Langmuir-Blodgett films of laurylamine-modified hydrophobic gold nanoparticles organized at the air-water interface

Langmuir-Blodgett Deposition of Cellulose Nanocrystal Surfactants into Ordered Monolayers

The cellulose nanocrystals (CNCs) are shown to interact with amine-functionalized polyhedral oligomeric silsesquioxane (POSS-NH₂) strongly at the water/oil interface, forming the CNC-POSS assemblies, that is, CNC surfactants that decrease the interfacial tension of the water/chloroform greatly. When bringing the CNC aqueous solution and POSS chloroform solution into a Langmuir trough, they form a monolayer of the CNC surfactants. Upon applying a continuous compression, a distinct transition appears in the surface pressure-area curves, and during this transition, the packing of the CNC surfactants in the produced monolayers transits from network-like patterns to ordered alignment.

Crystalline Gibbs monolayers of DNA-capped nanoparticles at the air-liquid interface

Using grazing-incidence small-angle X-ray scattering in a special configuration (parallel SAXS, or parSAXS), we mapped the crystallization of DNA-capped nanoparticles across a sessile droplet, revealing the formation of crystalline Gibbs monolayers of DNA-capped nanoparticles at the air-liquid interface. We showed that the spatial crystallization can be regulated by adjusting both ionic strength and DNA sequence length and that a modified form of the Daoud-Cotton model could describe and predict the resulting changes in interparticle spacing. Gibbs monolayers at the air-liquid interface provide an ideal platform for the formation and study of equilibrium nanostructures and may afford exciting routes toward the design of programmable 2D plasmonic materials and metamaterials.

Enhanced ordering in gold nanoparticles self-assembly through excess free ligands

Self-assembly of nanometer-sized particles is an elegant and economical approach to achieve dense patterns over large areas beyond the resolution and throughput capabilities of electron-beam lithography. In this paper, we present results of self-assembly of oleylamine-capped gold nanoparticles with 8.0 ± 0.3 nm diameter into densely packed and well-ordered monolayers with center-to-center distance of ∼11 nm. Self-assembly was done in a Langmuir-Blodgett trough and picked up onto Si substrates. The nanoparticles undesirably assembled within micrometer-sized "droplets" that were organic in nature. However, within these droplets, we observed that the addition of the excess ligand, oleylamine, drastically enhanced the self-assembly of the nanoparticles into monolayers with near-perfect ordering. This approach has the potential use in templated self-assembly of nanoparticles for rearranging poorly ordered assembly into a commensurate prepatterned substrate.

Nanoassemblies of colloidal gold nanoparticles by oxygen-induced inorganic ligand replacement

This article reports a novel method of the fabrication of floating ultrathin nanoporous films and superlattice-like bottom sediment flakes of colloidal gold nanoparticles (Au NPs) by the oxygen-induced ligand replacement of inorganic species. The two nanoassemblies were realized in a weighing bottle simply by aging the Au colloid, which was synthesized and stabilized using more divalent tin Sn(II) than required for the reduction of HAuCl(4). In situ Raman spectroscopy was employed to trace the assembly process, and we found that the protective Sn(II) species (mostly SnCl(3)(-)) of the gold colloid could be gradually replaced by Cl(-) ions in the solution, while the strongly chemically adsorbed Sn(II) species on the Au NPs was oxidized by O(2) from the air contact. The destabilized colloidal Au NPs by the ligand replacement of SnCl(3)(-) with Cl(-) first assembled into an ultrathin nanoporous film at the air-water interface and then sedimentated to the bottom. Superlattice-like sediment flakes of Au NPs can be obtained at lower temperature (approximately 5 degrees C). Particularly, this method does not involve any organic substances, providing clean ultrathin nanoporous films and superlattice-like flakes of Au NPs. The ultrathin nanoporous films and superlattice-like flakes of Au NPs can serve as SERS substrates with strong and long activity.

Monolayer of amphiphilic functionalized gold nanoparticles at an air-water interface

Langmuir films at the air-water interface exhibit a variety of surface phases which arise primarily due to the molecular interaction governed by intermolecular separation. We have studied the thermodynamical aspects of Langmuir monolayers of amphiphilic functionalized gold nanoparticles (AGNs) at the air-water (A-W) interface. Interestingly, the AGN monolayer exhibits phases like gas, a low-ordered liquid (L1) , a high-ordered liquid (L2) , and a collapsed state. We find that the first-order phase transition between L1 and L2 vanishes above a critical temperature of 28.4 degrees C . Surprisingly, for a range of higher temperatures ( > or = 29.4 degrees C and < or = 36.3 degrees C ), the L1 phase undergoes a transition to a bilayer of the L2 phase before entering into the collapsed state.

Adsorption of carboxylate-modified gold nanoparticles on an octadecylamine monolayer at the air/liquid interface

Gold nanoparticles (Au NPs) were prepared and surface-modified by mercaptosuccinic acid (MSA) to render a surface with carboxylic acid groups (MSA-Au). Octadecylamine (ODA) was used as a template monolayer to adsorb the Au NPs dispersed in the subphase. The effect of MSA concentration on the incorporation of Au NPs on the ODA monolayer and the relevant behavior of the mixed monolayer were studied using the pressure-area (pi-A) isotherm and transmission electron microscopy (TEM) observations. The experimental results showed that the adsorbed density of Au NPs is low without the surface modification by MSA. When MSA was added into the Au NP-containing subphase, the incorporation amount of Au NPs increased with increasing MSA concentration up to approximately 1 x 10-5 M for the particle density of 1.3 x 1011 particles/mL. With a further increase in the MSA concentration, the adsorbed particle density decreases due to competitive adsorption between the free MSA molecules and the MSA-Au NPs. It is inferred that free MSA molecules adsorb more easily than the MSA-Au NPs on the ODA monolayer. Therefore, an excess amount of MSA present in the subphase is detrimental to the incorporation of gold particles. The study on the monolayer behavior also shows that the pi-A isotherm of the ODA monolayer shifts right when small amounts of Au NPs or free MSA molecules are incorporated. However, when larger amounts of particles are adsorbed at the air/liquid interface, a left shift of the pi-A isotherm appears, probably due to the adsorption of ODA molecules onto the particle surface and the transferring of the particles from beneath the ODA monolayer to the air/water interface. According to the present method, it is possible to prepare uniform particulate films of controlled densities by controlling the particle concentration in the subphase, the MSA concentration, and the surface pressure of a mixed monolayer.

Effects of organic ligands, electrostatic and magnetic interactions in formation of colloidal and interfacial inorganic nanostructures

This paper discusses effects of organic ligands, electrostatic and magnetic interactions involved in morphological control of chemically synthesized inorganic nanostructures including colloid and planar systems. The special attention was concentrated on noble metal (gold and palladium) nanoparticles and nanostructures formed at the gas-liquid interface. The analysis of experimental data showed that electrostatic and ligand-related interactions influence very strongly on the metal nanostructure morphology. The hydrophobicity of ligand, charge and binding affinity to inorganic phase are important factors influencing the morphology of inorganic nanostructures formed in a layer at the gas/liquid interface by the interfacial synthesis method. The important point of this method is the quasi two-dimensional character of reaction area and possibilities to realize ultimately thin and anisotropic dynamic monomolecular reaction system with two-dimensional diffusion and interactions of precursors, intermediates and ligands resulting in planar growth and organization of inorganic nanoparticles and nanostructures in the plain of Langmuir monolayer. The morphology of resulting inorganic nanostructures can be controlled efficiently by variations of growth conditions via changes in state and composition of interfacial planar reaction media with the same precursor, and by variations of composition of adjacent bulk phases. The extreme anisotropy and heterogeneity of two-dimensional interfacial reaction system allows creating conditions when growing inorganic particles floating on the aqueous phase surface interact selectively with hydrophobic water-insoluble ligands in interfacial monolayer or with hydrophilic bulk-phase ligands, or at the same time with ligands of different nature present in monolayer and in aqueous phase. The spatial anisotropy of interfacial reaction system and non-homogeneity of ligand binding to inorganic phase gives possibilities for growth of integrated anisotropic nanostructures with unique morphologies, in particularly those characterized by very high surface/volume ratio, high effective perimeter, and labyrinth-like structure. In a case of magnetic nanoparticles dispersed in colloids specific magnetic dipolar interactions can result in formation of chains, rings and more complex nanoparticulate structures or separated highly anisotropic nanoparticles. Theoretical considerations indicate to the importance of system dimensionality in relation to the energy balance which determines specific features of structure organization in planar charged metallic and magnetic nanostructures. For example, a requirement of Coulomb energy minimum, the possibility of free electron redistribution and strengthened attractive interactions between particles in metallic nanostructures can explain formation of very branchy systems with extremely high "effective perimeter". The obtained experimental and literature data show that system dimensionality, organic ligand nature along with electrostatic and magnetic interactions are most important factors of morphological control of chemically synthesized inorganic nanomaterials. The understanding and appropriate exploitation of these factors can be useful for further developments of efficient nanofabrication techniques based on colloidal and interfacial synthetic methods.

Langmuir-Blodgett monolayers of gold nanoparticles with amphiphilic shells from V-shaped binary polymer arms

Gold nanoparticles functionalized with amphiphilic polybutadiene-poly(ethylene glycol) (PB-PEG) V-shaped arms formed stable Langmuir monolayers at the air-water and the air-solid interfaces. At these interfaces, the binary arms vertically segregated into a dense polymer corona, which surrounded the gold nanoparticles, preventing their large-scale agglomeration and keeping individual nanoparticles well-separated from each other and forming flattened, pancake nanostructures. The presence of both PEG and PB chains in the close proximity to the gold core was confirmed by surface enhanced Raman spectroscopy, whereas the AFM phase contrast images revealed the presence of 2 nm gold cores surrounded by the polymer shell with the diameter of 11 nm. We suggest that the amphiphilic shell drives their spontaneous organization into discrete 2D pancake-like hybrid structures that measured up to 10 microm in diameter and had a high packing density of gold clusters.

Synthesis of aqueous Au core-Ag shell nanoparticles using tyrosine as a pH-dependent reducing agent and assembling phase-transferred silver nanoparticles at the air-water interface

We demonstrate that the amino acid tyrosine is an excellent reducing agent under alkaline conditions and may be used to reduce Ag+ ions to synthesize stable silver nanoparticles in water. The tyrosine-reduced silver nanoparticles may be separated out as a powder that is readily redispersible in water. The silver ion reduction at high pH occurs due to ionization of the phenolic group in tyrosine that is then capable of reducing Ag+ ions and is in turn converted to a semi-quinone structure. These silver nanoparticles can easily be transferred to chloroform containing the cationic surfactant octadecylamine by an electrostatic complexation process. The now hydrophobic silver nanoparticles may be spread on the surface of water and assembled into highly ordered, linear superstructures that could be transferred as multilayers onto suitable supports by the versatile Langmuir-Blodgett technique. Further, tyrosine molecules bound to the surface of Au nanoparticles through amine groups in the amino acid may be used to selectively reduce silver ions at high pH on the surface of the Au nanoparticles, thus leading to a simple strategy for realizing phase-pure Au core-Ag shell nanostructures.

Computer simulation of the microstructure of a nanoparticle monolayer formed under interfacial compression

The uniaxial compression of a monolayer of nanosized monodisperse spherical particles adsorbed at a planar interface is simulated using the Brownian dynamics technique. Initially, the particles spread at the interface form crystalline loosely interconnected clusters. As the interface is compressed, the gaps between the clusters are removed and a close-packed monolayer is formed. At this stage, the structure of the interface consists of two-dimensional crystalline grains separated by defect boundaries. Further interfacial compression induces desorption of nanoparticles at these boundaries and creates striplike patterns of a secondary adsorbed layer. The structural features observed show remarkable agreement with recent experimental studies of the compression of gold nanoparticles in a Langmuir trough.