Monodisperse Gold Nanorods Research Articles

Spatiotemporally controlled emergence of nanoparticle microvortices under electric field

AbstractControlled assembly of nanoparticles (NPs) has garnered much interest over the past two decades. Beyond established techniques, new methods utilizing local short‐range or large‐scale long‐range interactions remain to be explored to achieve diverse micro‐ and nanoscale structures. Here, we report the controlled emergence of vortex‐pair arrays within monodispersed gold nanorods by applying a direct current electric field across a pair of sawtooth electrodes. By employing in situ darkfield microscopy and particle collective analysis, we elucidate the mechanism behind the formation and stabilization of the NP vortices, attributing it to the combined effects of the electrode shape, high NP density, and high solution viscosity. We further explore the controllability of the vortex‐pair arrays and obtain multiple complex vortice patterns. Our findings will facilitate the investigation of efficient and controlled dynamic assembly of NPs under external fields and help manufacture next‐generation optoelectronic functional materials.

Microfluidic device-based synthesis of highly monodispersed dumbbell-shaped gold nanorods and their biocompatibility evaluation

Anisotropic gold nanomaterials have always fascinated the scientific community; however, controlling their synthesis to achieve desired properties and monodispersity is challenging in a conventional batch reactor. Here, we report the synthesis of highly monodispersed dumbbell-shaped gold nanorods (Ds-Au NRs) in a droplet microfluidic device. The device comprises a tapered T-junction for reagent droplet production, a winding structure to promote nucleation, and lengthy parallel microchannels to support the slow growth required for dumbbell shape morphology. After optimising the flow parameters and device geometry for uniform droplet production, the aspect ratio of Ds-Au NRs was tuned by controlling the total reagent flow rate, reagent flow rate ratio, and concentration of shaping reagent and reducing agent. Compared to batch counterparts, reaction time was reduced to one-fifth and revealed higher monodispersity. The cytotoxicity analysis showed that microfluidic synthesised Ds-Au NRs are non-toxic and have great potential for usage in biomedical applications.

Controlled synthesis of monodisperse gold nanorods with a small diameter of around 10 nm and largest plasmon wavelength of 1200 nm.

Gold nanorods have been widely used in various fields due to their tunable anisotropic localized surface plasmon resonance (SPR) property. The facile preparation of gold nanorods with a tunable SPR wavelength extending to a near-infrared window, and at the same time, a relatively small particle size for facilitating applications especially in the biomedical field is of great value yet highly challenging. In this work, a new reducing agent, 1,6-dihydroxynaphthalene, is proposed for the synthesis of gold nanorods. The results indicate that gold nanorods with good monodispersity, high shape yield, maximum SPR wavelength of 1200 nm, and especially small diameter of around 10 nm can be acquired simultaneously. In terms of spectral and size controls, by respectively varying the experimental parameters including the amount of silver ions, reducing agents, and gold seeds not only can a good linear correlation be acquired corresponding to a SPR wavelength ranging from around 600 nm to 1200 nm, but a regular change in the particle diameter from 10.5 nm to 7.5 nm could also be observed. The structural and morphological evolutions of the particle for each changed parameter were carefully studied, and insights were gained into the growth mechanism based on the detailed analysis of particle evolution at a specific stage of the growth process.

PH-mediated synthesis of monodisperse gold nanorods with quantitative yield and molecular level insight

Although gold nanorods (GNRs) have been produced with different dimensions and aspect ratios, the current synthesis methods through seed-mediated growth are far from ideal, for instance, the quality (rod yield) and the quantity (gold conversion) cannot be simultaneously satisfied. More critically, there is no molecular level understanding of the growth mechanism. Here, we solved the problem by employing the stoichiometric ratio of reactants and tuning the reactivity of the reductant through adjusting the initial pH value of the growth solution to achieve both good quality and high quantity simultaneously. We also extended our strategy to other enols besides ascorbic acid, such as phenolic compounds, and found that the optimal pH for GNRs synthesis depends on the structure of the individual compound. The mechanistic insight greatly enriches the toolbox of reductants for GNRs growth and makes it possible to synthesize GNRs at both acidic and basic conditions. An interesting phenomenon is that for most of the phenolic compounds we tested, the morphology of the final products follows the same sphere-rod-sphere trend as the initial pH value of the reaction increases, whether it is under acidic or basic conditions, which cannot be explained by any previously proposed mechanism. The effect of pH is mainly attributed to the regulation of the reduction potential of the reductants, and thus the reaction rate. A model has been proposed to explain the dependence of anisotropic growth of GNRs on the concentration gradient of reactants around the seeds, which is decided by both the reaction rate and diffusion rate.

A novel electrochemical sensing platform based on mono-dispersed gold nanorods modified graphene for the sensitive determination of topotecan

In this research, we developed a novel design to fabricate a sensitive and selective electrochemical sensor based on high-quality graphene decorated with mono-dispersed gold nanorods (AuNRDs) for the determination of topotecan (TPC) in human blood serum. For this purpose, the screen-printed electrode (SPE) was firstly modified with graphene prepared by metal intercalation process (MI-GR), and then followed by the decoration of mono-dispersed AuNRDs synthesized by seed mediated growth method. The synthesized MI-GR and AuNRDs samples were characterized by Raman, XPS, UV-Vis and TEM techniques. Electrochemical studies indicated that the electrocatalytic activity of MI-GR has superior than that of chemically reduced graphene (CR-GR) prepared by conventional approach due to its faster electron transfer ability. Moreover, AuNRDs/MI-GR modified SPE (AuNRDs/MI-GR/SPE) demonstrated a wide linearity in the range of 0.1 – 16.0 μM with a low detection limit of 22.0 nM. The analytical reliability of AuNRDs/MI-GR/SPE was successfully confirmed by the detection of TPC in human blood serum with the satisfactory recoveries at the range of 99.8–100.07%. AuNRDs/MI-GR/SPE could be utilized as an alternative analytical tool for the determination of anti-cancer agents such as TPC in the next generation clinical applications.

Kinetic and plasmonic properties of gold nanorods adsorbed on glass substrates

Monodisperse gold nanorods with different sizes were synthesized and adsorbed on chemically modified glass substrates. Influence of surfactant molar concentration on nanorod adsorption was studied and the optimum range was determined. During substrate coverages we monitored the growth of longitudinal localized surface plasmon resonances at short times due to density increase of isolated nanorods and, at longer times, their subsequent decrease and a concurrent growth of coupling resonances owing to nanoparticle surface mobility and aggregation. Temporal evolution of amplitudes of resonance peaks in extinction spectra and nanorod counting statistics in electron micrographs were used to model both coverage and aggregation processes, as exponential-like functions of time. Their characteristic times and saturation values were analyzed and related to kinetic parameters, nanorod dimensions and extinction coefficients. This work can be used as a predictive tool to prepare plasmonic substrates with desired optical resonances.

Structural transitions and bilayer formation of CTAB aggregates

Controlled CTAB self-assembly is an essential prerequisite for the formation of gold nanorods with tailored shape and monodispersity. In this paper, we exploit the use of salt concentration and co-surfactant decanol for the preparation of CTAB aggregates with different morphologies. To this end we use a model of CTAB recently developed by ourselves, and perform electronic structure calculations in order to improve current point charge parametrization of decanol. Using molecular dynamics simulations with the new models, we find a sequence of structural transitions of CTAB aggregates induced by salt concentration and added cosurfactant. In pure solutions, CTAB aggregates form spherical micelles with a compact ionic shell and a diffuse double layer that can be qualitatively described with Poisson–Boltzmann theory. Addition of decanol as a cosurfactant induces a sequence of dramatic structural transitions at low surfactant concentration and allows the stabilization of compact ordered bilayers in a well defined range of intermediate decanol/CTAB ratios. At low and high decanol/CTAB ratios spherical micelles are transformed into wormlike cylindrical micelles. At intermediate decanol/CTAB ratios, fully formed bilayers are observed, with surfactants exhibiting a compact structure with strong positional and orientational order. We discuss how the controlled self-assembly of compact CTAB bilayers at low global CTAB concentration can pave the way for the selective passivation of gold facets and the controlled formation of monodisperse gold nanorods.

Monodisperse Gold Nanorods for High-Pressure Refractive Index Sensing.

The effects of hydrostatic pressure on the surface plasmon resonances (SPRs) of aqueous dispersions of monodisperse gold nanorods (AuNRs)were determined up to 9 GPa. The ultranarrow longitudinal SPR band of monodisperse AuNRs allows us to monitor a gradual red shift with pressure, which shows abrupt jumps at the liquid to ice phase VI and ice phase VII transitions. Despite solidifying at low pressure (∼1.8 GPa), water displays a regime of quasi-hydrostaticity in said phases VI and VII, up to ca. 5 GPa. Above this pressure, nonhydrostatic effects manifest themselves through broadening of the SPR bands, but barely any effect is observed on the position of the surface plasmon mode. The variation in the SPR peak wavelength with pressure allowed us to determine the pressure dependence of the refractive index of water. Unlike Brillouin scattering or interferometric techniques, this plasmon-spectroscopy-based method leads to a more direct determination of the refractive index, which is well described empirically by Murnaghan-type equations in the three explored phases. We report herein the obtained analytical functions providing the pressure dependence of refractive index in the liquid, ice VI, and ice VII phases of water.

High-yield synthesis of monodisperse gold nanorods with a tunable plasmon wavelength using 3-aminophenol as the reducing agent.

Facile synthesis of high quality gold nanorods (AuNRs) with a tunable size is of great value for applications of AuNRs in various fields and for the study of the growth mechanism of such anisotropic nanostructures. However, limitations usually exist in a specific synthetic protocol. In this work, using 3-aminophenol as the reducing agent, we present a AuNR synthetic strategy with an excellent comprehensive performance, which includes an exceptional monodispersity, a AuNR shape purity of around 99%, a conversion ratio of the gold precursor of about 91%, and an easily tuned longitudinal surface plasmon resonance wavelength ranging from 580 to ∼1050 nm. Studies on the impacts of the experimental parameters including silver ions, gold seeds, reducing agent, and cetyltrimethylammonium bromide (CTAB) revealed a profound recognition of the significant effect of the reductive atmosphere, in synergy with other parameters, in directing the growth and structural evolution of the gold seeds, thus deeply affecting the size, shape yield, monodispersity, and morphology of the final structure. These results could be immensely useful for the application and revelation of the growth mechanism of AuNRs.

Chloride ions assisted synthesis of tunable gold nanorods: Seedless synthesis, characterization and in vitro toxicity studies

Gold nanorods are interesting due to their capability to absorb light in the near infrared (NIR) wavelength range of 600 nm–1200 nm which is ideal for optical therapeutic imaging. Previous reports in synthesis used cetyltrimethylammonium bromide (CTAB) as capping agent via two-step seed-mediated method done in a laborious manner and major disadvantage is batch to batch variations. To address this issue, we have developed a novel synthetic procedure of synthesizing gold nanorods using cetyltrimethylammonium chloride (CTAC) via seedless, one-step method. The seedless method enables rapid formation of monodisperse gold nanorods with tunable aspect ratios (2–4). Gold nanorods have average lengths in range of 40 nm–55 nm and width of 12 nm–18 nm. Characterization of gold nanorods was done using optical spectroscopy, zeta potential, NTA analysis, FTIR and electron microscopy. Toxicity evaluation of the synthesized CTAC capped gold nanorods was studied on liver carcinoma cell line using MTT assay, Propidium Iodide and Trypan Blue Exclusion assay. The biocompatibility of gold nanorods was evaluated on liver cancer cell line (HepG2) in order to determine their safety using various cytotoxicity assays. We observed a size dependent toxicity of gold nanorods when exposed to liver cancer cells.

Dual Applicability of Polyaniline Coated Gold Nanorods: A Study of Antibacterial and Redox Activity

Non-agglomerated, dual-purpose monodispersed gold nanorods (GNRs) were synthesized by using a seed-mediated method, and subsequent in situ chemical polymerization yielded polyaniline-coated gold nanorods (PANI-GNRs). The synthesized GNRs and PANI-GNRs were characterized by electron microscopic analyses, selected-area electron diffraction patterns, and element analyses. The prepared nanomaterials had an average length of 40±0.4 nm and a diameter of 15±0.2 nm. Furthermore, the PANI coating around the GNRs had a thickness of ~21 nm. These nanomaterials were also characterized by ultraviolet- visible and Fourier transform-infrared spectroscopies. While the absorption peaks of GNRs were observed at 520 and 675 nm, those of PANI-GNRs showed absorption maxima at 325 and 665 nm. The combination of 0.91 vol% of aniline with 5 mL of GNRs provided better electrochemical properties at 0.52 V and 1.30 V. The minimum inhibitory concentrations of laser-irradiated PANI-GNRs, PANI-GNRs, and laser-irradiated GNRs against Escherichia coli and Staphylococcus aureus were determined by using the micro-dilution method and compared to those of kanamycin (standard drug), revealing the significant bactericidal activity of laser-irradiated PANI-GNRs. This activity of the PANI-GNRs was also supported by the results of fluorescence and atomic force microscopy imaging. Thus, the synthesized nanomaterials can be potentially utilized for waste water treatment and biomedical/ pharmaceutical applications.

Highly Concentrated Seed-Mediated Synthesis of Monodispersed Gold Nanorods

The extremely large optical extinction coefficient of gold nanorods (Au-NRs) enables their use in a diverse array of technologies, rnging from plasmonic imaging, therapeutics and sensors, to large area coatings, filters, and optical attenuators. Development of the latter technologies has been hindered by the lack of cost-effective, large volume production. This is due in part to the low reactant concentration required for symmetry breaking in conventional seed-mediated synthesis. Direct scale up of laboratory procedures has limited viability because of excessive solvent volume, exhaustive postsynthesis purification processes, and the generation of large amounts of waste (e.g., hexadecyltrimethylammonium bromide(CTAB)). Following recent insights into the growth mechanism of Au-NRs and the role of seed development, we modify the classic seed-mediated synthesis via temporal control of seed and reactant concentration to demonstrate production of Au-NRs at more than 100-times the conventional concentration, while maintaining independent control and narrow distribution of nanoparticle dimensions, aspect ratio, and volume. Thus, gram scale synthesis of Au-NRs with prescribed aspect ratio and volume is feasible in a 100 mL reactor with 1/100th of organic waste relative to conventional approaches. Such scale-up techniques are crucial to cost-effectively meet the increased demand for large quantities of Au-NRs in emerging applications.

Synthesis of less toxic gold nanorods by using dodecylethyldimethylammonium bromide as an alternative growth-directing surfactant

Gold nanoparticles having a rod-like morphology are regularly investigated as potential nano-therapeutic materials owing to their interesting optical properties, facile surface modification, tunable aspect ratios, and low cytotoxicity. Gold nanorods are historically prepared starting from HAuCl4 in the presence of ascorbic acid, silver nitrate, and the growth directing surfactant, cetyltrimethylammonium bromide (CTAB). While CTAB drives a rod-like morphology, it is known to be cytotoxic. This inherent toxicity is often addressed by removing or masking the native CTAB surfactant present on the nanorod surface. In the current study we have investigated a less toxic alternative surfactant, dodecylethyldimethylammonium bromide (C12EDMAB), as a possible growth-directing agent. Monodisperse gold nanorods having various lengths have been grown in the presence of C12EDMAB. SEM data suggests that the quantity of C12EDMAB on the rod’s surface is much higher than that of CTAB. Toxicity assays were performed on HEp-2 and A549 cells showing lower toxicity at select concentrations for C12EDMAB coated rods.

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

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

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.

Small Gold Nanorods with Tunable Absorption for Photothermal Microscopy in Cells.

The synthesis, sorting, and characterization of monodisperse gold nanorods with dimensions around 10 nm in length and below 6 nm in diameter is reported. They display tunable plasmon resonance in the near infrared, a region where cellular absorption is reduced. A dual color photothermal microscope is developed to demonstrate that they are promising single molecule probes for bioimaging.

High colloidal stability of gold nanorods coated with a peptide-ethylene glycol: Analysis by cyanide-mediated etching and nanoparticle tracking analysis

The stability of gold nanorods was assessed following coating with various charged or uncharged ligands, mostly peptides. Highly stable monodispersed gold nanorods were obtained by coating CTAB-stabilized gold nanorods with a pentapeptide with C-terminal ethylene glycol units (peptide-EG). UV⿿vis spectroscopy of these nanorods suspended in saline solutions indicated no signs of aggregation, and they were easily purified using size-exclusion chromatography. A more stringent measure of nanorod stability involved observing changes in the UV⿿vis absorbance of gold nanorods subjected to etching with cyanide. The λmax absorbance of peptide-EG coated nanorods red-shifted in etchant solution. The hypothesis that changes in the nanorod aspect ratio led to this red-shift was confirmed by TEM analysis, which showed pit formation along the transverse axis. The etching process was followed in solution using nanoparticle tracking analysis. The red-shift was shown to occur while the particles remained mono-dispersed, and so was not due to aggregation. Adding both etchant solution and peptide-EG to the nanorods was further shown to allow modulation of the ολmax red-shift and increase the etchant resistance of peptide-EG nanorods. Thus, very stable gold nanorods can be produced using the peptide-EG coating approach and their optical properties modulated with etchant.

Preparation of Silver Nanoparticles Loaded Photoresponsive Composite Microgels and Their Light-Controllable Catalytic Activity

A new type of smart composite microgels, which are able to control the catalytic activity of their loaded silver nanoparticles by light, was designed and fabricated based on the idea of function transfer between their constituent components. First, the surfaces of monodisperse gold nanorods (AuNRs) with strong photothermal effect were coated with poly(N-isopropylacrylamide) (PNIPAM) hydrogel by seed precipitation polymerization to prepare the two-component composite microgels with core–shell structure (AuNR@PNIPAM microgels). Then, Ag+ ions coordinated into the shell of AuNR@PNIPAM microgels were in situ reduced by sodium borohydride to produce silver nanoparticles (AgNPs) loaded three-component composite microgels (AuNR@(AgNPs/PNIPAM) microgels). The characterization results obtained by transmission electron microscopy show that the gold nanorod is located at the center of the three-component composite microgels and AgNPs with the average particle diameters of 6–10 nm are evenly distributed within its sh...

Template-assisted colloidal self-assembly of macroscopic magnetic metasurfaces.

We demonstrate a template-assisted colloidal self-assembly approach for magnetic metasurfaces on macroscopic areas. The choice of anisotropic colloidal particle geometry, assembly pattern and metallic film is based on rational design criteria, taking advantage of mirror-charge effects for gold nanorods placed on gold film. Monodisperse gold nanorods prepared utilizing wet-chemistry are arranged with high precision on wrinkled templates to form linear array-type assemblies and subsequently transferred to a thin gold film. Due to the obtained particle-to-film distance of 1.1 nm, the plasmonic mode of the nanorod is able to couple efficiently with the supporting metallic film, giving rise to a magnetic mode in the visible spectrum (721 nm). Conventional UV-vis-NIR measurements in close correlation with electromagnetic simulations provide evidence for the presence of a magnetic resonance on the macroscopic area. The herein presented scalable lithography-free fabrication process paves the road towards colloidal functional metasurfaces with an optical response in the effective magnetic permeability.

Recubrimiento controlado de sustratosde vidrio con nanobastones metálicos

RESUMENSe realizó la síntesis de nanobastones de oro monodispersos con una eficiencia superior al 80% sobre el total de nanopartículas, caracterizado por una resonancia plasmónica longitudinal cercana a los 800 nm. Se modificaron superficialmente sustratos de vidrio y se los recubrió con los nanobastones sintetizados, con control de la densidad superficial. Se monitoreó la dinámica del recubrimiento a través de espectros de extinción, y se observó una densidad máxima de saturación dada por repulsión electrostática y un tiempo característico del proceso. Luego de alcanzada la saturación de nanobastones por unidad de área se observa un ensanchamiento de las resonancias hacia el infrarrojo, debido a interacciones entre los nanobastones por producirse agregaciones sobre la superficie. Estos resultados tienen aplicación inmediata en el diseño y fabricación de dispositivos plasmónicos, por ejemplo en el sensado molecular.