Kinetics of Ion Transfer at the Ionic Liquid/Water Nanointerface

Ion transfer (IT) processes in ionic liquids (ILs) are essential for their applications in electrochemical systems and chemical separations. In this Article, the first measurements of IT kinetics at the IL/water interface are reported. Steady-state voltammetry was performed at the nanometer-sized polarizable interface between water and ionic liquid, [THTDP+][C4C4N−], immiscible with it that was formed at the tip of a nanopipet. Kinetic measurements at such interfaces are extremely challenging because of slow mass-transfer rates in IL, which is ∼700 times more viscous than water. The recently developed new mode of nanopipet voltammetry, common ion voltammetry, was used to overcome technical difficulties and ensure the reliability of the extracted kinetic parameters of IT. The results suggest that the rate of interfacial IT depends strongly on solution viscosity. Voltammetric responses of nanopipets of different radii were analyzed to evaluate the effect of the electrical double layer at the liquid/liquid interface on IT kinetics. The possibility of the influence of the charged pipet wall on ion transport was investigated by comparing currents produced by cationic and anionic species. Possible effects of relaxation phenomena at the IL/water interface on IT voltammograms have also been explored

Plasmonic Imaging of Surface Electrochemical Reactions of Single Gold Nanowires

Nanomaterials have been widely used in energy and sensing applications because of their unique chemical and physical properties, especially their surface reactions. Measuring the local reactions of individual nanomaterials, however, has been an experimental challenge. Here we report on plasmonic imaging of surface electrochemical reactions of individual gold nanowires (AuNWs). We coated a gold thin film (plasmonic sensing layer) with a dielectric layer (Cytop) with refractive index close to that of water, and then a graphene layer for electrical contact. This design removed the interference from the sensing layer while preserving sharp surface plasmon resonance, which allowed us to obtain cyclic voltammograms of surface electrochemistry of individual AuNWs for the first time. We also investigated the difference in the electrochemical reactions of AuNWs and Au surfaces, and local distribution of electrochemical activities within a single AuNW

Nanopipet Voltammetry of Common Ions across the Liquid−Liquid Interface. Theory and Limitations in Kinetic Analysis of Nanoelectrode Voltammograms

Finite element simulations of ion transfer (IT) reactions at the nanopipet-supported interface between two immiscible electrolyte solutions (ITIES) were carried out, and the numerical results were generalized in the form of an analytical approximation. The developed theory is the basis of a new approach to kinetic analysis of steady-state voltammograms of rapid IT reactions. Unlike the conventional voltammetric protocol, our approach requires the initial addition of a transferable ion to both liquid phases, i.e., to the filling solution inside a nanopipet and the external solution. The resulting steady-state IT voltammogram comprises two waves corresponding to the ingress of the common ion into the pipet and its egress into the external solution. We demonstrate that both ingress and egress waves are required for characterization of pipet geometry and precise determination of thermodynamic and kinetic parameters for rapid IT reactions. In this way, one can eliminate large uncertainties in kinetic parameters, which are inherent in the previously reported approaches to analysis of nearly reversible steady-state voltammograms of either IT at pipet-supported ITIES or electron transfer at solid electrodes. Numerical simulations also suggest that higher current density at the edge of the nanoscale ITIES increases the significance of electrostatic effects exerted by the charged inner surface of a pipet on IT processes

Application of Electrochemically Reduced Graphene Oxide on Screen-Printed Ion-Selective Electrode

In this study, a novel disposable all-solid-state ion-selective electrode using graphene as the ion-to-electron transducer was developed. The graphene film was prepared on screen-printed electrode directly from the graphene oxide dispersion by a one-step electrodeposition technique. Cyclic voltammetry and electrochemical impedance spectroscopy were employed to demonstrate the large double layer capacitance and fast charge transfer of the graphene film modified electrode. On the basis of these excellent properties, an all-solid-state calcium ion-selective electrode as the model was constructed using the calcium ion-selective membrane and graphene film modified electrode. The mechanism about the graphene promoting the ion-to-electron transformation was investigated in detail. The disposable electrode exhibited a Nernstian slope (29.1 mV/decade), low detection limit (10^–5.8 M), and fast response time (less than 10 s). With the high hydrophobic character of graphene materials, no water film was formed between the ion-selective membrane and the underlying graphene layer. Further studies revealed that the developed electrode was insensitive to light, oxygen, and redox species. The use of the disposable electrode for real sample analysis obtained satisfactory results, which made it a promising alternative in routine sensing applications

Plasmonic Imaging of Surface Electrochemical Reactions of Single Gold Nanowires

Nanomaterials have been widely used in energy and sensing applications because of their unique chemical and physical properties, especially their surface reactions. Measuring the local reactions of individual nanomaterials, however, has been an experimental challenge. Here we report on plasmonic imaging of surface electrochemical reactions of individual gold nanowires (AuNWs). We coated a gold thin film (plasmonic sensing layer) with a dielectric layer (Cytop) with refractive index close to that of water, and then a graphene layer for electrical contact. This design removed the interference from the sensing layer while preserving sharp surface plasmon resonance, which allowed us to obtain cyclic voltammograms of surface electrochemistry of individual AuNWs for the first time. We also investigated the difference in the electrochemical reactions of AuNWs and Au surfaces, and local distribution of electrochemical activities within a single AuNW

Plasmonic Imaging of Surface Electrochemical Reactions of Single Gold Nanowires

Nanomaterials have been widely used in energy and sensing applications because of their unique chemical and physical properties, especially their surface reactions. Measuring the local reactions of individual nanomaterials, however, has been an experimental challenge. Here we report on plasmonic imaging of surface electrochemical reactions of individual gold nanowires (AuNWs). We coated a gold thin film (plasmonic sensing layer) with a dielectric layer (Cytop) with refractive index close to that of water, and then a graphene layer for electrical contact. This design removed the interference from the sensing layer while preserving sharp surface plasmon resonance, which allowed us to obtain cyclic voltammograms of surface electrochemistry of individual AuNWs for the first time. We also investigated the difference in the electrochemical reactions of AuNWs and Au surfaces, and local distribution of electrochemical activities within a single AuNW

Eye-Readable and Wearable Colorimetric Sensor Arrays for <i>In Situ</i> Monitoring of Volatile Organic Compounds

Wearable sensors utilize changes in color as a response to physiological stimuli, making them easily recognizable by the naked eye. These colorimetric wearable sensors offer benefits such as easy readability, rapid responsiveness, cost-effectiveness, and straightforward manufacturing techniques. However, their applications in detecting volatile organic compounds (VOCs) in situ have been limited due to the low concentration of complex VOCs and complicated external interferences. Aiming to address these challenges, we introduced readable and wearable colorimetric sensing arrays with a microchannel structure and highly gas-sensitive materials for in situ detection of complex VOCs. The highly gas-sensitive materials were designed by loading gas-sensitive dyes into the porous metal–organic frameworks and further depositing the composites on the electrospun nanofiber membrane. The colorimetric sensor arrays were fabricated using various gas-sensitive composites, including eight dye/MOF composites that respond to various VOCs and two Pd2+/dye/MOF composites that respond to ethylene. This enables the specific recognition of multiple characteristic VOCs. A microfluidic channel made of polydimethylsiloxane (PDMS) was integrated with different colorimetric elements to create a wearable sensor array. It was attached to the surface of fruits to collect and monitor VOCs using the DenseNet classification method. As a proof of concept, we demonstrated the feasibility of the wearable sensing system in monitoring the ripening process of fruits by continuously measuring the VOC emissions from the skin of the fruit

Video4_Plasmonic Imaging of Electrochemical Reactions at Individual Prussian Blue Nanoparticles.AVI

Prussian blue is an iron-cyanide-based pigment steadily becoming a widely used electrochemical sensor in detecting hydrogen peroxide at low concentration levels. Prussian blue nanoparticles (PBNPs) have been extensively studied using traditional ensemble methods, which only provide averaged information. Investigating PBNPs at a single entity level is paramount for correlating the electrochemical activities to particle structures and will shed light on the major factors governing the catalyst activity of these nanoparticles. Here we report on using plasmonic electrochemical microscopy (PEM) to study the electrochemistry of PBNPs at the individual nanoparticle level. First, two types of PBNPs were synthesized; type I synthesized with double precursors method and type II synthesized with polyvinylpyrrolidone (PVP) assisted single precursor method. Second, both PBNPs types were compared on their electrochemical reduction to form Prussian white, and the effect from the different particle structures was investigated. Type I PBNPs provided better PEM sensitivity and were used to study the catalytic reduction of hydrogen peroxide. Progressively decreasing plasmonic signals with respect to increasing hydrogen peroxide concentration were observed, demonstrating the capability of sensing hydrogen peroxide at a single nanoparticle level utilizing this optical imaging technique.</p

Video1_Plasmonic Imaging of Electrochemical Reactions at Individual Prussian Blue Nanoparticles.AVI

Prussian blue is an iron-cyanide-based pigment steadily becoming a widely used electrochemical sensor in detecting hydrogen peroxide at low concentration levels. Prussian blue nanoparticles (PBNPs) have been extensively studied using traditional ensemble methods, which only provide averaged information. Investigating PBNPs at a single entity level is paramount for correlating the electrochemical activities to particle structures and will shed light on the major factors governing the catalyst activity of these nanoparticles. Here we report on using plasmonic electrochemical microscopy (PEM) to study the electrochemistry of PBNPs at the individual nanoparticle level. First, two types of PBNPs were synthesized; type I synthesized with double precursors method and type II synthesized with polyvinylpyrrolidone (PVP) assisted single precursor method. Second, both PBNPs types were compared on their electrochemical reduction to form Prussian white, and the effect from the different particle structures was investigated. Type I PBNPs provided better PEM sensitivity and were used to study the catalytic reduction of hydrogen peroxide. Progressively decreasing plasmonic signals with respect to increasing hydrogen peroxide concentration were observed, demonstrating the capability of sensing hydrogen peroxide at a single nanoparticle level utilizing this optical imaging technique.</p

Video3_Plasmonic Imaging of Electrochemical Reactions at Individual Prussian Blue Nanoparticles.AVI

Prussian blue is an iron-cyanide-based pigment steadily becoming a widely used electrochemical sensor in detecting hydrogen peroxide at low concentration levels. Prussian blue nanoparticles (PBNPs) have been extensively studied using traditional ensemble methods, which only provide averaged information. Investigating PBNPs at a single entity level is paramount for correlating the electrochemical activities to particle structures and will shed light on the major factors governing the catalyst activity of these nanoparticles. Here we report on using plasmonic electrochemical microscopy (PEM) to study the electrochemistry of PBNPs at the individual nanoparticle level. First, two types of PBNPs were synthesized; type I synthesized with double precursors method and type II synthesized with polyvinylpyrrolidone (PVP) assisted single precursor method. Second, both PBNPs types were compared on their electrochemical reduction to form Prussian white, and the effect from the different particle structures was investigated. Type I PBNPs provided better PEM sensitivity and were used to study the catalytic reduction of hydrogen peroxide. Progressively decreasing plasmonic signals with respect to increasing hydrogen peroxide concentration were observed, demonstrating the capability of sensing hydrogen peroxide at a single nanoparticle level utilizing this optical imaging technique.</p