Jinkee Hong

Facilitated Ion Transport in All-Solid-State Flexible Supercapacitors

The realization of highly flexible and all-solid-state energy-storage devices strongly depends on both the electrical properties and mechanical integrity of the constitutive materials and the controlled assembly of electrode and solid electrolyte. Herein we report the preparation of all-solid-state flexible supercapacitors (SCs) through the easy assembly of functionalized reduced graphene oxide (f-RGO) thin films (as electrode) and solvent-cast Nafion electrolyte membranes (as electrolyte and separator). In particular, the f-RGO-based SCs (f-RGO-SCs) showed a 2-fold higher specific capacitance (118.5 F/g at 1 A/g) and rate capability (90% retention at 30 A/g) compared to those of all-solid-state graphene SCs (62.3 F/g at 1A/g and 48% retention at 30 A/g). As proven by the 4-fold faster relaxation of the f-RGO-SCs than that of the RGO-SCs and more capacitive behavior of the former at the low-frequency region, these results were attributed to the facilitated ionic transport at the electrical double layer by means of the interfacial engineering of RGO by Nafion. Moreover, the superiority of all-solid-state flexible f-RGO-SCs was demonstrated by the good performance durability under the 1000 cycles of charging and discharging due to the mechanical integrity as a consequence of the interconnected networking structures. Therefore, this research provides new insight into the rational design and fabrication of all-solid-state flexible energy-storage devices as well as the fundamental understanding of ion and charge transport at the interface.

Innovative polymer nanocomposite electrolytes: nanoscale manipulation of ion channels by functionalized graphenes.

The chemistry and structure of ion channels within the polymer electrolytes are of prime importance for studying the transport properties of electrolytes as well as for developing high-performance electrochemical devices. Despite intensive efforts on the synthesis of polymer electrolytes, few studies have demonstrated enhanced target ion conduction while suppressing unfavorable ion or mass transport because the undesirable transport occurs through an identical pathway. Herein, we report an innovative, chemical strategy for the synthesis of polymer electrolytes whose ion-conducting channels are physically and chemically modulated by the ionic (not electronic) conductive, functionalized graphenes and for a fundamental understanding of ion and mass transport occurring in nanoscale ionic clusters. The functionalized graphenes controlled the state of water by means of nanoscale manipulation of the physical geometry and chemical functionality of ionic channels. Furthermore, the confinement of bound water within the reorganized nanochannels of composite membranes was confirmed by the enhanced proton conductivity at high temperature and the low activation energy for ionic conduction through a Grotthus-type mechanism. The selectively facilitated transport behavior of composite membranes such as high proton conductivity and low methanol crossover was attributed to the confined bound water, resulting in high-performance fuel cells.

Osteophilic Multilayer Coatings for Accelerated Bone Tissue Growth

Osteophilic modular nanostructured multilayers containing hydroxyapatite nanoparticles complexed with a natural polymer chitosan create an osteoconductive surface for mesenchymal stem cells (MSCs). Coupled with the sustained release of physiological amounts of osteoinductive bone morphogenetic protein over several days from degradable poly(β-amino ester) based multilayers, this single coating results in a synergistic accelerated and upregulated differentiation of MSCs into osteoblasts laying down new bone tissue on orthopedic implants.

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.

Layer-by-Layer Assembled Antisense DNA Microsponge Particles for Efficient Delivery of Cancer Therapeutics

Antisense oligonucleotides can be employed as a potential approach to effectively treat cancer. However, the inherent instability and inefficient systemic delivery methods for antisense therapeutics remain major challenges to their clinical application. Here, we present a polymerized oligonucleotides (ODNs) that self-assemble during their formation through an enzymatic elongation method (rolling circle replication) to generate a composite nucleic acid/magnesium pyrophosphate sponge-like microstructure, or DNA microsponge, yielding high molecular weight nucleic acid product. In addition, this densely packed ODN microsponge structure can be further condensed to generate polyelectrolyte complexes with a favorable size for cellular uptake by displacing magnesium pyrophosphate crystals from the microsponge structure. Additional layers are applied to generate a blood-stable and multifunctional nanoparticle via the layer-by-layer (LbL) assembly technique. By taking advantage of DNA nanotechnology and LbL assembly, functionalized DNA nanostructures were utilized to provide extremely high numbers of repeated ODN copies for efficient antisense therapy. Moreover, we show that this formulation significantly improves nucleic acid drug/carrier stability during in vivo biodistribution. These polymeric ODN systems can be designed to serve as a potent means of delivering stable and large quantities of ODN therapeutics systemically for cancer treatment to tumor cells at significantly lower toxicity than traditional synthetic vectors, thus enabling a therapeutic window suitable for clinical translation.

Rapid fabrication of thick spray-layer-by-layer carbon nanotube electrodes for high power and energy devices

Rapid fabrication of layer-by-layer (LbL) electrodes is essential to expand their utility in energy storage applications. Herein, we address challenges in developing thick LbL electrodes of multi-wall carbon nanotubes (MWNTs) using conventional dip- and spray-LbL processes, and present a solution to overcome these challenges. The vacuum-assisted spray-LbL process using porous carbon substrates enabled a linear growth of LbL-MWNT electrodes with a 600 time decrease in their fabrication time. This result was attributed to the enhanced surface interactions between MWNTs and substrate via increased surface areas, enhanced capillary forces, physical entrapment in pores, and changes in hydrodynamic drag forces. Scanning electron microscopy (SEM) revealed high surface area carbon nanotube networks comprised of individual MWNTs. The spray MWNT electrodes delivered a high gravimetric energy of 100 W h kg−1 at high gravimetric power of 50 kW kg−1, which is higher than those of most carbon nanotube electrodes reported. Moreover, the spray MWNT electrodes delivered the highest energy capacity per unit area (up to 300 μW h cm−2 at 0.4 mW cm−2 among the LbL electrodes reported, and showed excellent retention of energy capacity up to 100 μW h cm−2 at high power capacity of 200 mW cm−2. These performance values are higher or comparable to the most advanced battery electrodes for high energy capacity per unit area.

Inherent charge-shifting polyelectrolyte multilayer blends: a facile route for tunable protein release from surfaces.

Recent research has highlighted degradable multilayer films that enable the programmed release of different therapeutics. Multilayers constructed by the layer-by-layer (LbL) deposition that can undergo disassembly have been demonstrated to be of considerable interest, particularly for biomedical surface coatings due to their versatility and mild aqueous processing conditions, enabling the inclusion of biologic drugs with high activity. In this study, we examine the controlled release of a protein using a different mechanism for film disassembly, the gradual dissociation of film interactions under release conditions. Poly(β-amino ester)s and poly(L-lysine) (PLL) were used as the positively charged multilayer components coassembled with a model negatively charged antigen protein, ovalbumin (Ova). The release of the protein from these multilayer films is dominated by the slow shift in the charge of components under physiological pH conditions rather than by hydrolytic degradative release. The time scale of release can be varied over almost 2 orders of magnitude by varying the ratio of the two polyamines in the deposition solution. The highly versatile and tunable properties of these films form a basis for designing controlled and sequential delivery of drug coatings using a variety of polyions.

Controlled release of an anti-cancer drug from DNA structured nano-films

We demonstrate the generation of systemically releasable anti-cancer drugs from multilayer nanofilms. Nanofilms designed to drug release profiles in programmable fashion are promising new and alternative way for drug delivery. For the nanofilm structure, we synthesized various unique 3-dimensional anti cancer drug incorporated DNA origami structures (hairpin, Y, and X shaped) and assembled with peptide via layer-by-layer (LbL) deposition method. The key to the successful application of these nanofilms requires a novel approach of the influence of DNA architecture for the drug release from functional nano-sized surface. Herein, we have taken first steps in building and controlling the drug incorporated DNA origami based multilayered nanostructure. Our finding highlights the novel and unique drug release character of LbL systems in serum condition taken full advantages of DNA origami structure. This multilayer thin film dramatically affects not only the release profiles but also the structure stability in protein rich serum condition.

Multilayer Thin Film Coatings Capable of Extended Programmable Drug Release: Application to Human Mesenchymal Stem Cell Differentiation

The promise of cellular therapy lies in healing damaged tissues and organs in vivo as well as generating tissue constructs in vitro for subsequent transplantation. Postnatal stem cells are ideally suited for cellular therapies due to their pluripotency and the ease with which they can be cultured on functionalized substrates. Creating environments to control and successfully drive their differentiation toward a lineage of choice is one of the most important challenges of current cell-based engineering strategies. In recent years, a variety of biomaterials platforms have been prepared for stem cell cultures, primarily to provide efficient delivery of growth or survival factors to cells and a conductive microenvironment for their growth. Here, we demonstrate that repeating tetralayer structures composed of biocompatible poly(methacrylic acid), poly(acrylamide), and poly(ethylene oxide)-block-poly(ε-caprolactone) micelles arrayed in layer-by-layer films can serve as a payload region for dexamethasone delivery to human mesenchymal stem cells (MSCs). This architecture can induce MSC differentiation into osteoblasts in a dose-dependent manner. The amount of Dex loaded in the films is controlled by varying the deposition conditions and the film thickness. Release of Dex is tuned by changing the amount of covalent cross-linking of multilayers via thermal treatments. The multilayer architecture including payload and cell-adhesion region introduced here are well suited for extended cell culture thus affording the important and protective effect of both Dex release and immobilization. These films may find applications in the local delivery of immobilized therapeutics for biomedical applications, as they can be deposited on a wide range of substrates with different shapes, sizes, and composition.

Hollow capsules prepared from all block copolymer micelle multilayers

We introduce a novel and versatile approach for preparing hollow multilayer capsules containing functional hydrophobic components. Protonated polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) and anionic polystyrene-block-poly(acrylic acid) (PS-b-PAA) block copolymer micelles (BCM) were used as building blocks for the layer-by-layer assembly of BCM multilayer films onto polystyrene (PS) colloids. After removing the PS colloids, the stabilities of the formed BCM hollow capsules were found to be strongly dependent on the charge density of the hydrophilic corona segments (i.e., P4VP and PAA block segments) as well as the relative molecular weight ratio of hydrophobic core (i.e., PS segments) blocks and hydrophilic corona shells. Furthermore, in the case of incorporating hydrophobic fluorescent dyes into the PS core blocks of micelles, the hairy/hairy BCM multilayers showed well-defined fluorescent images after colloidal template removal process. These phenomena are mainly caused by the relatively high degree of electrostatic interdigitation between the protonated and anionic corona block shells.