Hiner, BMC Bioinformatics, 18: 529 (2017) Figure 1

Scanning Ion Conductance Microscopy: Quantitative Nanopipette Delivery–Substrate Electrode Collection Measurements and Mapping

Scanning ion conductance microscopy (SICM) is becoming a powerful multifunctional tool for probing and analyzing surfaces and interfaces. This work outlines methodology for the quantitative controlled delivery of ionic redox-active molecules from a nanopipette to a substrate electrode, with a high degree of spatial and temporal precision. Through control of the SICM bias applied between a quasi-reference counter electrode (QRCE) in the SICM nanopipette probe and a similar electrode in bulk solution, it is shown that ionic redox species can be held inside the nanopipette, and then pulse-delivered to a defined region of a substrate positioned beneath the nanopipette. A self-referencing hopping mode imaging protocol is implemented, where reagent is released in bulk solution (reference measurement) and near the substrate surface at each pixel in an image, with the tip and substrate currents measured throughout. Analysis of the tip and substrate current data provides an improved understanding of mass transport and nanoscale delivery in SICM and a new means of synchronously mapping electrode reactivity, surface topography, and charge. Experiments on Ru(NH3)63+ reduction to Ru(NH3)62+ and dopamine oxidation in aqueous solution at a carbon fiber ultramicroelectrode (UME), used as the substrate, illustrate these aspects. Finite element method (FEM) modeling provides quantitative understanding of molecular delivery in SICM. The approach outlined constitutes a new methodology for electrode mapping and provides improved insights on the use of SICM for controlled delivery to interfaces generally.

Fabrication of Tip-Protected Polymer-Coated Carbon-Fiber Ultramicroelectrodes and pH Ultramicroelectrodes

This work presents a method to easily construct nanometer and micrometer size electrodes and pH ultramicroelectrodes, which are important tools for scientists who desire to examine electrochemical events at small scales. Consistent construction of poly(oxyphenylene)-coated carbon fiber ultramicroelectrodes (CF-UMEs) with tip diameters less than 1.5 μm proceeds by epoxy-sealing a carbon-fiber filament within a pulled-glass capillary, electrochemical etching of the protruding fiber of an CF into a conical tip and insulating the protruding fiber by electropolymerized poly(oxyphenylene) while the conical tip is protected within a conductive silicon rubber membrane. The CF-UMEs were characterized by cyclic voltammetry and successfully employed as an amperometric SECM tip to study the correlation between crystal structure orientation of an iridium/iridium oxide substrate and its electrocatalytic activity for oxygen evolution reaction. To prepare a pH ultramicroelectrode (pH-UME), a thin film of IrOx is electrochemically deposited on the surface of a CF-UME. The pH-UME shows a response time of < 5 s over the pH range of 2–12 with a sensitivity of 65.3 mV/pH. As a potentiometric SECM-tip, the pH-UME was successfully used to map the pH changes near the surface of a negatively biased Au disk electrode in buffered solutions. Results indicate that these pH-UME tips are well-suited for SECM studies requiring high-resolution pH imaging.

Fabrication of Glass-Insulated Ultramicrometer to Submicrometer Carbon Fiber Electrodes to Support a Single Nanoparticle and Nanoparticle Ensembles in Electrocatalytic Investigations.

We report a procedure to fabricate glass-insulated carbon fiber ultramicroelectrodes (CF UMEs) with sizes of 10 and 1 μm in diameter. CF UMEs of theses sizes are commonly insulated with polymer or epoxy resins, which typically have pinholes and are less stable and difficult to polish. Through judicious choice of polishing materials, the fabrication procedure reported here leads to CF UMEs with a surface geometry that, within experimental error, is close to that of an inlaid disk. We demonstrate how the hardness factor of commonly used polishing materials may alter the geometry of CF UMEs, in which carbon has a larger hardness factor compared to other metals (i.e., Pt or Au). Atomic force microscopy (AFM) and steady-state voltammetry (SSV) were used to characterize the CF UME surface and record the electrochemical response, respectively. These results were compared to theoretical values for an inlaid disk UME. Pt nanoparticle (NP) collision experiments using electrocatalytic amplification were used to deposit an exact number of Pt NPs on a CF UME surface. Surface roughness was found to significantly decrease the sticking of Pt NPs on the CF surface, compared to the theoretical collision frequency. However, decreasing surface roughness through judicious polishing led to good agreement between experimental and theoretical collision frequencies of NPs on a CF UME surface, leading to the fabrication of NP ensembles of UME dimensions (UME-NPE) and a single-nanoparticle UME (SNP-UME). These electrodes were used to record and analyze SSVs for the hydrogen evolution reaction (HER) in acidic media.

Direct Electrochemical Observation of Single Platinum Cluster Electrocatalysis on Ultramicroelectrodes.

We demonstrate a method to electrodeposit and observe the electrocatalysis of small platinum clusters and nanoparticles (NPs) in real time as they form on an ultramicroelectrode (UME). Water droplets ( rdrop ∼ 700 nm), stabilized by sodium dodecyl sulfate (SDS), were suspended in a solution of dichloromethane (DCM) and tetrabutylammonium perchlorate ([TBA][ClO4]), which was used to mitigate charge balance during droplet electrolysis. When droplets collided with an UME biased sufficiently negative to drive water reduction, large blips of current were observed. Droplets were synthesized with varying concentrations of H2PtCl6 (from 24.4 mM to 32 nM), which can be reduced to Pt0 at 0.8 V more positive than water reduction on a Au or C UME. The observation of current blips synthesized with mM amounts of H2PtCl6 indicated water droplets deliver H2PtCl6 to the electrode surface, where a cathodic potential caused Pt NPs to form. The formation of clusters was observed by biasing the electrode potential more negative than water reduction on Pt but more positive than water reduction on Au, giving current blips for droplets containing μM to nM amounts of H2PtCl6. These blips corresponded to the electrocatalysis of thousands to tens of atoms (clusters). Droplet electrolysis allows for a large amplification such that the electrocatalysis of clusters can be observed in real time. Single, isolated clusters were further characterized voltammetrically on carbon fiber UMEs. The isolation step used the amperometric method, which unambiguously depicted cluster formation in real time. Carbon was chosen due to its large potential window and slow kinetics toward many inner-sphere reactions, such as proton reduction, used in this study. Voltammetric characterization of proton reduction in HClO4 and NaClO4 allowed for cluster size analysis using the limiting current. The reduction of proton on the clusters ( E1/2 ∼ -0.6 V vs Ag/AgCl) occurred at ca. 400 mV more negative than bulk, polycrystalline platinum.