Jinbo He

Self-directed self-assembly of nanoparticle/copolymer mixtures

The organization of inorganic nanostructures within self-assembled organic or biological templates is receiving the attention of scientists interested in developing functional hybrid materials. Previous efforts have concentrated on using such scaffolds to spatially arrange nanoscopic elements as a strategy for tailoring the electrical, magnetic or photonic properties of the material. Recent theoretical arguments have suggested that synergistic interactions between self-organizing particles and a self-assembling matrix material can lead to hierarchically ordered structures. Here we show that mixtures of diblock copolymers and either cadmium selenide- or ferritin-based nanoparticles exhibit cooperative, coupled self-assembly on the nanoscale. In thin films, the copolymers assemble into cylindrical domains, which dictate the spatial distribution of the nanoparticles; segregation of the particles to the interfaces mediates interfacial interactions and orients the copolymer domains normal to the surface, even when one of the blocks is strongly attracted to the substrate. Organization of both the polymeric and particulate entities is thus achieved without the use of external fields, opening a simple and general route for fabrication of nanostructured materials with hierarchical order.

Self-assembly of nanoparticles at interfaces

Developments in the assembly of nanoparticles at liquid-liquid interfaces are reviewed where the assemblies can be controlled by tuning the size of the nanoparticles and the chemical characteristics of the ligands. Both synthetic and biological nanoparticles are discussed. By controlling the type of ligands, uniform and Janus-type nanoparticles can be produced where, at liquid-liquid interfaces, subsequent reactions of the ligands can be used to generate crosslinked sheets of nanoparticles at the interface that have applications including novel encapsulants, filtration devices with well-defined porosities, and controlled release materials. By controlling the size and volume fraction of the nanoparticles and the chemical nature of the ligands, nanoparticle-polymer composites can be generated where either enthalpy or entropy can be used to control the spatial distribution of the nanoparticles, thereby, producing auto-responsive materials that self-heal, self-corral assemblies of nanoparticles, or self-direct morphologies. Such systems hold great promise for generating novel optical, acoustic, electronic and magnetic materials.

Synthesis of Nano/Microstructures at Fluid Interfaces

The generation of novel multifunctional materials with hierarchical ordering is a major focus of current materials science and engineering. For such endeavors, fluid interfaces, such as air-liquid and liquid-liquid interfaces, offer ideal platforms where nanoparticles or colloidal particles can accumulate and self-assemble. Different assembly processes and reactions have been performed at fluid interfaces to generate hierarchical structures, including two-dimensional crystalline films, colloidosomes, raspberry-like core-shell structures, and Janus particles, which lead to broad applications in drug delivery and controlled release, nanoelectronics, sensors, food supplements, and cosmetics.

On the kinetics of nanoparticle self-assembly at liquid/liquid interfaces

We investigate the concentration and size dependent self-assembly of cadmium selenide nanoparticles at an oil/water interface. Using a pendant drop tensiometer, we monitor the assembly kinetics and evaluate the effective diffusion coefficients following changes in the interfacial tension for the early and late stages of nanoparticle adsorption. Comparison with the coefficients for free diffusion reveals the energy barrier for particle segregation to the interface. The formation of a nanoparticle monolayer at the oil/water interface is characterised by transmission electron microscopy.

Self-Assembly of Tobacco Mosaic Virus at Oil/Water Interfaces

The oil/water interfacial assembly of tobacco mosaic virus (TMV) has been studied in situ by tensiometry and small-angle X-ray and neutron scattering (SAXS and SANS). TMV showed different orientations at the perfluorodecalin/water interface, depending on the initial TMV concentration in the aqueous phase. At low TMV concentration, the rods oriented parallel to the interface, mediating the interfacial interactions at the greatest extent per particle. At high TMV concentrations, the rods were oriented normal to the interface, mediating the interfacial interactions and also neutralizing inter-rod electrostatic repulsion. We found that the inter-rod repulsive forces between TMVs dominated the in-plane packing, which was strongly affected by the ionic strength and the bulk solution but not by the pH in the range of pH = 6-8.

Interfacial assembly of turnip yellow mosaic virus nanoparticles.

An extensive study of the factors that affect the interfacial assembly of bionanoparticles at the oil/water (O/W) interface is reported. Bionanoparticles, such as viruses, have distinctive structural properties due to the unique arrangement of their protein structures. The assembly process of such bionanoparticles at interfaces is governed by factors including the ionic strength and pH of the aqueous layer, concentration of the particles, and nature of the oil phase. This study highlights the impact of these factors on the interfacial assembly of bionanoparticles at the O/W interface using native turnip yellow mosaic virus (TYMV) as the prototype. Robust monolayer assemblies of TYMV were produced by self-assembly at the O/W interface using emulsions and planar interfaces. TYMV maintained its structure and integrity under different assembly conditions. For the emulsion droplets, they were fully covered with TYMV as evidenced by transmission electron microscopy (TEM) and scanning force microscopy (SFM). Tensiometry and small-angle neutron scattering (SANS) further supported this finding. Although the emulsions offered a complete coverage by TYMV particles, they lacked long-range ordering due to rapid exchange at the interface. By altering the assembly process, highly ordered, hexagonal arrays of TYMV were obtained at planar O/W interfaces. The pH, ionic strength, and viscosity of the solution played a crucial role in enhancing the lateral ordering of TYMV assembled at the planar O/W interface. This interfacial ordering of TYMV particles was further stabilized by introduction of a positively charged dehydroabietyl amine (DHAA) in the organic phase which held the assembly together by electrostatic interactions. The long-range array formation was observed using TEM and SFM. The results presented here illustrate that the interfacial assembly at the O/W interface is a versatile approach to achieve highly stable self-assembled structures.

Connecting quantum dots and bionanoparticles in hybrid nanoscale ultra-thin films

Ligand-functionalized CdSe quantum dots and nanorods, and horse spleen ferritin bionanoparticles, were co-assembled at an oil-water interface, then used in polymerization at the interface, effectively cross-linking the assembled mixtures of nanoparticles into robust structures. Both ring-opening metathesis polymerization (ROMP), and imine formation, proved suitable for preparation of the desired ultra-thin films in the form of capsules and sheets. The nanoparticle-based films prepared by ROMP exhibit chemical stability, while those prepared by aldehyde-amine coupling could be disrupted by addition of acid. Characterization of these hybrid nanoparticle-based materials, using transmission electron microscopy (TEM) and fluorescence confocal microscopy, confirmed the presence of both synthetic and naturally derived nanoparticles in the hybrid materials.