Jin-Woong Kim

Biologically templated photocatalytic nanostructures for sustained light-driven water oxidation

Over several billion years, cyanobacteria and plants have evolved highly organized photosynthetic systems to shuttle both electronic and chemical species for the efficient oxidation of water. In a similar manner to reaction centres in natural photosystems, molecular and metal oxide catalysts have been used to photochemically oxidize water. However, the various approaches involving the molecular design of ligands, surface modification and immobilization still have limitations in terms of catalytic efficiency and sustainability. Here, we demonstrate a biologically templated nanostructure for visible light-driven water oxidation that uses a genetically engineered M13 virus scaffold to mediate the co-assembly of zinc porphyrins (photosensitizer) and iridium oxide hydrosol clusters (catalyst). Porous polymer microgels are used as an immobilization matrix to improve the structural durability of the assembled nanostructures and to allow the materials to be recycled. Our results suggest that the biotemplated nanoscale assembly of functional components is a promising route to significantly improved photocatalytic water-splitting systems.

Fabrication of monodisperse thermosensitive microgels and gel capsules in microfluidic devices

We use droplet-based microfluidic techniques to produce monodisperse poly(N-isopropylacrylamide) gel particles in the size range of 10–1000 µm. Our techniques offer exquisite control over both outer dimensions and inner morphology of the particles. We demonstrate this control by fabricating conventional microgels, microgels with embedded materials and voids, and gel microcapsules with single- and multi-phase cores. These techniques should be applicable for the synthesis of particles and capsules of a variety of chemical compositions and for the generation of higher order “supraparticles” by directed assembly of colloidal particles in droplets.

Morphological effect of lipid carriers on permeation of lidocaine hydrochloride through lipid membranes

We have studied how the transdermal delivery of lidocaine hydrochloride (LHC) is affected by the morphology of lipid carriers, liposomes and micelles, having the same lipid composition of 1-stearoyl-sn-glycero-3-phosphocholine (LPC) and cholesteryl hemisuccinate (CHEMS). In vitro drug permeation study, carried out on guinea pig skin, has revealed that the liposomes made of LPC and CHEMS significantly enhance the permeation rate of entrapped LHC; by contrast, the mixed micelles with the same composition decrease the degree of delivering co-existing LHC. Basically, we have also investigated the release kinetics of LHC through the cellulose membrane and found that both liposomes and micelles have a similar releasing profile. To experimentally demonstrate this unique behavior, we have observed the fluidity of stratum corneum liposomal membranes in the presence of either our liposomes or micelles. From this study, we have found that LPC/CHEMS liposomes fluidize the lipid membrane of stratum corneum lipids; however, lipid micelles rather make the membrane rigid. These findings highlight that controlling the morphology of drug carriers provides us with a means to modulate the permeability of encapsulated drug molecules.

Silicone oil emulsions stabilized by semi-solid nanostructures entrapped at the interface

Oil-in-water (O/W) emulsions are typically stabilized using water-soluble surfactants, which anchor to the surface of oil droplets dispersed in an aqueous solution. The structure of the anchored surfactants is often susceptible to physical and chemical stresses because of their highly mobile properties. Here we introduce a new approach to prepare stable silicone oil emulsions under various external stresses using a water-insoluble amphiphilic block copolymer, poly(ethylene oxide)-b-poly(epsilon-caprolactone) (PEO-b-PCL). Above the melting temperature (around 60 degrees C) of the hydrophobic segment (PCL), PEO-b-PCL can be dissolved in silicone oil. When the polymer/oil mixture is dispersed in water, PEO-b-PCL is irreversibly reorganized into solid nanostructures at the interface of the aqueous/organic phases. The resulting interfacial structures provide a robust physical barrier to the emulsion coarsening processes. Accordingly, the prepared emulsions exhibit excellent structural tolerance against external stresses, including variations in pH, ionic strength, and temperature.

Fluctuations in flow produced by competition between apparent wall slip and dilatancy

Dense suspensions can exhibit a dramatic stress-induced transition from liquid-like to solid-like behavior. In many materials, the solid-like flow state is characterized by large flow fluctuations and instabilities. Although various experiments have been performed to characterize flow fluctuations, the mechanisms that govern the flow instabilities remain poorly understood. To elucidate these mechanisms, we characterize a system that rapidly fluctuates between two flow states. One of the flow states is dominated by apparent wall slip, and the other is dominated by dilatancy. The dilatant regime occurs at elevated stresses and is associated with reduced wall slip, whereas the wall slip-dominated regime occurs at lower stresses. At stresses that are intermediate between these two regimes, the material fluctuates between the two regimes in a semi-regular fashion. Our analysis of the fluctuations at millisecond timescales shows that fluctuations occur because neither regime is capable of supporting a constant stress in a stable manner. We rationalize our results in terms of the differences in the shear-induced particle pressure between regions that are particle-rich and regions of slip that are particle-depleted.

Flexible magnetic microtubules structured by lipids and magnetic nanoparticles.

This study presents a microtubule that responds to a magnetic field. We made such a structure by incorporating iron oxide nanoparticles during the preparation of the microtubule. We found that the microtubule stretches its body when the magnetic field is applied and easily aligns with the direction of the applied magnetic field by rotating its body. When the magnetic field is removed, it loses its orientation and goes back to its original state by contraction. From the analysis of its magnetic response, we estimated that the magnetic microtubule had an elastic modulus of 33 MPa. Further analysis showed that the stretching and contracting of its body are due to its flexibility.