Kyoung Taek Kim

Monosaccharide-Responsive Release of Insulin from Polymersomes of Polyboroxole Block Copolymers at Neutral pH

We synthesized a boroxole-containing styrenic monomer that can be polymerized by the reversible addition-fragmentation and chain transfer (RAFT) method. Poly(styreneboroxole) (PBOx) and its block copolymers with a poly(ethylene glycol) (PEG) as a hydrophilic block displayed binding to monosaccharides in phosphate buffer at neutral pH, as quantified by Wangs competitive binding experiments. By virtue of a controlled radical polymerization, we were able to adjust the degree of polymerization of the PBOx block to yield sugar-responsive block copolymers that self-assembled into a variety of nanostructures including spherical and cylindrical micelles and polymer vesicles (polymersomes). Polymersomes of these block copolymers exhibited monosaccharide-responsive disassembly in a neutral-pH medium. We demonstrated the possibility of using these polymersomes as sugar-responsive delivery vehicles for insulin in neutral phosphate buffer (pH 7.4). Encapsulated insulin could be released from the polymersomes only in the presence of sugars under physiologically relevant pH conditions.

Carbon Nanotubes/Heteroatom‐Doped Carbon Core–Sheath Nanostructures as Highly Active, Metal‐Free Oxygen Reduction Electrocatalysts for Alkaline Fuel Cells

A facile, scalable route to new nanocomposites that are based on carbon nanotubes/heteroatom-doped carbon (CNT/HDC) core-sheath nanostructures is reported. These nanostructures were prepared by the adsorption of heteroatom-containing ionic liquids on the walls of CNTs, followed by carbonization. The design of the CNT/HDC composite allows for combining the electrical conductivity of the CNTs with the catalytic activity of the heteroatom-containing HDC sheath layers. The CNT/HDC nanostructures are highly active electrocatalysts for the oxygen reduction reaction and displayed one of the best performances among heteroatom-doped nanocarbon catalysts in terms of half-wave potential and kinetic current density. The four-electron selectivity and the exchange current density of the CNT/HDC nanostructures are comparable with those of a Pt/C catalyst, and the CNT/HDC composites were superior to Pt/C in terms of long-term durability and poison tolerance. Furthermore, an alkaline fuel cell that employs a CNT/HDC nanostructure as the cathode catalyst shows very high current and power densities, which sheds light on the practical applicability of these new nanocomposites.

Polymersome stomatocytes:controlled shape transformation in polymer vesicles

We report here a controllable shape transformation of polymer vesicles (polymersomes) constructed from block copolymers of which the hydrophobic part is a high-molecular-weight glassy segment. Control over the shape transformation is obtained by kinetic manipulation of the phase behavior of this glassy hydrophobic segment. Kinetic manipulation of the phase behavior of polymer membranes allows for different shapes of polymersomes to be captured at specific times, which directly translates into physically robust nanostructures that are otherwise unobtainable. Combining the morphological diversity of giant liposomes and the physical robustness of polymersomes, our finding can be a general way to realize unusual nanostructures in a predictable manner.

Polymeric Monosaccharide Receptors Responsive at Neutral pH

We present the first detailed report of the synthesis of Wulff-type styrenic monomers and their polymerization by radical addition-fragmentation chain transfer (RAFT) methods. The resulting polymers and block copolymers exhibit sugar-responsive solubilization in aqueous buffer solutions (pH = 7.4-7.8) in the presence of monosaccharides such as D-fructose and D-glucose.

Colloidal inverse bicontinuous cubic membranes of block copolymers with tunable surface functional groups.

Analogous to the complex membranes found in cellular organelles, such as the endoplasmic reticulum, the inverse cubic mesophases of lipids and their colloidal forms (cubosomes) possess internal networks of water channels arranged in crystalline order, which provide a unique nanospace for membrane-protein crystallization and guest encapsulation. Polymeric analogues of cubosomes formed by the direct self-assembly of block copolymers in solution could provide new polymeric mesoporous materials with a three-dimensionally organized internal maze of large water channels. Here we report the self-assembly of amphiphilic dendritic-linear block copolymers into polymer cubosomes in aqueous solution. The presence of precisely defined bulky dendritic blocks drives the block copolymers to form spontaneously highly curved bilayers in aqueous solution. This results in the formation of colloidal inverse bicontinuous cubic mesophases. The internal networks of water channels provide a high surface area with tunable surface functional groups that can serve as anchoring points for large guests such as proteins and enzymes.

Synthesis and self-assembly of dendritic-helical block copolypeptides

Dendritic-helical diblock copolypeptides, dendritic poly(-lysine)--poly(γ-benzyl--glutamate) (PBLG-Lys) were synthesized up to 4th generation of dendritic poly(-lysine). PBLG was synthesized by conventional ring-opening polymerization of γ-benzyl--glutamate--carboxyanhydride with heptyl amine as an initiator. The -terminus of this PBLG was used for further coupling reactions with ,-bis(-butoxycarbonyl)--lysine pentafluorophenylester. These block copolypeptides possess well-defined 3-D structures in solution, and they self-assemble into a fibrous structure a nanoribbon mechanism in toluene. Amphiphilic block copolypeptides were obtained after deprotecting BOC groups at the periphery of the lysine dendrimer block. Due to the well-defined size and structure of the dendritic lysine block and well-defined α-helical conformation of the PBLG block, the block copolypeptides showed a generation-dependent self-assembly behavior in aqueous solution.

Organometallic–Polypeptide Block Copolymers: Synthesis and Self‐Assembly of Poly(ferrocenyldimethylsilane)‐b‐Poly(ε‐benzyloxycarbonyl‐L‐Lysine)

A new type of metallopolymer-polypeptide block copolymer poly(ferrocenyldimethylsilane)-b-poly (epsilon-benzyloxycarbonyl-L-lysine) was synthesized by ring-opening polymerization of epsilon-benzyloxycarbonyl-L-lysine N-carboxyanhydride initiated by using a primary amino-terminated poly(ferrocenyldimethylsilane). Studies on the self-organization behavior of this polypeptide block copolymer in both the bulk state and in solution were performed. In the bulk state, a cylindrical-in-lamellar structure was observed in a compositionally asymmetric sample. Rod-like micelles with a polyferrocenylsilane core formed in a polypeptide-selective solvent alone or in the presence of a common solvent. Significantly, an additional small quantity of the common solvent assisted the formation of longer micelles and micelles with better shape-regularity. This is attributed to a decrease in the number of nucleation events and PFS core reorganization effects.

Mesoporous monoliths of inverse bicontinuous cubic phases of block copolymer bilayers

Solution self-assembly of block copolymers into inverse bicontinuous cubic mesophases is a promising new approach for creating porous polymer films and monoliths with highly organized bicontinuous mesoporous networks. Here we report the direct self-assembly of block copolymers with branched hydrophilic blocks into large monoliths consisting of the inverse bicontinuous cubic structures of the block copolymer bilayer. We suggest a facile and scalable method of solution self-assembly by diffusion of water to the block copolymer solution, which results in the unperturbed formation of mesoporous monoliths with large-pore (>25 nm diameter) networks weaved in crystalline lattices. The surface functional groups of the internal large-pore networks are freely accessible for large guest molecules such as protein complexes of which the molecular weight exceeded 100 kDa. The internal double-diamond (Pn3m) networks of large pores within the mesoporous monoliths could be replicated to self-supporting three-dimensional skeletal structures of crystalline titania and mesoporous silica.

Solution Self-Assembly of Block Copolymers Containing a Branched Hydrophilic Block into Inverse Bicontinuous Cubic Mesophases

Solution self-assembly of amphiphilic block copolymers into inverse bicontinuous cubic mesophases is an emerging strategy for directly creating highly ordered triply periodic porous polymer nanostructures with large pore networks and desired surface functionalities. Although there have been recent reports on the formation of highly ordered triply periodic minimal surfaces of self-assembled block copolymer bilayers, the structural requirements for block copolymers in order to facilitate the preferential formation of such inverse mesophases in solution have not been fully investigated. In this study, we synthesized a series of model block copolymers, namely, branched poly(ethylene glycol)-block-polystyrene (bPEG-PS), to investigate the effect of the architecture of the block copolymers on their solution self-assembly into inverse mesophases consisting of the block copolymer bilayer. On the basis of the results, we suggest that the branched architecture of the hydrophilic block is a crucial structural requirement for the preferential self-assembly of the resulting block copolymers into inverse bicontinuous cubic phases. The internal crystalline lattice of the inverse bicontinuous cubic structure can be controlled via coassembly of branched and linear block copolymers. The results presented here provide design criteria for amphiphilic block copolymers to allow the formation of inverse bicontinuous cubic mesophases in solution. This may contribute to the direct synthesis of well-defined porous polymers with desired crystalline order in the porous networks and surface functionalities.

Carboxylated Pillar[5]arene-Coated Gold Nanoparticles with Chemical Stability and Enzyme-like Activity

A facile synthesis of gold nanoparticles (AuNPs) covered with a multidentate macrocycle, carboxylated pillar[5]arene (CP), via a one-pot hydrothermal process is reported. The resulting AuNPs are highly stable against salts and pH variations, while their traditional counterparts are not stable at the same conditions. For the stabilization, multiple carboxylate groups of CP might contribute to electrostatic or steric stabilization. In addition, we found that CP-coated AuNPs exhibit greater peroxidase-like activity than citrate-stabilized AuNPs in the presence of silver cations. The system presented herein would provide a new scheme to fabricate unique sensory systems in combination with enzymes, which can bind to the pocket of CP.