Ho Yeon Son

Bioinspired templating synthesis of metal-polymer hybrid nanostructures within 3D electrospun nanofibers.

Novel metal nanostructures immobilized within three-dimensional (3D) porous polymeric scaffolds have been utilized for catalysts and biosensors. However, efficient, robust immobilization of the nanostructures both outside and inside of the 3D scaffolds is a challenging task. To address the challenge, we synthesized a redox-active polymer, catechol-grafted poly(vinyl alcohol), PVA-g-ct. The grafted catechol is inspired by the adhesion mechanism of marine mussels, which facilitates binding and reduction of noble metal ions. Electrospinning the PVA-g-ct polymer results in highly open porous, 3D nanostructures, on which catechol mediates the spontaneous reduction of silver ions to solid silver nanocubes at an ambient temperature. Yet, gold and platinum ions are partially reduced and complexed with the nanofiber template, requiring an additional thermal treatment for complete reduction into solid metal nanostructures. Furthermore, silver-gold and silver-platinum hybrid nanostructures are generated by sequential treatments with metal ion precursor solutions of each. This study suggests that catechol-grafted polymer nanofibers are an attractive reactive template for the facile synthesis and immobilization of noble metal nanostructures within a 3D porous matrix for the potential applications to sensors, catalysis, and tissue engineering.

Bioinspired Design of an Immobilization Interface for Highly Stable, Recyclable Nanosized Catalysts.

Immobilization of nanometer-sized metal catalysts into porous substrates can stabilize the catalysts and allow their recycled uses, while immobilization often sacrifices the active surface of catalysts and degenerates the local microenvironments, resulting in the reduction of the catalytic activity. To maintain a high activity of immobilized nanocatalysts, it is critically important to design an interface that minimizes the contact area and favors reaction chemistry. Here we report on the application of mussel-inspired adhesion chemistry to the formation of catalytic metal nanocrystal-polydopamine hybrid materials that exhibit a high catalytic efficiency during recycled uses. Electrospun polymer nanofibers are used as a template for in situ formation and immobilization of gold nanoparticles via polydopamine-induced reduction of ionic precursors. The prepared hybrid nanostructures exhibit a recyclable catalytic activity for the reduction of 4-nitrophenol with a turnover frequency of 3.2-5.1 μmol g(-1) min(-1). Repeated uses of the hybrid nanostructures do not significantly alter their morphology, indicating the excellent structural stability of the hybrid nanostructures. We expect that the polydopamine chemistry combined with the on-surface synthesis of catalytic nanocrystals is a promising route to the immobilization of various colloidal nanosized catalysts on supporting substrates for long-term catalysis without the physical instability problem.

In situ functionalization of highly porous polymer microspheres with silver nanoparticles via bio-inspired chemistry

This study introduces a simple method to functionalize porous polymer microspheres with in situ synthesis of metal nanoparticles via mussel-inspired polydopamine chemistry. Highly open porous poly(methyl methacrylate) (PMMA) microspheres are prepared by a single oil-in-water emulsion solvent evaporation technique using Pluronic F127 as an extractable porogen. The internal pore surfaces of the prepared microspheres are coated with polydopamine via the oxidative polymerization of dopamines in an aqueous solution. The deposited polydopamine mediates the spontaneous reduction of silver nitrates into solid silver nanoparticles (Ag NPs) within the pores of the prepared microspheres, resulting in porous PMMA microspheres decorated with Ag NPs. Anti-bacterial experiments show that the Ag NP-decorated PMMA microspheres can be used as an excellent anti-bacterial platform. This study suggests that highly open porous microspheres can be used as a template to synthesize functional metal–polymer hybrid materials using the mussel-inspired polydopamine chemistry in an aqueous solution under ambient conditions.

Bioinspired Synthesis of Mesoporous Gold-silica Hybrid Microspheres as Recyclable Colloidal SERS Substrates

Noble metal nanostructures have been intensively investigated as active substrates for surface-enhanced Raman spectroscopy (SERS) from visible to near-IR wavelengths. However, metal nanoparticle-based SERS analysis in solutions is very challenging due to uncontrollable and irreproducible colloid aggregation. Here we report the templated synthesis of porous gold-silica hybrid microspheres and their application as reusable colloidal SERS substrates. Mesoporous polymer microspheres are synthesized and used as templates for the synthesis of non-aggregated gold nanoparticles, followed by polydopamine-mediated silicification to fabricate mesoporous gold-silica hybrid microspheres. The mesoporous hybrid particles detect crystal violet in the order of 10–8 M and provide the structural durability of the immobilized gold nanoparticles, allowing them to be recycled for repeated SERS analyses for analytes in a solution with the similar sensitivity. This work suggests that the mesoporous gold-silica hybrid microspheres are attractive SERS substrates in terms of reusability, sensitivity, and stability.

Multilayered Plasmonic Heterostructure of Gold and Titania Nanoparticles for Solar Fuel Production

Solar fuel production via photoelectrochemical (PEC) water splitting has attracted great attention as an approach to storing solar energy. However, a wide range of light-harvesting materials is unstable when exposed to light and oxidative conditions. Here we report a robust, multilayered plasmonic heterostructure for water oxidation using gold nanoparticles (AuNPs) as light-harvesting materials via localized surface plasmon resonance (LSPR). The multilayered heterostructure is fabricated using layer-by-layer self-assembly of AuNPs and TiO2 nanoparticles (TNPs). Plasmon-induced hot electrons are transferred from AuNPs to TNPs over the Au/TiO2 Schottky barrier, resulting in charge separation of hot carriers. Plasmonic photoanodes for water oxidation are completed by incorporating a Co-based oxygen-evolving catalyst on the multilayered heterostructure to scavenge hot holes. Light absorption capability and PEC properties of the photoanodes are investigated as a function of the number of AuNP/TNP bilayers. The PEC properties exhibits dependence on the number of the bilayers, which is affected by charge transport within the multilayered heterostructures. Photocurrent density and decrease in impedance by irradiation indicates significant photoactivity by LSPR excitation.

Layer-by-layer siRNA/poly(L-lysine) Multilayers on Polydopamine-coated Surface for Efficient Cell Adhesion and Gene Silencing

For tissue engineering applications, small interfering RNA (siRNA) is an attractive agent for controlling cellular functions and differentiation. Although polyionic condensation of nucleic acids with polycations has been widely used for gene delivery, siRNA is not strongly associated with cationic carriers due to its low charge density and rigid molecular structures. The use of an excess amount of cationic carriers is often used for siRNA condensation, though they can induce severe cytotoxicity. Here we introduce the self-assembly of siRNA with mild polyelectrolytes into multilayers for efficient gene silencing during cell proliferation. The multilayers were prepared through the sequential layer-by-layer deposition of siRNA and poly-L-lysine (PLL) on a polydopamine-coated substrate. The cells, grown on the siRNA/PLL multilayers, exhibited a remarkable inhibition of the expression of target genes as compared to the use of scrambled siRNA. The gene silencing efficiency depends on the number of siRNA layers within a multilayer. This result indicates that siRNA/PLL multilayers can be potentially utilized for efficient surface-mediated siRNA delivery.

Tannin–Titanium Oxide Multilayer as a Photochemically Suppressed Ultraviolet Filter

UV filters can initiate redox reactions of oxygen and water when exposed to sunlight, generating reactive oxygen species (ROS) that deteriorate the products containing them and cause biological damages. This photochemical reactivity originates from the high chemical potential of UV filters, which also determines the optical properties desirable for sunscreen applications. We hypothesize that this dilemma can be alleviated if the photochemical pathway of UV filters is altered to coupling with redox active molecules. Here, we employ tannic acid (TA) as a key molecule for controlling the photochemical properties of titanium dioxide nanoparticles (TiO2 NPs). TA provides an unusual way for layer-by-layer assembly of TiO2 NPs by the formation of a ligand-to-metal charge transfer complex that alters the nature of UV absorption of TiO2 NPs. The galloyl moieties of TA efficiently scavenge ROS due to the stabilization of ROS by intramolecular hydrogen bonding while facilitating UV screening through direct charge injection from TA to the conduction band of TiO2. The TiO2-TA multilayers assembled in open porous polymer microspheres substantially increased sun protection while dramatically reducing ROS under UV exposure. The assembled structure exhibits excellent in vivo anti-UV skin protection against epidermal hyperplasia, inflammation, and keratinocyte apoptosis without long-term toxicity.