Won San Choi

Production of graphene by exfoliation of graphite in a volatile organic solvent

The production of unfunctionalized and nonoxidized graphene by exfoliation of graphite in a volatile solvent, 1-propanol, is reported. A stable homogeneous dispersion of graphene was obtained by mild sonication of graphite powder and subsequent centrifugation. The presence of a graphene monolayer was observed by atomic force microscopy and transmission electron microscopy. The solvent, 1-propanol, from the deposited dispersion was simply and quickly removed by air drying at room temperature, without the help of high temperature annealing or vacuum drying, which shortens production time and does not leave any residue of the solvent in the graphene sheets.

Sandwich-like graphene nanocomposites armed with nanoneedles

A novel strategy for preventing aggregation, increasing surface area, and maintaining catalytic activity of water-soluble graphene nanosheet (GNS) catalysts is introduced, and use of the synthesized nanocatalyst [GNS (matrix)/metal (catalyst)/metal oxide (spacer)/ceramic (protector)] as recyclable nanocatalysts without loss of performance is demonstrated. Our graphene nanocomposite (GNC) armed with nanoneedles is fabricated by the synthesis of Au on GNS, followed by stepwise synthesis of needle-like IO and SiO2, in order. The distinctive structures of the needle-like layers prevent aggregation and contamination of the GNS catalysts, and allow recycling, both of which enhanced catalytic ability. The research demonstrates the possibility of a safe coating for water-soluble GNS catalysts, which are reactive to ionic species, and their application as nanocatalysts for the chemo-selective transformation of toxic substances. Our sandwich-like GNCs for recyclable nanocatalysts provide a new design concept for reusable GNS catalysts.

Polyelectrolyte-mediated hierarchical mesoporous calcium silicates: a platform for drug delivery carrier with ultrahigh loading capacity and controlled release behavior

We have developed a facile method for the poly(allylamine hydrochloride) (PAH)-assisted synthesis of mesoporous calcium silicate hydrates (PAH-CS) with a large specific surface area (BET = 348.4 m2 g-1) and pore volume (Vp = 1.42 cm3 g-1). Tetraethyl orthosilicate (TEOS) was employed as a silicon source, which was rapidly hydrolyzed and reacted with the amine groups of PAH to form spherical SiO2 nanoparticles (PAH-Si). Subsequently, Ca2+ ions reacted with the silicate anions produced during the dissolution of SiO2 in basic media, leading to the formation of the highly porous 3D networks of PAH-CS that were synthesized only under optimized reaction conditions. The PAH-CS containing an excess of Ca2+ and NH3 + enriched the surfaces with a very high cationic charge (ζ = +65.66 mV)and resulted in an extremely high loading capacity for anionic drugs and proteins. Ibuprofen (IBU) and FITC-labeled bovine albumin (FITC-Albumin) were chosen as a model drug and model protein, respectively, to test the loading and delivery efficiencies of the PAH-CS carriers. The ultrahigh drug loading capacities (DLC) and their release patterns were investigated under controlled pH conditions. Strikingly, the highest DLC reported to date (IBU or FITC-Albumin/carrier (3.35 g or 1 g g-1) was achieved in this work. The PAH-CS had no cytotoxic effect on osteoblast-like MC3T3-E1 cell lines evaluated by the LDH (Lactate dehydrogenase) assay in supernatant medium. Furthermore, the PAH-CS carriers could be entirely transformed to hydroxyapatite after releasing the drug in simulated body fluid (SBF), indicating good bioactivity and biodegradability of the PAH-CS carriers.

Structure-controllable superhydrophobic Cu meshes for effective separation of oils with different viscosities and aqueous pollutant purification

Micropothole-based oxidized (MPO) Cu meshes with various surface morphologies, such as needle-like (NL), hair-like (HL), arch-like (AL), and pine needle-like (PNL) structures, were prepared by controlling the pH and temperature of the oxidation reaction solution. The separation efficiencies of 1H,1H,2H,2H-perfluorooctyltriethoxysilane (PFTOS)-coated MPO-Cu meshes were found to be 96–99% for oil/water separation, and the meshes maintained this performance over 20 separation cycles without deterioration. The HL mesh with long and flexible structures was outstanding for the separation of low viscosity oils, while the NL mesh with short and rigid structures was optimal for the separation of highly viscous oils. Double-layered meshes consisting of photocatalytic and superhydrophobic meshes exhibited high catalytic performance after the oil/water separation, demonstrating purification of an aqueous pollutant separated from the oil/water mixture.

Polyelectrolyte complex particle-based multifunctional freestanding films containing highly loaded bimetallic particles

A facile approach to the development of multifunctional freestanding films consisting of polyelectrolytes (PEs) and polyelectrolyte complexes (nPECs) with highly embedded metal nanoparticles (NPs) was demonstrated. The composite films (nPEC/PE) containing NPs exhibited controllable properties that can be exploited by varying the type and content of NPs with high loading density. The approach described here enables the facile fabrication of conducting and transparent freestanding films with tunable optical/electrical properties, color, and large lateral dimensions with minimal effort in terms of the number of layers and ease of operation.

Free-standing polymer nanoactuators, nanoshutters, and nanofilters

Our study demonstrates a facile approach for the synthesis of metal NP-embedded freestanding polyelectrolyte complexes (PECs) without sacrificial templates, which can be used as nanoactuators, nanoshutters, and nanofilters. The metal NP-PECs were prepared by mixing metal precursor-preadsorbed cation and anion PEs, using a nonstoichiometric ratio, followed by a controlled heat treatment. The metal NP-PECs showed reversible structural changes such as shrinking or swelling when they were exposed to various pH conditions. During the changes, they were also able to reversibly control the interparticle distance of the metal NPs embedded in the PECs, which has allowed the easy tuning of their catalytic and optical properties. By the reversible control of structural changes, the PECs can also be used as nanoshutters to reversibly control the flow of nanomaterials in the channels or pores. Furthermore, metal NP-embedded PECs, Au/Ag-PECs, were demonstrated to be useful as nanofilters for removing or transforming unwanted materials.

Magnesium hydroxide nanoplate/graphene oxide composites as efficient adsorbents for organic dyes

Nanocomposites comprised of magnesium hydroxide (Mg(OH)2) and graphene oxide (GO) were prepared by the controlled precipitation of a magnesium salt on the GO surface. The population of Mg(OH)2 nanocrystals on the GO surface could be varied by varying the Mg(OH)2 precursor amount; the surface area of the resulting Mg(OH)2/GO nanocomposites varied from 75.2 m2 g−1 to 465 m2 g−1. The Mg(OH)2/GO nanocomposite with a surface area of 465 m2 g−1 showed the best performance. Owing to the synergistic effect of the nanoplate structure of Mg(OH)2 and the 2D structure of GO, the obtained Mg(OH)2/GO nanocomposites exhibited high performance in methylene blue adsorption (adsorption capacity: 779.4 mg g−1).

Rattle-type hierarchical particles containing multilevel cores (Ag@AgCl@SiO2 and Au/Ag@AgCl@SiO2) as versatile catalysts

A protocol for the synthesis of rattle-type core@shell particles containing Ag@AgCl or Au/Ag@AgCl core structures was developed, and the use of these particles as catalysts for the decomposition of toxic materials was demonstrated. A monometallic Ag or bimetallic Au/Ag core was incorporated into the interior of SiO2 capsules via controlled heat treatment of metal nanoparticle/SiO2-coated polymer particles, resulting in the formation of rattle-type core@shell structures. By appropriate treatments, it was possible to transform the Ag or Au/Ag core into multilevel cores (Ag@AgCl or Au/Ag@AgCl) within the SiO2 capsules (Ag@AgCl@SiO2 or Au/Ag@AgCl@SiO2). This method for the synthesis of rattle-type core@shell particles is useful for further introducing AgCl fused with plasmonic materials into the capsule structures. The rattle-type core@shell structures were used as photocatalysts for the decomposition of organic pollutants such as methyl orange. Furthermore, these nanocatalysts containing semiconductors such as AgCl were also applied toward the reduction of nitrophenol (NPh) to aminophenol (APh). The Ag@AgCl@SiO2 or Au/Ag@AgCl@SiO2 catalysts showed excellent catalytic properties in the decomposition of toxic substances in terms of their activity and reusability.

Self-assembly of individual polymer chain–metal nanoparticles for polymer cargo nanocomposites with tunable properties

We report a simple synthetic method and the unique properties of four types of individual NPs that are well-dispersed in polyelectrolyte complexes (PECs) from the core to the shell without the formation of alloy or core–shell NPs. Additionally, this result cannot be achieved by conventional methods. By mixing and heating PEs with metal precursors, individual metal NPs stabilized by PEs were synthesized. The four types of individual metal NP-containing PECs were prepared by mixing anionic and cationic PEs involving individual metal NPs. The individual metal NPs were homogeneously distributed within the polymer particles, and their contents were easily tuned by varying the loading fractions of the individual metal NP-containing PEs. The as-obtained PEC–Au/Pt/Ag/Pd (separate entities) exhibited promising performance for surface enhanced Raman scattering (SERS) and catalytic activities, which was further enhanced when the four types of metals demonstrated different optimal composition in comparison to the mixed types, such as the alloy or core–shell NPs. Our results also show that it is necessary to tune the types or individual metal contents of the multi-metallic complexes (individual, alloy, or core–shell) for the control or application of certain properties.

Facile synthesis of mesoporous SiO2 nanoparticles using the mobility differences of etchants

In this paper, we report the preparation of mesoporous silica nanoparticles (MSNPs) using the mobility differences of sulfonate or sulfate-containing materials as etchants. The MSNPs were synthesised by treating silica nanoparticles (SNPs) with styrene sulfonate (SS) or sodium dodecyl sulfate (SDS) under heat. The simple treatment of the SNPs with SS or SDS led to surface etching of the SNPs, resulting in surface roughening and pore generation within the silica structure. The surface structuring of the SNPs could also be controlled by varying the concentration of counter ions of the etchants. This one-step process is very simple, facile, and scalable. The MSNPs appeared almost transparent in an aqueous solution due to their unique surface morphology. The resulting MSNPs also exhibited excellent adsorption and desorption properties for toxic organic pollutants.