Examination of Oil Structural Motifs and Temperature in Promoting Reversible Self-Assembly of Gold Nanoparticle Langmuir Films.

The interaction between gold nanoparticles (AuNPs) with other components and phases has important consequences on their use in materials and devices as well as their fate in the environment or at biological interfaces. Previously we determined that long oil chain lengths and lower temperatures optimized the mixing of n-alkanes with alkanethiol-capped AuNPs which improved nanoparticle self-assembly into superlattices at aqueous interfaces. In this study, a variety of liquid phase hydrocarbon oils with structural and functional variations were surveyed for their mixing efficacy and propensity to enable reversible self-assembly of nanoparticle domains. Transmission electron microscopy (TEM) images and pressure vs area isotherms across this series reveal isotherm features that distinguish between the mixing and inclusion of the oil at the interface and that which enables reversible self-assembly. Structural and functional characteristics of the oil for promoting reversible self-assembly are identified which surpass the importance of chain length previously described. Temperatures below the ligand order-disorder transition were found to improve the reversibility of AuNP domains and are understood by application of a reparametrized x-DLVO model.

Ligand Shell Composition-Dependent Effects on the Apparent Hydrophobicity and Film Behavior of Gold Nanoparticles at the Air-Water Interface.

Nanoparticles with well-defined interfacial energy and wetting properties are needed for a broad range of applications involving nanoparticle self-assembly including the formation of superlattices, stability of Pickering emulsions, and for the control of nanoparticle interactions with biological membranes. Theoretical, simulated, and recent experimental studies have found nanometer-scale chemical heterogeneity to have important effects on hydrophobic interactions. Here we report the study of 4 nm gold nanoparticles with compositionally well-defined mixed ligand shells of hydroxyl-(OH) and methyl-(CH3) terminated alkylthiols as Langmuir films. Compositions ranging from 0-25% hydroxyl were examined and reveal nonmonotonic changes in particle hydrophobicity at the air-water interface. Unlike nanoparticles capped exclusively with a methyl-terminated alkylthiol, extensive particle aggregation is found for ligand shells containing <2% hydroxyl-terminated chains. This aggregation was lessened upon increasing the quantity of OH-terminated chains. Nanoparticles capped with 25% OH yield films of well-separated nanoparticles exhibiting a fluid-phase regime in the surface pressure vs area isotherm. Compression-expansion hysteresis, monolayer collapse, and mean nanoparticle area measurements support the TEM-observed changes in film morphology. Such clear changes in the hydrophobicity of nanoparticles based on very small changes in the ligand shell composition are shown to impact the process of interfacial nanoparticle self-assembly and are an important demonstration of nanoscale wetting with consequences in both materials and biological applications of nanoparticles that require tunable hydrophobicity.

Effects on the self-assembly of n-alkane/gold nanoparticle mixtures spread at the air-water interface.

Nanoparticle films formed at the air-water interface readily form rigid films, where the nanoparticles irreversibly associate into floating "islands", often riddled with voids and defects, upon solvent evaporation. Improving the nanoparticle mobility in these films is key to achieving control over the nanoparticle packing parameters, which is attractive for a variety of applications. In this study, a variety of n-alkanes were mixed with tetradecanethiol-capped 2 nm gold nanoparticles and studied as Langmuir films at 18 and 32 °C. Pressure-area isotherms at 18 °C reveal a mixed liquid-expanded phase of nanoparticles and alkane at the air-water interface, but only for n-alkanes that are equal to or exceed the nanoparticle capping ligand in carbon chain length. Transmission electron microscopy images of the corresponding films suggest that the nanoparticles are mixed with a continuous hydrocarbon phase at 0 mN/m and that the hydrocarbon is squeezed out of the nanoparticle film during compression.

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.

Effect of alkanethiol chain length on gold nanoparticle monolayers at the air-water interface.

In this study, we report the effects of the alkyl chain length on alkanethiol-capped gold nanoparticle Langmuir films. Gold nanoparticles (2-3 nm) capped with C(n)H(2n+1)SH (n = 5-12, 14-16, 18) were prepared via a two-phase synthesis. The films were sampled by Langmuir-Schaefer horizontal transfer at various points in the pressure-area isotherm and monitored with transmission electron microscopy. Changes in surface pressure, temperature, and alkyl chain length did not lead to observable differences in the mesoscale film morphology. Pressure-area isotherms at 22 °C, however, revealed that the work of compression and the collapse pressure are directly dependent on alkyl chain lengths of 14 carbons or greater. Variable temperature isotherms suggest that the work of compression is strongly affected by the phase state (i.e., crystalline vs liquid-like) of the gold-thiolate self-assembled monolayer (SAM) capping the nanoparticles.

Multiparametric cytotoxicity assessment: the effect of gold nanoparticle ligand functionalization on SKOV3 ovarian carcinoma cell death.

Gold nanoparticles (AuNP) are promising anti-cancer agents because of their modifiable properties and high biocompatibility. This study used multiple parallel analyses to investigate the cytotoxic properties of 5 nm AuNP conjugated to four different ligands with distinct surface chemistry: polyethylene glycol (PEG), trimethylammonium bromide (TMAB), 4-dimethylaminopyridine (DMAP), and carboxyl (COOH). We used a range of biochemical and high-content microscopy methods to evaluate the metabolic function, oxidative stress, cell health, cell viability, and cell morphology in SKOV3 ovarian cancer cells. Each AuNP displayed a distinct cytotoxicity profile. All AuNP species assessed exhibited signs of dose-dependent cytotoxicity when morphology, clonogenic survival, lysosomal uptake, or cell number were measured as the marker of toxicity. All particles except for AuNP-COOH increased SKOV3 apoptosis. In contrast, AuNP-TMAB was the only particle that did not alter the metabolic function or induce significant signs of oxidative stress. These results demonstrate that AuNP surface chemistry impacts the magnitude and mechanism of SKOV3 cell death. Together, these findings reinforce the important role for multiparametric cytotoxicity characterization when considering the utility of novel particles and surface chemistries.

Tuning the magnetism of gold nanoparticles by changing the thiol coating.

Room-temperature ferromagnetic behaviour has been reported in nanoscale materials expected to be diamagnetic, including gold. However, it is yet unclear which factors (size, shape, surface coating) predominantly influence the magnitude of the magnetic response. In this work, we study the magnetic and electronic properties of similarly-sized gold nanoparticles (Au NPs) coated with four different n-alkanethiols, as well as hydroxyl- and carboxyl-functionalized alkanethiols using superconducting quantum interference device (SQUID) magnetometry and ultraviolet photoelectron spectroscopy (UPS). We find room-temperature behaviour (hysteresis in magnetization vs. field strength loops) in all samples, as well as large effective magnetic anisotropy. Importantly, we find the nanoparticles coated with polar chain end-groups (-OH and -COOH) show markedly higher magnetization; this increased magnetization correlates with a higher work function. This work establishes chemical handles to enhance magnetism in nanoscale gold particles.

Tunable Mechanical Properties of Nanoparticle Monolayer Membranes via Ligand Phase Control and Defect Distribution.

In this study, the effects of ligand phase, morphology, and temperature on the elastic modulus of free-standing alkanethiol-capped gold nanoparticle membranes are reported. Langmuir films of 2.5 nm gold nanoparticles capped with tetradecanethiol were prepared at temperatures above and below the phase transition temperature (T m) of the ligand shell and transferred to holey carbon grids (containing 1.2 µm holes) to form free-standing membranes. Force-indentation measurements are used to measure the elastic modulus of the membranes using an atomic force microscope in the temperature range 10-40 °C. These films are compared with membranes of dodecanethiol-capped gold nanoparticles, which do not undergo a ligand order-disorder transition in the temperature range investigated. The ligand phase effect is observed in the tetradecanethiol-capped gold nanoparticle films, where an abrupt change in the elastic modulus is seen near T m. The temperature (relative to T m) during the fabrication of the films is determined to play an important role in tuning the mechanical strength of these films in this temperature range by both changing the nature of the interparticle interactions and by affecting microscale film morphology.

Rapid isolation of extracellular vesicles from cell culture and biological fluids using a synthetic peptide with specific affinity for heat shock proteins.

Recent studies indicate that extracellular vesicles are an important source material for many clinical applications, including minimally-invasive disease diagnosis. However, challenges for rapid and simple extracellular vesicle collection have hindered their application. We have developed and validated a novel class of peptides (which we named venceremin, or Vn) that exhibit nucleotide-independent specific affinity for canonical heat shock proteins. The Vn peptides were validated to specifically and efficiently capture HSP-containing extracellular vesicles from cell culture growth media, plasma, and urine by electron microscopy, atomic force microscopy, sequencing of nucleic acid cargo, proteomic profiling, immunoblotting, and nanoparticle tracking analysis. All of these analyses confirmed the material captured by the Vn peptides was comparable to those purified by the standard ultracentrifugation method. We show that the Vn peptides are a useful tool for the rapid isolation of extracellular vesicles using standard laboratory equipment. Moreover, the Vn peptides are adaptable to diverse platforms and therefore represent an excellent solution to the challenge of extracellular vesicle isolation for research and clinical applications.