Self-Assembly of Nanocrystals into Ring-like Superstructures: When Shape, Size, and Material Do Not Matter.

This manuscript describes a universal method for the spontaneous self-assembly of nanostructures ranging from 2-4 nm spherical particles to ∼440 nm long anisotropic nanorods into ring-like superstructures. The nanostructures composed of Au, Pt, and Pd as surface materials were synthesized in an aqueous cetyltrimethyl ammonium bromide (CTAB) solution. The ligand exchange technique with 4-mercaptophenol was applied to replace CTAB from the surface of nanostructures with a functional thiol. The esterification reaction was carried out to covalently attach carboxy-terminated long-chain polystyrene (PS) molecules to the surface of nanostructures. The high grafting density of PS chains around nanocrystals made them highly soluble in a wide range of organic solvents. When a drop of nanostructure solution in a volatile nonpolar solvent was dried on a solid surface, the nanostructures spontaneously arranged themselves in the form of ring-like assemblies. The condensation of microscopic water droplets from the atmosphere on the surface of an evaporating solvent creates templates for the self-assembly of nanostructures into rings. We demonstrate that this self-assembly method is highly universal and can be extended to various nanostructures regardless of their shapes, sizes, and surface materials.

Synthesis of Asymmetric One-Dimensional Pd on Au Bimetallic Nanostructures.

Nanostructures composed of a gold nanorod (AuNR) core and a Pd/Pt shell are of great interest due to their potential application as plasmon resonance-enhanced catalysts. However, the synthesis of well-defined one-dimensional bimetallic nanostructures with precise control over shell thickness and length remains a challenge. In this study, we report a detailed and systematic study on the chemical synthesis of a uniform Pd shell on single crystalline and pentahedrally twinned (PHT) AuNRs of various lengths. AuNRs were used as a template, and the slow and controlled reduction of Pd(II) ions on preformed AuNRs was carried out for the formation of rectangular-shaped Au@Pd bimetallic nanorods. The Pd shell thickness around the AuNRs was controlled by the supply of Pd(II) ions in the growth solution. We were able to grow a ∼20 nm uniform Pd shell around the AuNR, keeping the rod-like morphology intact without local nucleation to form irregular shapes and randomly overgrown nanostructures. The formation of bimetallic nanorods was also extended beyond typical single crystalline nanorods to PHT high aspect ratio gold nanorods and nanowires, using them as templates. To our surprise, unusually curved asymmetric nanorods were formed when the Pd deposition was carried out on AuNRs longer than ∼800 nm which could be possibly due to a Pd and Au lattice mismatch at the interface and higher flexibility of the nanorods when they exceeded certain lengths.

Gold Nanowires from Nanorods.

Gold nanowires (AuNWs) possess strong potential application in micro- and nanoelectronics as well as in plasmonic waveguides because of their low electrical resistance. However, the synthesis of pure solvent-dispersible AuNWs with full control over their length still remains a challenge. All the previously reported methods produce AuNWs with other impurities such as smaller nanorods, platelets, and spherical particles and are limited to a certain length (typically below 10 µm). This article describes a one-step synthesis of extremely long AuNWs (up to 25 µm) with great control over their dimensions by using pentahedrally twinned gold nanorods (AuNRs) as seed particles. To induce the AuNW growth, the reduction of Au(I) to Au(0) was carried out on the surface of AuNRs at a very low pH by introducing HCl into the growth solution. The slow conversion of Au(I) to Au(0) due to the increase in reduction potential at lower pH promoted the preferential deposition of metallic gold on the more reactive tips of AuNRs compared to their sides, resulting in the formation of AuNWs. In analogy to the "living" polymerization reaction, the length of the AuNWs was proportional to the amount of Au(I) added to the growth solution; thus, the desired length of AuNWs was achieved by controlling the supply of Au(I) ions in the reaction mixture. The AuNWs longer than 6 µm were found to be responsive to microwave radiation. When an aqueous solution of AuNWs was exposed to microwaves, the formation of sharp kinks was observed in several locations of AuNWs without their disintegration into smaller pieces.

Gold Nanorod Synthesis with Small Thiolated Molecules.

Size and shape tunability have been widely demonstrated for gold nanorods (AuNRs), but reproducible and reliable protocols for the synthesis of small nanocrystals with high yield are still needed for potential biomedical applications. Here, we present novel seed-mediated and seedless protocols for gold nanorods by incorporating bioadditives or small thiolated molecules during the growth stage. The bioadditives glutathione (GSH), oxidized glutathione (GSSG), l-cysteine (l-cys), and l-methionine (l-met) are utilized in nanomolar and micromolar concentrations to modify the aspect ratio of AuNRs in a reproducible form. Overall, smaller aspect ratios are achieved for both synthetic approaches due to reduction in length or increment in length and width depending on the method, type of bioadditive and the strength of its interaction with the nanorod surface. For the seeded synthesis, only GSSG produces large nanorods in high yield, whereas for the seedless method GSH and GSSG form small nanorods with higher quality when compared to controls.

Gram-Scale Synthesis of Isolated Monodisperse Gold Nanorods.

Although gold nanorods (AuNRs) have strong potential applications in nanotechnology, plasmonics, and sensing, the scale-up synthesis of isolated AuNRs in gram quantities remains a challenge. Nearly all previously reported methods produce aqueous solutions of cetyltrimethylammonium bromide (CTAB)-coated AuNRs in milligram quantities with yields of approximately 20-30 % in terms of AuI to Au0 conversion. In addition, it is difficult to remove the CTAB bilayer from the surface of AuNRs and yet make them soluble and functionalized for further processing and chemical modification. This report describes the synthesis of monodisperse functionalized AuNRs (standard deviation, σ≈5 %) in gram quantities. Our approach involved increasing the concentration of HAuCl4 ⋅3 H2 O in the growth solution to produce larger quantities of starting AuNRs and further reducing the remaining AuI ions onto the surface of AuNRs. The slow and controlled addition of ascorbic acid as a reducing agent continued the conversion of AuI into Au0 (through a disproportionation reaction) onto the surface of the nanorods, which maintained their uniform morphology without creating any unwanted impurities of various shapes. In addition, this approach significantly narrowed the size distribution owing to continuous growth of the partially grown AuNRs during the initial stage of the synthesis. To isolate a 1 g quantity of the AuNRs and to make them functionalized for further chemical reactions, a ligand-exchange approach was utilized, in which the CTAB surfactant was replaced with 4-mercaptophenol. The thiol group from 4-mercaptophenol formed a covalent bond with the surface of the AuNRs, leaving free functional OH groups available for further chemical coupling reactions. For the ligand-exchange process, a concentrated solution of 4-mercaptophenol in tetrahydrofuran solution was introduced into the AuNRs solution. Pure AuNRs functionalized with 4-mercaptophenol were isolated by dispersion and rinsing with an excess amount of THF, followed by centrifugation.

Improving the Shape Yield and Long-Term Stability of Gold Nanoprisms with Poly(vinylpyrrolidone).

Gold nanoprisms (AuNPRs) are anisotropic nanostructures that have gained great attention in recent years because of their interesting and unique optical properties that can be tailored for biomedical, energy, and sensing applications. At present, several protocols have reported the high yield synthesis of AuNPRs of different dimensions using a seed-mediated approach. However, there is a need to develop reproducible and scalable methods with the goal of a controllable synthesis. Here, we report an improved seed-mediated synthesis of small monodisperse AuNPRs of distinct sizes in high yield using poly(vinylpyrrolidone) (PVP) as an additive in nanomolar concentrations. We show optimal synthetic parameters for a blue-shifting of the surface plasmon resonance band which correlates with the reduction in the edge length (L) of AuNPRs from 75 to 35 nm. Using measured extinction coefficients for AuNPRs of different sizes, a linear equation is proposed to estimate the concentration of unknown samples by using Beer's law. Interestingly, the use of nanomolar amounts of PVP during the growth of AuNPRs significantly improves the shape yield. The surface chemistry properties of AuNPRs were measured by X-ray photoelectron spectroscopy and attenuated total reflectance infrared spectroscopy and revealed that PVP chains interact with AuNPRs through the carbonyl oxygen. This method is reproducible and scalable and enables the synthesis of AuNPRs with long-term shape stability (1 year) in aqueous solution.

Accelerating Gold Nanorod Synthesis with Nanomolar Concentrations of Poly(vinylpyrrolidone).

A novel modification for the seedless synthesis of gold nanorods (AuNRs) has been developed. Nanomolar concentrations of 10 kDa poly(vinylpyrrolidone) (PVP) can be introduced to a growth solution containing 25, 50, or 100 mM cetyltrimethylammonium bromide (CTAB) to significantly reduce the dimensions of AuNRs. We found that PVP accelerates the growth rate of AuNRs by more than two times that of nanorods grown in 50 and 100 mM CTAB solutions. Additionally, there is a time-dependent effect of adding PVP to the nanorod growth solution that can be utilized to tune their aspect ratio. Because the concentration of PVP is far below the concentration of HAuCl4 in the reaction mixture, PVP primarily functions not as a reducing agent, but as a capping or templating ligand to stabilize the growing nanorods. Our reproducible protocol enables the synthesis of AuNRs in high yield with tunable sizes: 45 × 6.7, 28 × 5.5, and 12 × 4.5 nm for 100, 50, and 25 mM CTAB, respectively. We estimated the number of PVP chains per nanorod in growth solutions to be around 30, which suggests that the effect on the aspect ratio is caused by a direct interaction between the AuNR surface and the PVP.

Low absorption losses of strongly coupled surface plasmons in nanoparticle assemblies.

Coupled surface plasmons in one-dimensional assemblies of metal nanoparticles have attracted significant attention because strong interparticle interactions lead to large electromagnetic field enhancements that can be exploited for localizing and amplifying electromagnetic radiation in nanoscale structures. Ohmic loss (i.e., absorption by the metal), however, limits the performance of any application due to nonradiative surface plasmon relaxation. While absorption losses have been studied theoretically, they have not been quantified experimentally for strongly coupled surface plasmons. Here, we report on the ohmic loss in one-dimensional assemblies of gold nanoparticles with small interparticle separations of only a few nanometers and hence strong plasmon coupling. Both the absorption and scattering cross-sections of coupled surface plasmons were determined and compared to electrodynamic simulations. A lower absorption and higher scattering cross-section for coupled surface plasmons compared to surface plasmons of isolated nanoparticles suggest that coupled surface plasmons suffer smaller ohmic losses and therefore act as better antennas. These experimental results provide important insight for the design of plasmonic devices.

Gold nanorods 3D-supercrystals as surface enhanced Raman scattering spectroscopy substrates for the rapid detection of scrambled prions.

Highly organized supercrystals of Au nanorods with plasmonic antennae enhancement of electrical field have made possible fast direct detection of prions in complex biological media such as serum and blood. The nearly perfect three-dimensional organization of nanorods render these systems excellent surface enhanced Raman scattering spectroscopy substrates with uniform electric field enhancement, leading to reproducibly high enhancement factor in the desirable spectral range.

Shape-dependent oriented trapping and scaffolding of plasmonic nanoparticles by topological defects for self-assembly of colloidal dimers in liquid crystals.

We demonstrate scaffolding of plasmonic nanoparticles by topological defects induced by colloidal microspheres to match their surface boundary conditions with a uniform far-field alignment in a liquid crystal host. Displacing energetically costly liquid crystal regions of reduced order, anisotropic nanoparticles with concave or convex shapes not only stably localize in defects but also self-orient with respect to the microsphere surface. Using laser tweezers, we manipulate the ensuing nanoparticle-microsphere colloidal dimers, probing the strength of elastic binding and demonstrating self-assembly of hierarchical colloidal superstructures such as chains and arrays.

Starfruit-shaped gold nanorods and nanowires: synthesis and SERS characterization.

Recently, branched and star-shaped gold nanoparticles have received significant attention for their unique optical and electronic properties, but most examples of such nanoparticles have a zero-dimensional shape with varying numbers of branches coming from a quasi-spherical core. This report details the first examples of higher-order penta-branched gold particles including rod-, wire-, and platelike particles which contain a uniquely periodic starfruitlike morphology. These nanoparticles are synthesized in the presence of silver ions by a seed-mediated approach based on utilizing highly purified pentahedrally twinned gold nanorods and nanowires as seed particles. The extent of the growth can be varied, leading to shifts in the plasmon resonances of the particles. In addition, the application of the starfruit rods for surface-enhanced Raman spectroscopy (SERS) is demonstrated.

Robust multilayer thin films containing cationic thiol-functionalized gold nanorods for tunable plasmonic properties.

Gold nanorods have great potential in a variety of applications because of their unique physical properties. In this article, we present the layer-by-layer (LbL) assembly of thin films containing positively charged gold nanorods that are covalently functionalized by cationic thiol molecules. The cationic gold nanorods are uniformly distributed in ultrathin nanocomposite LbL thin films. We studied the collective surface plasmon resonance coupling in the LbL films via UV-visible spectroscopy and evaluated their application in the surface-enhanced Raman scattering detection of rhodamine 6G probe molecules. Furthermore, we successfully manufactured freestanding nanoscale thin films containing multilayers of gold nanorods with a total thickness of less than 50 nm. The surface morphology and their optical and mechanical properties were systematically investigated, and the polycationic gold nanorods were found to play an important role in manipulating the properties of the nanocomposite thin films. Our findings reveal that such nanorods are excellent building blocks for constructing functional LbL films with tunable plasmonic behavior and robust mechanical properties.

Pancreatic tumor microenvironmental acidosis and hypoxia transform gold nanorods into cell-penetrant particles for potent radiosensitization.

Coating nanoparticles with stealth epilayers increases circulation time by evading opsonization, macrophage phagocytosis, and reticuloendothelial sequestration. However, this also reduces internalization by cancer cells upon reaching the tumor. We designed gold nanorods (GNRs) with an epilayer that retains stealth properties in circulation but transforms spontaneously in the acidotic tumor microenvironment to a cell-penetrating particle. We used a customized stoichiometric ratio of l-glutamic acid and l-lysine within an amphiphilic polymer of poly(l-glutamic acid-co-l-lysine), or P(Glu-co-Lys), to effect this transformation in acidotic environments. P(Glu-co-Lys)-GNRs were internalized by cancer cells to facilitate potent in vitro radiosensitization. When administered intravenously in mice, they accumulate in the periphery and core of tumors without any signs of serum biochemical or hematological alterations, normal organ histopathological abnormalities, or overt deterioration in animal health. Furthermore, P(Glu-co-Lys)-GNRs penetrated the tumor microenvironment to accumulate in the hypoxic cores of tumors to potently radiosensitize heterotopic and orthotopic pancreatic cancers in vivo.

Liquid-crystalline polymer composites with CdS nanorods: structure and optical properties.

We report on the structure, uniaxial orientation, and photoluminescent properties of CdS nanorods that form stable nanocomposites with smectic C hydrogen-bonded polymers from the family of poly(4-(n-acryloyloxyalkoxy)benzoic acids. TEM analysis of microtomed films of nanocomposites reveals that CdS nanorods form small domains that are homogeneously distributed in the LC polymer matrix. They undergo long-range orientation with the formation of one-dimensional aggregates of rods when the composite films are uniaxially deformed. The Stokes photoluminescence was observed from CdS NRs/LC polymer composites with emission peak located almost at the same wavelength as that of NRs solution in heptane. An anti-Stokes photoluminescence (ASPL) in polymer nanocomposites was found under the excitation below the nanoparticles ground state. The mechanism of ASPL was interpreted in terms of thermally populated states that are involved in the excitation process. These nanocomposites represent an unusual material in which the optical properties of anisotropic semiconductor nanostructures can be controlled by mechanical deformation of liquid-crystalline matrix.

Bleach-imaged plasmon propagation (BlIPP) in single gold nanowires.

Here, we present a novel approach to visualize propagating surface plasmon polaritons through plasmon-exciton interactions between single gold nanowires and a thin film of a fluorescent polymer. A plasmon polariton was launched by exciting one end of a single gold nanowire with a 532 nm laser. The local near-field of the propagating plasmon modes caused bleaching of the polymer emission. The degree of photobleaching along the nanowire could be correlated with the propagation distance of the surface plasmon polaritons. Using this method of bleach-imaged plasmon propagation (BlIPP), we determined a plasmon propagation distance of 1.8 +/- 0.4 mum at 532 nm for chemically grown gold nanowires. Our results are supported by finite difference time domain electromagnetic simulations.

Solution synthesis of anisotropic gold microcrystals.

Wet chemistry methods have proven to be successful for the synthesis of nanocrystals. However, the size and shape control is often lost when the dimensions of the particles exceed several hundred nanometers. Therefore, a synthetic transformation of nano- to microcrystals in solution remains to be a challenge. Here we report a very efficient chemical synthesis of anisotropic microcrystals of gold and the set of experimental conditions that enables precise control over their size and aspect ratio.

Chemical Transformation of Nanorods to Nanowires: Reversible Growth and Dissolution of Anisotropic Gold Nanostructures.

This manuscript describes a reversible wet chemical process for the tip-selective one-dimensional (1D) growth and dissolution of gold nanorods (AuNRs) and gold nanowires (AuNWs). Tip-selective dissolution was achieved by oxidation of AuNRs with a Au(III)/CTAB complex, whereas the growth of AuNRs was carried out by the reduction of Au(I) ions on the AuNR surface with a mild reducing agent, ascorbic acid (AA). Both the dissolution and growth processes are highly tip selective and proceed exclusively in one dimension. A decrease in the aspect ratio (AR) of AuNRs during the dissolution resulted in a blue shift in the longitudinal plasmon band (LPB) position, and red shifts in the LPB position were achieved by increasing the AR by 1D growth of AuNRs. Both growth and dissolution processes are fully controllable and can be stopped and resumed at any given time when the desired AR and/or LPB position is achieved. In addition, the tip-selective 1D growth of AuNRs can be continued with the additional supply of Au(I)/CTAB/AA solution to produce extremely long AuNWs.

High yield synthesis and surface chemistry exchange of small gold hexagonal nanoprisms.

A new seed-mediated synthesis of AuHNPs in high yield is described using hydroquinone as a weak reductant and poly(vinylpyrrolidone) as a shape-directing additive. We obtain distinct and small edge lengths of AuHNPs with long-term shape stability. Also, PVP enhances the monodispersity and enables the higher stability of functionalized nanoprisms.

Purification of high aspect ratio gold nanorods: complete removal of platelets.

Because physical and chemical properties of nanostructures strongly depend on their shape, it is of great importance to find either synthetic or separation techniques that can produce objects of a particular shape in a pure state. This manuscript describes a solution to a long standing problem of separating 2D platelets from 1D nanorods. The key aspect of our approach relies on the partial dissolution of faceted platelets with Au(III)/CTAB complex that transforms them into smaller nanodisks. Because of the reduction in size, the 2D structures become fully soluble in water and can be separated from the nanorods that undergo slow precipitation. In addition, the isolated nanodisks can be converted back into initial faceted platelets upon treatment with Au(I)/ascorbic acid mixture. As a result of these simple procedures, a seemingly inseparable mixture of rods, platelets, and spheres is converted into nearly pure individual components.