Electrochemical Detection of Single Aqueous Droplets in Organic Solvents via Pitting Collisions.

We report a novel detection method for single aqueous droplets in organic solvents by the collisional contact of the droplet, inducing the partial deformation of the ultramicroelectrode (UME) surface. For various chemical reactions in organic solvents, water impurities affect the catalytic activity, leading to a loss of productivity and selectivity. Therefore, it is necessary to monitor the water content of organic solvents in real time between many chemical production processes, from the laboratory to the industrial scale. Our method enables the detection of water contamination by real-time monitoring of the electrochemical signals or observing morphological changes in the microelectrode. When an aqueous droplet collides with the UME, the contact area of the electrode is electrolyzed, forming pits on the surface where the droplet falls. Current transient analysis shows a unique current spike corresponding to the reaction inside the adsorbed single aqueous droplet, which differs from those detected by the faradaic/nonfaradaic reaction of collision of other particles. Moreover, this analytical method can record the history of collision events from pits on the UME surface, implying that inspecting the UME surface could be a quick screening method for solvent contamination. Based on a comparison of the electrochemical signals and morphological changes of the electrode after each event, the sizes of the pits and droplets are related. A COMSOL simulation is performed to explain the shape of the peak current and pit formation during collision events. This experimental concept elucidates the dynamic behavior of aqueous droplets on a positively biased metal electrode.

Electrochemical Characterization of Neurotransmitters in a Single Submicron Droplet.

Single-entity electrochemistry, which employs electrolysis during the collision of single particles on ultramicroelectrodes, has witnessed significant advancements in recent years, enabling the observation and characterization of individual particles. Information on a single aqueous droplet (e.g., size) can also be studied based on the redox species contained therein. Dopamine, a redox-active neurotransmitter, is usually present in intracellular vesicles. Similarly, in the current study, the electrochemical properties of neurotransmitters in submicron droplets were investigated. Because dopamine oxidation is accompanied by proton transfer, unique electrochemical properties of dopamine were observed in the droplet. We also investigated the electrochemical properties of the adsorbed droplets containing DA and the detection of oxidized dopamine by the recollision phenomenon.

Highly emissive 4-carbazole-appended salen-indium complex: the effect of strong donor-acceptor interaction.

Herein, we report our findings on 4-carbazole (CBZ)-appended salen-based indium complexes, CBZIn1 and CBZIn2, which feature diimine bridges exhibiting different electron-accepting properties. Notably, CBZIn2 exhibited a significantly higher photoluminescence quantum efficiency (PLQY, ΦPL) in toluene than CBZIn1, with a value over 15 times greater (ΦPL = 57.7% for CBZIn2; ΦPL = 3.7% for CBZIn1). In particular, in the rigid state of THF at 77 K, CBZIn2 exhibited a near-unity PLQY of 98.2%. Even in the PMMA film, CBZIn2 maintained a high level of PLQY (ΦPL = 70.2%). These results can be attributed to the highly efficient radiative decay process based on intramolecular charge-transfer (ICT) transition between the moderately twisted CBZ, characterized by its conformational rigidity and the 1,2-dicyanoethylene-bridged salen, which exhibits a strong electron-accepting ability. Furthermore, these findings are supported by theoretical calculations.

The Development of a Permanent Implantable Spacer with the Function of Size Adjustability for Customized Treatment of Regurgitant Heart Valve Disease.

The Pivot Mandu is an innovative device featuring a leak-tight adjustable 3D balloon spacer, incorporating inner mesh support, an outer e-PTFE layer, and a compliant balloon in the middle layer with a specialized detachable system. To assess its feasibility, proof of concept was rigorously evaluated through bench testing and survival porcine animal experiments. The results demonstrated successful remote inflation of the balloon system, with the balloon spacer exhibiting sustained patent and functional integrity over an extended observation period of up to 6 months. A noteworthy feature of the newly designed 3D balloon spacer is its capability for easy size adjustment during procedures, enhancing its adaptability and practicality in clinical settings. This three-layered 3D balloon spacer, with its established long-term patency, exhibits highly encouraging outcomes that hold promise in overcoming the current limitations of spacer devices for heart valve diseases. Given the compelling results from preclinical investigations, the translation of the Pivot Mandu into human trials is strongly warranted.

Electrochemical Analysis of Attoliter Water Droplets in Organic Solutions through Partitioning Equilibrium.

Herein, we report the electrochemical monitoring of attoliters of water droplets in an organic medium by the electrolysis of an extracted redox species from the continuous phase upon collisional events on an ultramicroelectrode. To obtain information about a redox-free water droplet in an organic solvent, redox species with certain concentrations need to be contained inside it. The redox species inside the droplet were delivered by a partitioning equilibrium between the organic phase and the water droplets. The mass transfer of the redox species from the surrounding organic phase to the droplet is very fast because of the radial diffusion, which resultantly establishes the equilibrium. Upon the collisional contact between the droplet and the electrode, the extracted redox species in the water droplets were selectively electrolyzed, even though the redox species in the organic continuous phase remained unreacted because of the different solvent environments. The electrolysis of the redox species in the droplets, where the concentration is determined by the equilibrium constant of the redox species in water/oil, can be used to estimate the size of single water droplets in an organic solution.

In Situ Probing Liquid/Liquid Interfacial Kinetics through Single Nanodroplet Electrochemistry.

In this study, we report the new application of single nanodroplet electrochemistry to in situ monitor the interfacial transfer kinetics of electroactive species across liquid/liquid interface. Interfacial kinetic information is crucial in drug delivery and membrane transport. However, interfacial information has been mainly studied thermodynamically, such as partition coefficient, which could not manifest a speed of transfer. Herein, we measure the phase-transfer kinetic constant via the steady-state electrochemistry of an extracted redox species in a single nanodroplet. The redox species were transferred from the continuous oil phase to the water nanodroplet by partition equilibrium. The transferred redox species are selectively electrolyzed within the droplet when the droplet contacts with an ultramicroelectrode, while the electrochemical reaction of the redox species outside the droplet (i.e., organic solvent) is effectively suppressed by adjusting the electrolyte composition. The redox species in the water droplets can quickly attain a steady state during electrolysis owing to an extensive mass transfer by radial diffusion, and the steady-state current can be analyzed to obtain kinetic information with help from the finite-element method. Finally, a quick calculation method is suggested to estimate the kinetic constant of phase transfer without simulation.

In Situ Monitoring of Collision and Recollision Events of Single Attoliter Droplets via Single-Entity Electrochemistry.

We describe a simple method for real-time observation of collision and recollision behavior of a single aqueous attoliter droplet in an organic solvent through single-entity electrochemistry. The dynamics and morphology of the droplet after the collision event at the Au ultramicroelectrode (Au-UME) were monitored by consecutive cyclic voltammetry and amperometric current-time measurements. By sequentially applying oxidative potential and reductive potential at the Au-UME in the presence of attoliter droplets containing reversible redox species (e.g., ferrocyanide), we successfully detected the oxidative collision spike and its reductive recollision spike successively owing to the reversible redox reactions inside the droplet. Because the redox species was dissolved in a reduced form, the reductive collision spikes observed are the direct evidence that the water droplets colliding at the electrode surface are detached after the oxidation reaction. The collided droplet properties, such as size change and contact area, are also investigated and discussed.

Determining mean corpuscular volume and red blood cell count using electrochemical collision events.

Blood tests (e.g., red blood cell (RBC) count) are crucial for detecting, diagnosing, and monitoring the progression of blood disorders. Here, we report the development of a new and rapid method for electrochemically detecting RBCs using single-particle collision events. The principle of this method relies on the electrochemical oxidation of an electroactive redox species (potassium ferrocyanide) hindered by an RBC attached to an electrode surface. A decrease in staircase current, caused by the collision of RBCs on the electrode, was observed. The magnitude of this current decrease could provide quantitative information on the size and concentration of RBCs, which could be converted into the mean corpuscular volume (MCV) and used for diagnosis. Anemia-related diseases caused by abnormal count of RBCs (e.g., erythrocytosis, pernicious anemia) or abnormal RBC size (e.g. megaloblastic anemia, microcytic anemia) could be detected easily and quickly using this electrochemical collision method, potentially leading to extensive applications in hematology and point-of-care blood testing devices.

Programmable Electrochemical Rectifier Based on a Thin-Layer Cell.

A programmable electrochemical rectifier based on thin-layer electrochemistry is described here. Both the rectification ratio and the response time of the device are programmable by controlling the gap distance of the thin-layer electrochemical cell, which is easily controlled using commercially available beads. One of the electrodes was modified using a ferrocene-terminated self-assembled monolayer to offer unidirectional charge transfers via soluble redox species. The thin-layer configuration provided enhanced mass transport, which was determined by the gap thickness. The device with the smallest gap thickness (∼4 µm) showed an unprecedented, high rectification ratio (up to 160) with a fast response time in a two-terminal configuration using conventional electronics.

Label-Free Detection of Single Living Bacteria via Electrochemical Collision Event.

We detected single living bacterial cells on ultramicroelectrode (UME) using a single-particle collision method and optical microscopic methods. The number of collision events involving the bacterial cells indicated in current-time (i-t) curves corresponds to the number of bacterial cells (i.e., Escherichia coli) on the UME surface, as observed visually. Simulations were performed to determine the theoretical current response (75 pA) and frequency (0.47 pM(-1) s(-1)) of single Escherichia coli collisions. The experimental current response (83 pA) and frequency (0.26 pM(-1) s(-1)) were on the same order of magnitude as the theoretical values. This single-particle collision approach facilitates detecting living bacteria and determining their concentration in solution and could be widely applied to studying other bacteria and biomolecules.

Electrochemical Detection of Hydrazine Using Poly(dopamine)-Modified Electrodes.

We have developed a simple and selective method for the electrochemical detection of hydrazine (HZ) using poly(dopamine) (pDA)-modified indium tin oxide (ITO) electrodes. Modification with pDA was easily achieved by submerging the ITO electrode in a DA solution for 30 min. The electrocatalytic oxidation of HZ on the pDA-modified ITO electrode was measured by cyclic voltammetry. In buffer solution, the concentration range for linear HZ detection was 100 µM-10 mM, and the detection limit was 1 µM. The proposed method was finally used to determine HZ in tap water to simulate the analysis of real samples. This method showed good recovery (94%-115%) and was not affected by the other species present in the tap water samples.

Synthesis of gold coated magnetic microparticles and their application for electrochemical glucose sensing by the enzymatically precipitated prussian blue.

An enzyme stimulated deposition of prussian blue onto the gold-coated magnetic microparticles is described. We propose to synthesize the continuous outer gold layer on the magnetic particle for a gold working electrode and its superparamagnetic property. In-depth characterization of the gold shell formation was studied with scanning electron microscopy, energy-dispersive X-ray spectroscopy, cyclic voltammetry. The gold-coated magnetic microparticles offered adhesive layer for the immobilization of glucose oxidase catalyzing the generation of prussian blue in the presence of glucose. The assembled prussian blue on the gold shell surfaces was detected with electrochemical measurements depending on the glucose concentration. With accomplishing the linear response range from 0.2 mM to 10 mM of glucose, this approach successfully proposed the applicability of the magnetic core-gold shell structures to the electrochemical bioassay area.

Single particle detection by area amplification: single wall carbon nanotube attachment to a nanoelectrode.

We describe the electrochemical detection of single nanoparticle (NP) attachment on a nanoelectrode by the increase in the active electrode area. The attachment of gold NP-decorated single wall carbon nanotubes (Au-SWCNTs) was observed by their current-time transients for ferrocenemethanol (FcMeOH) oxidation. Since the attached Au-SWCNT increases the electroactive area available for FcMeOH oxidation, the current increases after attachment of the particle. The "staircase" shape of the current response establishes that the particles do not become deactivated for the outer-sphere electron transfer reaction after attachment. Au-SWCNTs migrate to and are held at the nanoelectrode by an electric field. However, SWCNTs that are not decorated with a gold NP produce only a sharp transient ("blip") response.

Open circuit (mixed) potential changes upon contact between different inert electrodes-size and kinetic effects.

We investigate the principle of the open circuit potential (OCP) change upon a particle collision event based on mixed potential theory and confirmed by a mimic experiment in which we studied the changes in the OCP when two different electrodes (Pt and Au) are brought into contact in a solution that contains some irreversible redox couples. A micrometer-sized Au ultramicroelectrode, when connected in parallel to a Pt micro- or nanoelectrode, showed clearly measurable OCP changes whose magnitude matches well with that predicted by a simplified mixed potential theory for a pair of different electrode materials. On the basis of the study, each electrode establishes a different mixed potential involving two or more half reactions that have different heterogeneous electron transfer kinetics at different electrodes and the OCP changes are very sensitive to the relative ratio of the rate constant of the individual half reaction at different materials.

Observation of single metal nanoparticle collisions by open circuit (mixed) potential changes at an ultramicroelectrode.

Single nanoparticle (NP) collisions were successfully observed by a potentiometric measurement. The open circuit potential (OCP) of a measuring Au ultramicroelectrode (UME) changes when Pt NPs collide with the UME in a hydrazine solution. The OCP change is related to the redox processes, the concentration of particles, particle size, and electrode size. Compared with the amperometric technique, this approach has several advantages: higher sensitivity, simpler apparatus, fewer problems with NP decomposition, and contamination.

Ordered polymeric microhole array made by selective wetting and applications for electrochemical microelectrode array.

In this paper, we report the microelectrode array fabrication using selective wetting/dewetting of polymers on a chemical pattern which is a simple and convenient method capable of creating negative polymeric replicas using polyethylene glycol (PEG) as a clean and nontoxic sacrificial layer. The fabricated hole-patterned polypropylene film on gold demonstrated enhanced electrochemical properties. The chemical pattern is fabricated by microcontact printing using octadecanethiol (ODT) as an ink on gold substrate. When PEG is spin-cast on the chemical pattern, PEG solution selectively dewets the ODT patterned areas and wets the remaining bare gold areas, leading to the formation of arrayed PEG dots. A negative replicas of the PEG dot array is obtained by spin-coating of polypropylene (PP) solution in hexane which preferentially interacts with the hydrophobic ODT region on the patterned gold surface. The arrayed PEG dots are not affected the during PP spin-coating step because of their intrinsic immiscibility. Consequently, the hole-patterned PP film is obtained after PEG removal. The electrochemical signal of the PP film demonstrates the negligible leakage current by high dielectric and self-healing of defects on the chemical pattern by the polymer. This method is applicable to fabrication of microelectrode arrays and possibly can be employed to fabricate a variety of functional polymeric structures, such as photomasks, arrays of biomolecules, cell arrays, and arrays of nanomaterials.

Nanosieving of anions and cavity-size-dependent association of cyclodextrins on a 1-adamantanethiol self-assembled monolayer.

In this paper, we studied charge transfer through a self-assembled monolayer (SAM) of 1-adamantanethiol on gold. Charge transfer through the 1-adamantanethiol SAM depended on the type of anion present when [Fe(CN)6]3- was used as a redox probe. The sluggish charge transfer process was monitored by cyclic voltammetry using the relatively large and hydrophobic perchlorate and hexafluorophosphate ions as the supporting electrolyte. In contrast, the charge transfer kinetics were nearly identical to those measured on bare gold with chloride, sulfate, and nitrate ions as the supporting electrolyte. We investigated the adsorption of alpha- and beta-cyclodextrin on the 1-adamantanethiol SAM via a host-guest interaction. The 1-adamantanethiol SAM could not bind beta-cyclodextrin via a host-guest interaction, probably due to the proximity of neighboring adamantine molecules on the surface. Immobilization of alpha-cyclodextrin by formation of an exterior complex with the SAM suppressed charge transfer. The adsorbed alpha-cyclodextrin was quantified using faradaic impedance experiments. The obtained adsorption isotherm was in good agreement with the Langmuir isotherm with a binding constant of 39.53 M(-1).

Characterization and electrocatalytic properties of Prussian blue electrochemically deposited on nano-Au/PAMAM dendrimer-modified gold electrode.

Gold electrode was modified with 3-mercaptopropionic acid (MPA) and further reacted with poly(amidoamine) (PAMAM) dendrimer (generation 4.0) then attached the nano-Au to obtain films on which Prussian blue (PB) was electrochemically deposited to afford much wider pH adaptive range, much better electrochemical stability and excellent electrochemical response. The microstructure and electrochemical behavior of Au/MPA/PAMAM/nano-Au/PB electrode were investigated by scanning electron microscopy (SEM) and cyclic voltammetry. The electrochemical response of the Au/MPA/PAMAM/nano-Au/PB-modified electrode for the electrocatalytic reduction of hydrogen peroxide was investigated, and it was found that the sensitivity as well as the corresponding detection limits were improved as compared to the voltammetric response of a Au/PB-modified electrode and Au/MPA/PAMAM/PB electrode. Based on this, a new electrochemical sensor for determination of hydrogen peroxide has been developed.