Hydrophobic coating- and surface active solvent-mediated self-assembly of charged gold and silver nanoparticles at water–air and water–oil interfaces

Abstract

We report self-assembly of charge-stabilized gold and silver nanoparticles at water-air and water-oil interfaces, via manipulation of the interactions between the interfaces and the adsorbing nanoparticles. Nanoparticle adsorption from bulk colloids to an interface is an energy-favored, but finite sorption barrier-restrained (kinetics-controlled) process. Consequently, to successfully mediate self-assembly of nanoparticles, the finite sorption barrier should be decreased. That can be accomplished by manipulating its three controlling forces: the repulsive electrostatic force, the repulsive van der Waals force, and the attractive hydrophobic force between the interface and the adsorbing nanoparticles. It was found that hydrophobic coatings change nanoparticle hydrophobicity and greatly increase the attractive hydrophobic force. Surface active organic solvents (methanol, ethanol, isopropanol, and acetone) decrease the attractive hydrophobic force to some extent. However, they decrease the repulsive electrostatic force to a larger extent, via a "charge dilution" mechanism, due to their positive adsorption at the charged water-air and water-oil interfaces. Hydrophobic coatings and organic solvents consequently decrease the sorption barrier, facilitate nanoparticles overcoming the sorption barrier, and mediate the self-assembly of nanoparticles.

Light welding nanoparticles: from metal colloids to free-standing conductive metallic nanoparticle film

“…2 shows the SEM images for the Ag NP thin film welded by light irradiation for different duration. As observed previously, the NP thin film assembled at the water-air interface shows a typical monolayer structure dotted with small voids [9,28,30]. Before light welding, most NPs only physically contact with each other, with abundant inter-particle junctions in the range of 1-5 nm.…”

“…After their synthesis, cyclohexane (5 mL) was gently covered on the surface of the metal colloids (20 mL) to produce a sharp water-oil interface. Upon quick addition of ethanol (1.5 mL), the metallic NPs in the colloids were induced to the wateroil interface, where they were trapped and spontaneously self-assembled into an NP thin film with metallic luster [9,28]. After careful removal of the oil using a syringe, the metallic NP thin film was left at the water-oil interface.…”

“…Once arriving at the interface, the NPs are irreversibly trapped therein, unable being detached by thermal activation from the Brownian motion. This is because the detaching energy caused by the interfacial tension is usually higher than the thermal energy by a few orders of magnitude [28,29]. This irreversible trapping constitutes the basis for the self-assembly of robust NP thin films at the fluid interface.…”

Synthesis of Nano/Microstructures at Fluid Interfaces

“…Jaeger and co-workers demonstrated that highly ordered two-dimensional gold nanoparticle films could be fabricated by a simple direct-drying method, either at the toluene-water interface or toluene-air interface (Figure 7 e, f). [66,67] Other ligand-capped gold nanoparticles, [26,68] FePt nanoparticles, [69] silver nanoparticles, [70] CoPt 3 nanoparticles, [71] and g-Fe 2 O 3 nanoparticles [26] can also self-assemble at the oil-water interface and form thin films.…”

Modulation of surface bio-functionality by using gold nanostructures on protein repellent surfaces

“…13 According to the extended DLVO model (Derjaguin and Landau, Verwey and Overbeek), [14][15][16] the attractive long-range forces might be electrostatic forces (if the nanoparticles and the surface have opposite charges) or acid-base interaction forces (due to the interaction of polar groups in water). As the zeta-potential of the Au NPs and the surface were both negative (see ESI for details †), the attractive forces could be attributed mainly to the acid-base and Lifshitz van der Waals interactions.…”