Interface-directed self-assembly of gold nanoparticles and fabrication of hybrid hollow capsules by interfacial cross-linking polymerization

In Situ Engineering of Conducting Polymer Nanocomposites at Liquid/Liquid Interfaces: A Perspective on Fundamentals to Technological Significance

The conducting polymers have continuously been hybridized with their counterparts to overcome the intrinsic functional limitations compared to the metallic or inorganic analogs. Remarkably, the liquid/liquid interface-assisted methods represent an efficient and facile route for developing fully tunable metamaterials for various applications. The spontaneous adsorption of nanostructures at a quasi-two-dimensional interface is energetically favorable due to the reduction in interfacial tension, interfacial area, and interfacial energy (Helmholtz free energy). This Perspective highlights the fundamentals of nanostructure adsorption leading to hierarchical architecture generation at the interface from an experimentalist's point of view. Thereafter, the essential applications of the conducting polymer/nanocomposites synthesized at the interface emphasize the capability of the interface to tune functional materials. This Perspective also summarizes the future challenges and the use of the known fundamental aspects in overcoming the functional limitations of polymer/nanomaterial composites and also provides some future research directions.

Fabrication of PεCL-AuNP-BSA core-shell-corona nanoparticles for flexible spatiotemporal drug delivery and SERS detection

Nanoparticles with protein coronae can be used as promising multifunctional platforms for nanomedicine due to the possibility of performing surface functionalization on protein molecules and the achievement of biomedical properties. In this research, nanoparticles (NPs) with poly(ε-caprolactone) (PεCL) cores, gold NP (AuNP) shells and BSA coronae were fabricated by a self-assembly approach. The hydrophobic PεCL cores were used to encapsulate curcumin (CUR), the AuNP shells were decorated with a Raman probe, and the protein molecules in the coronae were functionalized with folic acid (FA). The self-assembly behaviors, drug delivery and the surface-enhanced Raman scattering (SERS) effect of the hybrid NPs were investigated in this research. The sizes of the core-shell-corona NPs (CSCNPs) are dependent on the initial concentrations of PεCL and AuNPs. The CUR in CSCNPs show enzyme-triggered release properties. The added lipase or trypsin can facilitate the CUR release from the hybrid NPs. The functionalization of CSCNPs with FA can significantly improve the internalization of NPs into 4T1 tumor cells due to the overexpressed folate receptors on the cells. In addition, the SERS effect of CSCNPs can be achieved when the AuNPs are decorated with 2-naphthalenethiol. The hybrid CSCNPs can be used as a promising platform for spatiotemporal drug delivery, cell imaging, and theranostics. Based on the same CSCNP platform, flexible functions can be adjusted according to the application needs.

Colloidal Capsules Assembled from Gold Nanoparticles Using Small-Molecule Hydrophobic Cross-linkers

Colloidal capsules (or colloidosomes) have been studied for various applications such as therapeutic agent encapsulation, photothermal therapy, imaging, and energy storage. Emulsion-based synthesis is the most common approach for preparing colloidal capsules as it is relatively straightforward and scalable. However, while the initial formation requires only introducing the colloidal subunits into an emulsion and letting them assemble at the interface, a second step is required in order to prepare stable, covalently linked colloidal capsules, and preparing submicron colloidal capsules is quite challenging. Here, we describe a simple and quick one-step method to synthesize covalently linked, stable nanoscale colloidal capsules consisting of gold nanoparticles (NPs) (AuNP) and thiol-containing cross-linkers. Gold nanoparticle capsules (AuNCs) were formed by coating emulsion droplets containing thiol-containing cross-linkers with citrate-stabilized AuNPs. The physicochemical properties of the colloidal capsules can be tailored by changing the building blocks. In order to demonstrate this, colloidal capsules were assembled from AuNPs ranging from 5 to 20 nm in size. The use of the larger 20 nm starting particles resulted in AuNCs with a sufficiently pronounced red shift for λ_max to be suitable for biological photothermal applications, where use of a near-infrared laser is strongly preferred. The AuNCs were found to be biocompatible and stable in cell culture conditions and to provide moderate heating. This demonstrates the modularity of the synthesis and the potential advantages of a one-step synthesis to prepare nanoscale gold colloidal capsules.

Nanoparticle Assembly at Liquid-Liquid Interfaces: From the Nanoscale to Mesoscale

In the past few decades, novel syntheses of a wide range of nanoparticles (NPs) with well-defined chemical composition and structure have opened tremendous opportunities in areas ranging from optical and electronic devices to biomedical markers. Controlling the assembly of such well-defined NPs is important to effectively harness their unique properties. The assembly of NPs at liquid-liquid interfaces is becoming a central topic both in surface and colloid science. Hierarchical structures, including 2D films, 3D capsules, and structured liquids, have been generating significant interest and are showing promise for physical, chemical, and biological applications. Here, a brief overview of the development of the self-assembly of NPs at liquid-liquid interfaces is provided, from theory to experiment, from synthetic NPs to bio-nanoparticles, from water-oil to water-water, and from "liquid-like" to "solid-like" assemblies.

Submicron Colloidosomes of Tunable Size and Wall Thickness

We present a simple method for the fabrication of colloidal capsules of different sizes ranging from below 100 nm up to one micron. The capsules are produced by self-assembling 5 nm gold nanoparticles at the interface of oil-in-water emulsion droplets. The size of the capsules is regulated by tuning the wetting properties of the nanoparticles by changing either the composition of their ligand shell or the composition of the oil phase. The modified wettability affects not only the size but also the thickness of capsule walls. The wall can be thin if it is made of a single layer of the nanoparticles or thick when composed of multilayers. The durability of such capsules is quite high, although it can be improved by chemical cross-linking with UV light. Such capsules have low permeability, so they can store a molecular cargo and then release it on demand.

Colloidal capsules: nano- and microcapsules with colloidal particle shells

This review provides a comprehensive overview of the synthesis strategies and the progress made so far of bringing colloidal capsules closer to technical and biomedical applications.

One-Step Generation of Cell-Encapsulating Compartments via Polyelectrolyte Complexation in an Aqueous Two Phase System

Diverse fields including drug and gene delivery and live cell encapsulation require biologically compatible encapsulation systems. One widely adopted means of forming capsules exploits cargo-filled microdroplets in an external, immiscible liquid phase that are encapsulated by a membrane that forms by trapping of molecules or particles at the drop surface, facilitated by the interfacial tension. To eliminate the potentially deleterious oil phase often present in such processes, we exploit the aqueous two phase system of poly(ethylene glycol) (PEG) and dextran. We form capsules by placing dextran-rich microdroplets in an external PEG-rich phase. Strong polyelectrolytes present in either phase form complexes at the drop interface, thereby forming a membrane encapsulating the fluid interior. This process requires considerable finesse as both polyelectrolytes are soluble in either the drop or external phase, and the extremely low interfacial tension is too weak to provide a strong adsorption site for these molecules. The key to obtaining microcapsules is to tune the relative fluxes of the two polyelectrolytes so that they meet and complex at the interface. We identify conditions for which complexation can occur inside or outside of the drop phase, resulting in microparticles or poor encapsulation, respectively, or when properly balanced, at the interface, resulting in microcapsules. The resulting microcapsules respond to the stimuli of added salts or changes in osmotic pressure, allowing perturbation of capsule permeability or triggered release of capsule contents. We demonstrate that living cells can be sequestered and interrogated by encapsulating Pseudomonas aeruginosa PAO1 and using a Live/Dead assay to assess their viability. This method paves the way to the formation of a broad variety of versatile functional membranes around all aqueous capsules; by tuning the fluxes of complexing species to interact at the interface, membranes comprising other complexing functional moieties can be formed.

Surfactant Behavior of Amphiphilic Polymer-Tethered Nanoparticles

In recent years, an emerging research area has been the surfactant behavior of polymer-tethered nanoparticles. In this feature article, we have provided a general introduction to the synthesis, self-assembly, and interfacial activity of polymer-tethered inorganic nanoparticles, polymer-tethered organic nanoparticles, and polymer-tethered natural nanoparticles. In addition, applications of the polymer-tethered nanoparticles in colloidal and materials science are briefly reviewed. All research demonstrates that amphiphilic polymer-tethered nanoparticles exhibit surfactant behavior and can be used as elemental building blocks for the fabrication of advanced structures by the self-assembly approach. The polymer-tethered nanoparticles provide new opportunities to engineer materials and biomaterials possessing specific functionality and physical properties.

Secretion and Reversible Assembly of Extracellular-like Matrix by Enzyme-Active Colloidosome-Based Protocells

The secretion and reversible assembly of an extracellular-like matrix by enzyme-active inorganic protocells (colloidosomes) is described. Addition of N-fluorenyl-methoxycarbonyl-tyrosine-(O)-phosphate to an aqueous suspension of alkaline phosphatase-containing colloidosomes results in molecular uptake and dephosphorylation to produce a time-dependent sequence of supramolecular hydrogel motifs (outer membrane wall, cytoskeletal-like interior and extra-protocellular matrix) that are integrated and remodelled within the microcapsule architecture and surrounding environment. Heat-induced disassembly of the extra-protocellular matrix followed by cooling produces colloidosomes with a densely packed hydrogel interior. These procedures are exploited for the fabrication of nested colloidosomes with spatially delineated regions of hydrogelation.

Polystyrene-Core-Silica-Shell Hybrid Particles Containing Gold and Magnetic Nanoparticles

Polystyrene-core-silica-shell hybrid particles were synthesized by combining the self-assembly of nanoparticles and the polymer with a silica coating strategy. The core-shell hybrid particles are composed of gold-nanoparticle-decorated polystyrene (PS-AuNP) colloids as the core and silica particles as the shell. PS-AuNP colloids were generated by the self-assembly of the PS-grafted AuNPs. The silica coating improved the thermal stability and dispersibility of the AuNPs. By removing the "free" PS of the core, hollow particles with a hydrophobic cage having a AuNP corona and an inert silica shell were obtained. Also, Fe3O4 nanoparticles were encapsulated in the core, which resulted in magnetic core-shell hybrid particles by the same strategy. These particles have potential applications in biomolecular separation and high-temperature catalysis and as nanoreactors.

Synthesis of amphiphilic block-type macromolecular brushes with cleavable pendant chains and fabrication of micelle-templated polymer nanocapsules

Macromolecular brushes with cleavable pendant chains were synthesized by controlled free radical polymerizations and functional nanocapsules were fabricated on the basis of the brush polymers.

Polyphenol–gelatin nanoparticles as reductant and stabilizer for one-step synthesis of gold nanoparticles and their interfacial behavior

Polyphenol and gelatin self-assembled nanoparticles acted as a reductant for the synthesis of AuNPs and then stabilized oil–water interfaces.

Palladium encapsulated within magnetic hollow mesoporous TiO2 spheres (Pd/Fe3O4@hTiO2) as a highly efficient and recyclable catalysts for hydrodechlorination of chlorophenols

Herein, we introduced a method to encapsulate Pd(0) within TiO₂ hollow mesoporous nanostructures.

Membrane engineering of colloidosome microcompartments using partially hydrophobic mesoporous silica nanoparticles

Covalent coupling of hexadecyltrimethoxysilane to the surface of as-synthesized silica-surfactant mesostructured nanoparticles followed by chemical removal of the surfactant template is used to prepare partially hydrophobic mesoporous silica nanoparticles with uniform shape and size comprising both hydroxyl and hexadecyl surface moieties. The mesoporous nanoparticles are used as surface-active building blocks for the preparation of water-in-oil Pickering emulsion droplets consisting of shell-like inorganic microarchitectures (colloidosomes) that exhibit a high adsorption capacity for water/oil-soluble dyes and support interfacial catalytic activity. The colloidosomes can be transferred to water by cross-linking to produce microcapsules delineated by a continuous inorganic nanoparticle-based membrane showing type IV mesoporosity and a high adsorption capacity for gaseous N2. The design and construction of colloidosomes based on mesoporous nanoparticle building blocks should offer a general route to inorganic microcompartments with tunable porosities, host-guest membrane chemistry, and surface reactivity.

Fabrication of biconcave discoidal silica capsules and their uptake behavior by smooth muscle cells

Biconcave discoidal silica microcapsules were fabricated by reaction of tetraethoxysilane (TEOS) on biconcave discoidal Ca(OH)2 templates, followed by core removal. The biconcave discoidal morphology of microcapsules was characterized by confocal laser scanning microscopy (CLSM) and transmission electron microscopy (TEM). The thickness of silica capsule shell can be tuned by two methods, "Gradient concentration" method and "Multi-step growth" method. Through the latter one, the shell thickness can be controlled more effectively. Compared with spherical microcapsules, the biconcave discoidal ones were internalized into smooth muscle cells (SMCs) with a slower rate.

A simple way to prepare Au@polypyrrole/Fe3O4 hollow capsules with high stability and their application in catalytic reduction of methylene blue dye

Metal nanoparticles are promising catalysts for dye degradation in treating wastewater despite the challenges of recycling and stability. In this study, we have introduced a simple way to prepare Au@polypyrrole (PPy)/Fe3O4 catalysts with Au nanoparticles embedded in a PPy/Fe3O4 capsule shell. The PPy/Fe3O4 capsule shell used as a support was constructed in one-step, which not only dramatically simplified the preparation process, but also easily controlled the magnetic properties of the catalysts through adjusting the dosage of FeCl2·4H2O. The component Au nanoparticles could catalyze the reduction of methylene blue dye with NaBH4 as a reducing agent and the reaction rate constant was calculated through the pseudo-first-order reaction equation. The Fe3O4 nanoparticles permitted quick recycling of the catalysts with a magnet due to their room-temperature superparamagnetic properties; therefore, the catalysts exhibited good reusability. In addition to catalytic activity and reusability, stability is also an important property for catalysts. Because both Au and Fe3O4 nanoparticles were wrapped in the PPy shell, compared with precursor polystyrene/Au composites and bare Fe3O4 nanoparticles, the stability of Au@PPy/Fe3O4 hollow capsules was greatly enhanced. Since the current method is simple and flexible to create recyclable catalysts with high stability, it would promote the practicability of metal nanoparticle catalysts in industrial polluted water treatment.

One-step interfacial thiol-ene photopolymerization for metal nanoparticle-decorated microcapsules (MNP@MCs)

We herein reported a one-step strategy to prepare the noble metal nanoparticle-decorated microcapsules (MNP@MCs) through the interfacial thiol-ene photopolymerization. In the presence of amphiphlic polyhedral oligomeric silsesquioxane (POSS) containing thiol groups (PTPS) as a reactive surfactant and trimethylolpropane triacrylate (TMPTA) as a cross-linker, the oil phase of toluene dissolved with a photoinitiator was emulsified into a water phase containing a metal precursor to form an oil-in-water (O/W) emulsion. Upon irradiation of ultraviolet (UV) light, the thiol-ene photoploymerization and photoreduction at the interface of toluene/water lead to the formation of the cross-linked wall and metal nanoparticles, respectively. A series of gold, silver, and platinum nanoparticle-decorated microcapsules (AuNP@MC, AgNP@MC, and PtNP@MC) were prepared through this one-step interfacial thiol-ene photopolymerization and were characterized carefully by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM). The results revealed that the obtained MNP@MCs were 2.2-2.7 μm in diameter with a wall of 40-70 nm in thickness, which was covered with the metal nanoparticles. The size and amount of metal nanoparticles increased with the increasing concentration of the metal precursor in water. Furthermore, the catalyst performance of AuNP@MC was studied by reduction of aromatic nitro compounds and exhibited the enhanced catalytic activity and good stability in the reduction of hydrophobic nitrophenol. It is believed that this robust, convenient, simple strategy based on the one-step interfacial thiol-ene photopolymerization will provide an important alternative to fabricate the functional metal nanoparticle-modified microcapsules.

Fe₃O₄@rGO doped molecularly imprinted polymer membrane based on magnetic field directed self-assembly for the determination of amaranth

Based on magnetic field directed self-assembly (MDSA) of Fe3O4@rGO composites, a novel magnetic molecularly imprinted electrochemical sensor (MIES) was fabricated and developed for the determination of the azo dye amaranth. Fe3O4@rGO composites were obtained by a one-step approach involving the initial intercalating of iron ions between the graphene oxide layers via the electrostatic interaction, followed by the reduction with hydrazine hydrate to deposit Fe3O4 nanoparticles onto the reduced oxide graphene nanosheets. In molecular imprinting, the complex including the function monomer of aniline, the template of amaranth and Fe3O4@rGO was pre-assembled through π-π stacking and hydrogen bonding interactions, and then was self-assembled on the surface of magnetic glassy carbon electrode (MGCE) with the help of magnetic field induction before electropolymerization. The structures and morphologies of Fe3O4@rGO and the doped molecularly imprinted polymers (MIPs) were investigated by Fourier transform infrared spectrometer (FT-IR), Raman spectra and scanning electron microscope (SEM). Besides, the characterization by differential pulse voltammetry (DPV) showed that Fe3O4@rGO composites promoted the electrical conductivity of the imprinted sensors when doped into the MIPs. The adsorption isotherms and adsorption kinetics were employed to evaluate the performances of MIES. The detection of amaranth was achieved via the redox probe K3[Fe(CN)6] by blocking the imprinted cavities, which avoided the interferences of oxidation products and analogs of amaranth. Furthermore, the prepared MIES exhibited good sensitivity, selectivity, reproducibility and efficiency for detecting amaranth in fruit drinks. The average recoveries were 93.15-100.81% with the RSD <3.0%.