Enhanced stability of Janus nanoparticles by covalent cross-linking of surface ligands

Amide-Assisted Polymerization of 1,3-Butadiyne Containing Thiolate Ligands on Small Gold Nanoparticles

Incorporation of directing amide groups has been shown to facilitate the topochemical polymerization of 1,3-butadiyne (diacetylene) groups in noncrystalline phases such as gels, amorphous solids, and liquid crystals. It remains challenging to polymerize 1,3-butadiyne-containing alkylthiolate ligands within their self-assembled monolayers on gold nanoparticles (AuNPs), which enhances their stability and adds new optical and electronic properties. Especially smaller AuNPs of sizes below 5 nm in diameter have been reported to display sluggish photopolymerization and are susceptible to photodegradation under UV irradiation. To probe the effectiveness of the amide-directed photopolymerization of 1,3-butadiyne ligands, small AuNPs in the 2-4 nm range were synthesized that contain alkylthiolate ligands with and without amide and 1,3-butadiyne groups. Their photopolymerization and photostability were studied by transmission electron microscopy (TEM), UV-vis spectroscopy, and Raman spectroscopy. AuNP with amide-free 1,3-butadiyne ligands templated the polymerization of the 1,3-butadiyne ligands but fused to large and insoluble particles during the polymerization process. AuNPs with ligands containing both 1,3-butadiyne and amide groups polymerized significantly faster, which slowed down photodegradation. A UV irradiation (254 nm and 176 W/m²) for 5-10 min was found to be optimal for the AuNPs with directing amide groups studied here, although their average core sizes grew from 3.8 to 4.0 nm in diameter and about 20% of the attached 1,3-butadiyne ligands remained unreacted after 10 minutes of irradiation. About 75% of the attached 1,3-butadiyne ligands were already polymerized during the first 5 min of UV irradiation. This decrease in reactivity is reasoned with a fast polymerization of ligands attached to facet sites and slower polymerization rates for ligands attached to edge and corner sites. Unexpectedly, photopolymerization occurred only in the presence of solvent, whereas no polydiacetylene was generated when dry powders of any of the diacetylene-containing gold nanoparticles were irradiated.

Plasmonic circular dichroism of vesicle-like nanostructures by the template-less self-assembly of achiral Janus nanoparticles

Chiral nanostructures have been attracting extensive interest in recent years primarily because of the unique materials properties that can be exploited for diverse applications. In this study, gold Janus nanoparticles, with hexanethiolates and 3-mercapto-1,2-propanediol segregated on the two hemispheres of the metal cores (dia. 2.7 ± 0.4 nm), self-assembled into vesicle-like, hollow nanostructures in both water and organic media, and exhibited apparent plasmonic circular dichroism (PCD) absorption in the visible range. This was in contrast to individual Janus nanoparticles, bulk-exchange nanoparticles where the two ligands were homogeneously mixed on the nanoparticle surface, or nanoparticles capped with only one kind of ligand. The PCD signals were found to become intensified with increasing coverage of the 3-mercapto-1,2-propanediol ligands on the nanoparticle surface. This was accounted for by the dipolar property of the structurally asymmetrical Janus nanoparticles, and theoretical simulations based on first principles calculations showed that when the nanoparticle dipoles self-assembled onto the surface of a hollow sphere, a vertex was formed which gave rise to the unique chiral characteristics. The resulting chiral nanoparticle vesicles could be exploited for the separation of optical enantiomers, as manifested in the selective identification and separation of d-alanine from the l-isomer.

Gold nanoparticles with patterned surface monolayers for nanomedicine: current perspectives

Molecular self-assembly is a topic attracting intense scientific interest. Various strategies have been developed for construction of molecular aggregates with rationally designed properties, geometries, and dimensions that promise to provide solutions to both theoretical and practical problems in areas such as drug delivery, medical diagnostics, and biosensors, to name but a few. In this respect, gold nanoparticles covered with self-assembled monolayers presenting nanoscale surface patterns-typically patched, striped or Janus-like domains-represent an emerging field. These systems are particularly intriguing for use in bio-nanotechnology applications, as presence of such monolayers with three-dimensional (3D) morphology provides nanoparticles with surface-dependent properties that, in turn, affect their biological behavior. Comprehensive understanding of the physicochemical interactions occurring at the interface between these versatile nanomaterials and biological systems is therefore crucial to fully exploit their potential. This review aims to explore the current state of development of such patterned, self-assembled monolayer-protected gold nanoparticles, through step-by-step analysis of their conceptual design, synthetic procedures, predicted and determined surface characteristics, interactions with and performance in biological environments, and experimental and computational methods currently employed for their investigation.

Confinement effects on the properties of Janus dimers

We show how the confinement between two parallel walls affects the self-assembly, and dynamic and thermodynamic properties of Janus dumbbells.

Self-Assembly and Water-like Anomalies in Janus Nanoparticles

We explore the pressure versus temperature phase diagram of a system of dimeric Janus nanoparticles using molecular dynamics simulations. Each nanoparticle is modeled as a dumbbell which has one monomer that interacts by a standard Lennard-Jones potential while the other monomer interacts by a core-softened potential. The systems composed by particles interacting only by core-softened potential exhibit the density and the diffusion anomalous behavior observed in water while if the particles interact only by the Lennard-Jones potential no anomaly is present. Here we explore if the anomalous behavior is present when half of the particles are modeled by a core-softened potential and half with Lennard-Jones potential. We show that the diffusion anomaly is preserve, while the density anomaly can disappear depending on the nonanomalous monomer characteristics. We also show that the self-assembly structures characteristics of the dumbbell systems are affected by the balance between core-softened and non-core-softened monomers.

Topochemical polymerization of unsymmetrical aryldiacetylene supramolecules with nitrophenyl substituents utilizing C-H∙∙∙π interactions

Diacetylenes are versatile building blocks, in which many functional groups can be incorporated for the construction of new materials with desirable properties. In this study, 6-(p or m-nitrophenyl)-3,5-hexadiyne-1-ol (4a or 4b) containing nitrophenyl groups (host) and 2-hydroxyethyl groups (guest) in different diacetylene terminals were designed to establish an ordered supramolecular assembly that is complied with the strict requirements for the topochemical polymerization of diacetylenes. Crystal film and bulk crystals of compound 4b were obtained successfully by cast film and re-precipitation methods. Both of these could photopolymerize to the corresponding regular poly(diacetylene) polymer, as evidenced by UV-vis, IR, FL and Raman spectroscopy. The electrochemical properties and behaviors of 4a and 4b were also investigated, and the results show that the differences between the para and meta positions of the mono-phenylacetylene substituents probably result from the topochemical polymerization. Thus, m-nitrophenylbutadiyne derivatives with sizeable C-H∙∙∙π interactions seemed to be effective for the formation of a polymerizable packing, which is appropriate for topochemical polymerization.

Polydiacetylene stabilized gold nanoparticles – extraordinary high stability and integration into a nanoelectrode device

A new diacetylene containing photopolymerizable ligand molecule was developed, and tailored for applications in nanoelectronic devices based on gold nanoparticles.

Janus nanoparticles as versatile phase-transfer reagents

Janus nanoparticles were prepared by interfacial ligand-exchange reactions of hexanethiolate-protected gold (AuC6) nanoparticles with 3-mercapto-1,2-propanediol (MPD) at the air|water interface. As nanoscale analogues to conventional amphiphilic molecules, the resulting Janus nanoparticles were found to form oil-in-water micelle-like or water-in-oil reverse micelle-like superparticulate structures depending on the solvent media. These unique characteristics were exploited for the effective transfer of diverse guest nanoparticles between organic and water phases. The transfer of hydrophobic nanoparticles from organic to water media or water-soluble nanoparticles to the organic phase was evidenced by transmission electron microscopy, dynamic light scattering, UV-vis, and photoluminescence measurements. In particular, line scans based on energy-dispersive X-ray analysis showed that the vesiclelike structures consisted of multiple layers of Janus nanoparticles which encapsulated the guest nanoparticles in the cores. The results highlight the unique effectiveness of using Janus nanoparticles in the formation of functional nanocomposites.

Cross-linked carboxybetaine SAMs enable nanoparticles with remarkable stability in complex media

A photo-cross-linkable carboxybetaine (CB)-terminated thiol with only one CB headgroup was introduced to modify gold nanoparticles (GNPs) via self-assembled monolayers (SAMs). This CB-terminated thiol consists of three moieties: (a) an anchoring thiol group, which binds directly to the GNP surface, (b) a CB terminal group, which is highly resistant to protein adsorption, and (c) a diacetylene group in the middle, which is converted to a poly(enyne) structure during UV irradiation via 1,4-topochemical polymerization. Results show that, after cross-linking, CB-modified GNPs are highly resistant to protein adsorption from undiluted human blood serum and cell uptake, and are stable at low pH and high temperature. This cross-linkable CB thiol holds tremendous potentials for biomedical applications where stable and thin coatings are needed.

Oxygen reduction catalyzed by Au-TiO2 nanocomposites in alkaline media

Au-TiO2 nanocomposites were prepared by chemical deposition of gold nanoparticles onto TiO2 nanocolloids that were synthesized by a hydrothermal method. Transmission electron microscopic measurements showed that the TiO2 colloids exhibited an average diameter of about 5 nm and clearly defined lattice fringes that were consistent with those of anatase TiO2 and formed rather large agglomerates that spanned a few hundred nanometers in length. Additionally, gold nanoparticles were found to be embedded within the TiO2 matrices, and the size increased with increasing gold loading but all ranged from 10 to 50 nm in diameter. Consistent results were obtained in X-ray diffraction measurements. Electrochemical studies demonstrated that the resulting Au-TiO2 nanocomposites exhibited apparent electrocatalytic activity in oxygen reduction that was markedly improved as compared to that of TiO2 particles alone, as reflected in the onset potential, number of electron transfers involved, and kinetic current density. Among the series, the best catalyst for oxygen reduction was identified with the Au/Ti atomic ratio of 5.2%. The enhanced oxygen reduction kinetics was ascribed to the dissociation of water and formation of surface-adsorbed hydroxyl moieties that was facilitated by the loading of gold nanoparticles onto the TiO2 colloids.

Trimetallic Ag@AuPt Neapolitan nanoparticles

Trimetallic Ag@AuPt Neapolitan nanoparticles were prepared by two sequential galvanic exchange reactions of 1-hexanethiolate-capped silver nanoparticles (AgC6, 5.70 ± 0.82 nm in diameter) with gold(I)-thiomalic acid (Au(I)TMA) and platinum(II)-hexanethiolate (Pt(II)C6) complexes. The first reaction was carried out at the air-water interface by the Langmuir method where the AgC6 nanoparticles formed a compact monolayer and water-soluble Au(I)TMA was injected into the water subphase; the nanoparticles were then deposited onto a substrate surface in the up-stroke fashion and immersed into an acetone solution of Pt(II)C6. As both reactions were confined to an interface, the Au and Pt elements were situated on two opposite poles of the original Ag nanoparticles. The tripatchy structure was clearly manifested in elemental mapping of the nanoparticles, and consistent with the damping and red-shift of the nanoparticle surface plasmon resonance. Further characterizations by X-ray photoelectron spectroscopy showed that the reactions were mostly confined to the top layers of the Ag metal cores, and contact angle and infrared spectroscopic measurements confirmed the incorporation and segregated distribution of the organic capping ligands on the nanoparticle surface.

AgAu bimetallic Janus nanoparticles and their electrocatalytic activity for oxygen reduction in alkaline media

Bimetallic AgAu Janus nanoparticles were prepared by galvanic exchange reactions of 1-hexanethiolate-passivated silver (AgC6) nanoparticles with gold(I)-mercaptopropanediol complex. The AgC6 nanoparticles were deposited onto a solid substrate surface by the Langmuir-Blodgett method such that the galvanic exchange reactions were limited to the top face of the nanoparticles that was in direct contact with the gold(I) complex solution. The resulting nanoparticles exhibited an asymmetrical distribution not only of the organic capping ligands on the nanoparticle surface but also of the metal elements in the nanoparticle cores, in contrast to the bulk-exchange counterparts where these distributions were homogeneous within the nanoparticles, as manifested in contact angle, UV-vis, XPS, and TEM measurements. More interestingly, despite a minimal loading of Au onto the Ag nanoparticles, the bimetallic AgAu nanoparticles exhibited enhanced electrocatalytic activity in oxygen reduction reactions, as compared to the monometal AgC6 nanoparticles. Additionally, the electrocatalytic performance of the Janus nanoparticles was markedly better than the bulk-exchange ones, suggesting that the segregated distribution of the polar ligands from the apolar ones might further facilitate charge transfer from Ag to Au in the nanoparticle cores, leading to additional improvement of the adsorption and reduction of oxygen.