Se Gyu Jang

Controlled Supramolecular Assembly of Micelle-Like Gold Nanoparticles in PS-b-P2VP Diblock Copolymers via Hydrogen Bonding

We report a facile strategy to synthesize amphiphilic gold (Au) nanoparticles functionalized with a multilayer, micelle-like structure consisting of a Au core, an inner hydroxylated polyisoprene (PIOH) layer, and an outer polystyrene shell (PS). Careful control of enthalpic interactions via a systematic variation of structural parameters, such as number of hydroxyl groups per ligand (N(OH)) and styrene repeating units (N(PS)) as well as areal chain density of ligands on the Au-core surface (Σ), enables precise control of the spatial distribution of these nanoparticles. This control was demonstrated in a lamellae-forming poly(styrene-b-2-vinylpyridine) (PS-b-P2VP) diblock copolymer matrix, where the favorable hydrogen-bonding interaction between hydroxyl groups in the PIOH inner shell and P2VP chains in the PS-b-P2VP diblock copolymer matrix, driving the nanoparticles to be segregated in P2VP domains, could be counter balanced by the enthalphic penalty of mixing of the PS outer brush with the P2VP domains. By varying N(OH), N(PS), and Σ, the nanoparticles could be positioned in the PS or P2VP domains or at the PS/P2VP interface. In addition, the effect of additives interfering with the hydrogen-bond formation between hydroxyl groups on Au nanoparticles and P2VP chains in a diblock copolymer matrix was investigated, and an interesting pea-pod-like segregation of Au nanoparticles in PS domains was observed.

Striped, Ellipsoidal Particles by Controlled Assembly of Diblock Copolymers

Control of interfacial interactions leads to a dramatic change in shape and morphology for particles based on poly(styrene-b-2-vinylpyridine) diblock copolymers. Key to these changes is the addition of Au-based surfactant nanoparticles (SNPs) which are adsorbed at the interface between block copolymer-containing emulsion droplets and the surrounding amphiphilic surfactant to afford asymmetric, ellipsoid particles. The mechanism of formation for these novel nanostructures was investigated by systematically varying the volume fraction of SNPs, with the results showing the critical nature that the segregation of SNPs to specific interfaces plays in controlling structure. A theoretical description of the system allows the size distribution and aspect ratio of the asymmetric block copolymer colloidal particles to be correlated with the experimental results.

A Facile Synthesis of Dynamic, Shape‐Changing Polymer Particles

We herein report a new facile strategy to ellipsoidal block copolymer nanoparticles that exhibit a pH-triggered anistropic swelling profile. In a first step, elongated particles with an axially stacked lamellae structure are selectively prepared by utilizing functional surfactants to control the phase separation of symmetric polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) in dispersed droplets. In a second step, the dynamic shape change is realized by cross-linking the P2VP domains, thereby connecting glassy PS discs with pH-sensitive hydrogel actuators.

Size-Controlled Nanoparticle-Guided Assembly of Block Copolymers for Convex Lens-Shaped Particles

The tuning of interfacial properties at selective and desired locations on the particles is of great importance to create the novel structured particles by breaking the symmetry of their surface property. Herein, a dramatic transition of both the external shape and internal morphology of the particles of polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) was induced by precise positioning of size-controlled Au nanoparticle surfactants (Au NPs). The size-dependent assembly of the Au NPs was localized preferentially at the interface between the P4VP domain at the particle surface and the surrounding water, which generated a balanced interfacial interaction between two different PS/P4VP domains of the BCP particles and water, producing unique convex lens-shaped BCP particles. In addition, the neutralized interfacial interaction, in combination with the directionality of the solvent-induced ordering of the BCP domains from the interface of the particle/water, generated defect-free, vertically ordered porous channels within the particles. The mechanism for the formation of these novel nanostructures was investigated systemically by varying the size and the volume fraction of the Au NPs. Furthermore, these convex lens-shaped particles with highly ordered channels can be used as a microlens, in which the light can be concentrated toward the focal point with enhanced near-field signals. And, these particles can possess additional optical properties such as unique distribution of light scattering as a result of the well-ordered Au cylinders that filled into the channels, which hold great promise for use in optical, biological-sensing, and imaging applications.

Gold-Decorated Block Copolymer Microspheres with Controlled Surface Nanostructures

Gold-decorated block copolymer microspheres (BCP-microspheres) displaying various surface morphologies were prepared by the infiltration of Au precursors into polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) microspheres. The microspheres were fabricated by emulsifying the PS-b-P4VP polymers in chloroform into a surfactant solution in water, followed by the evaporation of chloroform. The selective swelling of the P4VP domains in the microspheres by the Au precursor under acidic conditions resulted in the formation of Au-decorated BCP-microspheres with various surface nanostructures. As evidenced by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) measurements, dotted surface patterns were formed when microspheres smaller than 800 nm were synthesized, whereas fingerprint-like surface patterns were observed with microspheres larger than 800 nm. Au nanoparticles (NPs) were located inside P4VP domains near the surfaces of the prepared microspheres, as confirmed by TEM. The optical properties of the BCP-microspheres were characterized using UV-vis absorption spectroscopy and fluorescence lifetime measurements. A maximum absorption peak was observed at approximately 580 nm, indicating that Au NPs are densely packed into P4VP domains on the microspheres. Our approach for creating Au-NP-hybrid BCP-microspheres can be extended to other NP systems such as iron-oxide or platinum NPs. These precursors can also be selectively incorporated into P4VP domains and induce the formation of hybrid BCP-microspheres with controlled surface nanostructures.

Synthesis of Multifunctional Micrometer‐Sized Particles with Magnetic, Amphiphilic, and Anisotropic Properties

Well-defi ned, functional colloids provide critical model systems for a variety of fundamental phenomena in materials and soft matter, and enable a broad range of technological applications from optics [ 1 ] to biotechnology. [ 2 ] The ability to fabricate large quantities of colloids with high uniformity and specifi ed shape and function has thus been greatly desired. For example, magnetic colloids can be externally manipulated and controlled, and have been used in applications ranging from photonic crystals, [ 3 ] cell sorting, [ 2 ] biosensors, [ 4 ] drug delivery, [ 5 ] biomedical applications, [ 6 ] and single-molecule biophysics. [ 7 ] Magnetic colloids that can be remotely controlled by applied fi elds have also been exploited in m-ink, [ 8 ] microrheological probes, [ 9–11 ] and a variety of self-organizing systems. [ 12 , 13 ] In a similar fashion, ferrofl uids and magneto-rheological fl uids are frequently used to make useful materials because of their tuneable dynamic response. [ 14 , 15 ]

Synthesis of thermally stable Au-core/Pt-shell nanoparticles and their segregation behavior in diblock copolymer mixtures

We report a facile strategy for the preparation of sub-5 nm gold/platinum (Au-Pt) nanoparticles which are thermally stabilized by a crosslinked polymer shell. Diblock copolymer (PS-b-PI-SH) ligands on the Au nanoparticles were used to crosslink the vinyl functionalities on PIviahydrosilylation with 1,1,3,3-tetramethyldisiloxane in the presence of a platinum catalyst. The Pt catalyst was reduced on the Au nanoparticles during the hydrosilylation reaction resulting in the formation of a Pt-shell on the Au nanoparticle. The hydrosilylation reaction on the Au nanoparticles as well as their positioning in films of a poly(styrene-b-2-vinylpyridine) (PS-b-P2VP) block copolymer were thoroughly characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM), UV-VIS spectroscopy, and X-ray photoelectron spectroscopy (XPS). Variables such as number of vinyl groups on PS-b-PI-SH ligands, the areal density of these ligands on the Au nanoparticle as well as the concentrations of the reactive species were varied systematically to obtain thermally stable nanoparticles. Au-Pt nanoparticles were stable in organic solvents up to 130 °C, and in polymer films at 190 °C for several days. This increased stability allowed the nanoparticles to be thermally annealed in films of PS-b-P2VP where their strong interfacial activity and localization were observed.

Multicolor Emission of Hybrid Block Copolymer–Quantum Dot Microspheres by Controlled Spatial Isolation of Quantum Dots

Multicolor or white light-emitting systems have attracted great attention because of their potential uses as lighting sources and full-color displays. [ 1 ] In recent years, numerous efforts have been devoted to the development of low-cost, simple processes for the fabrication of pure white-light sources as an alternative for conventional lighting sources. [ 2–4 ] Fluorescent quantum dots (QDs) have been explored as one of the promising candidate materials due to their pure color emission spectra, high fl uorescence quantum yield, and photochemical stability. [ 5 , 6 ] However, the emission spectrum produced by each individual QD is narrow, thus requiring simultaneous emission from more than two colored QDs to illuminate across the visible regime. Typically, this is achieved through a combination of either three different QDs emitting red, green and blue light or two different ones emitting orange/red and green/blue light. In such a system, undesired Förster resonance energy transfer (FRET) between QDs often occurs, decreasing the effi ciency of the white light emission. [ 7 , 8 ] The controlled spatial isolation of multiple QDs is made necessary by the strong dependence of the energy-transfer process on the donor-acceptor distance on the nanometer scale. [ 9 , 10 ]

Perfectly Hydrophobic Surfaces with Patterned Nanoneedles of Controllable Features

In this Letter, we present a simple and reproducible method for generating polystyrene (PS) nanoneedle arrays by utilizing the trapping of inorganic silica particles at the polystyrene/air interface via capillary wetting of a thermoplastic polystyrene polymer and SF(6) reactive-ion etching. A monolayer of silica microspheres was directly formed and trapped on the smooth PS film, and subsequent wet etching with HF and reactive-ion etching with SF(6) left behind hexagonal arrays of protruding tips with tip diameters around 20 nm. The patterned PS surface possessed a well-defined nanoneedle array with the pattern density as high as 2.5 x 10(8)/cm(2) and exhibited advancing and receding water contact angles of 180 degrees. The surface showed no affinity for water as confirmed by a series of contact, compression, and release tests. Finally, the perfect hydrophobicity of the fabricated surface is explained in terms of its surface morphology and chemical composition.

High-fidelity optofluidic on-chip sensors using well-defined gold nanowell crystals.

Recent advances in nanofabrication techniques have enabled the creation of various metallic nanostructures in order to engineer the location and properties of electromagnetic hot spots in a controlled manner. However, most previous methods usually require complicated and time-consuming techniques, and the integration of metallic nanostructures into simple, low-cost devices for chemical or biological sensing is still challenging. Here, we report a promising new strategy for the fabrication of large-area gold nanowell arrays with novel geometric features that makes use of the trapping of self-assembled colloidal particles on a polymer surface. Through both systematic experimental and theoretical analysis, we confirm that the strong plasmon resonances of the proposed nanowell structures are associated with localized surface plasmon resonance (LSPR) on the brims of the nanoholes in the top gold films as well as in the bottom gold disks. In addition, we demonstrate a novel optofluidic platform with built-in subwavelength nanowell arrays that exhibits strong plasmon resonances within microfluidic chips. In our optofluidic systems, the plasmon coupling between the brims and the disks of nanowells makes the plasmon resonance more sensitive to surrounding materials. The dependence of the plasmon resonance on the refractive index of the surrounding medium is found to be as high as 570 nm RIU(-1) (refractive index units). These data lead to a figure of merit (FOM), the slope of refractive index sensitivity in eV RIU(-1)/line width (eV), as high as 4.1.