Ji-Hyung Han

Ionic Circuits Based on Polyelectrolyte Diodes on a Microchip

Green means go: A polyelectrolyte diode on a microchip exhibits well-defined nonlinear rectifying behavior. This system visualizes the dynamic distribution of ions in a charged polymer phase under an electric field on a real-time basis using fluorescence images (see picture). Multiple polyelectrolyte diodes are integrated on a microchip to produce a variety of logic gates based on ionic circuits.

Polyelectrolyte junction field effect transistor based on microfluidic chip

This study developed the first polyelectrolyte junction field effect transistor capable of operating in an aqueous microfluidic network on a chip. In this system, polydiallyldimethylammonium chloride and poly-2-acrylamido-2-methyl-1-propanesulfonic acid are used for the elaborate control of the ion flow by selective extraction of anions and cations from the microchannel. The rate of ion extraction can be regulated by simply adjusting the gate voltage, and it results in ion depletion in the vicinity of the polyelectrolyte plugs. The extent of ion depletion between the polyelectrolyte plugs is a sensitive function of the ion resistance of the microchannel; therefore, the current between the source and the drain can be controlled by regulating the gate voltage.

Dynamic Preconcentration of Gold Nanoparticles for Surface‐Enhanced Raman Scattering in a Microfluidic System

As considerable attention was recently paid to label-free technology in bioanalysis field,[1–3] analytical techniques employing no label such as surface plasmon resonance,[4] piezoelectrics,[5] cantilevers,[6] and transistors[7] have been increasingly suggested. Surface-enhanced Raman scattering (SERS) is one of the promising candidates for label-free analysis because it can provide molecular fingerprint information in the aqueous phase. It is attractive in that it provides sufficient enhancement, as high as 1010 times that of normal Raman signals, allowing quick detection of low-concentration analytes. Currently, SERS is expected to enable even realtime monitoring of molecular secretions in biological systems[8,9] or intermediate species temporarily present during chemical/electrochemical reactions without labeling.[10,11] SERS, employing visible incident light, is observed at nanoparticles, nanoshells, and the rough surfaces of a few metals like Au, Ag, or Cu, as well as narrow interstices among them (“hot spots”). Since SERS originates from nanostructures per se, its intensity is sensitive to the geometry, size, and shape of the substrates.[12] Sophisticated engineering of nanomaterials became a hot issue in nanoscience and in due course has driven significant recent progress in the development of better SERS substrates.[13] As a consequence, many metallic nanostructures with highly elaborate morphologies have been reported, such as Au nanorods[14] and Ag

Ion flow crossing over a polyelectrolyte diode on a microfluidic chip.

Key evidences are reported for the rectification mechanism of an aqueous ion diode based on polyelectrolytic plugs on a microfluidic chip by monitoring the ion flow crossing over the junction. The ion flow penetrating the polyelectrolyte junction is visualized by employing a fluorescent chemodosimeter, rhodamine B hydrazide and the pH-dependent dye, carboxy-fluorescein. How hysteresis phenomena, exhibited through the nonlinear behavior of the polyelectrolyte diode, are affected by a variety of parameters (e.g., switching potential, scan rate, and electrolyte concentration) is also investigated. The insights and knowledge from this study provide a crucial foundation for ion control at the iontronic diode in the aqueous phase, leading to more advanced aqueous organic computing devices and more diverse applications for microfluidic logic devices.

Effects of adsorption and confinement on nanoporous electrochemistry.

Characteristic molecular dynamics of reactant molecules confined in the space of the nanometer scale augments the frequency of collisions with the electrified surface so that a given faradaic reaction can be enhanced at nanoporous electrodes, the so-called nano-confinement effect. Since this effect is grounded on diffusion inside nanopores, it is predicted that adsorption onto the surface will seriously affect the enhancement by nano-confinement. We experimentally explored the correlation between adsorption and the confinement effect by examining the oxidation of butanol isomers at platinum and gold nanoporous electrodes. The results showed that electrooxidation of 2-butanol, which is a non-adsorption reaction, was enhanced more than that of 1-butanol, which is an adsorption reaction, at nanoporous platinum in acidic media. In contrast, the nanoporous gold electrode, on which 1-butanol is less adsorptive than it is on platinum, enhanced the electrooxidation of 1-butanol greatly. Furthermore, the electrocatalytic activity of nanoporous gold for oxygen reduction reaction was improved so much as to be comparable with that of flat Pt. These findings show that the nano-confinement effect can be appreciable for electrocatalytic oxygen reduction as well as alcohol oxidation unless the adsorption is extensive, and suggests a new strategy in terms of material design for innovative non-noble metal electrocatalysts.

Grand-canonical Monte Carlo simulation study of polyelectrolyte diode

In this study we have performed grand canonical Monte Carlo (GCMC) simulations to understand the working mechanism of polyelectrolyte diodes, which have the electrostatic junction between two oppositely charged polyelectrolyte. Polyelectrolyte gels are modeled as melts of freely-jointed hard chains with charged monomers and small ions as charged hard spheres. We examined the effects of applied voltage and gel concentration on the current flow. We observed that as positive voltage is applied (forward state), small ions start moving across the gel to induce electric current flow. On the other hand, with negative voltage (reverse state) current flow is significantly weak. These simulation results are in good agreement with experimental results. We also examined the effects of applied voltage, gel structure, ion concentration.

Electrooxidation of saccharides at platinum electrode

Saccharides have been emerging as promising fuels for future energy industry because they possess high energy density and tremendous amount of them can be obtained from abundant biomass. Direct electrochemical oxidation of saccharides to generate electricity is a potentially competitive approach in terms of the demand for small, handy, and cost-effective electric power sources. To develop efficient sugar fuel cell, it is necessary to understand mechanism of electrooxidation of saccharide at electrode surface. Although glucose oxidation at platinum surface has been well known, fundamental mechanism study on electrooxidation of other sugars is still in its infancy. Based on research of glucose oxidation, we will predict the electrooxidation of other saccharides such as fructose.