Seong Il Yoo

Single layers of diblock copolymer micelles for the fabrication of arrays of nanoparticles

Recent advances in the process of using single layers of diblock copolymer micelles for the fabrication of arrays of nanoparticles were highlighted. The technique using a monolayer of diblock copolymer micelles as an effective nanostructured template allowed precise control over the type, size, location, and ordering regularity of nanoparticles. The approach using copolymer micelles was fully compatible with top-down lithographical methods for micropatterning of nanoparticles. In addition, an array of two types of nanoparticles in specific positions was effectively fabricated for a multifunctional array. A near-perfect hexagonal array of nanoparticles was also created by solvent annealing in situ on a single layered film of copolymer micelles prior to synthesizing nanoparticles.

Switching Off FRET in the Hybrid Assemblies of Diblock Copolymer Micelles, Quantum Dots, and Dyes by Plasmonic Nanoparticles

Recently, it has been noticed that surface plasmon resonance of metal nanoparticles can alter the intrinsic properties of nearby fluorophores. Field enhancement and radiative decay engineering are major principles for understanding a number of experimental observations such as enhanced and quenched emission of fluorophores in the vicinity of metal nanoparticles. At the same time, there are apparent similarities between surface-plasmon-coupled fluorescence and fluorescence resonance energy transfer (FRET), as both are near-field through-space interactions. From this perspective, we hypothesize that donor-acceptor interaction in the FRET can be altered by metal nanoparticles. Our approach is based on diblock copolymer micelles, which have been widely applied for nanoscale arrangement of functionalities. By applying self-assembling techniques of copolymer micelles to organize the spatial location of semiconductor quantum dots, fluorescent dyes, and metal nanoparticles, the FRET in hybrid assemblies can be switched off by plasmonic effects.

Highly Ordered Hexagonal Arrays of Hybridized Micelles from Bimodal Self‐Assemblies of Diblock Copolymer Micelles

We demonstrate the formation of highly ordered hexagonal arrays of hybridized polystyrene-poly(4-vinyl pyridine), PS-PVP, micelles with controllable size by solvent annealing techniques. Because the formation of hybridized micelles was prohibited in the mixture solutions of two different-sized PS-PVP micelles, single-layered films with bimodal self-assemblies of small and large micelles were fabricated from the mixture solutions by adjusting their mixing ratios. When the single-layered films were solvent annealed by saturated vapor of tetrahydrofuran (THF), on the other hand, small and large PS-PVP micelles in the bimodal self-assemblies merged together to form hybridized micelles. In addition, the hybridized micelles arranged themselves in a highly ordered hexagonal array, the diameter and center-to-center distance of which were precisely adjusted by varying the mixing ratio of small to large micelles in the bimodal assemblies.

Nanostructures of diblock copolymer micelles for controlled fluorescence resonance energy transfer

Since fluorescence resonance energy transfer (FRET) between fluorophores strongly depends on the distance between and position of donors and acceptors at the nanometer scale, the accurate organization of multiple fluorophores in a specific arrangement plays a critical role in controlling their energy-transferring processes. Herein, we highlight our recent development on the utilization of nanostructures of diblock copolymer micelles for nanoscale arrangement of multiple fluorophores including quantum dots (QDs) to adjust FRET for tuning emissions from a single emitting layer.

Correlation of micellar structures with surface-plasmon-coupled fluorescence in a strategy for fluorescence enhancement

The understanding of underlying phenomena of localized surface plasmon resonance of metal nanoparticles (NPs) enables the unique control on the intrinsic properties of fluorophores in the proximity of metal NPs. While other parameters have to be considered, the near-field interactions between metal NPs and fluorophores are strongly interconnected with their nanoscale organization. In this perspective, many researchers are struggling to find a better assembling method to engineer NP–fluorophore interactions and also to discover a new opportunity in many fields of science. In this study, we applied the self-segregating property of block copolymer micelles to organize the spatial location of fluorophores in the proximity of metal NPs. The strong correlation of micellar structures with surface-plasmon-coupled fluorescence has been discussed with an emphasis on the strategy for fluorescence enhancement.

Ordered complex nanostructures from bimodal self-assemblies of diblock copolymer micelles with solvent annealing.

We report the formation of ordered complex nanostructures from single-layered films of mixtures of polystyrene-poly(2-vinylpyridine) (PS-P2VP) and polystyrene-poly(4-vinylpyridine) (PS-P4VP) diblock copolymer micelles by THF (tetrahydrofuran) annealing. We first examined the influence of THF vapor on PS-P2VP and PS-P4VP micelles in their single-layered films. Due to the different solubility of PS-P2VP and PS-P4VP copolymers in THF, a hexagonal array of PS-P2VP micelles was changed into cylindrical nanodomains, but that of PS-P4VP micelles was not changed. The different influence of THF on PS-P2VP and PS-P4VP micelles was combined in single-layered films of mixtures of both micelles. For the purpose, we prepared mixture solutions of independently prepared small PS-P2VP and large PS-P4VP micelles. Then, bimodal self-assemblies of micelles were prepared from the mixtures, for which the hexagonal array of large PS-P4VP micelles was surrounded by small PS-P2VP micelles. When the bimodal self-assembly was annealed by THF vapor, PS-P2VP micelles were transformed into cylindrical nanodomains, but their reorganization was guided by hexagonally arranged PS-P4VP micelles. As a result, we were able to produce ordered complex nanostructures in the form of a hexagonal array of PS-P4VP micelles surrounded by PS-P2VP cylinders, which was further utilized for the synthesis of Au nanoparticles.

Bimodal arrays of two types of nanoparticles by mixtures of diblock copolymer micelles

For multiple functionalities with nanoparticles, controlled assembly of several types of nanoparticles on a solid substrate is desirable. In this study, we demonstrate the fabrication of a bimodal array of two types of nanoparticles and switching of their sizes and locations by using a mixture of copolymer micelles. In proof of concept, we first synthesized a bimodal array of large gold nanoparticles and small platinum oxide nanoparticles from a mixture of large and small micelles containing precursors of nanoparticles. By switching the location of the precursors, the array was converted to one of large platinum oxide nanoparticles and small gold nanoparticles with the preservation of the order of the array. The methodology demonstrated here can be applied to the fabrication and control of bimodal arrays of nanoparticles with a wide choice of types and sizes for multifunctional arrays of nanoparticles.

Bimodal assembly of two different-sized diblock copolymer micelles by stepwise coating process

Phase-separation of block copolymers into periodic nanostructures is of great interests in nanoscience because the size and morphology of the nanodomains of block copolymer can be engineered by experimental factors such as degree of polymerization and composition of each block. In particular, thin films of block copolymer in lamellar, cylindrical, and spherical nanodomains have been extensively studied for the past to produce nanoscale functionalities with controllable size and shape. In comparison with top-down lithographic techniques, block-copolymer approach is quite attractive in terms of simplicity, scalability, and cost effectiveness; however, more precise control on the orientation and placement of the nanodomains of block copolymers is necessary for practical applications such as electric and data storage devices. In this context, many research groups reported the directed self-assemblies of block copolymers on the pre-defined chemical or physical patterns. It has been found that the ordering regularity, orientation, or alignment of nanodomains of block copolymers can be extensively improved by engineering a specific interaction between pre-defined patterns and block copolymers. For examples, Ross and coworkers applied sphereforming polystyrene-poly(ferrocenyl dimethylsilane) copolymers to lithographic patterns to improve the ordering regularity of the nanodomains. Nealey and coworkers utilized chemical patterns of self-assembled monolayers to induce arbitrarily angled lamellar structures from ternary blends of polystyrene-poly(methyl methacrylate)/polystyrene/poly(methyl methacrylate). Steinhart and coworkers reported the confinement of block copolymers in anodized aluminum oxide to create interesting structures such as helices and stacked doughnuts. There have been many other publications reporting periodic patterns of parallel lines, bent lines, and junctions by using patterned substrates. Interestingly, the nanodomains of block copolymers themselves can be alternatively employed as pre-defined patterns to assist the second level of self-assembly of block copolymers. For instance, Kim and coworkers recently reported the formation of multi-component patterns, in which cylindrical nanodomains of polystyrene-poly(4-vinyl pyridine), PS-P4VP, were utilized as a physical pattern to further direct subsequent assemblies of block copolymers. Since nanodomains of block copolymers have been used as nanoreactors for the particle synthesis or etching mask for the nanolithography, the directed self-assemblies of two different kinds of block copolymers can reveal interesting opportunities in multifunctional patterns. With this regard, we demonstrate the formation of bimodal assembly of two different-sized diblock copolymer micelles via a stepwise coating process. Here, two dissimilar PS-P4VP diblock copolymers having the same chemical structure but different molecular weights have been employed to produce spherical micelles with two different sizes. Subsequently, large micelles were spin coated onto solid substrate to produce a mono-layered film of micelles having short-ranged hexagonal order. As a second level of assembly, small micelles were preferentially deposited onto interstitial sites of hexagonallyarranged large micelles by dip coating, which resulted in the formation of bimodal patterns of small and large micelles. In fact, this assembling behavior of copolymer micelles could be quite analogous to that found in superlattice formation from binary colloidal particles. However, unlike colloidal assemblies, block copolymer micelles having a broad size distribution assembled into short-range ordered structure with a number of defects instead of the formation of crystal lattice.

Core-Corona Functionalization of Diblock Copolymer Micelles by Heterogeneous Metal Nanoparticles for Dual Modality in Chemical Reactions.

Nanoscale assemblies composed of different types of nanoparticles (NPs) can reveal interesting aspects about material properties beyond the functions of individual constituent NPs. This research direction may also represent current challenges in nanoscience toward practical applications. With respect to the assembling method, synthetic or biological nanostructures can be utilized to organize heterogeneous NPs in specific sites via chemical or physical interactions. However, those assembling methods often encounter uncontrollable particle aggregation or phase separation. In this study, we anticipated that the self-segregating properties of block copolymer micelles could be particularly useful for organizing heterogeneous NPs, because the presence of chemically distinct domains such as the core and the corona can facilitate the selective placement of constituent NPs in separate domains. Here, we simultaneously functionalized the core and the corona of micelles by Au NPs and Ag NPs, which exhibited plasmonic and catalytic functions, respectively. Our primary question is whether these plasmonic and catalytic functions can be combined in the assembled structures to engineer the kinetics of a model chemical reaction. To test this hypothesis, the catalytic reduction of 4-nitrophenol was selected to evaluate the collective properties of the micellar assemblies in a chemical reaction.

Directed Self-Assembly of Block Copolymer Micelles onto Topographically Patterned Surface

We report a facile method to control directed self-assembly (DSA) of spherical micelles of block copolymers (BCPs) by topographically patterned surface. A cylinder-forming polystyrene-block-poly(2-vinylpyridine) copolymer [Mn,PS = 175 kg/mol, Mn,P2VP = 70 kg/mol, and polydipersity index (PDI) = 1.08] was phase-separated on a thin film of poly(vinyl alcohol) (PVA) by solvent annealing. By additional treatment with ethanol as a preferential solvent for P2VP block, the surface of BCP thin film was reconstructed to produce nanopores. Nanoporous structures in BCP thin films were transferred to the underlying hydrophilic PVA film by reactive ion etching (RIE). Then spherical BCP micelles were quickly self-assembled within the nanopores in the PVA layer due to topographical contrast and surface energy difference during spin-coating. Consequently, the site-selective array of BCP micelles was utilized as templates to achieve heterogeneous organization of nanoparticles and organic fluorescent dyes over a large area. In addition, it was observed that those heterogeneous assemblies showed a remarkable decrease in fluorescence intensity of organic dyes.