Short gold nanorod growth revisited: the critical role of the bromide counterion

Curcumin-capped gold nanorods as optical sensing platform for sequence specific detection of DNA based on their self-assembly

We are reporting curcumin-induced gold nanorods as an optical sensing platform for the detection of sequence-specific DNA target through their self-assembly. The combined effect of eco-friendly reducing agent (i.e., curcumin) and silver nitrate in a basic medium (i.e., pH 10) has been attributed for the formation of small gold nanorods (AuNRs) having approximate length and diameter i.e., 19.7 ± 0.8 nm and 6.0 ± 0.5 nm, respectively, and lower longitudinal surface plasmon resonance (SPR) enable to detect and analyse different biomarkers. Further, for evaluating cellular uptake of as-synthesized AuNRs, the cytotoxicity study has been carried out by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) assay on A549 cells and HEPG2 cell lines, respectively, and shown approximately similar cytotoxicity. Interestingly, as-synthesized optically and electronically active AuNRs based nanobiosensing platform enable to detect sequence-specific DNA targets with low level of detection limit i.e., LOD 8.6 ± 0.15 pM for complimentary target (CT) DNA with higher sensitivity and better selectivity. Finally, this study is suggesting a simplistic bio-mediated approach of tuning the shape and size of AuNRs for sensitive, selective and reliable nanobiosensing platform for sequence-specific DNA detection related to cancer cells.

Spectroelectrochemical Studies of CTAB Adsorbed on Gold Surfaces in Perchloric Acid

The behaviour of CTAB adsorbed on polycrystalline gold electrodes has been studied using a combination of spectroelectrochemical methods. The results indicate that the formation of the layer is the consequence of the precipitation of the CTAB micelles on the electrode surface as bromide ions, which stabilize the micelles, are replaced by perchlorate anions. This process leads to the formation of CTA⁺ layers in which perchlorate ions are intercalated, in which the adlayer suffers a continuous rearrangement that leads to the formation of micro-dominions of different types of hydrogen-bonded water populations throughout the adlayer. After prolonged cycling, a stable situation is reached. Under these conditions, water molecules permeate through the adlayer toward the electrode surface at potentials positive of the potential of zero charge, due to the repulsion between the CTA⁺ layer and the positive charge of the electrode.

Gold nanocrystals: optical properties, fine-tuning of the shape, and biomedical applications

Noble metal nanomaterials with special physical and chemical properties have attracted considerable attention in the past decades. In particular, Au nanocrystals (NCs), which possess high chemical inertness and unique surface plasmon resonance (SPR), have attracted extensive research interest. In this study, we review the properties and preparation of Au NCs with different morphologies as well as their important applications in biological detection. The preparation of Au NCs with different shapes by many methods such as seed-mediated growth method, seedless synthesis, polyol process, ultrasonic method, and hydrothermal treatment has already been introduced. In the seed-mediated growth method, the influence factors in determining the final shape of Au NCs are discussed. Au NCs, which show significant size-dependent color differences are proposed for preparing biological probes to detect biomacromolecules such as DNA and protein, while probe conjugate molecules serves as unique coupling agents with a target. Particularly, Au nanorods (NRs) have some unique advantages in the application of biological probes and photothermal cancer therapy compared to Au nanoparticles (NPs).

Effect of Bromide on the Surfactant Stabilization Layer Density of Gold Nanorods

Some studies have speculated that the concentration of bromide ions plays a crucial role in the surfactant density surrounding gold nanorods (AuNR). Small-angle X-ray and neutron scattering (SAXS and SANS) experiments were conducted to analyze any influence the bromide ions might have on the stabilization layer and the aggregation behavior of the ligand CTAB molecules in general. The AuNR were immersed in solutions containing a fixed CTA⁺ concentration of 2 mM and varying bromide ion concentrations from 0 to 22 mM. A patchy AuNR stabilization shell at low bromide ion concentrations was found, contrary to previously published SANS studies on the AuNR stabilization shell. However, with increasing bromide ion concentration, the density of the stabilization shell increases asymptotically toward a closed/collapsed bilayer configuration. AuNR grown under similar conditions show higher anisotropy with larger bromide ion concentrations. Both results indicate that anisotropic growth strongly depends on a sufficiently dense stabilization layer established by high bromide ion concentrations.

Anticoagulation and antibacterial properties of heparinized nanosilver with different morphologies

Synthesis and characterization of nanoparticles with different morphologies coupled to minimal chemical interventions for sustainable applications is one of the contemporary topics in the field of nanotechnology. In the current study, heparinized silver nanoparticles were synthesized using a chemical reduction method. Different concentrations of heparin were used to investigate its role in the stability and morphological properties of silver nanoparticles. Interestingly, it has been observed that the concentration of the stabilizing agent heparin plays a pivotal role in dictating the size and shape of the nanosilver. As visualized under a transmission electron microscope, nanosilver with different morphological states such as triangles, truncated triangles, hexagon, and spheres has been experimentally trapped. Such modular property of heparin coated nanosilver has also exhibited substantial differences in their anticoagulation and antimicrobial activities.

Improvement of Gold Nanorods in Photothermal Therapy: Recent Progress and Perspective

Cancer is a life-threatening disease, and there is a significant need for novel technologies to treat cancer with an effective outcome and low toxicity. Photothermal therapy (PTT) is a noninvasive therapeutic tool that transports nanomaterials into tumors, absorbing light energy and converting it into heat, thus killing tumor cells. Gold nanorods (GNRs) have attracted widespread attention in recent years due to their unique optical and electronic properties and potential applications in biological imaging, molecular detection, and drug delivery, especially in the PTT of cancer and other diseases. This review summarizes the recent progress in the synthesis methods and surface functionalization of GNRs for PTT. The current major synthetic methods of GNRs and recently improved measures to reduce toxicity, increase yield, and control particle size and shape are first introduced, followed by various surface functionalization approaches to construct a controlled drug release system, increase cell uptake, and improve pharmacokinetics and tumor-targeting effect, thus enhancing the photothermal effect of killing the tumor. Finally, a brief outlook for the future development of GNRs modification and functionalization in PTT is proposed.

Graphene Oxide-Coated Gold Nanorods: Synthesis and Applications

The application of gold nanorods (AuNRs) and graphene oxide (GO) has been widely studied due to their unique properties. Although each material has its own challenges, their combination produces an exceptional material for many applications such as sensor, therapeutics, and many others. This review covers the progress made so far in the synthesis and application of GO-coated AuNRs (GO-AuNRs). Initially, it highlights different methods of synthesizing AuNRs and GO followed by two approaches (ex situ and in situ approaches) of coating AuNRs with GO. In addition, the properties of GO-AuNRs composite such as biocompatibility, photothermal profiling, and their various applications, which include photothermal therapy, theranostic, sensor, and other applications of GO-AuNRs are also discussed. The review concludes with challenges associated with GO-AuNRs and future perspectives.

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.

Effective replacement of cetyltrimethylammonium bromide (CTAB) by mercaptoalkanoic acids on gold nanorod (AuNR) surfaces in aqueous solutions

The highly packed cetyltrimethylammonium bromide (CTAB) bilayer built up on the surface of gold nanorods (AuNRs) when synthesized by the seed-mediated procedure hampers the complete ligand exchange under experimental conditions that preserves the stability of the dispersions. In the present work, a ligand exchange protocol by using carboxy-terminated alkanethiols of different chain lengths by means of a green approach that uses only aqueous solutions is presented. The protocol is based on the knowledge of the stability in the aqueous solution of both the starting CTAB-AuNRs and the final products that help in the choice of the experimental conditions used for ligand exchange. The characterization of the CTAB protective layer as well as the study of its colloidal stability in solution has helped us to design an appropriate methodology. Cyclic voltammetry of CTAB-AuNRs demonstrates the high stability of the bilayer showing the existence of a two-dimensional phase transition from a highly ordered to a less organized phase. Other techniques such as XPS, FT-IR and Raman spectroscopy provide information about the structure of the layer and UV-visible-NIR spectroscopy establishes the stability conditions in aqueous solution. We have chosen an exchange procedure for 11-mercaptoundecanoic acid (MUA) and 16-mercaptohexadecanoic acid (MHDA) based on a one-pot methodology under conditions where all the species involved are stable. The protocol, however, can be extended to different chemical functionalities that are considered useful to be applied in living systems. Under these conditions the complete exchange of CTAB by the mercaptoderivatives was successful as demonstrated by the different characterization techniques used: UV-visible-NIR, FT-IR, Raman, XPS spectroscopy, cyclic voltammetry and transmission electron microscopy (TEM).

Gold nanorods: from anisotropy to opportunity. An evolution update

Gold nanoparticles have drawn attention to nanomedicine for many years due to their physicochemical properties, which include: good stability; biocompatibility; easy surface chemistry and superior magnetic; and last, electronic properties. All of these properties distinguish gold nanoparticles as advantageous carriers to be exploited. The challenge to develop new gold nanostructures has led to anisotropy, a new property to exploit for various medical applications: diagnostic and imaging strategies as well as therapeutic options. Gold nanorods are the most studied anisotropic gold nanoparticles because of the presence of two absorption peaks according to their longitudinal and transversal plasmon resonances. The longitudinal surface plasmonic resonance can provide the absorption in the near-infrared region and this is an important aspect of using gold nanorods for medical purposes.

Controlled etching and tapering of Au nanorods using cysteamine

While gold nanorods (AuNRs) have found many applications due to their unique optical properties, a few challenges persist in their synthesis. Namely, it is often difficult to reproducibly synthesize AuNRs with specific and monodisperse sizes, especially at shorter aspect ratios. Here, we report a method of post-synthesis precise tailoring of AuNRs by etching with cysteamine. Cysteamine selectively etches AuNRs from their ends while preserving the initial rod shape and monodispersity, making this a viable means of obtaining highly monodisperse short AuNRs down to aspect ratio 2.3. Further, we explore the effect of this etching method on two types of silica-coated AuNRs: silica side-coated and silica end-coated AuNRs. We find that the etching process is cysteamine concentration-dependent and can lead to different degrees of sharpening of the silica-coated AuNRs, forming elongated tips. We also find that cysteamine behaves only as a ligand at concentrations above 200 mM, as no etching of the AuNRs is observed in this condition. Simulations show that excitation of plasmon resonances in these sharpened AuNRs produces local electric fields twice as strong as those produced by conventional AuNRs. Thus, cysteamine etching of AuNRs is shown to be an effective means of tailoring both the size and shape of AuNRs along with their corresponding optical properties. At the same time, the resulting cysteamine coating on the etched AuNRs displays terminal amino groups that allow for further functionalization of the nanorods.

Growing gold nanostructures for shape-selective cellular uptake

With development in the synthesis of shape- and size-dependent gold (Au) nanostructures (NSs) and their applications in nanomedicine, one of the biggest challenges is to understand the interaction of these shapes with cancer cells. Herein, we study the interaction of Au NSs of five different shapes with glioblastoma-astrocytoma cells. Three different shapes (nanorods, tetrahexahedra, and bipyramids), possessing tunable optical properties, have been synthesized by a single-step seed-mediated growth approach employing binary surfactant mixtures of CTAB and a secondary surfactant. By the use of two-step seed-mediated approach, we obtained new NSs, named nanomakura (Makura is a Japanese word used for pillow) which is reported for the first time here. Spherical Au nanoparticles were prepared by the Turkevich method. To study NS-cell interactions, we functionalized the NSs using thiolated PEG followed by 11-Mercaptoundecanoic acid. The influence of shape and concentration of NSs on the cytotoxicity were assessed with a LIVE/DEAD assay in glioblastoma-astrocytoma cells. Furthermore, the time-dependent uptake of nanomakura was studied with TEM. Our results indicate that unlike the other shapes studied here, the nanomakura were taken up both via receptor-mediated endocytosis and macropinocytosis. Thus, from our library of different NSs with similar surface functionality, the shape is found to be an important parameter for cellular uptake.

The Many "Facets" of Halide Ions in the Chemistry of Colloidal Inorganic Nanocrystals

Over the years, scientists have identified various synthetic "handles" while developing wet chemical protocols for achieving a high level of shape and compositional complexity in colloidal nanomaterials. Halide ions have emerged as one such handle which serve as important surface active species that regulate nanocrystal (NC) growth and concomitant physicochemical properties. Halide ions affect the NC growth kinetics through several means, including selective binding on crystal facets, complexation with the precursors, and oxidative etching. On the other hand, their presence on the surfaces of semiconducting NCs stimulates interesting changes in the intrinsic electronic structure and interparticle communication in the NC solids eventually assembled from them. Then again, halide ions also induce optoelectronic tunability in NCs where they form part of the core, through sheer composition variation. In this review, we describe these roles of halide ions in the growth of nanostructures and the physical changes introduced by them and thereafter demonstrate the commonality of these effects across different classes of nanomaterials.

Morphological control of gold nanorods via thermally driven bi-surfactant growth and application for detection of heavy metal ions

We report a modified synthesis route of colloidal gold nanorods (AuNRs) by combining the thermal re-shaping treatment and bi-surfactant modification using hexadecyltrimethylammonium bromide (CTAB) and sodium oleate (NaOL). Aspect ratios down to 1.3 ± 0.1 can be achieved in addition to good monodispersity, uniformity, and chemical stability of the materials. Furthermore, without needing post-treatment, metal ions directly interact with the AuNRs efficiently, allowing rapid and sensitive colorimetric detection of heavy metal ions such as Pb²⁺ and Cu²⁺ with a low concentration down to 2.5 μM. The detection performance in terms of selectivity, sensitivity and stability is systematically evaluated. The AuNRs with tunable aspect ratios as well as chemical stability have potential in surface-plasmon-based applications such as biochemical sensing, biochemical imaging, medical diagnostics, and cancer therapy.

Effect of Cationic Surfactants with Different Counterions on the Growth of Au Nanoclusters

The influence of a series of cationic surfactants composed of cetyltrimethylammonium cations with different counterions (Br^-, Cl^-, OH^-, C₇H₈O₃S^-, [CeCl₃Br]^-, and NO₃^-) on the aging process of gold nanoclusters (Au NCs) was studied. The finely different points of Au NCs treated by different surfactants were demonstrated by UV-vis and fluorescence spectra, transmission electron microscopy images, etc. Because of the difference of counterions, these surfactants have diverse physicochemical properties in surface activity, specific conductivity, pH, and viscosity, which may account for the difference of Au NCs in the aging process. In addition, the affinity of the counterions in surfactants to the surface of Au has also been demonstrated completely. This affinity may further guide the difference of the synthesized Au nanomaterials.

Silver-Assisted Synthesis of Gold Nanorods: the Relation between Silver Additive and Iodide Impurities

Seed-mediated methods employing cetyltrimethylammonium bromide (CTAB) as a surfactant, and silver salts as additives, are the most common synthetic strategies for high-yield productions of quality Au nanorods. However, the mechanism of these reactions is not yet fully understood and, importantly, significant lab-to-lab reproducibility issues still affect these protocols. In this study, the direct correlation between the hidden content of iodide impurities in CTAB reagents, which can drastically differ from different suppliers or batches, and the optimal concentration of silver required to maximize the nanorods yield is demonstrated. As a result, high-quality nanorods are obtained at different iodide contents. These results are interpreted based on the different concentrations of CTAB and cetyltrimethylammonium iodide (CTAI) complexes with Ag⁺ and Au⁺ metal ions in the growth solution, and their different binding affinity and reduction potential on distinct crystallographic planes. Notably, the exhaustive conversion of CTAI-Au⁺ to CTAI-Ag⁺ appears to be the key condition for maximizing the nanorod yield.

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.

Efficient seed-mediated method for the large-scale synthesis of Au nanorods

Seed-mediated methods are widely followed for the synthesis of Au nanorods (NRs). However, mostly dilute concentrations of the Au precursor (HAuCl₄) are used in the growth solution, which leads to a low final concentration of NRs. Attempts of increasing the concentration of NRs by simply increasing the concentration of HAuCl₄, other reagents in the growth solution and seeds lead to a faster growth kinetics which is not favourable for NR growth. Herein, we demonstrate that the increase in growth kinetics for high concentrations of reagents in growth solution can be neutralised by decreasing the pH of the solution. The synthesis of the NRs can be scaled up by using higher concentrations of reagents and adding an optimum concentration of HCl in the growth solution. The concentration of HAuCl₄ in the growth solution can be increased up to 5 mM, and 10-20 times more NRs can be synthesised for the same reaction volume compared to that of the conventional seed-mediated method. We have also noticed that a cetyltrimethylammonium bromide (CTAB)-to-HAuCl₄ molar ratio of 50 is sufficient for obtaining high yield of NRs.

Seedless Synthesis of Monodispersed Gold Nanorods with Remarkably High Yield: Synergistic Effect of Template Modification and Growth Kinetics Regulation

Gold nanorods (AuNRs) are versatile materials due to their broadly tunable optical properties associated with their anisotropic feature. Conventional seed-mediated synthesis is, however, not only limited by the operational complexity and over-sensitivity towards subtle changes of experimental conditions but also suffers from low yield (≈15 %). A facile seedless method is reported to overcome these challenges. Monodispersed AuNRs with high yield (≈100 %) and highly adjustable longitudinal surface plasmon resonance (LSPR) are reproducibly synthesized. The parameters that influence the AuNRs growth were thoroughly investigated in terms of growth kinetics and soft-template regulation, offering a better understanding of the template-based mechanism. The facile synthesis, broad tunability of LSRP, high reproducibility, high yield, and ease of scale-up make this method promising for the future mass production of monodispersed AuNRs for applications in catalysis, sensing, and biomedicine.

Anisotropic Nanoparticles and Anisotropic Surface Chemistry

Anisotropic nanoparticles are powerful building blocks for materials engineering. Unusual properties emerge with added anisotropy-often to an extraordinary degree-enabling countless new applications. For bottom-up assembly, anisotropy is crucial for programmability; isotropic particles lack directional interactions and can self-assemble only by basic packing rules. Anisotropic particles have long fascinated scientists, and their properties and assembly behavior have been the subjects of many theoretical studies over the years. However, only recently has experiment caught up with theory. We have begun to witness tremendous diversity in the synthesis of nanoparticles with controlled anisotropy. In this Perspective, we highlight the synthetic achievements that have galvanized the field, presenting a comprehensive discussion of the mechanisms and products of both seed-mediated and alternative growth methods. We also address recent breakthroughs and challenges in regiospecific functionalization, which is the next frontier in exploiting nanoparticle anisotropy.

Shape control of inorganic nanoparticles from solution

Inorganic materials with controllable shapes have been an intensely studied subject in nanoscience over the past decades. Control over novel and anisotropic shapes of inorganic nanomaterials differing from those of bulk materials leads to unique and tunable properties for widespread applications such as biomedicine, catalysis, fuels or solar cells and magnetic data storage. This review presents a comprehensive overview of shape-controlled inorganic nanomaterials via nucleation and growth theory and the control of experimental conditions (including supersaturation, temperature, surfactants and secondary nucleation), providing a brief account of the shape control of inorganic nanoparticles during wet-chemistry synthetic processes. Subsequently, typical mechanisms for shape-controlled inorganic nanoparticles and the general shape of the nanoparticles formed by each mechanism are also expounded. Furthermore, the differences between similar mechanisms for the shape control of inorganic nanoparticles are also clearly described. The authors envision that this review will provide valuable guidance on experimental conditions and process control for the synthesis of inorganic nanoparticles with tunable shapes in the solution state.

Silver-Overgrowth-Induced Changes in Intrinsic Optical Properties of Gold Nanorods: From Noninvasive Monitoring of Growth Kinetics to Tailoring Internal Mirror Charges

We investigate the effect of surfactant-mediated, asymmetric silver overgrowth of gold nanorods on their intrinsic optical properties. From concentration-dependent experiments, we established a close correlation of the extinction in the UV/vis/NIR frequency range and the morphological transition from gold nanorods to Au@Ag cuboids. Based on this correlation, a generic methodology for in situ monitoring of the evolution of the cuboid morphology was developed and applied in time-dependent experiments. We find that growth rates are sensitive to the substitution of the surfactant headgroup by comparison of benzylhexadecyldimethylammonium chloride (BDAC) with hexadecyltrimethylammonium chloride (CTAC). The time-dependent overgrowth in BDAC proceeds about 1 order of magnitude slower than in CTAC, which allows for higher control during silver overgrowth. Furthermore, silver overgrowth results in a qualitatively novel optical feature: Upon excitation inside the overlap region of the interband transition of gold and intraband of silver, the gold core acts as a retarding element. The much higher damping of the gold core compared to the silver shell in Au@Ag cuboids induces mirror charges at the core/shell interface as shown by electromagnetic simulations. Full control over the kinetic growth process consequently allows for precise tailoring of the resonance wavelengths of both modes. Tailored and asymmetric silver-overgrown gold nanorods are of particular interest for large-scale fabrication of nanoparticles with intrinsic metamaterial properties. These building blocks could furthermore find application in optical sensor technology, light harvesting, and information technology.

One-pot synthesis of long twin gold nanorods in a gelled surfactant solution

A new one-pot synthesis method was developed for producing long twin gold nanorods with lengths of ca. 1755 nm, which have the potential to be arranged on a substrate using anisotropic surface modification.

EPR and rheological study of hybrid interfaces in gold-clay-epoxy nanocomposites

With the aim to obtain new materials with special properties to be used in various industrial and biomedical applications, ternary "gold-clay-epoxy" nanocomposites and their nanodispersions were prepared using clay decorated with gold nanoparticles (AuNPs), at different gold contents. Nanocomposites structure was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Rheology and electron paramagnetic resonance (EPR) techniques were used in order to evaluate the molecular dynamics in the nanodispersions, as well as dynamics at interfaces in the nanocomposites. The percolation threshold (i.e., the filler content related to the formation of long-range connectivity of particles in the dispersed media) of the gold nanoparticles was determined to be ϕp = 0.6 wt % at a fixed clay content of 3 wt %. The flow activation energy and the relaxation time spectrum illustrated the presence of interfacial interactions in the ternary nanodispersions around and above the percolation threshold of AuNPs; these interfacial interactions suppressed the global molecular dynamics. It was found that below ϕp the free epoxy polymer chains ratio dominated over the chains attracted on the gold surfaces; thus, the rheological behavior was not significantly changed by the presence of AuNPs. While, around and above ϕp, the amount of the bonded epoxy polymer chains on the gold surface was much higher than that of the free chains; thus, a substantial increase in the flow activation energy and shift in the spectra to higher relaxation times appeared. The EPR signals of the nanocomposites depended on the gold nanoparticle contents and the preparation procedure thus providing a fingerprint of the different nanostructures. The EPR results from spin probes indicated that the main effect of the gold nanoparticles above ϕp, was to form a more homogeneous, viscous and polar clay-epoxy mixture at the nanoparticle surface. The knowledge obtained from this study is applicable to understand the role of interfaces in ternary nanocomposites with different combinations of nanofillers.

Large-Scale Synthesis of Gold Nanorods through Continuous Secondary Growth

Gold nanorods (GNRs) exhibit a tunable longitudinal surface plasmon resonance (LSPR) that depends on the GNR aspect ratio (AR). Independently controlling the AR and size of GNRs remains challenging but is important because the scattering intensity strongly depends on the GNR size. Here, we report a secondary (seeded) growth procedure, wherein continuous addition of ascorbic acid (AA) to a stirring solution of GNRs, stabilized by cetyltrimethylammonium bromide (CTAB) and synthesized by a common GNR growth procedure, deposits the remaining (~70%) of the Au precursor onto the GNRs. The growth phase of GNR synthesis is often performed without stirring, since stirring has been believed to reduce the yield of rod-shaped nanoparticles, but we report that stirring coupled with continuous addition of AA during secondary growth allows improved control over the AR and size of GNRs. After a common primary GNR growth procedure, the LSPR of GNRs is ~820 nm, which can be tuned between ~700-880 nm during secondary growth by adjusting the rate of AA addition or adding benzyldimethylhexadecylammonium chloride hydrate (BDAC). This approach for secondary growth can also be used with primary GNRs of different ARs to achieve different LSPRs and can likely be extended to nanoparticles of different shapes and other metals.

Effects of surfactant concentration on formation of high-aspect-ratio gold nanorods

The effects of surfactant concentration in a growth solution on the elongation of gold nanorods were examined. Gold nanorods were synthesized in solutions with different concentrations of hexadecyltrimethylammonium bromide (HTAB): 100, 200, 300, 400, 500, and 600 mM. The nanorods grown in a solution with higher surfactant concentrations were longer (aspect ratio ~30) than those grown in that with lower concentrations (aspect ratio <10). The self-assembled surfactant structures in the solutions were analyzed using viscosity measurement and small-angle X-ray scattering. These results showed a decrease in the inter-micellar distance with increasing surfactant concentration. Taking the chemical equilibrium for the complex formation between Au ions and HTAB micelles into account, we found that the free Au ion concentration decreases accompanied with the increase in the surfactant concentration. This decrease in the free Au ion concentration suppresses undesirable secondary nucleation of gold crystals in a growth solution, resulting in gold nanorod elongation.

A simple millifluidic benchtop reactor system for the high-throughput synthesis and functionalization of gold nanoparticles with different sizes and shapes

Despite the continuing interest in the applications of functionalized nanomaterials, the controlled and reproducible synthesis of many important functionalized nanoparticles (NPs) above the milligram scale continues to be a significant challenge. The synthesis of functionalized NPs in automated reactors provides a viable approach to circumvent some of the shortcomings of traditional nanomaterial batch syntheses, providing superior control over reagent addition, improved reproducibility, the opportunity to interface real-time product monitoring, and a viable high-throughput synthetic approach. Here, we demonstrate the construction and operation of a simple millifluidic reactor assembled entirely from commercially available components found in almost any chemical laboratory. This reactor facilitates the aqueous gram-scale synthesis of a variety of functionalized gold nanoparticles, including the synthesis of gold nanospheres with tightly controlled core diameters and gold nanorods with controlled aspect ratios between 1.5 and 4.0. The absolute dimensions (i.e., the transverse diameter) of gold nanorods synthesized within the reactor can also be tailored to produce different gold nanorod shapes, including "small" gold nanorods and gold nanocubes. In addition, we show that the reactor can interface with existing purification and monitoring techniques in order to enable the high-throughput functionalization/purification of gold nanorods and real-time monitoring of gold nanoparticle products for quality control. We anticipate that this millifluidic reactor will provide the blueprint for a versatile and portable approach to the gram-scale synthesis of monodisperse, hydrophilically functionalized metal NPs that can be realized in almost any chemistry research laboratory.

Seeded growth of monodisperse gold nanorods using bromide-free surfactant mixtures

We demonstrate for the first time that monodisperse gold nanorods (NRs) with broadly tunable dimensions and longitudinal surface plasmon resonances can be synthesized using a bromide-free surfactant mixture composed of alkyltrimethylammonium chloride and sodium oleate. It is found that uniform gold NRs can be obtained even with an iodide concentration approaching 100 μM in the growth solution. In contrast to conventional wisdom, our results provide conclusive evidence that neither bromide as the surfactant counterion nor a high concentration of bromide ions in the growth solution is essential for gold NR formation. Correlated electron microscopy study of three-dimensional structures of gold NRs reveals a previously unprecedented octagonal prismatic structure enclosed predominantly by high index {310} crystal planes. These findings should have profound implications for a comprehensive mechanistic understanding of seeded growth of anisotropic metal nanocrystals.

Using binary surfactant mixtures to simultaneously improve the dimensional tunability and monodispersity in the seeded growth of gold nanorods

We report a dramatically improved synthesis of colloidal gold nanorods (NRs) using a binary surfactant mixture composed of hexadecyltrimethylammonium bromide (CTAB) and sodium oleate (NaOL). Both thin (diameter <25 nm) and thicker (diameter >30 nm) gold NRs with exceptional monodispersity and broadly tunable longitudinal surface plasmon resonance can be synthesized using seeded growth at reduced CTAB concentrations (as low as 0.037 M). The CTAB-NaOL binary surfactant mixture overcomes the difficulty of growing uniform thick gold NRs often associated with the single-component CTAB system and greatly expands the dimensions of gold NRs that are accessible through a one-pot seeded growth process. Gold NRs with large overall dimensions and thus high scattering/absorption ratios are ideal for scattering-based applications such as biolabeling as well as the enhancement of optical processes.

Functional gold nanorods: synthesis, self-assembly, and sensing applications

Gold nanorods have received much attention due to their unique optical and electronic properties which are dependent on their shape, size, and aspect ratio. This article covers in detail the synthesis, functionalization, self-assembly, and sensing applications of gold nanorods. The synthesis of three major types of rods is discussed: single-crystalline and pentahedrally-twinned rods, which are synthesized by wet chemistry methods, and polycrystalline rods, which are synthesized by templated deposition. Functionalization of these rods is usually necessary for their applications, but can often be problematic due to their surfactant coating. Thus, general strategies are provided for the covalent and noncovalent functionalization of gold nanorods. The review will then examine the significant progress that has been made in controllable assembly of nanorods into various arrangements. This assembly can have a large effect on measurable properties of rods, making it particularly applicable towards sensing of a variety of analytes. Other types of sensing not dependent on nanorod assembly, such as refractive-index based sensing, are also discussed.

Tuning the size of gold nanoparticles in the citrate reduction by chloride ions

A new approach was developed to control the size of gold nanoparticles in citrate reduction by altering the concentration of chloride ions. The size of the as-prepared gold nanoparticles could be tuned in the range 19-47 nm at a specific molar ratio of citrate and tetrachloroauric acid (5:1) by simply changing the concentration of chloride ions from 0 to 20 mM. UV-visible spectra and TEM observations showed that the increased size of the gold particles was primarily related to the promoted aggregation of the primary gold particles. The aggregation was attributed to their decreased surface charge as the chloride ion concentration in the reaction solutions increased. This approach could also be extended to other reaction systems, for example, the size of gold nanoparticles prepared by NaBH(4) reduction increased from 3 to 12 nm as the chloride ion concentration was increased from 0 to 20 mM.