Amphiphilic AuNPs Research Articles

Effect of ratio of hydrophilic and hydrophobic ligand length on the adsorption behaviors of amphiphilic gold nanoparticles at the liquid-liquid interface

Amphiphilic gold nanoparticles (AuNPs) covered with hydrophobic and hydrophilic ligands assemble at the interface between hexane and water into monolayer structures, forming microdroplets that are covered with amphiphilic AuNPs. Here, we focused on the effect of the ratio of hydrophilic to hydrophobic ligand lengths on the assembly of amphiphilic AuNPs at the interface surface. By controlling the ratio, the extent of amphiphilic AuNPs migrating to the interface was examined. The amphiphilic AuNPs tended to migrate to the interface with an increase in the length of hydrophobic ligands. More importantly, diverse adsorption isotherm models were applied to the systems to examine the adsorption behavior of amphiphilic AuNPs at the interface. Adsorption parameters such as equilibrium constant and maximum uptake depending on the ratio of ligand length were obtained from data fitting, and they had high values with an increase in the hydrophobic ligand length. These results suggest that the adsorption behavior of amphiphilic AuNPs at the interface can be controlled by adjusting the ligand length ratio. Furthermore, the closest distance between amphiphilic AuNPs at the interface by using the obtained maximum uptake values was estimated, and it also was confirmed by the small-angle X-ray scattering (SAXS) technique.

Amphiphilic Gold Nanoparticles: A Biomimetic Tool to Gain Mechanistic Insights into Peptide-Lipid Interactions.

Functional peptides are now widely used in a myriad of biomedical and clinical contexts, from cancer therapy and tumor targeting to the treatment of bacterial and viral infections. Underlying this diverse range of applications are the non-specific interactions that can occur between peptides and cell membranes, which, in many contexts, result in spontaneous internalization of the peptide within cells by avoiding energy-driven endocytosis. For this to occur, the amphipathicity and surface structural flexibility of the peptides play a crucial role and can be regulated by the presence of specific molecular residues that give rise to precise molecular events. Nevertheless, most of the mechanistic details regulating the encounter between peptides and the membranes of bacterial or animal cells are still poorly understood, thus greatly limiting the biomimetic potential of these therapeutic molecules. In this arena, finely engineered nanomaterials—such as small amphiphilic gold nanoparticles (AuNPs) protected by a mixed thiol monolayer—can provide a powerful tool for mimicking and investigating the physicochemical processes underlying peptide-lipid interactions. Within this perspective, we present here a critical review of membrane effects induced by both amphiphilic AuNPs and well-known amphiphilic peptide families, such as cell-penetrating peptides and antimicrobial peptides. Our discussion is focused particularly on the effects provoked on widely studied model cell membranes, such as supported lipid bilayers and lipid vesicles. Remarkable similarities in the peptide or nanoparticle membrane behavior are critically analyzed. Overall, our work provides an overview of the use of amphiphilic AuNPs as a highly promising tailor-made model to decipher the molecular events behind non-specific peptide-lipid interactions and highlights the main affinities observed both theoretically and experimentally. The knowledge resulting from this biomimetic approach could pave the way for the design of synthetic peptides with tailored functionalities for next-generation biomedical applications, such as highly efficient intracellular delivery systems.

Position Effects of Metal Nanoparticles on the Performance of Perovskite Light-Emitting Diodes.

Metal nanoparticles have been widely used for improving the efficiencies of many optoelectronic devices. Herein, position effects of gold nanoparticles (Au NPs) on the performance of perovskite light-emitting diodes (PeLEDs) are investigated. Amphiphilic Au NPs are synthesized so that they can be incorporated into different layers of the PeLEDs to enhance device efficiencies. The photoluminescent (PL) studies indicate apparent position effects; the strongest PL intensity occurs when the NPs are directly blended with the light-emitting perovskite layer. In contrast, the PeLEDs exhibit the highest luminance efficiency while the Au NPs are placed in the hole-transporting layer. The direct blending of the NPs in the perovskite layer might affect the electrical properties, resulting in inferior device performance. The results reported herein can help to understand the enhancing mechanism of the PeLEDs and may also lead to even better efficiencies in the near future.

Microscale droplets covered by amphiphilic gold nanoparticles with various ligand ratios and concentrations.

We describe a simple and reliable route for forming microscale droplets covered with amphiphilic gold nanoparticles (AuNPs) functionalized with mixed hydrophilic and hydrophobic ligands on the AuNP surface. When an aqueous solution of amphiphilic AuNPs was mixed with hexane, the hexane (or water) droplets covered with amphiphilic AuNPs in the water (or hexane) phase were created, in which amphiphilic AuNPs in water phase migrated to the hexane-water interface. The extent of AuNP migration to the interface could be tuned by the ratio of the hydrophobic to hydrophilic ligands on the AuNP surface and the overall concentration of AuNPs. Importantly, the extent of the AuNP migration to the interface depending on the AuNP concentration followed the Langmuir isotherm model well. The equilibrium constant and maximum uptake of AuNPs at the interface were estimated using the Langmuir isotherm model. To the best of our knowledge, this is the first report of the equilibrium constant determination of amphiphilic AuNPs at the liquid-liquid interface. Based on the maximum uptake of AuNPs at the hexane-water interface, we demonstrated that the AuNPs could be closely packed at the interface.

Surface property modification of well-dispersed amphiphilic gold nanoparticles as individuals

Amphiphilic gold nanoparticles (AuNPs) functionalized with mixed monolayers consisting of hydrophobic and hydrophilic ligands find widespread applications in biosensing, drug delivery, and bioimaging. One important aspect of amphiphilic AuNPs in such applications is the tuning of the surface properties of these AuNPs by modifying the composition of the ligands. In this study, well-dispersed AuNPs as individuals with mixed monolayers of hydrophobic and hydrophilic ligands were synthesized and the ratios of hydrophilic and hydrophobic ligands on the AuNP surfaces with varying ligand lengths were investigated by electrostatic titration. We demonstrated that longer hydrophobic ligands have higher affinity for the AuNP surface, and that the relative ligand length plays an important role in determining the maximum hydrophobic coverage on the AuNP surface at which the ratio of the amount of hydrophobic to that of hydrophilic ligands on the AuNP surface is the largest, for AuNPs to remain as individuals. We expect that the AuNPs synthesized with diverse ratios of hydrophobic and hydrophilic ligands on the surface can be useful in biological applications.

Crystallization-Driven Two-Dimensional Self-Assembly of Amphiphilic PCL-b-PEO Coated Gold Nanoparticles in Aqueous Solution.

This Letter reports the first formation of free-standing plasmonic monolayer nanosheets by the self-assembly of AuNPs without assistance from a planar interface. The strategy involves the coating of poly(caprolactone)-b-poly(ethylene oxide) (PCL-b-PEO) diblock copolymers on AuNPs, followed by two-dimensional (2D) self-assembly of the resultant amphiphilic AuNPs in aqueous phase. The crystallization of the PCL blocks, affected by their grafting density and radius of gyration, drives the formation of the AuNP nanosheets, which undergoes a growth process of individual micelles to small nanosheets and eventually to large sheets. Due to the plasmonic coupling of AuNPs in close proximity, the AuNP nanosheets exhibit near-infrared (NIR) absorption with the maximum at about 700 nm. This study not only brings a new approach toward polymer-AuNP hybrid superstructures in solution, but also provides a new system for fundamental study on 2D self-assembly of AuNPs.

Amphiphilic N-Heterocyclic Carbene-Stabilized Gold Nanoparticles and Their Self-Assembly in Polar Solvents.

Soft matter-directed self-assembly of amphiphilic inorganic nanoparticles (NPs) has recently emerged as a promising approach to access NP ensembles with superior collective properties. While thiol-terminated molecules are primarily employed to tether the amphiphilic ligand to the metal, concerns remain regarding the stabilities of the resulting NPs and their corresponding aggregates. As an alternative, we report amphiphilic N-heterocyclic carbene (NHC)-functionalized gold nanoparticles (AuNPs). To accomplish this, an amphiphilic NHC-AuI complex based on an asymmetric triethylene glycol-/dodecyl-functionalized benzimidazole was first synthesized and used to prepare the corresponding stable amphiphilic NHC-decorated AuNPs. The resulting NPs were comprehensively characterized using both solution- and solid-state-based techniques such as proton nuclear magnetic resonance spectroscopy, dynamic light scattering, transmission electron microscopy, thermogravimetric analysis, and X-ray photoelectron spectroscopy. By optimizing the self-assembly behavior of these amphiphilic AuNPs in deionized water, ethanol, and their mixtures, we were able to fine-tune the plasmonic properties of the AuNPs in the wide range of 525-640 nm. Furthermore, when treated with thiols, the ensembles showed greater stability compared to their parent discrete AuNP counterparts at room temperature.

Aromaticity/Bulkiness of Surface Ligands to Promote the Interaction of Anionic Amphiphilic Gold Nanoparticles with Lipid Bilayers.

The presence of large hydrophobic aromatic residues in cell-penetrating peptides or proteins has been demonstrated to be advantageous for their cell penetration. This phenomenon has also been observed when AuNPs were modified with peptides containing aromatic amino acids. However, it is still not clear how the presence of hydrophobic and aromatic groups on the surface of anionic AuNPs affects their interaction with lipid bilayers. Here, we studied the interaction of a range of anionic amphiphilic AuNPs coated by different combinations of hydrophobic and anionic ligands with four different types of synthetic lipid vesicles. Our results demonstrated the important role of the surface aromatic or bulky groups, relative to the hydrocarbon chains, in the interaction of anionic AuNPs with lipid bilayers. Hydrophobic interaction itself arising from the insertion of aromatic/bulky ligands on the surface of AuNPs into lipid bilayers is sufficiently strong to cause overt disruption of lipid vesicles and cell membranes. Moreover, by comparing the results obtained from AuNPs coated with aromatic ligands and cyclohexyl ligands lacking aromaticity respectively, we demonstrated that the bulkiness of the terminal groups in hydrophobic ligands instead of the aromatic character might be more important to the interaction of AuNPs with lipid bilayers. Finally, we further correlated the observation on model liposomes with that on cell membranes, demonstrating that AuNPs that are more disruptive to the more negatively charged liposomes are also substantially more disruptive to cell membranes. In addition, our results revealed that certain cellular membrane domains that are more susceptible to disruption caused by hydrophobic interactions with nanoparticle surfaces might determine the threshold of AuNP-mediated cytotoxicity.

Directed Self-Assembly and Infrared Reflection Absorption Spectroscopy Analysis of Amphiphilic and Zwitterionic Janus Gold Nanoparticles.

Here, we report an approach to use infrared reflection absorption spectroscopy (IRRAS) for the unambiguous proof of the presence as well as the spatial distribution of organic ligands on the Janus gold nanoparticle (AuNP) surface. For this purpose we synthesized amphiphilic and zwitterionic Janus AuNPs and immobilized these on pretreated gold surfaces by directed self-assembly, exploiting hydrophilic/hydrophobic or electrostatic interactions, respectively. Thus, we obtained macroscopic two-dimensional arrays of Janus AuNPs exhibiting a specific orientation. These arrays were investigated by IRRAS, and the obtained spectra revealed only peaks of the ligands facing the IR beam, while the ligands facing the gold substrate were not detected due to reflection of the IR beam on the AuNP cores. Thus, we describe a straightforward spectroscopic procedure to prove the Janus character of zwitterionic and amphiphilic AuNPs in the size range of 10-15 nm.

Light-triggered reversible self-assembly of gold nanoparticle oligomers for tunable SERS.

A photoresponsive amphiphilic gold nanoparticle (AuNP) is achieved through the decoration of AuNP with hydrophilic poly(ethylene glycol) (PEG) and hydrophobic photoresponsive polymethacrylate containing spiropyran units (PSPMA). Owing to the photoresponsive property of spiropyran units, the amphiphilic AuNPs can easily achieve the controllable assembly/disassembly behaviors under the trigger by light. Under visible light, spiropyran units provide weak intermolecular interactions between neighbored AuNPs, leading to isolated AuNPs in the solution. While under UV light irradiation, spiropyran units in the polymer brushes transform into merocyanine isomer with conjugated structure and zwitterionic state, promoting the integration of adjacent AuNPs through π-π stacking and electrostatic attractions, further leading to the formation of Au oligomers. The smart reversible AuNP oligomers exhibited switchable plasmonic coupling for tuning surface-enhanced Raman scattering (SERS) activity, which is promising for the application of SERS based sensors and optical imaging.

Nanoporous Bicontinuous Structures via Addition of Thermally-Stable Amphiphilic Nanoparticles within Block Copolymer Templates

Herein, we fabricated the bicontinuous structures from nanocomposites of poly(styrene-b-methyl methacrylate) (PS-b-PMMA) block copolymer and the shell-cross-linked, thermally stable gold nanoparticles (Au NPs). The surface property of Au NPs was controlled with ligands containing various compositions of PS and PMMA so that the resulting Au NPs were selective to PS or PMMA block or nonselective (i.e., neutral) to both blocks. The amphiphilic Au NPs were also prepared by coating the surface of Au NPs with equimolar mixtures of PS and PMMA selective ligands. Consequently, it was found that the morphological behaviors of thermally annealed nanocomposites containing amphiphilic Au NPs and PS-b-PMMA were dramatically different from the case of neutral Au NPs that were coated with nonselective ligands. With increasing the amount of amphiphilic Au NPs, a transition from lamellar to bicontinuous structures was observed, whereas the neutral Au NPs were aggregated within the PS-b-PMMA lamellae. Furthermore, the nanoporous bicontinuous thin films were fabricated on the silicon substrates and the morphological behaviors were quantitatively investigated by grazing-incidence small-angle X-ray scattering (GISAXS) analysis.

Amphiphilic gold nanoparticles formed at a liquid–liquid interface and fabrication of hybrid nanocapsules based on interfacial UV photodimerization

In this paper a novel method for fabrication of hybrid hollow capsules based on interface-directed self-assembly of gold nanoparticles (AuNPs) and interfacial photodimerization of anthracene is reported. A hydrophobic random copolymer with a disulfide group at the midpoint and pendant anthracene groups on the repeating units was synthesized by atom transfer radical polymerization. The hydrophobic random copolymer was dissolved in toluene and hydrophilic citrate-stabilized AuNPs were dispersed in water. An emulsion was obtained upon mixing of the two solutions. In the emulsion, hydrophilic AuNPs were able to undergo interface-directed self-assembly and amphiphilic AuNPs were produced at the liquid–liquid interface via ligand exchange. Hybrid nanocapsules with AuNPs on the surfaces were prepared after anthracene photodimerization at the liquid–liquid interface. Transmission electron microscopy, dynamic light scattering, UV-vis and scanning electron microscopy were used to characterize the structure of the hybrid nanocapsules. In the fabrication of the hybrid nanocapsules, the addition of small molecular surfactant has an important effect on the structures. The average size and size distribution of the hybrid nanocapsules can be controlled by controlling the concentration of the small molecular surfactant in the solution, and in the meanwhile the use of the small molecular surfactant will not reduce the surface density of AuNPs on the hybrid nanocapsules. The hybrid nanocapsules display effective catalytic activity in the reduction of 4-nitrophenol by NaBH4.