Chan Beum Park

Highly Photoactive, Low Bandgap TiO2 Nanoparticles Wrapped by Graphene

Highly photoactive, graphene-wrapped anatase TiO(2) nanoparticles are synthesized through one-step hydrothermal reduction of graphene oxide (GO) and TiO(2) crystallization from GO-wrapped amorphous TiO(2) NPs. Graphene-TiO(2) nanoparticles exhibit a red-shift of the band-edge and a significant reduction of the bandgap (2.80 eV). Graphene-TiO(2) nanoparticles possess excellent photocatalytic properties under visible light for the degradation of methylene blue.

General functionalization route for cell adhesion on non-wetting surfaces

We present a versatile route for promoting cell adhesion and viability on various non-wetting surfaces, inspired by mussel adhesion mechanism. The oxidative polymerization of dopamine, a small designer molecule of the DOPA-K motif found in mussels, results in the formation of a poly(dopamine) ad-layer on any material surface. We found that the poly(dopamine) coating can promote cell adhesion on any type of material surfaces including the well-known anti-adhesive substrate, poly(tetrafluoroethylene). According to our results, mammalian cells well adhered and underwent general cell adhesion processes (i.e., attachment to substrate, spreading, and cytoskeleton development) on poly(dopamine)-modified surfaces, while they barely adhered and spread on unmodified non-wetting surfaces. The mussel-inspired surface functionalization strategy is extremely useful because it does not require the time-consuming synthesis of complex linkers and the process is solvent-free and non-toxic. Therefore, it can be a powerful route for converting a variety of bioinert substrates into bioactive ones.

Pressure effects on intra- and intermolecular interactions within proteins

The effects of pressure on protein structure and function can vary dramatically depending on the magnitude of the pressure, the reaction mechanism (in the case of enzymes), and the overall balance of forces responsible for maintaining the proteins structure. Interactions between the protein and solvent are also critical in determining the response of a protein to pressure. Pressure has long been recognized as a potential denaturant of proteins, often promoting the disruption of multimeric proteins, but recently examples of pressure-induced stabilization have also been reported. These global effects can be explained in terms of pressure effects on individual molecular interactions within proteins, including hydrophobic, electrostatic, and van der Waals interactions, which can now be studied in greater detail than ever before. However, many uncertainties remain, and thorough descriptions of how proteins respond to pressure remain elusive. This review summarizes basic concepts and new findings related to pressure effects on intra- and intermolecular interactions within proteins and protein complexes, and discusses their implications for protein structure-function relationships under pressure.

Human endothelial cell growth on mussel-inspired nanofiber scaffold for vascular tissue engineering

The endothelialization of prosthetic scaffolds is considered to be an effective strategy to improve the effectiveness of small-diameter vascular grafts. We report the development of a nanofibrous scaffold that has a polymeric core and a shell mimicking mussel adhesive for enhanced attachment, proliferation, and phenotypic maintenance of human endothelial cells. Polycaprolactone (PCL) was chosen as a core material because of its good biodegradability and mechanical properties suitable for tissue engineering. PCL was electrospun into nanofibers with a diameter of approximately 700 nm and then coated with poly(dopamine) (PDA) to functionalize the surface of PCL nanofibers with numerous catechol moieties similar to mussel adhesives in nature. The formation of a PDA ad-layer was analyzed using multiple techniques, including scanning electron microscopy, Raman spectroscopy, and water contact angle measurements. When PDA-coated PCL nanofibers were compared to unmodified and gelatin-coated nanofibers, human umbilical vein endothelial cells (HUVECs) exhibited highly enhanced adhesion and viability, increased stress fiber formation, and positive expression of endothelial cell markers (e.g., PECAM-1 and vWF).

Inhibition of insulin amyloid formation by small stress molecules

Amyloidogenic proteins undergo an alternative folding pathway under stressful conditions leading to formation of fibrils having cross β‐sheet structure, which is the hallmark of many neurodegenerative diseases. As a means of surviving against external stress, on the other hand, many microorganisms accumulate small stress molecules to prevent abnormal protein folding and to contribute to protein stability, which hints at the efficacy of the solutes against amyloid formation. The current work demonstrates the effectiveness of small stress molecules such as ectoine, betaine, trehalose, and citrulline on inhibition of insulin amyloid formation in vitro. The inhibitory effects were analyzed by thioflavin T‐induced fluorescence, circular dichroism, and atomic force microscopy. This report suggests that naturally occurring small molecules may serve a function that is typically fulfilled by protein chaperones, and it provides a hint for designing inhibitors against amyloid formation associated with neurodegenerative disorders.

Spatial Control of Cell Adhesion and Patterning through Mussel-Inspired Surface Modification by Polydopamine

The spatial control and patterning of mammalian cells were achieved by using the universal adhesive property of mussel-inspired polydopamine (PDA). The self-polymerization of dopamine, a small molecule inspired by the DOPA motif of mussel foot proteins, resulted in the formation of a PDA adlayer when aqueous dopamine solution was continuously injected into poly(dimethylsiloxane) microchannels. We found that various cells (fibrosarcoma HT1080, mouse preosteoblast MC3T3-E1, and mouse fibroblast NIH-3T3) predominantly adhered to PDA-modified regions, maintaining their normal morphologies. The cells aligned in the direction of striped PDA patterns, and this tendency was not limited by the type of cell line. Because PDA modification does not require complex chemical reactions and is applicable to any type of material, it enables cell patterning in a simple and versatile manner as opposed to conventional methods based on the immobilization of adhesive proteins. The PDA-based method of cell patterning should be useful in many biomaterial research areas such as the fabrication of tissue engineering scaffolds, cell-based devices for drug screening, and the fundamental study of cell-material interactions.

Graphene-Based Chemiluminescence Resonance Energy Transfer for Homogeneous Immunoassay

We report on chemiluminescence resonance energy transfer (CRET) between graphene nanosheets and chemiluminescent donors. In contrast to fluorescence resonance energy transfer, CRET occurs via nonradiative dipole-dipole transfer of energy from a chemiluminescent donor to a suitable acceptor molecule without an external excitation source. We designed a graphene-based CRET platform for homogeneous immunoassay of C-reactive protein (CRP), a key marker for human inflammation and cardiovascular diseases, using a luminol/hydrogen peroxide chemiluminescence (CL) reaction catalyzed by horseradish peroxidase. According to our results, anti-CRP antibody conjugated to graphene nanosheets enabled the capture of CRP at the concentration above 1.6 ng mL(-1). In the CRET platform, graphene played a key role as an energy acceptor, which was more efficient than graphene oxide, while luminol served as a donor to graphene, triggering the CRET phenomenon between luminol and graphene. The graphene-based CRET platform was successfully applied to the detection of CRP in human serum samples in the range observed during acute inflammatory stress.

Self-Assembled Light-Harvesting Peptide Nanotubes for Mimicking Natural Photosynthesis†

Light-harvesting peptide nanotubes are synthesized by the self-assembly of diphenylalanine with THPP and platinum nanoparticles (nPt; see picture; TEOA = triethanolamine). The light-harvesting peptide nanotubes are suitable for mimicking photosynthesis because of their structure and electrochemical properties that are similar to the ones of photosystem I in natural photosynthesis.

Myoblast differentiation on graphene oxide

Graphene-based nanomaterials have received much attention in biomedical applications for drug/gene delivery, cancer therapy, imaging, and tissue engineering. Despite the capacity of 2D carbon materials as a nontoxic and implantable platform, their effect on myogenic differentiation has been rarely studied. We investigated the myotube formation on graphene-based nanomaterials, particularly graphene oxide (GO) and reduced graphene oxide (rGO). GO sheets were immobilized on amine-modified glass to prepare GO-modified glass, which was further reduced by hydrazine treatment for the synthesis of rGO-modified substrate. We studied the behavior, including adhesion, proliferation, and differentiation, of mouse myoblast C2C12 on unmodified, GO-, and rGO-modified glass substrates. According to our analyses of myogenic protein expression, multinucleate myotube formation, and expression of differentiation-specific genes (MyoD, myogenin, Troponin T, and MHC), myogenic differentiation was remarkably enhanced on GO, which resulted from serum protein adsorption and nanotopographical cues. Our results demonstrate the ability of GO to stimulate myogenic differentiation, showing a potential for skeletal tissue engineering applications.

Graphene-biomineral hybrid materials.

Scheme 1 . Illustration of GO/graphene–CaCO 3 hybrid material synthesis and its conversion to GO/graphene–hydroxyapatite (HAp) composites. The steps describe a) CO 2 mineralization to CaCO 3 in the presence of GO sheets and CaCl 2 , b) GO/graphene–CaCO 3 hybrid materials, and c) their conversion to GO/graphene–HAp hybrid materials. Images in the right column show a) an AFM image and sectional analysis of graphene oxide (GO) sheets used for this study, b) SEM images of GO-wrapped vaterite microspheres, and c) SEM images of GO–HAp hybrid fi lm that show HAp surrounding GO sheets and covering the entire fi lm surface. Graphene, an sp 2 -bonded carbon sheet with a thickness of single atom, has recently received attention from materials scientists because of its unique qualities, which include excellent thermal and mechanical properties and electrical conductivity resulting from long-range π -conjugation. [ 1–5 ] While graphene was originally developed for nanoelectronics applications, [ 1 , 6 , 7 ] research interests in graphene are continuously expanding to other fi elds. [ 1 , 2 ] For example, graphene is considered to be an adequate reinforcing component for composite materials. [ 3 , 5 , 8 , 9 ] However, hybridization or interaction of graphene with biominerals has so far been rarely reported. Biomineralization is the process that gives rise to small and large inorganic-based structures in biological systems, and it often results in sophisticated materials having elaborate morphologies, excellent mechanical and optical properties, and vital biological functions. [ 10–13 ] Thus, the convergence of the study of graphene with biomineralization is expected to widen the horizons of material science. We have successfully incorporated graphene and graphene oxide (GO) sheets into the crystals of the two most abundantly studied biominerals found in the hard tissues of invertebrates and vertebrtates: calcium carbonate [ 14–16 ] and calcium phosphate. [ 17 , 18 ] As illustratived in Scheme 1 , we used GO sheets for the synthesis of a graphene–CaCO 3 hybrid fi lm, which then underwent a transformation into graphene-incorporated hydroxyapaptite [Ca 10 (PO 4 ) 6 (OH) 2 ; HAp]. By applying CO 2 gas to a mixture of GO and CaCl 2 , we obtained spherical CaCO 3 vaterite microspheres that were wrapped and interconnected by a GO network. After the reduction of GO–CaCO 3 composite, we fabricated a conductive, biocompatible, and bone-bioactive hybrid fi lm that consisted of CaCO 3 microspheres interconnected with a graphene network. When incubated in simulated body fl uid (SBF), the graphene–CaCO 3 hybrid fi lm was transformed to