Choonghyeon Lee

Aqueous synthesis of silver nanoparticle embedded cationic polymer nanofibers and their antibacterial activity.

This paper describes the one-pot, aqueous synthesis of cationic polymer nanofibers with embedded silver nanoparticles. Poly[2-(tert-butylaminoethyl) methacrylate] (PTBAM) was used as a cationic polymer substrate to reinforce the antimicrobial activity of the embedded silver nanoparticles. Electron microscope analyses revealed that the as-synthesized nanofibers had diameters of approximately 40 nm and lengths up to about 10 μm. Additionally, silver nanoparticles of approximately 8 nm in diameter were finely embedded into the prepared nanofibers. The embedded silver nanoparticles had a lower tendency to agglomerate than colloidal silver nanoparticles of comparable size. In addition, the nanofibers with embedded silver nanoparticles exhibited excellent antibacterial performance against Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus. Interestingly, the prepared nanofibers exhibited enhanced bactericidal performance compared with the silver-embedded poly(methyl methacrylate) (PMMA) nanofibers, presumably because of the antibacterial properties of the PTBAM substrate.

Polypyrrole Nanotube Embedded Reduced Graphene Oxide Transducer for Field-Effect Transistor-Type H2O2 Biosensor

We report a rapid-response and high-sensitivity sensor with specificity toward H2O2 based on a liquid-ion-gated field-effect transistor (FET) using graphene-polypyrrole (PPy) nanotube (NT) composites as the conductive channel. The rGO, PPy, NTs, and nanocomposite materials were characterized using Raman spectroscopy, Fourier transform-infrared (FT-IR) spectroscopy, transmission electron microscopy (TEM), and scanning electron microscopy (SEM). On the basis of these results, a well-organized structure is successfully prepared owing to the specific interactions between the PPy NTs and the rGO sheet. Reliable electrical contacts were developed between the rGO/PPy NTs and the microelectrodes, which remained stable when exposed to the liquid-phase analyte. Liquid-ion-gated FETs composed of these graphene nanocomposites exhibited hole-transport behavior with conductivities higher than those of rGO sheets or PPy NTs. This implies an interaction between the PPy NTs and the rGO layers, which is explained in terms of the PPy NTs forming a bridge between the rGO layers. The FET sensor provided a rapid response in real time and high sensitivity toward H2O2 with a limit of detection of 100 pM. The FET-type biosensing geometry was also highly reproducible and stable in air. Furthermore, the liquid-gated FET-type sensor exhibited specificity toward H2O2 in a mixed solution containing compounds found in biological fluids.

Fe3O4/Carbon Hybrid Nanoparticle Electrodes for High-Capacity Electrochemical Capacitors

Fe3O4/carbon hybrid nanoparticles (FeCHNPs) were fabricated using dual-nozzle electrospraying, vapor deposition polymerization (VDP), and carbonization. FeOOH nanoneedles decorated with polypyrrole (PPy) nanoparticles (FePNPs) were fabricated by electrospraying pristine PPy mixed with FeCl3 solution, followed by heating stirring reaction. A PPy coating was then formed on the FeOOH nanoneedles through a VDP process. FeCHNPs were produced through carbonization of PPy and FeOOH phase transitions. These hybrid carbon nanoparticles (NPs) were used to build electrodes of electrochemical capacitors. The specific capacitance of the FeCHNPs was 455 F g(-1), which is larger than that of pristine PPy NPs (105 F g(-1)) or other hybrid PPy NPs. Furthermore, the FeCHNP-based capacitors exhibited better cycle stability during charge-discharge cycling than other hybrid NP capacitors. This is because the carbon layer on the Fe3 O4 surface formed a protective coating, preventing damage to the electrode materials during the charge-discharge processes. This fabrication technique is an effective approach for forming stable carbon/metal oxide nanostructures for energy storage applications.

A metal-oxide nanofiber-decorated three-dimensional graphene hybrid nanostructured flexible electrode for high-capacity electrochemical capacitors

Carbonized polypyrrole-coated SnO2/Co3O4 nanofiber-decorated three-dimensional graphene (CPSC-3rGO) was fabricated using single-nozzle co-electrospinning, freeze-drying, and thermal reduction. Polypyrrole-coated SnO2/Co3O4 nanofibers (PSCNFs) were fabricated using vapor deposition polymerization (VDP) of SnO2/Co3O4 nanofibers. The PSCNFs were then mixed with an aqueous graphene oxide (GO) solution and freeze dried to form a PSCNF-decorated 3D GO (CPSC-3GO) structure. The CPSC-3rGO was produced via thermal reduction to form a hybrid nanomaterial for use as the electrodes of electrochemical capacitors (ECs). The specific capacitance of the CPSC-3rGO was 446 F g−1, which is larger than that of the other 3D nanomaterials investigated (the specific capacitance of PSCNFs was 270 F g−1, that of PSC-3GO was 285 F g−1, and that of 3D rGO was 150 F g−1). In addition, ECs with two symmetrical CPSC-3rGO electrodes were fabricated from two layers of CPSC-3rGO separated by a polymer electrolyte gel and encapsulated in polyethylene terephthalate (PET) membranes. These devices exhibited excellent electrical performance, which was preserved following repeated mechanical deformation.

Three‐Dimensional Scaffolds of Carbonized Polyacrylonitrile for Bone Tissue Regeneration

Carbon-based materials have been extensively studied for stem cell culture. However, difficulties associated with engineering pure carbon materials into 3D scaffolds have hampered applications in tissue engineering and regenerative medicine. Carbonized polyacrylonitrile (cPAN) could be a promising alternative, as cPAN is a highly ordered carbon isomorph that resembles the graphitic structure and can be easily processed into 3D scaffolds. Despite the notable features of cPAN, application of cPAN in tissue engineering and regenerative medicine have not been explored. This study, for the first time, demonstrates the fabrication of microporous 3D scaffolds of cPAN and excellent osteoinductivity of cPAN, suggesting utility of 3D cPAN scaffolds as synthetic bone graft materials. The combination of excellent processability and unique bioactive properties of cPAN may lead to future applications in orthopedic regenerative medicine.

High-Performance Three-Dimensional Mesoporous Graphene Electrode for Supercapacitors using Lyophilization and Plasma Reduction

In this study, a three-dimensional (3D) mesoporous plasma-reduced graphene oxide web (mPrGO web) was fabricated via lyophilization of graphene oxide (GO) solution and subsequent plasma reduction. The lyophilized graphene oxide web (GO web) was successfully reduced by a short plasma treatment (<2 s) using a commercially available plasma apparatus. The degree of reduction of the mPrGO web was determined by the applied plasma power (W) of the apparatus; the optimum power level for effective reduction was identified. The as-synthesized mPrGO web showed a high degree of reduction and robust graphitic characteristics, with a unique crack-like mesoporous structure created on corrugated graphene sheets. In addition to the above characteristics, the mPrGO web possessed a 3D web-like architecture that provided enhanced surface area along with ion-transportable channels derived from lyophilization. Owing to the synergistic effect of lyophilization and plasma reduction, the mPrGO web exhibited high electrical conductivity (87 S cm-1) and increased surface area (642 m2 g-1). Accordingly, the mPrGO web showed outstanding specific capacitance of 253.8 F g-1 at 0.2 A g-1 along with the excellent rate capability (76% capacitance retention at 5 A g-1). Furthermore, the assembled all-solid-state symmetric supercapacitor also exhibited remarkable electrochemical performances, demonstrating the potential applicability of the mPrGO web as an effective supercapacitor electrode material.

Dissipative particle dynamics modeling of a graphene nanosheet and its self-assembly with surfactant molecules

A coarse-grained graphene nanosheet (CGNS) is modeled with dissipative particle dynamics to investigate its self-assembly with surfactant molecules adsorbing onto the graphene nanosheet in aqueous solution. The adsorption dynamics of coarse-grained sodium dodecyl sulfate (CSDS) and a micelle collapse are observed with the CGNS. Different morphologies for the equilibrium structures are obtained by varying the concentration of CSDS in a simulation box. It is found that the arrangement of the CSDS molecules onto the CGNS changed from the edge to the central region of the CGNS to minimize the exposure of the hydrophobic segments of the CSDS molecules to water. In particular, the CSDS surfactants tend to be located beside the CGNS edge at a high ratio of surfactant molecules to the graphene surface area.

Simultaneous Chemical and Optical Patterning of Polyacrylonitrile Film by Vapor-Based Reaction

The surface of polyacrylonitrile (PAN) film is treated with ethyleneamines (EDA) in a simple chemical vapor phase reaction. Successful introduction of amine functional groups on the cyano group of PAN backbone is verified by FT-IR and NMR measurements. Further UV-vis and photoluminescence analyses show a red shift of the emission peak after repeated EDA treatment, which might be attributed to the formation of imine conjugation from newly formed carbon-nitrogen bonds on the PAN backbone. Further confocal laser scanning microscopy reveals that selective patterning of EDA on PAN films is possible via local polydimethylsiloxane masking. The results indicate that both chemical and optical patterning on PAN film can be realized via a single reaction and show the potential of this novel methodology in selective patterning.

Enhancement of the rate performance of plasma-treated platelet carbon nanofiber anodes in lithium-ion batteries

The rate performances of lithium-ion battery (LIB) anodes using platelet carbon nanofiber (PCNF) and its graphitized version (GPCNF) are enormously enhanced by introducing carbon–fluorine (CmFn) functional groups on the nanofiber surfaces. The CmFn functional groups are selectively introduced through controlled plasma treatment in vacuo with C4F8 gas. Combined X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR) and time-of-flight secondary ion mass spectrometry (TOF-SIMS) analyses demonstrate that the CmFn functional groups had mainly semi-ionic C–F bonds, which are introduced only on the surface of PCNF and GPCNF. PCNF treated with the plasma for 60 s (PCNF–F60 s) exhibits the largest discharge capacity of 387 mA h g−1 with increased first-cycle coulombic efficiency and a discharge capacity of 293 mA h g−1 at the 10C rate. These are 1.13- and 1.63-fold higher, respectively, than those of pristine PCNF. The presence of CmFn functional groups on the PCNF and GPCNF surfaces can reduce the resistance of the anode, which is related to lithium-ion migration and charge transfer resistance. The improved migration and reduced resistance result in a marked increase in rate performance at discharge without deterioration of the first-cycle coulombic efficiency.