Correlation between Gas Bubble Formation and Hydrogen Evolution Reaction Kinetics at Nanoelectrodes

We report the correlation between H₂ gas bubble formation potential and hydrogen evolution reaction (HER) activity for Au and Pt nanodisk electrodes (NEs). Microkinetic models were formulated to obtain the HER kinetic information for individual Au and Pt NEs. We found that the rate-determining steps for the HER at Au and Pt NEs were the Volmer step and the Heyrovsky step, respectively. More interestingly, the standard rate constant (<i>k</i>⁰) of the rate-determining step was found to vary over 2 orders of magnitude for the same type of NEs. The observed variations indicate the HER activity heterogeneity at the nanoscale. Furthermore, we discovered a linear relationship between bubble formation potential (<i>E</i>_bubble) and log­(<i>k</i>⁰) with a slope of 125 mV/decade for both Au and Pt NEs. As log (<i>k</i>⁰) increases, <i>E</i>_bubble shifts linearly to more positive potentials, meaning NEs with higher HER activities form H₂ bubbles at less negative potentials. Our theoretical model suggests that such linear relationship is caused by the similar critical bubble formation condition for Au and Pt NEs with varied sizes. Our results have potential implications for using gas bubble formation to evaluate the HER activity distribution of nanoparticles in an ensemble

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Electrochemical Generation of a Hydrogen Bubble at a Recessed Platinum Nanopore Electrode

We report the electrochemical generation of a <i>single</i> hydrogen bubble within the cavity of a recessed Pt nanopore electrode. The recessed Pt electrode is a conical pore in glass that contains a micrometer-scale Pt disk (1–10 μm radius) at the nanopore base and a nanometer-scale orifice (10–100 nm radius) that restricts diffusion of electroactive molecules and dissolved gas between the nanopore cavity and bulk solution. The formation of a H₂ bubble at the Pt disk electrode in voltammetric experiments results from the reduction of H⁺ in a 0.25 M H₂SO₄ solution; the liquid-to-gas phase transformation is indicated in the voltammetric response by a precipitous decrease in the cathodic current due to rapid bubble nucleation and growth within the nanopore cavity. Finite element simulations of the concentration distribution of dissolved H₂ within the nanopore cavity, as a function of the H⁺ reduction current, indicate that H₂ bubble nucleation at the recessed Pt electrode surface occurs at a critical supersaturation concentration of ∼0.22 M, in agreement with the value previously obtained at (nonrecessed) Pt disk electrodes (∼0.25 M). Because the nanopore orifice limits the diffusion of H₂ out of the nanopore cavity, an anodic peak corresponding to the oxidation of gaseous and dissolved H₂ trapped in the recessed cavity is readily observed on the reverse voltammetric scan. Integration of the charge associated with the H₂ oxidation peak is found to approach that of the H⁺ reduction peak at high scan rates, confirming the assignment of the anodic peak to H₂ oxidation. Preliminary results for the electrochemical generation of O₂ bubbles from water oxidation at a recessed nanopore electrode are consistent with the electrogeneration of H₂ bubbles

Low-Voltage Origami-Paper-Based Electrophoretic Device for Rapid Protein Separation

We present an origami paper-based electrophoretic device (<i>o</i>PAD-Ep) that achieves rapid (∼5 min) separation of fluorescent molecules and proteins. Due to the innovative design, the required driving voltage is just ∼10 V, which is more than 10 times lower than that used for conventional electrophoresis. The <i>o</i>PAD-Ep uses multiple, thin (180 μm/layer) folded paper layers as the supporting medium for electrophoresis. This approach significantly shortens the distance between the anode and cathode, and this, in turn, accounts for the high electric field (>1 kV/m) that can be achieved even with a low applied voltage. The multilayer design of the <i>o</i>PAD-Ep enables convenient sample introduction by use of a slip layer as well as easy product analysis and reclamation after electrophoresis by unfolding the origami paper and cutting out desired layers. We demonstrate the use of <i>o</i>PAD-Ep for simple separation of proteins in bovine serum, which illustrates its potential applications for point-of-care diagnostic testing

Faradaic Ion Concentration Polarization on a Paper Fluidic Platform

We describe the design and characteristics of a paper-based analytical device for analyte concentration enrichment. The device, called a hybrid paper-based analytical device (hyPAD), uses faradaic electrochemistry to create an ion depletion zone (IDZ), and hence a local electric field, within a nitrocellulose flow channel. Charged analytes are concentrated near the IDZ when their electrophoretic and electroosmotic velocities balance. This process is called faradaic ion concentration polarization. The hyPAD is simple to construct and uses only low-cost materials. The hyPAD can be tuned for optimal performance by adjusting the applied voltage or changing the electrode design. Moreover, the throughput of hyPAD is 2 orders of magnitude higher than that of conventional, micron-scale microfluidic devices. The hyPAD is able to concentrate a range of analytes, including small molecules, DNA, proteins, and nanoparticles, in the range of 200–500-fold within 5 min

Unusual Activity Trend for CO Oxidation on Pd_<i>x</i>Au_{140–<i>x</i>}@Pt Core@Shell Nanoparticle Electrocatalysts

A theoretical and experimental study of the electrocatalytic oxidation of CO on Pd_<i>x</i>Au_{140–<i>x</i>}@Pt dendrimer-encapsulated nanoparticle (DEN) catalysts is presented. These nanoparticles are comprised of a core having an average of 140 atoms and a Pt monolayer shell. The CO oxidation activity trend exhibits an unusual koppa shape as the number of Pd atoms in the core is varied from 0 to 140. Calculations based on density functional theory suggest that the koppa-shaped trend is driven primarily by structural changes that affect the CO binding energy on the surface. Specifically, a pure Au core leads to deformation of the Pt shell and a compression of the Pt lattice. In contrast, Pd, from the pure Pd cores, tends to segregate on the DEN surface, forming an inverted configuration having Pt within the core and Pd in the shell. With a small addition of Au, however, the alloy PdAu cores stabilize the core@shell structures by preventing Au and Pd from escaping to the particle surface

Colorimetric Sensor Array for Discrimination of Heavy Metal Ions in Aqueous Solution Based on Three Kinds of Thiols as Receptors

In the present work, we report a novel colorimetric sensor array for rapid identification of heavy metal ions. The sensing mechanism is based on the competition between thiols and urease for binding with the metal ions. Due to the different metal ion-binding abilities between the thiols and urea, different percentages of urease are free of metal ions and become catalytically active in the presence of varied metal ions. The metal ion-free urease catalyzes the decomposition of urea releasing ammonia and changing the pH of the analyte solution. Bromothymol blue, the pH indicator, changes its color in response to the metal-caused pH change. Three different thiols (l-glutathione reduced, l-cysteine, and 2-mercaptoethanol) were used in our sensor array, leading to a unique colormetric repsonse pattern for each metal. Linear discriminant analysis (LDA) was employed to analyze the patterns and generate a clustering map for identifying 11 species of metal ions (Ni²⁺, Mn²⁺, Zn²⁺, Ag⁺, Cd²⁺, Fe³⁺, Hg²⁺, Cu²⁺, Sn⁴⁺, Co²⁺, and Pb²⁺) at 10 nM level in real samples. The method realizes the simple, fast (within 30 s), sensitive, and visual discrimination of metal ions, showing the potential applications in environmental monitoring

Tunable Negative Differential Electrolyte Resistance in a Conical Nanopore in Glass

Liquid-phase negative differential resistance (NDR) is observed in the <i>i–V</i> behavior of a conical nanopore (∼300 nm orifice radius) in a glass membrane that separates an external <i>low-conductivity</i> 5 mM KCl solution of dimethylsulfoxide (DMSO)/water (v/v 3:1) from an internal <i>high-conductivity</i> 5 mM KCl aqueous solution. NDR appears in the <i>i–V</i> curve of the negatively charged nanopore as the voltage-dependent electro-osmotic force opposes an externally applied pressure force, continuously moving the location of the interfacial zone between the two miscible solutions to a position just inside the nanopore orifice. An ∼80% decrease in the ionic current occurs over less that a ∼10 mV increase in applied voltage. The NDR turn-on voltage was found to be tunable over a ∼1 V window by adjusting the applied external pressure from 0 to 50 mmHg. Finite-element simulations based on solution of Navier–Stokes, Poisson, and convective Nernst–Planck equations for mixed solvent electrolytes within a negatively charged nanopore yield predictions of the NDR behavior that are in qualitative agreement with the experimental observations. Applications in chemical sensing of a tunable, solution-based electrical switch based on the NDR effect are discussed

Overcoming the Potential Window-Limited Functional Group Compatibility by Alternating Current Electrolysis

The functional group compatibility of an electrosynthetic method is typically limited by its potential reaction window. Here, we report that alternating current (AC) electrolysis can overcome such potential window-limited functional group compatibility. Using alkene heterodifunctionalization as a model system, we design and demonstrate a series of AC-driven reactions that add two functional groups sequentially and separately under the cathodic and anodic pulses, including chloro- and bromotrilfuoromethylation as well as chlorosulfonylation. We discovered that the oscillating redox environment during AC electrolysis allows the regeneration of the redox-active functional groups after their oxidation or reduction in the preceding step. As a result, even though redox labile functional groups such as pyrrole, quinone, and aryl thioether fall in the reaction potential window, they are tolerated under AC electrolysis conditions, leading to synthetically useful yields. The cyclic voltammetric study has confirmed that the product yield is limited by the extent of starting material regeneration during the redox cycling. Our findings open a new avenue for improving functional group compatibility in electrosynthesis and show the possibility of predicting the product yield under AC electrolysis from voltammogram features

Electrochemical Measurements of Single H₂ Nanobubble Nucleation and Stability at Pt Nanoelectrodes

Single H₂ nanobubble nucleation is studied at Pt nanodisk electrodes of radii less than 50 nm, where H₂ is produced through electrochemical reduction of protons in a strong acid solution. The critical concentration of dissolved H₂ required for nanobubble nucleation is measured to be ∼0.25 M. This value is ∼310 times larger than the saturation concentration at room temperature and pressure and was found to be independent of acid type (e.g., H₂SO₄, HCl, and H₃PO₄) and nanoelectrode size. The effects of different surfactants on H₂ nanobubble nucleation are consistent with the classic nucleation theory. As the surfactant concentration in H₂SO₄ solution increases, the solution surface tension decreases, resulting in a lower nucleation energy barrier and consequently a lower supersaturation concentration required for H₂ nanobubble nucleation. Furthermore, amphiphilic surfactant molecules accumulate at the H₂/solution interface, hindering interfacial H₂ transfer from the nanobubble into the solution; consequently, the residual current decreases with increasing surfactant concentration