Exceptional Electrical Detection of Trace NO via Mixed Metal MOF-on-MOF Film-Based Sensors.

The tunability of metal-organic frameworks (MOFs) makes them exceptional materials for the development of highly selective, low-power sensors for toxic gas detection. Herein, we demonstrate enhanced detection of NO gas by a MOF-based electrical impedance sensor made using a unique mixed metal MOF-on-MOF synthesis. A combined experimental and computational study was performed using the exemplar NiMg-MOF-74 to understand the fundamental structure-property relationships behind metal mixing and MOF film synthesis methods on sensor performance.

Nanoconfinement of Carbon Dioxide within Interfacial Aqueous/Ionic Liquid Systems.

Nanoporous, gas-selective membranes have shown encouraging results for the removal of CO from flue gas, yet the optimal design for such membranes is often unknown. Therefore, we used molecular dynamics simulations to elucidate the behavior of CO within aqueous and ionic liquid (IL) systems ([EMIM][TFSI] and [OMIM][TFSI]), both confined individually and as an interfacial aqueous/IL system. We found that within aqueous systems the mobility of CO is reduced due to interactions between the CO oxygens and hydroxyl groups on the pore surface.

Transport and Energetics of Carbon Dioxide in Ionic Liquids at Aqueous Interfaces.

A major hurdle in utilizing carbon dioxide (CO) lies in separating it from industrial flue gas mixtures and finding suitable storage methods that enable its application in various industries. To address this issue, we utilized a combination of molecular dynamics simulations and experiments to investigate the behavior of CO in common room-temperature ionic liquids (RTIL) when in contact with aqueous interfaces. Our investigation of RTILs, [EMIM][TFSI] and [OMIM][TFSI], and their interaction with a pure water layer mimics the environment of a previously developed ultrathin enzymatic liquid membrane for CO separation.

Impedance-Based Detection of NO Using Ni-MOF-74: Influence of Competitive Gas Adsorption.

Chemically robust, low-power sensors are needed for the direct electrical detection of toxic gases. Metal-organic frameworks (MOFs) offer exceptional chemical and structural tunability to meet this challenge, though further understanding is needed regarding how coadsorbed gases influence or interfere with the electrical response. To probe the influence of competitive gases on trace NO detection in a simulated flue gas stream, a combined structure-property study integrating synchrotron powder diffraction and pair distribution function analyses was undertaken, to elucidate how structural changes associated with gas binding inside Ni-MOF-74 pores correlate with the electrical response from Ni-MOF-74-based sensors.

Electrodeposition of Complex High Entropy Oxides via Water Droplet Formation and Conversion to Crystalline Alloy Nanoparticles.

A combination of electrodeposition and thermal reduction methods have been utilized for the synthesis of ligand-free FeNiCo alloy nanoparticles through a high-entropy oxide intermediate. These phases are of great interest to the electrocatalysis community, especially when formed by a sustainable chemistry method. This is successfully achieved by first forming a complex five element amorphous FeNiCoCrMn high-entropy oxide (HEO) phase via electrodeposition from a nanodroplet emulsion solution of the metal salt reactants.

Kinetically Controlled Linker Binding in Rare Earth-2,5-Dihydroxyterepthalic Acid Metal-Organic Frameworks and Its Predicted Effects on Acid Gas Adsorption.

In the pursuit of highly stable and selective metal-organic frameworks (MOFs) for the adsorption of caustic acid gas species, an entire series of rare earth MOFs have been explored. Each of the MOFs in this series (RE-DOBDC; RE = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu; DOBDC = 2,5-dihydroxyterepthalic acid) was synthesized in the tetragonal space group 4/. Crystallized MOF samples, specifically Eu-DOBDC, were seen to have a combination of monodentate and bidentate binding when synthesized under typical reaction conditions, resulting in a contortion of the structure.

Bio-inspired incorporation of phenylalanine enhances ionic selectivity in layer-by-layer deposited polyelectrolyte films.

The addition of a common amino acid, phenylalanine, to a Layer-by-Layer (LbL) deposited polyelectrolyte (PE) film on a nanoporous membrane can increase its ionic selectivity over a PE film without the added amino acid. The addition of phenylalanine is inspired by detailed knowledge of the structure of the channelrhodopsins family of protein ion channels, where phenylalanine plays an instrumental role in facilitating sodium ion transport. The normally deposited and crosslinked PE films increase the cationic selectivity of a support membrane in a controllable manner where higher selectivity is achieved with thicker PE coatings, which in turn also increases the ionic resistance of the membrane.

Evaluation of Electrodialysis Desalination Performance of Novel Bioinspired and Conventional Ion Exchange Membranes with Sodium Chloride Feed Solutions.

Electrodialysis (ED) desalination performance of different conventional and laboratory-scale ion exchange membranes (IEMs) has been evaluated by many researchers, but most of these studies used their own sets of experimental parameters such as feed solution compositions and concentrations, superficial velocities of the process streams (diluate, concentrate, and electrode rinse), applied electrical voltages, and types of IEMs. Thus, direct comparison of ED desalination performance of different IEMs is virtually impossible. While the use of different conventional IEMs in ED has been reported, the use of bioinspired ion exchange membrane has not been reported yet.

Continuous MOF Membrane-Based Sensors via Functionalization of Interdigitated Electrodes.

Three M-MOF-74 (M = Co, Mg, Ni) metal-organic framework (MOF) thin film membranes have been synthesized through a sensor functionalization method for the direct electrical detection of NO. The two-step surface functionalization procedure on the glass/Pt interdigitated electrodes resulted in a terminal carboxylate group, with both steps confirmed through infrared spectroscopic analysis. This surface functionalization allowed the MOF materials to grow largely in a uniform manner over the surface of the electrode forming a thin film membrane over the Pt sensing electrodes.

A High Entropy Oxide Designed to Catalyze CO Oxidation Without Precious Metals.

The chemical complexity of single-phase multicationic oxides, commonly termed high entropy oxides (HEOs), enables the integration of conventionally incompatible metal cations into a single-crystalline phase. However, few studies have effectively leveraged the multicationic nature of HEOs for optimization of disparate physical and chemical properties. Here, we apply the HEO concept to design robust oxidation catalysts in which multicationic oxide composition is tailored to simultaneously achieve catalytic activity, oxygen storage capacity, and thermal stability.

Polyelectrolyte layer-by-layer deposition on nanoporous supports for ion selective membranes.

This work demonstrates that the ionic selectivity and ionic conductivity of nanoporous membranes can be controlled independently layer-by-layer (LbL) deposition of polyelectrolytes and subsequent selective cross-linking of these polymer layers. LbL deposition offers a scalable, inexpensive method to tune the ion transport properties of nanoporous membranes by sequentially dip coating layers of cationic polyethyleneimine and anionic poly(acrylic acid) onto polycarbonate membranes. The cationic and anionic polymers are self-assembled through electrostatic and hydrogen bonding interactions and are chemically crosslinked to both change the charge distribution and improve the intermolecular integrity of the deposited films.

Ultra-Sensitive Potentiometric Measurements of Dilute Redox Molecule Solutions and Determination of Sensitivity Factors at Platinum Ultramicroelectrodes.

Open circuit potential (OCP) measurements can be very sensitive to small changes in the electrode environment and may allow detection of electron transfer events involving few, and maybe single, electrons. Factors affecting the overall sensitivity of OCP measurements were investigated to achieve the highest sensitivity. The OCP of platinum ultramicroelectrodes (UMEs) was determined in solutions that initially contained only supporting electrolyte where the OCP is a mixed potential governed by background faradaic processes.

Cathodically Dissolved Platinum Resulting from the O and HO Reduction Reactions on Platinum Ultramicroelectrodes.

The cathodic dissolution of platinum, resulting from the oxygen reduction reaction (ORR) or hydrogen peroxide reduction on platinum, has been investigated. Highly oxidizing hydroxyl radicals (OH) are believed to be the species responsible for the platinum dissolution phenomenon. These radicals are produced from the ORR byproduct, hydrogen peroxide, through a 1 electron reduction pathway (HO + e → OH + OH).

Electrochemical Characterization of Ultrathin Cross-Linked Metal Nanoparticle Films.

Here we report the preparation, characterization, and electrochemical study of conductive, ultrathin films of cross-linked metal nanoparticles (NPs). Nanoporous films ranging from 40 to 200 nm in thickness composed of gold and platinum NPs of ∼5 nm were fabricated via a powerful layer-by-layer spin coating process. This process allows preparation of uniform NP films as large as 2 × 2 cm(2) with precise control over thickness, structure, and electrochemical and electrocatalytic properties.

Study of the formation and quick growth of thick oxide films using platinum nanoelectrodes as a model electrocatalyst.

We report a study of the formation and quick growth of thick films of platinum oxide on platinum nanoelectrodes at low anodic potentials. Here, structurally well-defined platinum nanoelectrodes are used as a model platform for nanoscale platinum electrocatalysts. Platinum films are formed on the surface of the nanoelectrode upon application of a constant anodic potential in an acidic environment for an extended time period.

Chemically resolved transient collision events of single electrocatalytic nanoparticles.

Here we report the use of fast-scan cyclic voltammetry (FSCV) to study transient collision and immobilization events of single electrocatalytic metal nanoparticles (NPs) on an inert electrode. In this study, a fast, repetitive voltage signal is continuously scanned on an ultramicroelectrode and its faradaic signal is recorded. Electrocatalytically active metal NPs are allowed to collide and immobilize on the electrode resulting in the direct recording of the transient voltammetric response of single NPs.

Fluorescence coupling for direct imaging of electrocatalytic heterogeneity.

Here we report the use of fluorescence microscopy and closed bipolar electrodes to reveal electrochemical and electrocatalytic activity on large electrochemical arrays. We demonstrate fluorescence-enabled electrochemical microscopy (FEEM) as a new electrochemical approach for imaging transient and heterogeneous electrochemical processes. This method uses a bipolar electrode mechanism to directly couple a conventional oxidation reaction, e.

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.

Voltammetric behavior of gold nanotrench electrodes.

We report the fabrication and electrochemical response of a gold nanoband electrode located at the bottom of a glass/epoxy nanotrench, hereafter referred to as a gold nanotrench electrode. Gold nanotrench electrodes of 12.5 and 40 nm in width with various depths from a few tens of nanometers to approximately 4 μm are fabricated and further characterized by cyclic voltammetry.

Highly sensitive detection of exocytotic dopamine release using a gold-nanoparticle-network microelectrode.

Here we report a new type of microelectrode sensor for single-cell exocytotic dopamine release. The new microsensor is built by forming a gold-nanoparticle (AuNP) network on a carbon fiber microelectrode. First a gold surface is obtained on a carbon fiber microdisk electrode by partially etching away the carbon followed by electrochemical deposition of gold into the pore.

Au disk nanoelectrode by electrochemical deposition in a nanopore.

In this technical note, we report a process in scaling down the fabrication of Au disk nanoelectrodes as small as approximately 4 nm in radii. We have developed a bottom-up approach toward the fabrication of individual disk-shape Au nanoelectrodes. This new approach is based upon electrochemical deposition of Au in a silica nanopore electrode and involves the following four steps.