Self‐Organization of Gold Nanoparticle Assemblies with 3D Spatial Order and Their External Stimuli Responsiveness

Gold nanoparticles (AuNP) with pyridyl end‐capped polystyrenes (PS‐4VP) as “quasi‐monodentate” ligands self‐assemble into ordered PS‐4VP/AuNP nanostructures with 3D hexagonal spatial order in the dried solid state. The key for the formation of these ordered structures is the modulation of the ratio AuNP versus ligands, which proves the importance of ligand design and quantity for the preparation of novel ordered polymer/metal nanoparticle conjugates. Although the assemblies of PS‐4VP/AuNP in dispersion lack in high dimensional order, strong plasmonic interactions are observed due to close contact of AuNP. Applying temperature as an external stimulus allows the reversible distortion of plasmonic interactions within the AuNP nanocomposite structures, which can be observed directly by naked eye. The modulation of the macroscopic optical properties accompanied by this structural distortion of plasmonic interaction opens up very interesting sensoric applications.

     

Evaporation‐Induced Ordered Honeycomb Structures of Gold Nanoparticles at the Air/Water Interface

The breath figure method was used to prepare dodecanethiol‐capped gold nanoparticle macroporous structures with pore diameters from 1.7 to 3.5 μm on an air/water interface. A two‐step procedure is proposed for the fabrication of these macroporous structures, by forming a surfactant monolayer on water, and drop‐casting a gold nanoparticle dispersion in chloroform onto the surfactant monolayer. The self‐assembled films are easily transferred from the water surface onto different substrates and were characterized by TEM, SEM, and AFM. Ordered honeycomb structures with different pore arrays (perforated monolayer films, hexagonal networks and alveoli‐like porous films) were obtained. The change in morphology is concentration dependent, and deformed structures with elliptic honeycomb networks are also observed. In addition, honeycomb films using gold nanoparticles stabilized by a weakly bound ligand (dioctadecyldimethylammonium chloride) were formed by the same technique. These films have potential as substrates for surface‐enhanced Raman spectroscopy.     

Supramolecular Proteoglycan Aggregate Mimics: Cyclodextrin‐Assisted Biodegradable Polymer Assemblies for Electrostatic‐Driven Drug Delivery

Self‐assembled, noncovalent polymeric biodegradable materials mimicking proteoglycan aggregates were synthesized from inclusion complexes of cationic surfactants with γ‐cyclodextrin and the natural anionic polymer hyaluronan. The amorphous structure of this ternary system was proven by X‐ray diffraction and thermal analysis. Light‐scattering measurements showed that there was a competition between hyaluronic acid and the surfactant for the cyclodextrin cavity. These self‐assembled supramolecular matrices were loaded with both hydrophilic and lipophilic drug substances for dissolution studies. The release of the entrapped drugs was found to be controlled by cations in the surrounding media and by biodegradation. Slow drug release in an ion‐free medium became faster in physiological salt solution in which the macroscopic polymer matrix was disassembled. In contrast, the enzymatic degradation of hyaluronan was hindered in the polymeric matrix. The supramolecular systems consisting of γ‐cyclodextrin as a macrocyclic host, a cationic surfactant guest, and hyaluronic acid as the anionic polymer electrostatically cross‐linked by the inclusion complex of the first two was found to be a novel drug‐delivery system for the controlled release of traditional drugs such as curcumin and ketotifen and proteins such as bovine serum albumin.     

Laser‐Modulated Ordering of Gold Nanoparticles at the Air/Water Interface

Beam me up, Scotty! Laser irradiation of Langmuir monolayers of gold nanoparticles (NPs) and elaidic acid led to dramatic reorganization that was dependent on the laser power (see picture, scale bar=100 μm). Variable‐temperature experiments indicate that localized surface heating in an extremely small temperature range, induced by the laser beam, causes ordering of the NPs.

     

pH-Responsive Luminescent Lanthanide-Functionalized Gold Nanoparticles with "On-Off" Ytterbium Switchable Near-Infrared Emission

A pH indicator: Near-infrared emitting lanthanide-functionalized gold nanoparticles have been prepared through self-assembly at the gold surface between a ytterbium(III)-cyclen complex and xylenol orange. Excitation of the xylenol orange unit with visible light, up to 600?nm, results in the sensitization of the Yb(III) -centered near-infrared emission that can be reversibly switched "on-off" as a function of the pH value.     

Reversible Self‐Assembly of Carboxylated Peptide‐Functionalized Gold Nanoparticles Driven by Metal‐Ion Coordination

Carboxylated peptide‐functionalized gold nanoparticles (peptide‐GNPs) self‐assemble into two‐ and three‐dimensional nanostructures in the presence of various heavy metal ions (i.e. Pb²⁺, Cd²⁺, Cu²⁺, and Zn²⁺) in aqueous solution. The assembly process is monitored by following the changes in the surface plasmon resonance (SPR) band of gold nanoparticles in a UV/Vis spectrophotometer, which shows the development of a new SPR band in the higher‐wavelength region. The extent of assembly is dependent on the amount of metal ions present in the medium and also the time of assembly. TEM analysis clearly shows formation of two‐ and three‐dimensional nanostructures. The assembly process is completely reversible by addition of alkaline ethylenediaminetetraacetic acid (EDTA) solution. The driving force for the assembly of peptide‐GNPs is mainly metal ion/carboxylate coordination. The color and spectral changes due to this assembly can be used for detection of these heavy‐metal ions in solution.     

The Molecularly Controlled Synthesis of Ordered Bi‐dimensional C60 Arrays

Much of the research effort concerning the nanoscopic properties of clays has focused on its mechanical applications, for example, as nanofillers for polymer reinforcement. To broaden the horizon of what is possible by exploiting the richness of clays in nanoscience, herein we report a bottom‐up approach for the production of hybrid materials in which clays act as the structure‐directing interface and reaction media. This new method, which combines self‐assembly with the Langmuir–Schaefer technique, uses the clay nanosheets as a template for the grafting of C₆₀ into a bi‐dimensional array, and allows for perfect layer‐by‐layer growth with control at the molecular level. In contrast to the more‐common growth of C₆₀ arrays through nanopatterning, our approach can be performed under atmospheric conditions, can be upscaled to areas of tenths of cm², and can be applied to almost any hydrophobic substrate. Herein, we report a detailed study of this approach by using temperature‐dependent X‐ray diffraction, spectroscopic measurements, and STM.     

Seed‐Mediated Synthesis of Single‐Crystal Gold Nanospheres with Controlled Diameters in the Range 5–30 nm and their Self‐Assembly upon Dilution

Single‐crystal gold nanospheres with controlled diameters in the range 5–30 nm were synthesized by using a facile approach that was based on successive seed‐mediated growth. The key to the success of this synthesis was the use of hexadecyltrimethylammonium chloride (CTAC) as a capping agent and a large excess of ascorbic acid as a reductant to ensure fast reduction and, thus, single crystallinity and a spherical shape of the resultant nanoparticles. The diameters of the gold nanospheres could be readily controlled by varying the amount of seeds that were introduced into the reaction system. The gold nanospheres could be produced with uniform diameters of up to 30 nm; thus, their localized surface plasmon resonance properties could be directly compared with the results that were obtained from theoretical calculations. Interestingly, we also found that these gold nanospheres self‐assembled into dimers, larger aggregates, and wavy nanowires when they were collected by centrifugation, dispersed in deionized water, and then diluted to different volumes with deionized water.     

Seed‐Mediated and Iodide‐Assisted Synthesis of Gold Nanocrystals with Systematic Shape Evolution from Rhombic Dodecahedral to Octahedral Structures

We report the development of a seed‐mediated and iodide‐assisted method for the synthesis of monodisperse gold nanocrystals with systematic shape evolution from rhombic dodecahedral to octahedral structures. Particle growth is complete in 15 min at room temperature, so the process is fast and energy‐efficient. By progressively increasing the volume of KI used in a growth solution while keeping the amount of ascorbic acid added constant, nanocrystals with morphologies that vary from rhombic dodecahedral to rhombicuboctahedral, edge‐ and corner‐truncated octahedral, corner‐truncated octahedral, and octahedral structures were synthesized. The nanocrystals are monodisperse in size and readily form self‐assembled structures on substrates. By simply adjusting the volume of gold seed solution added to a growth solution, particle sizes of the octahedral gold nanocrystals can be tuned with average opposite corner‐to‐corner distances of 42, 48, 54, 60, 68, 93, 107, and 125 nm. In the presence of HAuCl₄, iodide may act as a reducing agent. Variation of its volume in the solution may slightly modulate the reduction rate and affect the final crystal morphology. Intermediate structures collected during crystal growth reveal the presence of many twisted structures that surround a developing nanocrystal core. This nanocrystal growth mechanism and the less important role of surfactant in directing the polyhedral nanocrystal morphology is discussed.     

Polymer‐Mediated Hierarchical and Reversible Emulsion Droplet Assembly

Making and breaking : Stable, functional micrometer‐sized emulsion droplets can be assembled into various complex macroscopic liquid structures (see picture). The hierarchical assembly process is mediated by interactions between polymeric surfactant molecules located on the droplet surfaces. These interdroplet interactions are reversible, therefore these “engineered emulsions” can be readily disassembled by using a simple pH switch.