Dayang Wang

Template-directed colloidal self-assembly – the route to ‘top-down’ nanochemical engineering

This article gives an overview of recent progress in template-directed colloidal self-assembly. We first highlight colloidal crystallization and clustering guided by using lithographic patterns as templates. Secondly, we also outline current efforts on binary colloidal crystallization, paving a way to direct self-assembly of colloids by templating with colloidal assemblies. Finally, the use of colloidal spheres as templates to direct the self-assembly of colloids is also outlined.

The water/oil interface: the emerging horizon for self-assembly of nanoparticles

This article highlights our recent achievement on directing nanoparticles to self-assemble at the water/oil interface. We demonstrate that the contact angle of 90° is prerequisite for nanoparticles to localize at the interface, which is determined by the terminal groups of the capping ligands. With this peculiar surface wettability, nanoparticles of different sizes and chemical composition may self-assemble into homogeneous or composite thin films at the water/oil interface. The interfacially active nanoparticles may be employed to stabilize water-in-oil or oil-in-water droplets, creating the microcapsules whose shells are composed of randomly close packed nanoparticles. The permeability of the resulting microcapsules is defined by the sizes of the nanoparticles used.

Synthesis of Monodisperse Quasi-Spherical Gold Nanoparticles in Water via Silver(I)-Assisted Citrate Reduction

We demonstrate a simple and reproducible way to produce quasi-spherical Au nanoparticles (NPs) with a fairly narrow size distribution in water by rapidly adding a mixture solution of HAuCl(4), sodium citrate, and a trace amount of silver nitrate into boiling water. The sizes of quasi-spherical Au NPs obtained increases from 12 +/- 1 nm to 18 +/- 3, 25 +/- 3, and 36 +/- 3 nm with decrease of the citrate concentration in a fairly linear way. The present protocol can efficiently minimize the effect of citrate to buffer the pH of the reaction media and thus change the type and reactive activity of auric ions and significantly speed up the nucleation and growth rate of Au NPs. The presence of Ag(+) ions can not only suppress the secondary nucleation but also reshape the polycrystalline Au NPs into a quasi-spherical shape. In the case of synthesis of Au NPs of sizes ranging from 10 to 36 nm, our approach efficiently makes up the shortages of the classical Turkevich method with respect to the reproducibility and uniformity of the NP size and shape.

Nanoparticle Cages for Enzyme Catalysis in Organic Media

Encapsulation of enzymes in Pickering emulsions results in a large interfacial area of the enzyme-containing aqueous phase for biocatalysis in organic media. This immobilization technique minimizes enzyme inactivation through stabilizing immiscible liquids by particles, facilitates separation processes, and significantly increases catalytic performance of both stable and vulnerable enzymes. Thus, a broad technical applicability can be envisioned.

Colloidal Lithography—The Art of Nanochemical Patterning

Nanochemical printmaking: Colloidal lithography paves a powerful nanochemical way for patterning on planar substrates and microparticles. The feature size can easily be scaled down to 100 nm by reducing the diameter of the microspheres and the feature shape diversified by the crystalline structure of a colloidal crystal mask, the mask etching time, the incidence angle of the vapor beam, and the mask registry (the azimuth angle of the vapor beam). Colloidal lithography relies on using colloidal crystals as masks for etching and deposition, and allows fabrication of various nanostructures on planar and non-planar substrates with low-cost, high-throughput-processing, large fabrication area, and a broad choice of materials. The feature size can easily shrink by decreasing the microsphere diameter in the colloidal mask. The feature shape can be diversified by varying the crystal structure of the colloidal mask, etching the mask, altering the incidence angle of the vapor beam, and stepwise manipulation of the mask registry. This nanochemical patterning strategy paves a complementary way to conventional top-down lithography. This focus review provides an overview of the principle of colloidal lithography, and surveys the recent developments as well as outlining the future challenges.

Stability of Interfacial Nanobubbles

Interfacial nanobubbles (INBs) on a solid surface in contact with water have drawn widespread research interest. Although several theoretical models have been proposed to explain their apparent long lifetimes, the underlying mechanism still remains in dispute. In this work, the morphological evolution of INBs was examined in air-equilibrated and partially degassed water with the use of atomic force microscopy (AFM). Our results show that (1) INBs shrank in the partially degassed water while they grew slightly in the air-equilibrated water, (2) the three-phase boundary of the INBs was pinned during the morphological evolution of the INBs. Our analyses show that (1) the lifetime of INBs was sensitive to the saturation level of dissolved gases in the surrounding water, especially when the concentration of dissolved gases was close to saturation, and (2) the pinning of the three-phase boundary could significantly slow down the kinetics of both the growth and the shrinkage of the INBs. We developed a one-dimensional version of the Epstein-Plesset model of gas diffusion to account for the effect of pinning.

Cleaning of Oil Fouling with Water Enabled by Zwitterionic Polyelectrolyte Coatings: Overcoming the Imperative Challenge of Oil–Water Separation Membranes

Herein we report a self-cleaning coating derived from zwitterionic poly(2-methacryloyloxylethyl phosphorylcholine) (PMPC) brushes grafted on a solid substrate. The PMPC surface not only exhibits complete oil repellency in a water-wetted state (i.e., underwater superoleophobicity), but also allows effective cleaning of oil fouled on dry surfaces by water alone. The PMPC surface was compared with typical underwater superoleophobic surfaces realized with the aid of surface roughening by applying hydrophilic nanostructures and those realized by applying smooth hydrophilic polyelectrolyte multilayers. We show that underwater superoleophobicity of a surface is not sufficient to enable water to clean up oil fouling on a dry surface, because the latter circumstance demands the surface to be able to strongly bond water not only in its pristine state but also in an oil-wetted state. The PMPC surface is unique with its described self-cleaning performance because the zwitterionic phosphorylcholine groups exhibit exceptional binding affinity to water even when they are already wetted by oil. Further, we show that applying this PMPC coating onto steel meshes produces oil-water separation membranes that are resilient to oil contamination with simply water rinsing. Consequently, we provide an effective solution to the oil contamination issue on the oil-water separation membranes, which is an imperative challenge in this field. Thanks to the self-cleaning effect of the PMPC surface, PMPC-coated steel meshes can not only separate oil from oil-water mixtures in a water-wetted state, but also can lift oil out from oil-water mixtures even in a dry state, which is a very promising technology for practical oil-spill remediation. In contrast, we show that oil contamination on conventional hydrophilic oil-water separation membranes would permanently induce the loss of oil-water separation function, and thus they have to be always used in a completely water-wetted state, which significantly restricts their application in practice.

Fabrication of Heterogeneous Binary Arrays of Nanoparticles via Colloidal Lithography

Heterogeneous binary arrays of metallic nanoparticles have been constructed by consecutively depositing gold and silver into monolayers of hexagonally close-packed latex spheres at the incidence angles of 15 and -15 degrees, followed by removal of the colloidal masks. The present approach is independent of the chemical nature of both colloidal masks and deposition materials. The pattern feature of the resulting binary nanoparticle arrays is dependent on the colloidal mask registry.

Novel lithium-loaded porous aromatic framework for efficient CO2 and H2 uptake

Novel porous aromatic frameworks, PAF-18-OH and its lithiated derivative PAF-18-OLi, have been successfully synthesized. In particular, PAF-18-OLi displays significant enhancement of H2 and CO2 adsorption capacity, especially for CO2 uptake (14.4 wt%). More valuably, the stable PAF-18-OLi material exhibits high CO2/N2 selectivity, as high as 129 in the case of CO2 capture from simulated post-combustion flue gas mixtures (85% N2 and 15% CO2). Furthermore, the PAF-18-OLi has shown improved H2 storage capacity after lithiation.

Probing the Surface Hydration of Nonfouling Zwitterionic and PEG Materials in Contact with Proteins

Zwitterionic polymers and poly(ethylene glycol) (PEG) have been reported as promising nonfouling materials, and strong surface hydration has been proposed as a significant contributor to the nonfouling mechanism. Better understanding of the similarity and difference between these two types of materials in terms of hydration and protein interaction will benefit the design of new and effective nonfouling materials. In this study, sum frequency generation (SFG) vibrational spectroscopy was applied for in situ and real-time assessment of the surface hydration of the sulfobetaine methacrylate (SBMA) and oligo(ethylene glycol) methacrylate (OEGMA) polymer brushes, denoted as pSBMA and pOEGMA, in contact with proteins. Whereas a majority of strongly hydrogen-bonded water was observed at both pSBMA and pOEGMA surfaces, upon contact with proteins, the surface hydration of pSBMA remained unaffected, but the water ordering at the pOEGMA surface was disturbed. The effects of free sulfobetaine, free PEG chains with two different molecular weights, and PEG coated gold nanoparticles on the surface hydration of proteins were investigated. The results indicated that free sulfobetaine could strengthen the protein hydration layer, but free PEG chains greatly disrupt the protein hydration layer and likely directly interact with the protein molecules. In contrast to free PEG, the PEG chains anchored on the nanoparticles behave similarly to the pOEGMA surface and could induce strong hydrogen bonding of the water molecules at the protein surfaces.