Nokyoung Park

Restacking-inhibited 3d reduced graphene oxide for high performance supercapacitor electrodes

Graphene has received considerable attention in both scientific and technological areas due to its extraordinary material properties originating from the atomically single- or small number-layered structure. Nevertheless, in most scalable solution-based syntheses, graphene suffers from severe restacking between individual sheets and thus loses its material identity and advantages. In the present study, we have noticed the intercalated water molecules in the dried graphene oxide (GO) as a critical mediator to such restacking and thus eliminated the hydrogen bonding involving the intercalated water by treating GO with melamine resin (MR) monomers. Upon addition of MR monomers, porous restacking-inhibited GO sheets precipitated, leading to the carbonaceous composite with an exceptionally large surface area of 1040 m(2)/g after a thermal treatment. Utilizing such high surface area, the final graphene composite exhibited excellent electrochemical performance as a supercapacitor electrode material: specific capacitance of 210 F/g, almost no capacitance loss for 20,000 cycles, and ~7 s rate capability. The current study delivers a message that various condensation reactions engaging GO sheets can be a general synthetic approach for restacking-inhibited graphene in scalable solution processes.

A mechanical metamaterial made from a DNA hydrogel

Metamaterials are artificial substances that are structurally engineered to have properties not typically found in nature. To date, almost all metamaterials have been made from inorganic materials such as silicon and copper, which have unusual electromagnetic or acoustic properties that allow them to be used, for example, as invisible cloaks, superlenses or super absorbers for sound. Here, we show that metamaterials with unusual mechanical properties can be prepared using DNA as a building block. We used a polymerase enzyme to elongate DNA chains and weave them non-covalently into a hydrogel. The resulting material, which we term a meta-hydrogel, has liquid-like properties when taken out of water and solid-like properties when in water. Moreover, upon the addition of water, and after complete deformation, the hydrogel can be made to return to its original shape. The meta-hydrogel has a hierarchical internal structure and, as an example of its potential applications, we use it to create an electric circuit that uses water as a switch.

pH-responsive assembly of gold nanoparticles and "spatiotemporally concerted" drug release for synergistic cancer therapy.

A challenge in using plasmonic nanostructure-drug conjugates for thermo-chemo combination cancer therapy lies in the huge size discrepancy; the size difference can critically differentiate their biodistributions and hamper the synergistic effect. Properly tuning the plasmonic wavelength for photothermal therapy typically results in the nanostructure size reaching ∼100 nm. We report a new combination cancer therapy platform that consists of relatively small 10 nm pH-responsive spherical gold nanoparticles and conjugated doxorubicins. They are designed to form aggregates in mild acidic environment such as in a tumor. The aggregates serve as a photothermal agent that can selectively exploit external light by their collective plasmon modes. Simultaneously, the conjugated doxorubicins are released. The spatiotemporal concertion is confirmed at the subcellular, cellular, and organ levels. Both agents colocalize in the cell nuclei. The conjugates accumulate in cancer cells by the rapid phagocytic actions and effective blockage of exocytosis by the increased aggregate size. They also effectively accumulate in tumors up to 17 times over the control because of the enhanced permeation and retention. The conjugates exhibit a synergistic effect enhanced by nearly an order of magnitude in cellular level. The synergistic effect is demonstrated by the remarkable reductions in both the therapeutically effective drug dosage and the photothermal laser threshold. Using an animal model, effective tumor growth suppression is demonstrated. The conjugates induce apoptosis to tumors without any noticeable damage to other organs. The synergistic effect in vivo is confirmed by qRT-PCR analysis over the thermal stress and drug-induced growth arrest.

DNA hydrogel-based supercapacitors operating in physiological fluids

DNA nanostructures have been attractive due to their structural properties resulting in many important breakthroughs especially in controlled assemblies and many biological applications. Here, we report a unique energy storage device which is a supercapacitor that uses nanostructured DNA hydrogel (Dgel) as a template and layer-by-layer (LBL)-deposited polyelectrolyte multilayers (PEMs) as conductors. Our device, named as PEM-Dgel supercapacitor, showed excellent performance in direct contact with physiological fluids such as artificial urine and phosphate buffered saline without any need of additional electrolytes, and exhibited almost no cytotoxicity during cycling tests in cell culture medium. Moreover, we demonstrated that the PEM-Dgel supercapacitor has greater charge-discharge cycling stability in physiological fluids than highly concentrated acid electrolyte solution which is normally used for supercapacitor operation. These conceptually new supercapacitors have the potential to be a platform technology for the creation of implantable energy storage devices for packageless applications directly utilizing biofluids.

Polypyrrole/Agarose-based electronically conductive and reversibly restorable hydrogel.

Conductive hydrogels are a class of composite materials that consist of hydrated and conducting polymers. Due to the mechanical similarity to biointerfaces such as human skin, conductive hydrogels have been primarily utilized as bioelectrodes, specifically neuroprosthetic electrodes, in an attempt to replace metallic electrodes by enhancing the mechanical properties and long-term stability of the electrodes within living organisms. Here, we report a conductive, smart hydrogel, which is thermoplastic and self-healing owing to its unique properties of reversible liquefaction and gelation in response to thermal stimuli. In addition, we demonstrated that our conductive hydrogel could be utilized to fabricate bendable, stretchable, and patternable electrodes directly on human skin. The excellent mechanical and thermal properties of our hydrogel make it potentially useful in a variety of biomedical applications such as electronic skin.

Light-responsible DNA hydrogel–gold nanoparticle assembly for synergistic cancer therapy

Assembled AuNPs in a DNA hydrogel (Dgel) showed strongly coupled plasmon modes, and the Dgel vehicle can co-load anticancer drugs such as doxorubicin (Dox) as a light-controlled releasing cargo by DNA intercalations. Upon laser excitation, local heat shock generation was accompanied by the release of Dox. A highly synergistic combination of thermo- and chemotherapy was demonstrated in cellular and animal models. Our Dgel vehicle can be fragmented after the excitation-induced heat generations, which subsequently causes the dispersion of the AuNPs. Our system may be less toxic because it uses small sizes of AuNPs, and the inherently biocompatible scaffold may reduce the long-term toxicity by rapid clearance.

DNA hydrogel templated carbon nanotube and polyaniline assembly and its applications for electrochemical energy storage devices

Functional nanocomposites comprising of biomaterials and non-biomaterials are one of the main subjects of recent research due to their wide range of potential applications. Here, we demonstrate that the porous DNA hydrogel (Dgel) can be an excellent template for combining carbon nanotubes (CNTs) and polyaniline (PANI) resulting in high performance supercapacitor electrodes. These hybrid supercapacitors have been constructed by electrostatic deposition of conductive CNTs on DNA hydrogel followed by coating of PANI pseudocapacitor. Performances of supercapacitors in terms of specific capacitance, cycling stability, power density, and energy density have been systematically investigated. The specific capacitance of these DNA hydrogel based supercapacitors has reached up to 146.4 F g−1 with a power density of 23.3 kW kg−1 and an energy density of 13.0 Wh kg−1 in acidic media which is higher than commercially available products. In addition, the cytotoxicity of our supercapacitors was evaluated in vitro in cell culture media during the charge–discharge processes. In both human and mouse skin cell culture media, our devices exhibited zero cytotoxicity. Our novel biological hybrid electrodes can be a platform towards biocompatible and implantable energy storage devices for in vivo applications.

A sub 6 nanometer plasmonic gold nanoparticle for pH-responsive near-infrared photothermal cancer therapy

A small (sub 6 nm hydrodynamic size) and pH-responsive gold nanoparticle photothermal agent is reported, which can respond to changes in pH and form aggregates. The coupled plasmon mode of aggregates can be efficiently exploited for photothermal cancer therapy using longer excitation wavelength.

Metal ion-induced dual fluorescent change for aza-crown ether acridinedione-functionalized gold nanorods and quantum dots

Aza-crown ether acridinedione-functionalized quantum dots (ACEADD-QDs) and aza-crown ether acridinedione-functionalized gold nanorods (ACEADD-GNRs) have been developed as a pair for a fluorescent chemosensor detecting metal ions. The ACEADD-QDs have dual emissions at a visible wavelength of ∼430 nm from the acridinedione dye moiety and at a near-infrared (NIR) wavelength of ∼775 nm from the CdTeSe QDs. In the presence of Ca2+ or Mg2+ ions, the ACEADD-QD and ACEADD-GNR pair can form a sandwich complex mediated by the metal ion. The ACEADD-QD and ACEADD-GNR complex pair shows visible fluorescence enhancement from the acridinedione dye and concurrent fluorescence quenching from the NIR QD. The aza-crown ether complex results in the suppression of photoinduced electron transfer from the aza-crown ether to the acridinedione dye moiety. At the same time, the QD fluorescence can be effectively quenched by the nanometal surface energy transfer from the QD to the GNR. This ACEADD-QD and ACEADD-GNR pair can effectively transduce the selective binding event of crown ethers with metal ions into the simultaneous modulation of the enhancement in dye fluorescence and the quenching of QD emission, which can open a new strategy for ratiometric sensors that are selective and robust against the environment conditions.

Three-Dimensional Structure and Thermal Stability Studies of DNA Nanostructures by Energy Transfer Spectroscopy

Structural changes and stability of DNA nanoarchitectures including Y-shaped DNA (see picture), dendrimer-like DNA, and DNA hydrogels are investigated. The results demonstrate the feasibility and flexibility of FRET and NSET (Forster resonance/ nanometal surface- energy transfer) in determining difficult-to-obtain 3D structures and characterizing the thermal responses of DNA nanoarchitectures in real time.