The structure and stability of hybrid lipid vesicles containing bilayer-encapsulated hydrophobic nanoparticles is dependent upon lipid phase behavior. By embedding stearylamine-stabilized gold nanoparticles in dipalmitoylphosphatidylcholine/dipalmitoylphosphatidylglycerol vesicles, we show that encapsulation at lipid to nanoparticle ratios from 10,000:1 to 5000:1 leads to bilayer thickening and hydrophobic mismatch, favoring nanoparticle inclusion in gel phase vesicles. High loadings lead to large increases in the gel to fluid melting temperature upon heating and significant hysteresis on cooling, which cannot be attributed solely to excess free ligand. This behavior is due to a cooperative effect of excess free SA ligand and nanoparticle embedment. Nanoparticle clustering was observed during lipid melting and could be reversed upon lipid freezing owing to lateral capillary forces within the bilayer. The impact of nanoparticle embedment on vesicle structure and properties at such low concentrations is reminiscent of hydrophobic proteins, suggesting that the underlying lipid biophysics between proteins and nanoparticle are similar and may provide a predictive design tool for therapeutic applications.

This paper details a facile approach for the synthesis of stable and monodisperse silver nanoparticles performed at ambient/low temperature where Allium sativum (garlic) extract functions as the silver salt reducing agent during nanoparticle synthesis as well as the post-synthesis stabilizing ligands. Varying the synthesis conditions provides control of particle size, size-distribution, and kinetics of particle formation. Infrared spectroscopy, energy dispersive x-ray chemical analysis, and high performance liquid chromatography indicated that the carbohydrates present in the garlic extract are the most likely nanoparticle stabilizing chemistry. The synthesized silver nanoparticles also demonstrate potential for biomeical applications, owing to the 1) enhanced stability in biological media, 2) resistance to oxidation by the addition of H2O2, 3) ease and scalability of synthesis, and 4) lack of harsh chemicals required for synthesis. Cytotoxicity assays indicated no decrease in cellular proliferation for vascular smooth muscle cells and 3T3 fibroblasts at a concentration of 25 μg/ml, confirming that garlic extract prepared silver nanoparticles are ideal candidates for future experimentation and implementation into biomedical applications.

Wet chemical nanoparticle synthesis is commonly employed because of the ability to tailor and control nanoparticle size, shape, polydispersity, and surface chemistry; however, excess ligands or free surfactants in the colloidal dispersion can be detrimental for many nanoparticle applications. Postsynthesis purification using antisolvent precipitation is a widely employed method to remove the excess ligands or precursors; however, there has been little in-depth fundamental investigation of the dynamics between the nanoparticle, ligand, and dispersing media as well as the morphology and fate of the nanoparticle and ligands during nanoparticle processing. In this paper, we investigate the changes in ligand surface coverage for dodecanethiol-stabilized gold and silver nanoparticles in response to repetitive antisolvent precipitation and redispersion using gas chromatography, thermogravimetric analysis, and small-angle neutron scattering. These techniques were each used to determine percent surface coverage and equilibrium ligand partitioning between the nanoparticle surface and bulk solution, which was then modeled with the Langmuir isotherm to determine the binding free energy. The binding free energy for dodecanethiol on 4.2 nm diameter gold nanoparticles was found to be −23 kJ/mol by gas chromatography (GC) and −34 kJ/mol by small-angle neutron scattering (SANS). The binding free energy for dodecanethiol on 7.7 nm diameter silver nanoparticles was found to be −21 kJ/mol by TGA and −29 kJ/mol by SANS. While these numbers demonstrate variability based on the method, they are comparable to literature values. Other notable results from this work demonstrate the optimization of the purification process and avoidance of using excess antisolvent which can lead to coprecipitation of the excess ligand with the nanoparticles, hindering the purification. Finally, multiple techniques for determining ligand binding free energy are demonstrated as well as evaluation of the pros and cons of each method.

Stabilizing ligands play a major role in the synthesis and stabilization of metallic nanoparticles, allowing dispersibility in various solvents including gas-expanded liquids (GXLs). Interaction energy modeling has been used to predict the dispersibility of hydrophobically stabilized metal nanoparticles in GXLs but often overpredicts the mean particle size dispersed at a given solvent composition. More accurate and robust interaction energy models can be developed if the changes to both the ligand length and ligand solvation as a response to the composition of GXLs are better understood. Small-angle neutron scattering (SANS) is a unique technique for nanoparticle characterization where in-situ ligand solvation measurements can be obtained by contrasting hydrogenated nanoparticle ligands with deuterated solvent. This study presents the first indepth SANS measurements of ligand length and ligand solvation variation during nanoparticle antisolvent precipitation. The focus of this investigation is dodecanethiol-stabilized silver nanoparticles in carbon dioxide (CO 2 )-expanded hexane. Upon pressurization with CO 2 antisolvent, the ligand length and ligand solvation for dodecanethiol-capped silver nanoparticles decrease as a function CO 2 composition in the GXL prior to nanoparticle precipitation. This work discusses the dependence of nanoparticle dispersibility as a function of CO 2 composition in n-hexane-d 14 GXL and the competing roles of ligand surface coverage, ligand length, and ligand solvation for nanoparticles with varying surface curvature.

Solution-based nanoparticle synthesis offers many benefits, primarily control over nanoparticle size, shape, and surface chemistry, as appropriate for different applications. Often, excess stabilizing ligand or surfactant is required during synthesis and remains in solution. This can be deleterious to end applications, and thus postsynthesis purification must be employed. Ethanol is a common liquid antisolvent used to purify and size-selectively fractionate hydrophobically modified metallic nanoparticles dispersed in organic solvents, particularly toluene or hexane. We have employed small-angle neutron scattering (SANS) to investigate the nanoparticle ligand response to antisolvent conditions for gold nanoparticles (GNPs) in toluene-d 8 and n-hexane-d 14 at varying ethanol-d 6 antisolvent compositions. These conditions are common in postsynthesis nanoparticle purification, fractionation, and surface deposition. The ligand lengths and ligand solvation for octadecanethiol and dodecanethiol modified GNPs were found to decrease from 16 to 8 Å and 13 to 7 Å, respectively, with increasing ethanol-d 6 concentrations, directly impacting their dispersibility in solution. Calculated FloryÀHuggins interaction parameters were found to support the trends determined by SANS. This research has led to a greater understanding of ligand structure and solvation during the nanoparticle precipitation process, providing critical results to model the ligand repulsion contributions to the interparticle interaction energy, which governs the nanoparticle behavior in solution. In addition, nanoparticle clustering was observed at dilute concentrations, and a fractal cluster model was used to interpret the SANS data.