Hee Uk Lee

Enzymatic fuel cells based on electrodeposited graphite oxide/cobalt hydroxide/chitosan composite–enzymeelectrode

Enzymatic fuel cells (EFCs) use redox enzymes with high electron transfer rates that lead to high power density from bioavailable substrates. However, EFCs are limited by the difficult electrical wiring of the enzymes to the electrode. Therefore, deposition of Co(OH)₂ onto graphite oxide (GO) was improved for efficient wiring of the enzymes. The GO/Co(OH)₂/chitosan composites were electrodeposited for immobilization of glucose oxidase (GOD) or laccase on an Au electrode, respectively. The electrical properties of the bioelectrode according to cyclic voltammetry were improved using GO/Co(OH)₂/chitosan composites. The anode and cathode system was composed of GOD and laccase as biocatalysts and glucose/oxygen as substrates under ambient conditions (pH 7.0 and 25 °C). The EFC using GO/Co(OH)₂/chitosan composites with a mediator delivered a high power density of up to 517±3.3 μW/cm² at 0.46 V and open circuit voltage of 0.60 V. These results provide a promising direction for further development and application of EFCs.

Batch and continuous synthesis of lactulose from whey lactose by immobilized β-galactosidase.

In this study, lactulose synthesis from whey lactose was investigated in batch and continuous systems using immobilized β-galactosidase. In the batch system, the optimal concentration of fructose for lactulose synthesis was 20%, and the effect of galactose, glucose and fructose on β-galactosidase activity was determined for hydrolysis of whey lactose and the transgalactosylation reaction for lactulose synthesis. Galactose and fructose were competitive inhibitors, and glucose acted as a noncompetitive inhibitor. The inhibitory effects of galactose and glucose were stronger in the transgalactosylation reaction than they were in the hydrolysis reaction. In addition, when immobilized β-galactosidase was reused for lactulose synthesis, its catalytic activity was retained to the extent of 52.9% after 10 reuses. Lactulose was synthesized continuously in a packed-bed reactor. We synthesized 19.1g/l lactulose during the continuous flow reaction at a flow rate of 0.5 ml/min.

Quantitative Detection of Glyphosate by Simultaneous Analysis of UV Spectroscopy and Fluorescence Using DNA-Labeled Gold Nanoparticles

A sandwich-type immunosensor composed of antigen-double target/probe DNA-coated gold nanoparticles (NPs) was developed for the measurement of fluorescence intensity and quantitative analysis of single-stranded DNA based on the concentration of free glyphosate. The reaction between the antigen-double DNA-gold NPs and immobilized antibody on the substrate was carried out for 2 h. The results of the antigen-antibody reaction were measured on the basis of the fluorescence intensity obtained from comparison with the free antigens at concentrations of 0.01-100 μg mL(-1) for the detection of immobilized antigen-double DNA-gold NPs. For the quantitative analysis based on the concentration of glyphosate(0.01-100 μg mL(-1)), the immunosensor response also revealed the same detection range of glyphosate using DNA detection.

Optimization of lactulose synthesis from whey lactose by immobilized β-galactosidase and glucose isomerase.

In the present study, commercially available whey was used as a lactose source, and immobilized β-galactosidase and glucose isomerase were used to synthesize lactulose from whey lactose in the absence of fructose. Optimal reaction conditions, such as lactose concentration, temperature, ionic strength of the buffer, and ratio of immobilized enzymes, were determined to improve lactulose synthesis using immobilized enzymes. Lactulose synthesis using immobilized enzymes improved markedly after optimizing the reaction conditions. When the lactulose synthesis was carried out at 53.5°C using 20% (w/v) whey lactose, 12U/ml of immobilized β-galactosidase and 60U/ml of immobilized glucose isomerase in 100mM sodium phosphate buffer at pH 7.5, the lactulose concentration and specific productivity were 7.68g/l and 0.32mg/Uh, respectively. Additionally, when the immobilized enzymes were reused for lactulose synthesis, their catalytic activity was 57.1% after 7 repeated uses.

Pen lithography for flexible microsupercapacitors with layer-by-layer assembled graphene flake/PEDOT nanocomposite electrodes

An energy device using an all solid-state microsupercapacitor (MSC) has to play the roles of both a current collector and an electrode material, as well as demonstrating properties of high charge storage, conductivity, and flexibility. Despite the complexity and processing costs, microfabrication techniques are being employed in fabricating a great variety of MSC devices. In this work, simpler and cheaper concepts are proposed to fabricate flexible MSCs based on graphene flakes and polyethylenedioxythiophene (PEDOT) with a layer-by-layer assembly method using pen lithography, without the need for complex processing, or a cleanroom environment. In order to fabricate interdigitated finger patterned electrodes for the MSC, we report the preparation of highly conductive graphene flakes and PEDOT on a polyethylene terephthalate (PET) film formed by inducing the polymerization of 3,4-ethylenedioxythiophene (EDOT) monomers. The sheet resistance of 15 Ω sq.−1 measured for 3-layer graphene/PEDOT is much lower than that displayed by the graphene flake/EDOT (14.8 ± 0.6 kΩ sq.−1). For a flexible MSC application, the MSC exhibits a maximum energy density of 1.5 mW h cm−3, a power density of 141 W cm−3, and a volumetric capacitance of 7.7 F cm−3 (at a current density of 0.02 A cm−3), which are higher than those values obtained for other solid state MSCs. The graphene/PEDOT MSC also shows good long-term cycling stability, with a capacitance retention rate of 81% after a large cycling number of 2500 times. The simplicity and wide scope of this innovative strategy can open up new avenues for easy and scalable fabrication of a wide variety of devices.

Three-Dimensional Flexible All-Organic Conductors for Multifunctional Wearable Applications

Wearable textile electrodes based on π-conjugated polymers are appealing alternatives to carbon fabrics, conductive yarns, or metal wires because of their design flexibility, low cost, flexibility, and high throughput. This provides the benefits of both electronics and textiles. Herein, a general and new method has been developed to produce tailorable, wearable energy devices that are based on three-dimensional (3D) poly(3,4-ethylenedioxythiophene) (PEDOT)-coated textile conductors. To obtain the desired electrode materials, both facile solution-dropping polymerization methods are used to fabricate a 3D flexible PEDOT conductor on a cotton textile (PEDOT/textile). PEDOT/textile shows a very low sheet resistance of 4.6-4.9 Ω·sq-1. Here, we employ the example of this 3D network-like structure and the excellent electrical conductivities under the large deformation of PEDOT/textiles to show that wearable and portable heaters have immense potential. A flexible textile heater with a large area (8 × 7.8 cm2) reached a saturation temperature of ∼83.9 °C when a bias of 7 V was applied for ∼70 s due to the good electrical conductivity of PEDOT. To demonstrate the performance of all-solid-state supercapacitors, nano-ascidian-like PEDOT (PEDOT-NA) arrays were prepared via a simple vapor-phase polymerization of 3,4-ethylenedioxythiophene on PEDOT/textile to increase both the surface area and the number of ion diffusion paths. The PEDOT-NA arrays on PEDOT/textile showed outstanding performance with an areal capacitance of 563.3 mF·cm-2 at 0.4 mA·cm-2 and extraordinary mechanical flexibility. The maximum volumetric power density and energy density of the nanostructured PEDOT on the textile were 1.75 W·cm-3 and 0.0812 Wh·cm-3, respectively. It is expected that the wearable nanostructured conducting polymers will have advantages when used as structures for smart textronics and energy conversion/storage.