Ultramicroelectrode Research Articles

Study on simplified strategies for procedure of rapid detection of water toxicity

In this work, a simplified procedure of detection of water toxicity based on Pt ultramicroelectrode (UME) and mixed microorganism cultured without sterilization was the first proposed. A stable Pt UME was successfully prepared with a special glass tube as insulation and support material, which was used as working electrode in the biosensor. The Pt UME exhibits the typical cyclic voltammogram (CV) of Pt UME with sigmoid shape and possesses good stability, enlarged current response and tunable dimension. In addition, it was an effective and simple method for toxicity biosensor using mixed microorganisms cultured in unsterilized lysogeny broth (LB) as the bioreceptor. K3[Fe(CN)6] was used as an electron mediator. Under the optimal conditions of 30 mM K3[Fe(CN)6], OD600 = 1 cell concentration, and 50 mM phosphate-buffered solution (PBS), the half-maximal inhibitory concentration (IC50) values measured for Cd2+, Cu2+ and Ni2+ were 3.99 mg/L, 1.16 mg/L and 2.37 mg/L, respectively. The results indicated that the biosensor with large diameter Pt UME and mixed microorganisms cultured in unsterilized LB realized rapid and simple detection of water toxicity.

Oxygen Absorption in Electrocatalyst Layers Detected by Scanning Electrochemical Microscopy.

Scanning electrochemical microscopy (SECM) is able to track the local electrochemical activity of an electrolyte‐immersed substrate employing an ultra‐micro‐electrode (UME) in micrometer‐scale spatial resolution. In this study, SECM is employed to investigate the presence of oxygen in the electrocatalyst layers of polymer electrolyte membrane fuel cells and electrolyzers. Approach curves on electrocatalyst layers with the tip potential set for oxygen reduction reveal that a significant amount of oxygen is absorbed in the catalyst layer. We confirm that the coexistence of Nafion ionomer and carbon black leads to oxygen confinement. It is suggested that this oxygen is confined within the hydrophobic parts of the self‐assembled Nafion on the graphitic surfaces of the carbon black.

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.

Temporally Resolved Electrochemical Interrogation for Stochastic Collision Dynamics of Electrogenerated Single Polybromide Droplets.

In this article, we present electrochemical interrogation for collision dynamics of electrogenerated individual polybromide ionic liquid (PBIL) droplets through chronoamperometry combined with fast scan cyclic voltammetry (CA-FSCV). In the CA mode of CA-FSCV, a Pt ultramicroelectrode (UME) acts as the electrochemical generator for PBIL droplets by holding the oxidation potential for Br- in a given time, while FSCV is repetitively performed at a certain frequency. In the FSCV mode of CA-FSCV, a Pt UME serves as the probe to electrochemically monitor Br3- reduction for an adsorbed PBIL droplet during collision with a high temporal resolution. Based on the newly introduced CA-FSCV, we can estimate the dynamic changes in the following parameters for a short collision time: the contact radius of a PBIL droplet on a Pt UME, the concentration of Br- in the droplet, and the apparent charge transfer rate constant for electro-reduction of Br3- to Br- in the droplet, koapp. Moreover, a computational calculation using molecular dynamics is presented that can explain the change in koapp as a function of time for Br- electrolysis in a PBIL droplet. Based on the quantitative estimation of the above parameters, we suggest a more advanced mechanism for the stochastic electrochemical collision process of a PBIL droplet. These findings are important for understanding QBr2n+1/QBr half redox reactions in aqueous energy storage systems, such as Zn-Br redox flow batteries and Br-related redox enhanced electrochemical capacitors.

Brain-implantable multifunctional probe for simultaneous detection of glutamate and GABA neurotransmitters

Glutamate (GLU) and gamma-aminobutyric acid (GABA) are neurotransmitters (NTs) with an essential role in signal transmission in the brain. Brain disorders, such as epilepsy, Alzheimer's and Parkinson's diseases, and traumatic brain injury can be linked to imbalances in the GLU-GABA homeostasis that occurs in sub-second to seconds time frames. Current measurement techniques can detect these two NT concentrations simultaneously only in vitro. The present work reports on the fabrication of a silicon multifunctional biosensor microarray probe for sub-second simultaneous GLU-GABA detection in real-time, with excellent analyte sensitivity and selectivity and in vivo capabilities. The novel Si probes feature four surface-functionalized platinum ultramicroelectrodes (UMEs) for simultaneous amperometric detection of GLU and GABA with a sentinel, and a built-in microfluidic channel for the introduction of neurochemicals in the proximity of the UMEs. The microchannel also allows functioning of an On-Demand In-situ Calibrator that runs in-situ biosensor calibration. The probe exhibited excellent robustness at insertion in agarose-gel brain-tissue-mimicking test, and remarkably high hydrogen peroxide sensitivity (a by-product of GLU-GABA enzyme biosensor) with values on the order of 5000 nA μM -1 cm -2 and maximum sensitivities of 204±15 nA μM -1 cm -2 and 37±7 nA μM -1 cm -2 for GLU and GABA, respectively. Furthermore, the limit of detection of the biosensors reached as low as 7 nM, 165 nM and 750 nM for H 2 O 2, GLU and GABA, respectively and a temporal resolution of hundreds of milliseconds during in vivo studies using freely moving rats.

(Invited) Potential Dependent Layering in the Electrochemical Double Layer of Highly Concentrated Electrolytes

Highly concentrated electrolytes are recently of great interest because of their possible relevance for batteries. The origin for their promising application has been ascribed to the formation of percolating nanodomains in the bulk. However, the interfacial structure of these concentrated electrolytes still remains to be understood. In this talk we characterize the potential dependent double layer of a LiTFSI-based electrolyte on a charged electrode surface. Ultramicroelectrode (UME) measurements reveal a surface-confinement effect for a ferricyanide redox species at the electrode/WiSE interface. Potential dependent atomic force microscopy (AFM) show the presence of layers, the structure of which changes with applied potential. Thicker layers (6.4 Å and 6.7 Å) are observed at positive potentials, associated with [Li(H2O)x]+([TFSI]-)y ion pairs, while thinner layers (2.8 Å and 3.3 Å) are found at negative potentials and associated with [Li(H2O)x]+ alone. Vibrational spectroscopy shows that the composition of the double layer also changes with potential, where [TFSI]- is enriched at positive and [Li(H2O)x]+ enriched at negative potentials, respectively. The talk will also discuss battery-related applications of these electrolytes.

Amorphous carbon nitride microband integrated in a microfluidic device for DNA biosensors applications

This study presents the use of new kind of carbon electrode materials as ultramicroelectrodes (UMEs) in the field of electrochemical DNA biosensors which has already been proven to be effective in protocols to DNA sequences hybridization. In contrast to other carbon materials such as diamond like carbon, that are difficult to integrate in microfluidic devices due to their high temperature deposition, amorphous carbon nitride (a-CNx) is easily synthesized at room temperature on various materials using sputtering techniques. Here, we report a-CNx use as microband electrodes in Glass/PDMS microfluidic devices. a-CNx electrodes were activated and then biofuntionalized by covalent grafting of a DNA probe as self-assembled monolayer (SAM) with a view to future development of a detection platform targeting circulating DNA or RNA sequences in microfluidic channels.

A Review: Scanning Electrochemical Microscopy (SECM) for Visualizing the Real-Time Local Catalytic Activity

Scanning electrochemical microscopy (SECM) is a powerful scanning probe technique for measuring the in situ electrochemical reactions occurring at various sample interfaces, such as the liquid-liquid, solid-liquid, and liquid-gas. The tip/probe of SECM is usually an ultramicroelectrode (UME) or a nanoelectrode that can move towards or over the sample of interest controlled by a precise motor positioning system. Remarkably, electrocatalysts play a crucial role in addressing the surge in global energy consumption by providing sustainable alternative energy sources. Therefore, the precise measurement of catalytic reactions offers profound insights for designing novel catalysts as well as for enhancing their performance. SECM proves to be an excellent tool for characterization and screening catalysts as the probe can rapidly scan along one direction over the sample array containing a large number of different compositions. These features make SECM more appealing than other conventional methodologies for assessing bulk solutions. SECM can be employed for investigating numerous catalytic reactions including the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen evolution reaction (HER), water oxidation, glucose oxidation reaction (GOR), and CO2 reduction reaction (CO2RR) with high spatial resolution. Moreover, for improving the catalyst design, several SECM modes can be applied based on the catalytic reactions under evaluation. This review aims to present a brief overview of the recent applications of electrocatalysts and their kinetics as well as catalytic sites in electrochemical reactions, such as oxygen reduction, water oxidation, and methanol oxidation.

Suspended highly 3D interdigitated carbon microelectrodes

3D interdigitated pyrolytic carbon microelectrodes (3D IDE) with high complexity and interconnectivity were fabricated taking advantage of suspended interdigitated microstructures. A novel fabrication method for a 3D interdigitated polymer precursor template was developed based on a dual photoresist process including multiple UV exposures at two different wavelengths. The precursor structures were subsequently pyrolyzed at 1100°C for 1 h in N2 environment to obtain 3D interdigitated carbon microelectrodes. Different 3D electrode designs were fabricated and the electrochemical performance was evaluated by cyclic voltammetry and electrochemical impedance spectroscopy (EIS). 3D IDE with smaller structural dimensions displayed ultra-microelectrode (UME)-like behavior with a sigmoidal shape of the cyclic voltammograms (CV) and the absence of the linear diffusion regime in EIS. Furthermore, the 3D IDE displayed 2-fold higher peak currents in CV and significantly reduced charge transfer resistance compared to 2D IDE.

Characterization of Electrodeposited Li Metal by Cryo-Scanning Transmission Electron Microscopy/Electron Energy Loss Spectroscopy

The Li metal anode is a promising key component for next-generation high-energy-density batteries. Understanding the charge/discharge mechanism of Li metal is therefore necessary for the effective utilization of Li metal anodes in commercial batteries. In this study, scanning transmission electron microscopy (STEM) combined with electron energy loss spectroscopy (EELS) was conducted to reveal the chemical state of the Li metal anode surface. Cryogenic techniques and ultramicroelectrodes (UMEs) enabled the observation of electrodeposited Li metal on the nanometer scale. The chemical compositions of several surface layers were revealed by cryo-STEM-EELS analysis, and these measurements gave crucial information regarding the surface layer of the electrodeposited Li metal.

Fabrication and electrochemical response of pristine graphene ultramicroelectrodes

Ultramicroelectrodes (UMEs) are electrochemical working electrodes with at least one dimension less than 25 μm. Here, we demonstrate a method to fabricate pristine (contamination and doping-free) monolayer graphene UMEs and characterise its electrochemical properties. Graphene is particularly sensitive to both contamination from device fabrication and doping from the substrate, and these effects are detrimental to accurate measurements of fundamental electrochemical properties as well as electrochemical electrode applications. We demonstrate a UME design and fabrication steps which ensure that the electro-active surface of the graphene never comes into contact with contaminants such as process polymers and solvents, has a well-defined area due to self-aligned fabrication, and we shield the converse side of the graphene monolayer with the highly inert dielectric hexagonal boron nitride (h-BN). Cyclic voltammetry demonstrates the expected steady-state behaviour for a UME in the hemispherical diffusion regime. The reduction of IrCl62− in weak KCl electrolytes was employed to investigate the electron transfer characteristics of the pristine graphene UMEs and the reproducibility of the measurements. Values of the standard rate constant, k0 and the transfer coefficient, α were measured to be 3.04 ± 0.78 × 10−3 cm s−1 and 0.272 ± 0.024 respectively. These values are consistent for monolayer graphene with reduced charge doping from contaminants and the substrate. This is further confirmed by Raman spectroscopy measurements of the graphene UME before and after the electrochemical measurements.

Intrinsic Electrocatalytic Activity of Single MoS2 Quantum Dot Collision on Ag Ultramicroelectrodes

It is still a challenge to identify the intrinsic electrocatalytic activity of single MoS2 quantum dots (QDs) because their characterization usually dissects contributions from a large amount of particles. Herein, we demonstrate a single-entity electrochemical method to help us investigate the enhancing hydrogen evolution reaction (HER) activity at the single MoS2 QD level by using silver ultramicroelectrodes (UMEs). At Ag UMEs, the i–t response for MoS2 QD collision consists of repeated current “spikes” that return to the background level as hydrogen formation and covering the QD surface. In contrast, a stepwise “staircase” i–t response is observed for MoS2 QDs colliding on carbon UMEs (C UMEs). The results also show that the current density of single MoS2 QD collision on Ag UMEs is much higher than on C UMEs, revealing the impact of the unique property of the metal–MoS2 interface on HER performance, which has been demonstrated by thermodynamic analysis and density functional theory (DFT) calculations. It has been found that the interactions between MoS2 QDs and substrate UMEs lead to significant changes in the hydrogen adsorption energies (ΔEads) and Gibbs free energy (ΔGH), resulting in the difference in HER activity.

Single Potential Scan Methods for Nanoporous Gold Formation on Ultramicroelectrode Surfaces

AbstractNanoporous gold (NPG) has been extensively investigated because of their applications. Here, we report a straightforward method for the preparation of NPG on micrometer‐scale electrodes. Well‐defined NPG structures were formed on Au surfaces by a single potential scan within 100 seconds. This method is applied to Au surfaces with small dimensions regardless of the electrode geometry, whereas it is not applicable to conventional millimeter‐scale electrodes. The effects of electrode sizes and scan rates on NPG formation were systematically examined, and the amperometric glucose detection with 20‐μL sample volumes using an ultramicroelectrode (UME) with NPG surfaces was demonstrated.

Monitoring single Au38 nanocluster reactions via electrochemiluminescence.

Herein, we report for the first time single Au38 nanocluster reaction events of highly efficient electrochemiluminescence (ECL) with tri-n-propylamine radicals as a reductive co-reactant at the surface of an ultramicroelectrode (UME). The statistical analyses of individual reactions confirm stochastic single ones influenced by the applied potential.

Electrochemical Amplification Methods in Observation of Single Nanoparticle Collisions

We describe the electrochemical detection of single nanoparticle (NP) collision at a less catalytic electrode. We could observe the various single NP collision through the enhanced current by electrocatalytic redox recycling. The current responses were caused by random individual events as a function of time rather than the usual continuous current caused by an ensemble of a large number of events. The overall current transient such as spike-like or staircase, changed depending on the reaction mechanism of collided NP. The height and frequency of the peak current are proportional to the applied potential and the particle concentration.Single iridium oxide (IrOx) NP collision at a less catalytic electrode surface such as the NaBH4-treated Pt or the bare Au ultramicroelectrode (UME) was investigated. The overall current transient consists of repeated current spikes, superimposed on a current decay to reach a steady-state value.Collisions of Ag NPs with a more electrocatalytic Au or Pt surface have been observed. Depending on the applied potential to the UME, the current response of Ag NPs collision onto the UME resulted in various shape changes. A staircase decrease, a blip decrease, and a blip increase of the hydrazine oxidation current were obtained at an applied potential of 0.33 V, 0.80 V, and 1.3 V, respectively.The current response of ascorbic acid stabilized Cu single NP collision on Au UME surface was observed. Here, the glucose oxidation electrocatalyzed by oxidized Cu NP was used as the indicating reaction. In this system, the NP collision signals were obtained simultaneously by both direct particle electrolysis and electrocatalytic amplification.

Electrochemical Detection Method Using Serotonin Oxidation inside a Single Human Platelet

An early diagnosis of platelet-related diseases is important for maintaining a healthy human life for blood clotting disorders caused by internal and/or external stimuli. Various technologies based on physics and chemistry have been developed for the detection of platelets. These methods are mainly set up in clinical laboratory and hospitals, cannot be portable, and require both complex pretreatment procedure and expensive detectors for platelet detection. Platelets are one of the human blood compositions and an important role in blood coagulation. Therefore, it is necessary to develop a method that can be adopted as a medical device capable of diagnosis in everyday life. We proposed an electrochemical collision method for direct detection of serotonin in a single platelet from human blood. This method is based on collisions of platelets with an ultramicroelectrode (UME) and observed the oxidation signal of serotonin without any modification of electrode through single-entity electrochemistry (SEE). When a platelet collides with the UME, secreted serotonin from the platelet gets oxidized at the electrode surface generating anodic currents. In our methods, the current for effective detection of platelets is very sensitive because there is no redox species. Studies on biomolecule detection through SEE can be useful for measuring the number of blood cells in our blood. Observing changes in the current that occur when platelets selectively collide with the UME, is medically useful information obtained cheaply and easily. The SEE method applied to blood analysis will be a new way to overcome the limitations of the existing sampling process, analysis time and environment. It is also expected to contribute to the development of high-performance sensors based on point-of-care-testing (POCT) devices capable of on-site and real-time diagnosis.

Electrochemical Detection of Inverse Emulsion Droplets.

We report electrochemical detection of the collisions of single water droplets in organic solvent using an ultramicroelectrode (UME). Water-in-oil droplets were made from the ultrasonic mixing of organic solvent (e.g., diethylene chloride) and aqueous solution containing the redox species. And emulsion stabilizer as well as the supporting electrolyte were carefully selected and added to perform the attoliter-droplet electrochemistry. With appropriately applied potentials at an UME, we could observe the single emulsion droplet collision phenomena from in the amperometric current-time (i-t) measurement. Emulsions that have established ionic conduction to the collided UME exhibit amperometric current spikes by instantaneous electrolysis of a redox species in single water-in-oil droplets. The size and contact area of the water droplets were also estimated by analyzing the collisional current peaks. On the basis of the collisional events, we could successfully establish a facile water droplets detection system in an organic solvent medium.

Observing the Current Change with Increasing Temperature in Thermo-Responsive Polymer Solution Using Electrochemical Method

The behavior of redox in thermo-responsive polymer solution while heating was electrochemically observed and analyzed in this research. Temperature responsive polymers are intensively applied to various research fields and industries such as drug carriers, lithium ion batteries, temperature sensitive films, and sensor technology. For this reason, measuring the critical point and analyzing the phase transition of temperature stimuli responsive polymer are very important.In previous studies, the phase behavior of polymers according to the temperature was examined by measuring the transmittance of solution using ultraviolet–visible (UV-vis) spectroscopy and the size of polymer particles in solution using dynamic light scattering (DLS). As an electrochemical method, a collision method was used for detecting current decrease caused by aggregated polymer which blocked the electrode surface. These methods, however, were unable to track the behavior of polymer in solution during continuous temperature increase in real time easily. The temperature was increased at intervals of 5 ℃ to measure the absorbance of solution in UV-vis spectroscopy and only specific temperature and time could be used to observe the blocking signal in the electrochemical collision method. In this study, we offer the easy and novel way to monitor the thermal induced change of polymer and redox with continuous temperature increase using the electrochemical system.Lower critical solution temperature (LCST) polymer was used as the thermo-responsive polymer in this research. Dissolving the LCST polymer in organic media containing the redox, we measured the current with increasing temperature in real time. The experiment was conducted using electrochemical method with an ultramicroelectrode (UME) and the current transition according to the temperature growth was monitored. As a result, the specific current changes were obtained. Analyzing these specific current changes can be used to confirm the phase transition behavior of the temperature responsive polymer and redox in polymer solution. This study will be applied for further research with controlling the various conditions.

Study on Collided Ag or Pt Single Nanoparticle’s Current Peak Shape Changing Depending on Applying Potential

Electrocatalytic amplification method was used for analyzing electrochemical properties about collided nanoparticles. Silver nanoparticles (Ag NPs) and platinum nanoparticles (Pt NPs)’s characteristics about hydrazine oxidation reaction were investigated. Those NPs have distinct electrocatalytic activation for hydrazine oxidation reaction. For detecting these NPs, gold (Au) ultramicroelectrode (UME) was used as working electrode which has appropriate catalytic activities comparing with both NPs. When applied potential was above 0 V, Pt NPs showed positive electrocatalytic characteristic, then collided Pt NPs produced amplified transient current signal. In contrast, Ag NPs produced decreased transient current signal when colliding onto Au UME applied above 0.25 V which potential range Ag NPs showed negative electrocatalytic characteristics. But the transient current signals by each collided NPs showed shape shifting as changing applied potential. When applying relatively low potential range, current signals produced by each NPs were staircase shape which means the collided NPs sustain stable state on Au UME after collision events. After changing high potential range, the current signals were shifting to blip or spike shape. As analyzing current peak shape changing phenomenon, electrocatalytic reaction mechanisms by each NPs could be understood by this method.