Electrochemical Behavior and Detection of Diclofenac at a Microporous Si3N4 Membrane Modified Water–1,6-dichlorohexane Interface System

The electrochemical behavior when the liquid–liquid interface was modified by commercially available, microporous silicon nitride membrane, was achieved using cyclic voltammetry with tetramethyl ammonium. The transfer characteristics of the ionizable drug diclofenac ( DCF − ), as an anti-inflammatory, anti-rheumatic, antipyretic, and analgesic treatment in common use in biomedical applications, were also investigated across microporous silicon nitride-modified liquid interface. Thus, some thermodynamic variables for DCF − , such as the standard Gibbs energy of transfer, the standard transfer potential and lipophilicity were estimated. Furthermore, the influence of possible interfering substances (ascorbic acid, sugar, amino acid, urea, and metal ions) on the detection of DCF − was investigated. An electrochemical DCF sensor is investigated using differential pulse voltammetry (DPV) as the quantification technique, a linear range of 8–56 µM and a limit of detection of 1.5 µM was possible due to the miniaturized interfaces formed within silicon nitride.

Coupled Electron- and Phase-Transfer Reactions at a Three-Phase Interface

Coupled electron- and phase-transfer reactions are fundamentally important in electrochemical energy conversion and storage, e.g., intercalation of Li⁺ in batteries and electrochemistry at the three-phase boundary in fuel cells. The mechanism, energetics, and kinetics of these complex reactions play an important role in device performance. Herein, we describe experimental methodology to quantitatively investigate coupled electron- and phase-transfer reactions at an individual, geometrically well-defined, three-phase interface. Specifically, a Pt-Ir wire electrode is placed across a H₂O/1,2-dichloroethane (DCE) interface, creating a Pt-Ir/H₂O/DCE boundary that is defined mathematically by a line around the surface of the wire. We investigated the oxidation of ferrocene (Fc), initially present in DCE (but essentially insoluble in water), at the three-phase boundary, and the transfer of its charged reaction product ferrocenium (Fc⁺) across the interface into the aqueous phase. In cyclic voltammetry, a reversible wave at E_1/2 ∼ 0.58 V is observed for Fc oxidation in DCE on the first scan. The Fc⁺ produced near the H₂O/DCE interface transfers into the aqueous phase. On the second and subsequent cycles, a second reversible wave at more negative potentials, E_1/2 ∼ 0.33 V, is observed, corresponding to the reduction of Fc⁺ (and reoxidation back to Fc) in the aqueous phase. Finite-element simulations quantitatively capture the voltammetric response of coupled electron and phase transfer at the three-phase interface and indicate that the electrochemical response observed in the aqueous phase occurs within ∼200 μm of the Pt-Ir/H₂O/DCE boundary. Finally, we demonstrate that the rate of transfer of Fc⁺ is strongly dependent on the concentration of supporting electrolyte, reaching a maximum at an intermediate electrolyte concentration, suggesting a critical role of the electric field distribution in determining the reaction rates at the three-phase interface.

Mechanistic Study of Oxygen Reduction at Liquid/Liquid Interfaces by Hybrid Ultramicroelectrodes and Mass Spectrometry

Proton-coupled electron transfer (PCET) reactions at various interfaces (liquid/membrane, solid/electrolyte, liquid/liquid) lie at the heart of many processes in biology and chemistry. Mechanistic study can provide profound understanding of PCET and rational design of new systems. However, most mechanisms of PCET reactions at a liquid/liquid interface have been proposed based on electrochemical and spectroscopic data, which lack direct evidence for possible intermediates. Moreover, a liquid/liquid interface as one type of soft interface is dynamic, making the investigation of interfacial reactions very challenging. Herein a novel electrochemistry method coupled to mass spectrometry (EC-MS) was introduced for in situ study of the oxygen reduction reaction (ORR) by ferrocene (Fc) under catalysis from cobalt tetraphenylporphine (CoTPP) at liquid/liquid interfaces. The key units are two types of gel hybrid ultramicroelectrodes (agar-gel/organic hybrid ultramicroelectrodes and water/PVC-gel hybrid ultramicroelectrodes), which were made based on dual micro- or nanopipettes. A solidified liquid/liquid interface can be formed at the tip of these pipettes, and it serves as both an electrochemical cell and a nanospray emitter for mass spectrometry. We demonstrated that the solidified L/L interfaces were very similar to typical L/L interfaces. Key CoTPP intermediates of the ORR at the liquid/liquid interfaces were identified for the first time, and the four-electron oxygen reduction pathway predominated, which provides valuable insights into the mechanism of the ORR. Theoretical simulation has further supported the possibility of formation of intermediates. This type of platform is promising for in situ tracking and identifying intermediates to study complicated reactions at liquid/liquid interfaces or other soft interfaces.

Short-chain oligonucleotide detection by glass nanopore using targeting-induced DNA tetrahedron deformation as signal amplifier

Glass capillary nanopore has been developed as a promising sensing platform for bioassay with single-molecule resolution. Although the diameter of glass capillary nanopore can be easily tuned, direct event-readouts of small biomacromolecules, like short-chain oligonucleotide fragments (within ∼20 nucleotides) remain great challenge, which limited by the configuration of the conical-shaped nanopore and the instrumental temporal resolution. Here, we exploit a smart strategy for glass nanopore detection of short-chain oligonucleotides by using relatively big-sized tetrahedral DNA nanostructures as a signal amplifier, which can amplify the signals and retard the translocation speed meanwhile. The tetrahedral DNA nanostructure with a hairpin loop sequence in one edge, undergoes a shape transformation upon the complementary combination of the target oligonucleotides, in which the presence of short-chain target oligonucleotide can be readout due to obvious variation in amplitude of ion current pulse that caused by volume change of the DNA tetrahedral. Therefore, this strategy is promising for extending glass nanopore sensing platform for sensitive detection of short-chain oligonucleotides.

Nanopore-Based Confined Spaces for Single-Molecular Analysis

The field of nanopore sensing at the single-molecular level is in a "boom" period. Such nanopores, which are either composed of biological materials or are fabricated from solid-state substrates, offer a unique confined space that is compatible with the single-molecular scale. Under the influence of an electrical field, such single-biomolecular interfaces can read single-molecular information and, if appropriately fine-tuned, each molecule plays its individual ionic rhythm to compose a "molecular symphony". Over the past few decades, many research groups have worked on nanopore-based single-molecular sensors for a range of thrilling chemical and clinical applications. Furthermore, for the past decade, we have also focused on nanopore-based sensors. In this Minireview, we summarize the recent developments in fundamental research and applications in this area, along with data algorithms and advances in hardware, which act as infrastructure for the electrochemical analysis.

Ionic Current Behaviors of Dual Nano- and Micropipettes

Ionic current rectification (ICR) phenomena within dual glass pipettes are investigated for the first time. We demonstrate that the ionic flow presents different behaviors in dual nano- and micropipettes when the two channels are filled with the same electrolyte KCl and hung in air. Bare dual nanopipettes cannot rectify the ionic current because of their geometric symmetry, but the ICR can be directly observed based on bare dual micropipettes. The phenomena based on dual micropipettes could be explained by the simulation of the Poisson-Nernst-Plank equation. After modification with different approaches, the dual nanopipettes have asymmetric charge patterns and show various ICR behaviors. They have been successfully employed to fabricate various nanodevices, such as ionic diodes and bipolar junction transistors. Due to the simple and fast fabrication with high reproducibility, these dual pipettes can provide a novel platform for controlling ionic flow in nano- and microfluidics, fabrication of novel nanodevices, and detection of biomolecules.

Electroactivity of Aptamer at Soft Microinterface Arrays

The electrochemical behavior of a synthetic oligonucleotide, thrombin-binding aptamer (TBA, 15-mer), was explored at a liquid-organogel microinterface array. TBA did not display any response when only background electrolytes were present in both phases. On the basis of literature reports that surfactants can influence nucleic acid detection, the response in the presence of cetyltrimethylammonium (CTA⁺) was examined. With both TBA and CTA⁺ in the aqueous phase, the transfer current for CTA⁺ was diminished, signifying the interaction of CTA⁺ with TBA. Experiments with CTA⁺ spiked into the organic phase revealed a sharp current peak, consistent with the interfacial formation of a CTA⁺-TBA complex. However, use of CTA⁺ as the organic phase electrolyte cation, as the salt with tetrakis(4-chlorophenyl)borate, greatly improved the response to TBA. In this case, a distinctive peak response (at ca. -0.25 V) was attributed to the transfer of CTA⁺ across the soft interface to complex with aqueous phase TBA. Employing this process as a detection step enabled a detection limit of 0.11 μM TBA (by cyclic voltammetry). Furthermore, the presence of magnesium cations at physiological concentration resulted in the disappearance of the TBA response because of Mg²⁺-induced folding of TBA. Also, the current response of TBA was decreased by the addition of thrombin, indicating TBA interacted with this binding partner. Finally, the interfacial surfactant-aptamer interaction was explored in a synthetic urine matrix that afforded a detection limit of 0.29 μM TBA. These results suggest that aptamer-binding interactions can be monitored by electrochemistry at aqueous-organic interfaces and open up a new possibility for detection in aptamer-binding assays.

Fabrication and Use of Nanopipettes in Chemical Analysis

This review summarizes progress in the fabrication, modification, characterization, and applications of nanopipettes since 2010. A brief history of nanopipettes is introduced, and the details of fabrication, modification, and characterization of nanopipettes are provided. Applications of nanopipettes in chemical analysis are the focus in several cases, including recent progress in imaging; in the study of single molecules, single nanoparticles, and single cells; in fundamental investigations of charge transfer (ion and electron) reactions at liquid/liquid interfaces; and as hyphenated techniques combined with other methods to study the mechanisms of complicated electrochemical reactions and to conduct bioanalysis.

Electrified Soft Interface as a Selective Sensor for Cocaine Detection in Street Samples

A straightforward, direct, and selective method is presented for electrochemical cocaine identification in street samples. The sensing mechanism is based on a simple ion transfer reaction across the polarized liquid–liquid interface. The interfacial behavior of a number of cutting agents is also reported. Interfacial miniaturization has led to improved electroanalytical properties of the liquid–liquid interface based sensor as compared with the macroscopic analogue. The reported method holds great potential to replace colorimetric tests with poor selectivity for on-site street sample analysis.

GABA Detection with Nano-ITIES Pipet Electrode: A New Mechanism, Water/DCE-Octanoic Acid Interface

Interface between two immiscible electrolyte solutions (ITIES) supported on the orifice of a pipet have become a powerful platform to detect a broad range of analytes. We present here the detection of γ-aminobutyric acid (GABA) with the nanoITIES pipet electrodes for the first time. GABA has a net charge of zero in an aqueous solution at pH ≈ 7, and it has not previously been detected at ITIES. In this work, we demonstrated GABA detection at ITIES in an aqueous solution at pH ≈ 7, where we introduced a novel detection strategy based on "pH modulation from the oil phase". To the best of our knowledge, this is the first report of such. Current increases linearly with increasing concentrations of GABA, ranging from 0.25 mM to 1.0 mM. The measured half-wave transfer potential of GABA is -0.401 ± 0.010 V ( n = 22) vs E1/2,TBA. The measured diffusion coefficient for GABA detection at nanoITIES pipet electrode is 6.09 (±0.58) × 10-10 m2/s ( n = 5). Experimental results indicate that protons generated from octanoic acid dissociation in the oil phase do not come out from the oil phase into the aqueous phase; neither were protons produced in the aqueous phase. NanoITIES pipet electrodes with radii of 320-340 nm were used in the current study. This new strategy and knowledge presented here lays the groundwork for the future development of ITIES pipet electrodes, especially for the detection of electrochemically nonredox active analytes.

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.

Portable Sensor for the Detection of Choline and Its Derivatives Based on Silica Isoporous Membrane and Gellified Nanointerfaces

A portable amperometric ion sensor was fabricated by integrating silica isoporous membrane (SIM) and organogel composed of polyvinyl chloride and 1,2-dichloroethane (PVC-DCE) on a 3D-printed polymer chip. The detection of ionic species in aqueous samples could be accomplished by adding a microliter of sample droplet to the sensor and by identifying the ion-transfer potential and current magnitude at the water/organogel interface array templated by SIM. Thanks to the ultrasmall channel size (2-3 nm in diameter), high channel density (4 × 10⁸ μm^-2), and ultrathin thickness (80 nm) of SIM, the ensemble of nanoscopic water/organogel (nano-W/Gel) interface array behaved like a microinterface with two back-to-back hemispherical mass diffusion zones. So, the heterogeneous ion-transfer across the nano-W/Gel interface array generated a steady-state sigmoidal current wave. The detection of choline (Ch) and its derivatives, including acetylcholine (ACh), benzoylcholine (BCh), and atropine (AP), in aqueous samples was examined with this portable sensor. Using differential pulse stripping voltammetry (DPSV), the quantification of these analytes was achieved with a limit of detection (LOD) down to 1 μM. Moreover, the portable ion sensor was insensitive to various potential interferents that might coexist in vivo, owing to size-/charge-based selectivity and antifouling capacity of SIM. With this priority, the portable ion sensor was able to quantitatively determine Ch and its derivatives in diluted urine and blood samples. The LODs for Ch, ACh, AP, and BCh in urine were 1.12, 1.30, 1.08, and 0.99 μM, and those for blood samples were 3.61, 3.38, 2.32, and 1.81 μM, respectively.

Electrochemistry-mass spectrometry for mechanism study of oxygen reduction at water/oil interface

Electrochemistry methods have been widely employed in the development of renewable energy, and involved in various processes, e.g. water splitting and oxygen reduction. Remarkable progress notwithstanding, there are still many challenges in further optimization of catalysts to achieve high performance. For this purpose, an in-depth understanding of reaction mechanism is needed. In this study, an electrochemistry-mass spectrometry method based on a Y-shaped dual-channel microchip as electrochemical cell and ionization device was demonstrated. Combined solutions of aqueous phase and oil phase were introduced into mass spectrometer directly when electrochemical reactions were happening to study the reduction of oxygen by decamethylferrocene or tetrathiafulvalene under the catalysis of a metal-free porphyrin, tetraphenylporphyrin, at water/1,2-dichloroethane interfaces. Monoprotonated and diprotonated tetraphenylporphyrin were detected by mass spectrometer, confirming the previously proposed mechanism of the oxygen reduction reaction. This work offers a new approach to study electrochemical reactions at liquid-liquid interface.

Advanced electroanalytical chemistry at nanoelectrodes

Nanoelectrodes, with dimensions below 100 nm, have the advantages of high sensitivity and high spatial resolution. These electrodes have attracted increasing attention in various fields such as single cell analysis, single-molecule detection, single particle characterization and high-resolution imaging. The rapid growth of novel nanoelectrodes and nanoelectrochemical methods brings enormous new opportunities in the field. In this perspective, we discuss the challenges, advances, and opportunities for nanoelectrode fabrication, real-time characterizations and high-performance electrochemical instrumentation.

Electrochemical behaviour at a liquid-organogel microinterface array of fucoidan extracted from algae

The electrochemical behaviour of fucoidan, a sulfated polysaccharide, was investigated, leading to a detection strategy by adsorptive stripping voltammetry.

Detection of Metoprolol in Human Biofluids and Pharmaceuticals via Ion-Transfer Voltammetry at the Nanoscopic Liquid/Liquid Interface Array

Metoprolol (MTP) is one of the most widely used antihypertensive drugs yet banned to use in sport competition. Therefore, there has been an increasing demand for developing simple, rapid, and sensitive methods suited to the identification and quantification of MTP in human biofluids. In this work, ultrathin silica nanochannel membrane (SNM) with perforated channels was employed to support nanoscale liquid/liquid interface (nano-ITIES) array for investigation of the ion-transfer voltammetric behavior of MTP and for its detection in multiple human biofluids and pharmaceutical formulation. Several potential interfering substances, including small molecules, d-glucose, urea, ascorbic acid, glycine, magnesium chloride, sodium sulfate and large molecules, bovine serum albumin (BSA), were chosen as models of biological interferences to examine their influence on the ion-transfer current signal of MTP. The results confirmed that the steady-state current wave barely changed in the presence of small molecules. Although BSA displayed an apparent blockade on the transfer of MTP, the accurate determination of MTP in multiple human biofluids (i.e., urine, serum and whole blood) and pharmaceutical formulation were still feasible, thanks to the molecular sieving and antifouling abilities of SNM. A limit of detection (LOD) within the physiological level of MTP during therapy could be achieved for all cases, i.e., 0.5 and 1.1 μM for 100 times diluted urine and serum, respectively, and 2.2 μM for 1000 times diluted blood samples. These results demonstrated that the nano-ITIES array behaved as a simplified and integrated detection platform for ionizable drug analysis in complex media.

Trends in Hydrophilicity/Lipophilicity of Phosphonium Ionic Liquids As Determined by Ion-Transfer Electrochemistry

Ionic liquids (ILs) have become valuable new materials for a broad spectrum of applications including additives or components for new hydrophobic/hydrophilic polymer coatings. However, fundamental information surrounding IL molecular properties is still lacking. With this in mind, the microinterface between two immiscible electrolytic solutions (micro-ITIES), for example, water|1,2-dichloroethane, has been used to evaluate the hydrophobicity/lipophilicity of 10 alkylphosphonium ILs. By varying the architecture around the phosphonium core, chemical differences were induced, changing the lipophilicity/hydrophilicity of the cations. Ion transfer (IT) within the polarizable potential window (PPW) was measured to establish a structure-property relationship. The Gibbs free energy of IT and the solubility of their ILs were also calculated. For phosphonium cations bearing either three butyl or three hydroxypropyl groups with a tunable fourth arm, the latter displayed a wide variety of easily characterizable IT potentials. The tributylphosphonium ILs, however, were too hydrophobic to undergo IT within the PPW. Utilizing a micro-ITIES (25 μm diameter) housed at the tip of a capillary in a uniquely designed pipet holder, we were able to probe beyond the traditional potential window and observe ion transfer of these hydrophobic phosphonium ILs for the first time. A similar trend in lipophilicity was determined between the two subsets of ILs by means of derived solubility product constants. The above results serve as evidence of the validation of this technique for the evaluation of hydrophobic cations that appear beyond the conventional PPW and of the lipophilicity of their ILs.

Evaluation of mRNA Localization Using Double Barrel Scanning Ion Conductance Microscopy

Information regarding spatial mRNA localization in single cells is necessary for a better understanding of cellular functions in tissues. Here, we report a method for evaluating localization of mRNA in single cells using double-barrel scanning ion conductance microscopy (SICM). Two barrels in a nanopipette were filled with aqueous and organic electrolyte solutions and used for SICM and as an electrochemical syringe, respectively. We confirmed that the organic phase barrel could be used to collect cytosol from living cells, which is a minute but sufficient amount to assess cellular status using qPCR analysis. The water phase barrel could be used for SICM to image topography with subcellular resolution, which could be used to determine positions for analyzing mRNA expression. This system was able to evaluate mRNA localization in single cells. After puncturing the cellular membrane in a minimally invasive manner, using SICM imaging as a guide, we collected a small amount cytosol from different positions within a single cell and showed that mRNA expression depends on cellular position. In this study, we show that SICM imaging can be utilized for the analysis of mRNA localization in single cells. In addition, we fully automated the pipet movement in the XYZ-directions during the puncturing processes, making it applicable as a high-throughput system for collecting cytosol and analyzing mRNA localization.

Electroanalytical Ventures at Nanoscale Interfaces Between Immiscible Liquids

Ion transfer at the interface between immiscible electrolyte solutions offers many benefits to analytical chemistry, including the ability to detect nonredox active ionized analytes, to detect ions whose redox electrochemistry is accompanied by complications, and to separate ions based on electrocontrolled partition. Nanoscale miniaturization of such interfaces brings the benefits of enhanced mass transport, which in turn leads to improved analytical performance in areas such as sensitivity and limits of detection. This review discusses the development of such nanoscale interfaces between immiscible liquids and examines the analytical advances that have been made to date, including prospects for trace detection of ion concentrations.

Electroanalytical Opportunities Derived from Ion Transfer at Interfaces between Immiscible Electrolyte Solutions

This review presents an introduction to electrochemistry at interfaces between immiscible electrolyte solutions and surveys recent studies of this form of electrochemistry in electroanalytical strategies. Simple ion and facilitated ion transfers across interfaces varying from millimetre scale to nanometre scales are considered. Target detection strategies for a range of ions, inorganic, organic, and biological, including macromolecules, are discussed.

Trapped in the coordination sphere: nitrate ion transfer driven by the cerium(iii/iv) redox couple

The electrochemical transfer of nitrate anions between oil and water phases—driven by the reduction and oxidation of cerium coordination complexes in oil phases—provides a new entry into chemical separations where an electrode potential tunes solute transfer between phases by ‘trapping’ the migrating anion on the cerium cation.

Imaging electrocatalytic processes on single gold nanorods

Imaging electrochemical processes has attracted increasing attention in past decades. Particularly, monitoring electrochemical reactions rapidly at the nano-scale is still a challenge due to the ultra-low current detection and long scanning time required. The development of optical techniques provide a new way to demonstrate electrochemical processes through optical signals which enhance sensitivity and spatial resolution. Herein, we developed a novel method to image electrocatalytic processes on single gold nanorods (GNRs) during Cyclic Voltammetry (CV) scanning based on plasmon resonance scattering information by using dark-field microscopy. The electrocatalytic oxidation of hydrogen peroxide was selected as a typical reaction and the catalytic mechanism was revealed using the obtained spectra. Notably, observation on single GNRs avoided the averaging effects in bulk systems and confirmed that the individual nanoparticles had variable catalytic properties with different spectral change during the reaction process. Furthermore, a color-amplified system was introduced to convert light intensity into imaging information via the Matlab program which was able to image thousands of nanoparticles simultaneously. This approach offered the statistical intensity distribution of all of the nanoparticles in a dark-field image which dramatically enhanced the detection accuracy and avoided random events.

Amperometric Ion Sensing Approaches at Liquid/Liquid Interfaces for Inorganic, Organic and Biological Ions

Electrochemistry at the interface between two immiscible electrolyte solutions (ITIES) has become an invaluable tool for the selective and sensitive detection of cationic and anionic species, including charged drug molecules and proteins. In addition, neutral molecules can also be detected at the ITIES via enzymatic reactions. This chapter highlights recent developments towards creating a wide spectrum of sensing platforms involving ion transfer across the ITIES. As well as outlining the basic principles needed for performing these sensing applications, the development of ITIES-based detection strategies for inorganic, organic, and biological ions is discussed.

Electrochemical nanoprobes for the chemical detection of neurotransmitters

Neurotransmitters, acting as chemical messengers, play an important role in neurotransmission, which governs many functional aspects of nervous system activity. Electrochemical probes have proven a very useful technique to study neurotransmission, especially to quantify and qualify neurotransmitters. With the emerging interests in probing neurotransmission at the level of single cells, single vesicles, as well as single synapses, probes that enable detection of neurotransmitters at the nanometer scale become vitally important. Electrochemical nanoprobes have been successfully employed in nanometer spatial resolution imaging of single nanopores of Si membrane and single Au nanoparticles, providing both topographical and chemical information, thus holding great promise for nanometer spatial study of neurotransmission. Here we present the current state of electrochemical nanoprobes for chemical detection of neurotransmitters, focusing on two types of nanoelectrodes, i.e. carbon nanoelectrode and nano-ITIES pipet electrode.