Tunable Approach to Induce the Formation of Flexible Nanofilms from Small (3 nm) Gold Nanoparticles at Oil-Water Interfaces

The self-assembly of gold nanoparticles (AuNPs) into thin films at the liquid-liquid interface has promising applications in industries such as catalysis, optics, and sensors. However, precise control over their formation is complex, influenced by several factors which scale differently with core size. Due to their small free energy of adsorption, there are few examples of AuNPs with core diameters <10 nm. The present research evaluates the adsorption of ∼3 nm AuNPs from either side of the oil-aqueous interface with variations in ligand shell composition, the oil phase composition, and the structure of alcohol additives to best drive thin-film formation. Film formation and quality are evaluated, and a recent thermodynamic model is used to gain insight into the primary forces promoting this adsorption. Results demonstrate that longer-chain alcohol additives (namely, n-butanol and n-hexanol) induced adsorption more efficiently than shorter-chain alcohols (ethanol). The volume of alcohol additive needed to induce adsorption was dependent upon the ligand composition, suggesting that the mechanism for induced interfacial adsorption is via interaction with the AuNP ligand shell. Comparison with the thermodynamic model indicates that the driving force for this induced adsorption is the alteration of the three-phase contact angle. Additionally, the use of various oils demonstrates that as oil-water interfacial tension increases, more AuNPs adsorb to the interface. This relationship is also supported by the model. Insight gained for favorable conditions of adsorption for AuNPs < 10 nm as well as the underlying thermodynamic mechanism is important in working toward the ability to fine-tune such films for industrial applications.

2D superlattices via interfacial self-assembly of polymer-grafted Au nanoparticles

Nanoparticle (NP) superlattices are periodic arrays of nanoscale building blocks. Because of the collective effect between functional NPs, NP superlattices can exhibit exciting new properties that are distinct from those of individual NPs or corresponding bulk materials. In particular, two-dimensional (2D) NP superlattices have attracted increasing attention due to their emerging applications in micro/opto-electronics, catalysis, sensing, and other fields. Among various preparation methods, evaporation-induced interfacial self-assembly has become the most popular method for preparing 2D NP superlattices because it is a simple, low-cost, and scalable process that can be widely applied to various NPs. Introducing soft ligands, such as polymers, can not only provide convenience in controlling the self-assembly process and tuning superlattice structures but also improve the properties of 2D NP superlattices. This feature article focuses on the methods of evaporation-induced self-assembly of polymer-grafted Au NPs into free-standing 2D NP superlattice films at air/liquid interfaces and 2D NP superlattice coatings on substrates, followed by studies on in situ tracking of the self-assembly evolution process through small-angle X-ray scattering. Their application in nano-floating gate memory devices is also included. Finally, the challenges and perspectives of this direction are discussed.

Electrostatic pair-interaction of nearby metal or metal-coated colloids at fluid interfaces

In this paper, we theoretically study the electrostatic interaction between a pair of identical colloids with constant surface potentials sitting in close vicinity next to each other at the fluid interface. By employing a simplified yet reasonable model system, the problem is solved within the framework of classical density functional theory and linearized as well as nonlinear Poisson-Boltzmann (PB) theory. Apart from providing a sound theoretical framework generally applicable to any such problem, our novel findings, all of which contradict common beliefs, include the following: first, quantitative and qualitative differences between the interactions obtained within the linear and the nonlinear PB theories; second, the importance of the electrostatic interaction between the omnipresent three-phase contact lines in interfacial systems; and, third, the occurrence of an attractive electrostatic interaction between a pair of identical metal colloids. The unusual attraction we report largely stems from an attractive line interaction, which although scales linearly with the size of the particle can compete with the surface interactions and can be strong enough to alter the nature of the total electrostatic interaction. Our results should find applications in metal or metal-coated particle-stabilized emulsions where densely packed particle arrays are not only frequently observed but also sometimes required.

Macroscopic two-dimensional monolayer films of gold nanoparticles: fabrication strategies, surface engineering and functional applications

In the last few decades, two-dimensional monolayer films of gold nanoparticles (2D MFGS) have attracted increasing attention in various fields, due to their superior attributes of macroscopic size and accessible fabrication, controllable electromagnetic enhancement, distinctive optical harvesting and electron transport capabilities. This review will focus on the recent progress of 2D monolayer films of gold nanoparticles in construction approaches, surface engineering strategies and functional applications in the optical and electric fields. The research challenges and prospective directions of 2D MFGS are also discussed. This review would promote a better understanding of 2D MFGS and establish a necessary bridge among the multidisciplinary research fields.

Metal Microcapsules Prepared via Electroless Plating at Liquid-Liquid Interface

We report the facile preparation of metal microcapsules via the formation of a Pickering emulsion, stabilized by catalytic palladium nanoparticles (PdNPs), and subsequent electroless plating at an oil-water interface induced by the adsorbed PdNPs. Metal microcapsules with smooth metal shells are formed by simply agitating the mixture of the plating solution and 1,2-dichloroethane containing poly(vinylpyrrolidone)-coated PdNPs, without external heating or electrical sources as energy inputs. We found that the metal microcapsules had thin copper metal shells with a relatively smooth surface and metallic luster. The metal shells were composed of more than 99 atom % copper in the form of Cu and Cu₂O. We believe that this simple metal microcapsule preparation method may be beneficial to produce novel functional microcapsules with metal shells.

Ionic Liquid-Responsive Phase Transfer of Gold Nanoparticles: Anionic Metathesis

In this work, a fresh approach has been proposed for the efficient transfer of gold nanoparticles (AuNPs) from an aqueous to organic phase by the metathesis reaction or anion exchange reaction. Here, we synthesized ionic liquid 1-butyl 3-hexadecyl imidazolium bromide [C₄C₁₆Im]Br-stabilized AuNPs which exhibit excellent stability in solution. Transfer of Au@[C₄C₁₆Im]Br from an aqueous to organic phase was investigated by the metathesis reaction with different hydrophobic ionic liquid-forming salts such as LiNTf₂, LiClO₄, and KPF₆. The anionic exchange process in ionic liquids at the AuNP surface to make hydrophilic to hydrophobic AuNPs is demonstrated. It was found that hydrophobic ionic liquids provide the most effective transfer of AuNPs from the aqueous to organic phase. Interestingly, we have noticed no change in color, size, and shape of AuNPs for more than a month, indicating more efficient transfer of AuNPs in organic solvents, which remained stable for over a month. The ionic liquids with anions NTf₂^-, ClO₄^-, and PF₆^- make the AuNP surface hydrophobic, indicating their good dispersibility in nonpolar solvents. Finally, these AuNPs exhibit excellent sensitivity toward the refractive index of organic solvents, which is correlated with the surface plasmon resonance (SPR) λ_SPR bands.

Spontaneous Formation of Cold-Welded Plasmonic Nanoassemblies with Refracted Shapes for Intense Raman Scattering

Nanostructures with concave shapes made from continuous segments of plasmonic metals are known to dramatically enhance Raman scattering. Their synthesis in solutions is hindered, however, by their thermodynamic instability due to large surface area and high curvature of refracted geometries with nanoscale dimensions. Herein, we show that nanostructures with concave geometries can spontaneously form via self-organization of gold nanoparticles (NPs) at the air-water interface. The weakly bound surface ligands on the particle surface make possible their spontaneous accumulation and self-assembly at the air-water interface, forming monoparticulate films. Upon heating to 80 °C, the NPs further assemble into concave nanostructures where NPs are cold-welded to each other. Furthermore, the nanoassemblies effectively adsorb molecular analytes during their migration from the bulk solution to the surface where they can be probed by laser spectroscopies. We demonstrate that these films with local concentration of analytes increased by orders of magnitude and favorable plasmonic shapes can be exploited for surface-enhanced Raman scattering for high-sensitivity analysis of aliphatic molecules.

New developments in liquid-liquid extraction, surface modification and agglomerate-free processing of inorganic particles

This review describes new methods for the particle extraction through liquid-liquid interface (PELLI). The discovery of new surface modification techniques, advanced extractors and new adsorption mechanisms enabled novel applications of PELLI in nanotechnology of metals, quantum dots, oxides and hydroxides. Colloidal and interface chemistry of PELLI is emerging as a new area of technological and scientific interest. The progress achieved in the understanding of particle behavior and interactions at the liquid-liquid interface, phase transfer and interface reactions allowed for the development of new extraction mechanisms. An important breakthrough was the development of surface modification techniques for extraction of functional oxides. Especially important is the possibility of particle transfer from the synthesis medium to the device processing medium, which facilitates agglomerate-free processing of functional nanoparticles. Multifunctional extractor molecules were discovered and used as capping and reducing agents for particle synthesis or dispersing and charging agents for colloidal processing. The progress achieved in the development of extractors and extraction mechanisms has driven the advances in the surface modification and functionalization of materials. New PELLI techniques were used for the development of advanced materials and devices for optical, photovoltaic, energy storage, electronic, biomedical, sensor and other applications.

Electrodeposition of Gold Nanostructures at the Interface of a Pickering Emulsion

The controlled electrodeposition of nanoparticles at the surface of an emulsion droplet offers enticing possibilities in regards to the formation of intricate structures or fine control over the locus or duration of nanoparticle growth. In this work we develop electrochemical control over the spontaneous reduction of aqueous phase Au(III) by heterogeneous electron transfer from decamethylferrocene present in an emulsion droplet - resulting in the growth of nanoparticles. As gold is a highly effective conduit for the passage of electrical current, even on the nanoscale, the deposition significantly enhances the current response for the single electron transfer of decamethylferrocene when acting as a redox indicator. The nanostructures formed at the surface of the emulsion droplets were imaged by cryo-TEM, providing an insight into the types of structures that may form when stabilised by the interface alone, and how the structures are able to conduct electrons.

Gold Raspberry-Like Colloidosomes Prepared at the Water-Nitromethane Interface

In this study, we propose a simple shake-flask method to produce micron-size colloidosomes from a liquid-liquid interface functionalized with a gold nanoparticle (AuNP) film. A step-by-step extraction process of an organic phase partially miscible with water led to the formation of raspberry-like structures covered and protected by a gold nanofilm. The distinctive feature of the prepared colloidosomes is a very thin shell consisting of small AuNPs of 12 or 38 nm in diameter instead of several hundred nanometers reported previously. The interesting and remarkable property of the proposed approach is their reversibility: the colloidosomes may be easily transformed back to a nanofilm state simply by adding pure organic solvent. The obtained colloidosomes have a broadband absorbance spectrum, which makes them of great interest in applications such as photothermal therapy, surface-enhanced Raman spectroscopy studies, and microreactor vesicles for interfacial electrocatalysis.