Taemin Lee

Ionic liquid modified graphene nanosheets anchoring manganese oxide nanoparticles as efficient electrocatalysts for Zn–air batteries

Ionic liquid (IL) modified reduced graphene oxide (rGO–IL) nanosheets anchoring manganese oxide (Mn3O4) are synthesized via a facile solution-based growth mechanism and applied to a Zn–air battery as an effective electrocatalyst for the oxygen reduction reaction (ORR). In this study, the IL moiety in these composites increases not only the conductivity of the system, but also the electrocatalytic activity compared to pristine rGO, together with the synergic effect of facilitating the ORR with the intrinsic catalytic activity of Mn3O4. Based on the Koutecky–Levich plot, we suggest that the ORR pathway of these composites is tunable with the relative amount of Mn3O4 nanoparticles supported onto the graphene sheets; for example, the ORR mechanism of the system with a lower Mn3O4 (19.2%) nanoparticle content is similar to a Pt/C electrode, i.e., a one-step, quasi-4-electron transfer, unlike that with a higher Mn3O4 (52.5%) content, which undergoes a classical two-step, 2-electron pathway. We also demonstrate the potential of these hybrid rGO–IL/Mn3O4 nanoparticles as efficient catalysts for the ORR in the Zn–air battery with a maximum peak power density of 120 mW cm−2; a higher performance than that from commercial cathode catalysts.

Stable aqueous dispersion of reduced graphene nanosheets via non-covalent functionalization with conducting polymers and application in transparent electrodes.

We developed a simple and facile method of producing a stable aqueous suspension of reduced graphene oxide (RGO) nanosheets through the chemical reduction of graphene oxide in the presence of a conducting polymer dispersant, poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS). This approach involves the cooperative interactions of strong π- π interactions between a two-dimensional graphene sheet and a rigid backbone of PEDOT and the intermolecular electrostatic repulsions between negatively charged PSS bound on the RGO sheets, which impart the colloidal stability of the resulting hybrid nanocomposite of RGO/PEDOT. Moreover, our one-step solution-based method allows preserving the intrinsic chemical and electronic properties of both components, yielding a hybrid film of RGO nanosheets of high conductivity of 2.3 kΩ/sq with a transmittance of 80%. By taking advantage of conducting network structure of conducting polymers which provides an additional flexibility and mechanical stability of RGO nanosheets, we demonstrate the potential application of hybrid RGO/PEDOT as highly flexible and transparent electrodes.

Hybrid multilayer thin film supercapacitor of graphene nanosheets with polyaniline: importance of establishing intimate electronic contact through nanoscale blending

A hybrid electrode consisting of an electric double-layer capacitor of graphene nanosheets and a pseudocapacitor of the conducting polymer polyaniline exhibits a synergistic effect with excellent electrochemical performance for flexible thin film supercapacitors. This hybrid supercapacitor is constructed by a nanoscale blending method of layer-by-layer (LbL) assembly based on the electrostatic interactions between positively charged polyaniline (PANi) and negatively charged graphene oxide (GO) nanosheets. The hybrid electrode provides not only improved electronic conductivity through the intimate contact with the graphene nanosheet, but also enhanced chemical stability during the charge–discharge process. We also investigated the dependence of the electrochemical performance on the various parameters of LbL assembly such as the number of bilayers and the post-thermal and chemical treatments that could affect the degree of reduction of GO and PANi. We found that after thermal treatment, the LbL-assembled thin film of PANi with GO nanosheets exhibited an excellent gravimetric capacitance of 375.2 F g−1 at a discharge current density of 0.5 A g−1 that outperformed many other hybrid supercapacitors reported to date. The hybrid supercapacitor maintained its capacity up to 90.7% over 500 cycles at a high current density of 3.0 A g−1. This study opens up the possibility for the production of diverse graphene-based hybrid nanocomposites that are promising for future flexible supercapacitors.

Drug delivery by a self-assembled DNA tetrahedron for overcoming drug resistance in breast cancer cells

A DNA tetrahedron is employed for efficient delivery of doxorubicin into drug-resistant breast cancer cells. The drug delivered with the DNA nanoconstruct is considerably cytotoxic, whereas free doxorubicin is virtually non-cytotoxic for the drug-resistant cells. Thus, the DNA tetrahedron, made of the inherently natural and biocompatible material, can be a good candidate for the drug carrier to overcome MDR in cancer cells.

Carbon-based layer-by-layer nanostructures: from films to hollow capsules

Over the past years, the layer-by-layer (LbL) assembly has been widely developed as one of the most powerful techniques to prepare multifunctional films with desired functions, structures and morphologies because of its versatility in the process steps in both material and substrate choices. Among various functional nanoscale objects, carbon-based nanomaterials, such as carbon nanotubes and graphene sheets, are promising candidates for emerging science and technology with their unique physical, chemical, and mechanical properties. In particular, carbon-based functional multilayer coatings based on the LbL assembly are currently being actively pursued as conducting electrodes, batteries, solar cells, supercapacitors, fuel cells and sensor applications. In this article, we give an overview on the use of carbon materials in nanostructured films and capsules prepared by the LbL assembly with the aim of unraveling the unique features and their applications of carbon multilayers prepared by the LbL assembly.

Interfacing Living Yeast Cells with Graphene Oxide Nanosheaths

The first example of the encapsulation of living yeast cells with multilayers of GO nanosheets via LbL self-assembly is reported. The GO nanosheets with opposite charges are alternatively coated onto the individual yeast cells while preserving the viability of the yeast cells, thus affording a means of interfacing graphene with living yeast cells. This approach is expanded by integrating other organic polymers or inorganic nanoparticles to the cells by hybridizing the entries with GO nanosheets through LbL self-assembly. It is demonstrated that incorporated iron oxide nanoparticles can deliver magnetic properties to the biological systems, allowing the integration of new physical and chemical functions for living cells with a combination of GO nanosheets.

Highly Tunable Aptasensing Microarrays with Graphene Oxide Multilayers

A highly tunable layer-by-layer (LbL)-assembled graphene oxide (GO) array has been devised for high-throughput multiplex protein sensing. In this array, the fluorescence of different target-bound aptamers labeled with dye is efficiently quenched by GO through fluorescence resonance energy transfer (FRET), and simultaneous multiplex target detection is performed by recovering the quenched fluorescence caused by specific binding between an aptamer and a protein. Thin GO films consisting of 10 bilayers displayed a high quenching ability, yielding over 85% fluorescence quenching with the addition of a 2 μM dye-labeled aptamer. The limit for human thrombin detection in the 6- and 10-bilayered GO array is estimated to be 0.1 and 0.001 nM, respectively, indicating highly tunable nature of LbL assembled GO multilayers in controlling the sensitivity of graphene-based FRET aptasensor. Furthermore, the GO chip could be reused up to four times simply by cleaning it with distilled water.

Sentinel lymph node imaging by a fluorescently labeled DNA tetrahedron.

Sentinel lymph nodes (SLNs) are the first lymph nodes which cancer cells reach after traveling through lymphatic vessels from the primary tumor. Evaluating the nodal status is crucial in accurate staging of human cancers and accordingly determines prognosis and the most appropriate treatment. The commonly used methods for SLN identification in clinics are based on employment of a colloid of radionuclide or injection of a small dye. Although these methods have certainly contributed to improve surgical practice, new imaging materials are still required to overcome drawbacks of the techniques such as inconvenience of handling radioactive materials and short retention time of small dyes in SLNs. Here, we prepare a fluorescence-labeled DNA tetrahedron and perform SLN imaging by using the DNA nanoconstruct. With a successful identification of SLNs by the DNA nanoconstruct, we suggest that DNA tetrahedron hold great promises for clinical applications.

Mussel-inspired nitrogen-doped graphene nanosheet supported manganese oxide nanowires as highly efficient electrocatalysts for oxygen reduction reaction

Electrocatalysts for oxygen reduction reaction (ORR) play a vital role in determining the performance of fuel cells and metal–air batteries. Carbon nanomaterials doped with heteroatoms are highly attractive by virtue of their excellent electrocatalytic activity, high conductivity and large surface area. This study reports the synthesis of a highly efficient electrocatalyst based on nitrogen-doped (N-doped) graphene nanosheets (NG) using mussel-inspired dopamine as a nitrogen source. Dopamine undergoes oxidative polymerization that can functionalize the surface of graphene and also introduces nitrogen atoms onto the graphene nanosheets upon pyrolysis. N-doping not only leads to improved catalytic activity, but it also provides anchoring sites for the growth of electroactive amorphous manganese oxide nanowires on the graphene nanosheets (NG/MnOx). On the basis of a Koutecky–Levich plot, it is found that the hybrid NG/MnOx catalyst exhibits excellent catalytic activity with a direct four-electron pathway in ORR. Furthermore, the hybrid electrocatalyst possesses superior stability and gives a low yield of peroxide compared to commercial Pt/C catalysts. This suggests that the unique combination of an N-doped graphene support and amorphous MnOx nanowires can synergistically improve the catalytic activity for ORR.

Facile synthesis of hybrid graphene and carbon nanotubes as a metal-free electrocatalyst with active dual interfaces for efficient oxygen reduction reaction

We report metal-free electrocatalysts to enhance utilization of dissolved and gaseous oxygen during oxygen reduction reaction (ORR). Proper balance between hydrophobicity and hydrophilicity is achieved using reduced graphene oxide (rGO) and polyelectrolyte functionalized multiwalled carbon nanotubes (pMWNTs). In this unique architecture, both two- and three-phase reactions in ORR can be maximized with a quasi-four-electron pathway.