Seed-mediated Growth Of Gold Nanorods Research Articles

Insight into the Role of Ag in the Seed-Mediated Growth of Gold Nanorods: Implications for Biomedical Applications

The synthesis of gold nanorods (AuNRs) with a specific length/diameter aspect ratio is crucial for their use in imaging, sensing, drug delivery, and biological applications. However, the most commo...

Seed-Mediated Growth of Gold Nanorods Using Silver Seeds: Effect of Silver Seeds Concentration and Growth Time

Seed-mediated growth method (SMGM) in preparation of gold nanoparticles becomes one of the most popular methods due to the simplicity of the experimental procedures and flexibility in structural modifications. In this paper, we report a new method for synthesizing gold nanoparticles using silver seeds. The effect of seed concentration and growth time are investigated in this work. By increasing the silver seed concentration, it is found that the color of the colloidal gold nanorods obtained are changed from light pink to reddish purple, the surface plasmon resonance band is shifted to the blue region whereas absorption spectra becomes narrower. The additional peak is also spotted when increasing silver seed concentration to 5 µl. Meanwhile, increasing the growth time from 5 to 240 minutes tends to increase the color concentration of the solution. Besides that, the absorbance of colloidal gold nanorods is also increased with an increase in the growth time whereas optimum growth time is found to be 45 minutes. FESEM characterization shows that gold nanoparticles shapes are dominated by nanorods with average length, width, and aspect ratio are 129.8 nm, 42.9 nm, and 3.4, respectively. The energy-dispersive x-ray spectroscopy (EDX) shows the chemical composition of the synthesized sample is Gold (Au) with weight % and atomic % are 32.23 and 5.98, respectively. Besides that, signals from Carbon (C), Oxygen (O), and Indium (In) atoms were also recorded from EDS spectra. The present approach thus provides new method for synthesis gold nanoparticles with additional plasmon resonance peak thus it has very potential for application in plasmonic sensing.

Reversible Tuning the Aspect Ratio and Plasmonic Shift of Gold Nanorods in Alkaline Environment: Growth, Etching and Rebuilding

The well-developed seed-mediated growth of gold nanorods (GNRs) is under the weakly acidic condition, and the aggregation of the GNRs usually takes place when the pH value is increased to alkalinity. In this study, GNRs have been successfully prepared in the alkaline environment using the reducibility of H2O2 under the alkaline condition (pH = 10), and the longitudinal localized surface plasmon resonance (LSPR) could be fine tuned within a wider wavelength range of 600 ~ 1050 nm by changing the concentration of silver ion. On the other hand, by using the strong oxidizability of the H2O2 under the acidic condition (pH = 3), the prepared GNRs could be etched completely, and the chloroauric acid, as the important raw material of preparing GNRs, could be retrieved. The recovered materials could be reused as the growth solution of GNRs, and the re-prepared GNRs have longer aspect ratio and stronger LSPR. This research reflects the cyclic utilization of noble metal along with preparation of GNRs in the weak alkaline environment.

Understanding the Seed-Mediated Growth of Gold Nanorods through a Fractional Factorial Design of Experiments.

Since the development of simple, aqueous protocols for the synthesis of anisotropic metal nanoparticles, research into many promising, valuable applications of gold nanorods has grown considerably, but a number of challenges remain, including gold-particle yield, robustness to minor impurities, and precise control of gold nanorod surface chemistry. Herein we present the results of a composite fractional factorial series of experiments designed to screen seven additional potential avenues of control and to understand the seed-mediated silver-assisted synthesis of gold nanorods. These synthesis variables are the amount of sodium borohydride used and the rate of stirring when producing seed nanoparticles, the age of and the amount of seeds added, the reaction temperature, the amounts of silver nitrate and ascorbic acid added, and the age of the reduced growth solution before seed nanoparticles are added to initiate rod formation. This statistical experimental design and analysis method, besides determining which experimental variables are important and which are not when synthesizing gold nanorods (and quantifying their effects), gives further insight into the mechanism of growth by measuring the degree to which variables interact with each other by mapping out their mechanistic connections. This work demonstrates that when forming gold nanorods by the reduction of auric ions by ascorbic acid onto seed nanoparticles, ascorbic acid determines how much gold is reduced, and the amount of seeds determine how it is divided, yet both influence the intrinsic growth rates, in both width and length, of the forming nanorods. Furthermore, this work shows that the reduction of gold proceeds via direct reduction on the surface of seeds and not through a disproportionation reaction. Further control over the length of gold nanorods can be achieved by tuning the amount of silver nitrate or the reaction temperature. This work shows that silver does not directly influence rod length or width, and a new primary role for silver is proposed as a catalyst promoting the reduction of gold on the ends of forming nanorods. Furthermore, this silver catalyst is removed from the reaction by adsorption onto the surface of the growing nanorod. This work also demonstrates the importance of freshly prepared silver nitrate and ascorbic acid solutions, free from even a few hours of photodegradation, in preparing gold nanorods with high shape purity and gold yield.

Anion-Mediated End-Shape Control in Seed-Mediated Growth of Gold Nanorods.

End-shape-controlled gold nanorods (GNRs) were synthesized at room-temperature by a seeded-growth method, in which hexadecyltrimethylammonium bromide (CTAB) was used as a stabilizer and capping agent. The average dimension of the GNRs was 46 nm in length and 15 nm in diameter, which corresponds to aspect ratio of c.a. 3.0. Then, their both ends were further grown at the presence of silver precursor (AgNO3), resulting in formation of arrow-head GNRs. By tuning the amount of the silver precursor, the end-shape of the GNRs was changed to dumbbell like shape. Moreover, the growth rate of gold could be controlled by tuning the amount of hydrochloric acid (HCl). While arrow-headed GNRs having sharp edges were produced without HCl, the GNRs having dog-bone like or round-head shape at both ends were obtained with HCl.

Seed-Mediated Growth of Gold Nanorods: Limits of Length to Diameter Ratio Control

The effects of the seed reaction conditions on the two-step seed-mediated growth of gold nanorods and the effect of gold and reducing agent content in the growth solution were evaluated. Results indicate that the reaction conditions used to produce the seeds have a significant impact on the aspect ratio of the gold nanorods produced. Increasing the concentration of gold or the reaction temperature in the seed production step results in lower length to diameter (aspect ratio) gold rods. In addition, the amount of prepared seed added to the growth solution impacts the rod aspect ratio, with increasing amounts of seed reducing the aspect ratio. The effects of reducing agent, ascorbic acid (AA), and gold content of the growth solution on the aspect ratio of the produced rods are strongly interrelated. There exists a minimum ascorbic acid to gold concentration below which rods will not form; however, increasing the ratio above this minimum results in shorter rods being formed. Characterization of nanorod growth is performed by UV-vis-NIR spectrophotometry and transmission electron microscopy (TEM).

Copper-Ion-Assisted Growth of Gold Nanorods in Seed-Mediated Growth: Significant Narrowing of Size Distribution via Tailoring Reactivity of Seeds

In the well-developed seed-mediated growth of gold nanorods (GNRs), adding the proper amount of Cu(2+) ions in the growth solution leads to significant narrowing in the size distribution of the resultant GNRs, especially for those with shorter aspect ratios (corresponding longitudinal surface plasmon resonance (LSPR) peaks shorter than 750 nm). Cu(2+) ions were found to be able to catalyze the oxidative etching of gold seeds by oxygen, thus mediating subsequent growth kinetics of the GNRs. At proper Cu(2+) concentrations, the size distribution of the original seeds is greatly narrowed via oxidative etching. The etched seeds are highly reactive and grow quickly into desired GNRs with significantly improved size distribution. A similar mechanism can be employed to tune the end cap of the GNRs. Except for copper ions, no observable catalytic effect is observed from other cations presumably due to their lower affinity to oxygen. Considering the widespread use of seed-mediated growth in the morphology-controlled synthesis of noble metal nanostructures, the tailoring in seed reactivity we presented herein could be extended to other systems.

Effective surface modification of gold nanorods for localized surface plasmon resonance-based biosensors

This paper presents a novel, pH-mediated protocol for the surface modification of gold nanorods (GNRs), which provides a robust platform for the assembly of localized surface plasmon resonance (LSPR)-based sensors, by facilitating functionalization at the surface of the nanoparticles. This is achieved by replacing the surface bilayer of cetyltrimethylammonium bromide (CTAB) with 11-mercaptoundecanoic acid (MUA) at alkaline pH after seed-mediated growth of GNRs. This method fully exploits the characteristics of MUA (i) the carboxylic acid functionality enables ligand exchange in an aqueous environment; (ii) the hydrophobic nature of the molecule stabilizes the nanorods and prevents aggregation via a self-assembled monolayer and (iii) the sulfur moiety binds to the gold surface. Critically, this procedure also simplifies subsequent functionalization, overcoming the limitation arising from commonly used CTAB-only preparation. XPS (X-ray photoelectron spectroscopy) was used to confirm the ligand exchange at the surface of the GNRs. The surface modification facilitates the preparation of a LSPR-based biosensor based on human IgG for the detection of anti-human IgG and a detection limit of 0.4nM was observed.

Improved Size-Tunable Synthesis of Monodisperse Gold Nanorods through the Use of Aromatic Additives

We report an improved synthesis of colloidal gold nanorods (NRs) by using aromatic additives that reduce the concentration of hexadecyltrimethylammonium bromide surfactant to ~0.05 M as opposed to 0.1 M in well-established protocols. The method optimizes the synthesis for each of the 11 additives studied, allowing a rich array of monodisperse gold NRs with longitudinal surface plasmon resonance tunable from 627 to 1246 nm to be generated. The gold NRs form large-area ordered assemblies upon slow evaporation of NR solution, exhibiting liquid crystalline ordering and several distinct local packing motifs that are dependent upon the NR's aspect ratio. Tailored synthesis of gold NRs with simultaneous improvements in monodispersity and dimensional tunability through rational introduction of additives will not only help to better understand the mechanism of seed-mediated growth of gold NRs but also advance the research on plasmonic metamaterials incorporating anisotropic metal nanostructures.

Potential-Controlled Electrochemical Seed-Mediated Growth of Gold Nanorods Directly on Electrode Surfaces

Here we compare the amount and the morphology of Au nanostructures electrodeposited from a solution containing 2.5 x 10(-4) M AuCl(4)(-) and 0.1 M cetyltrimethylammonium bromide (CTAB) onto nonseeded and Au-nanoparticle (NP)-seeded mercaptopropyltrimethoxysilane (MPTMS)-functionalized glass/indium tin oxide (glass/ITO) electrodes as a function of the electrode potential and deposition time. The method is similar to the previously reported seed-mediated chemical synthesis of Au nanorods (NRs) in solution and on surfaces, except that we replace the chemical reducing agent (ascorbic acid) with the electrochemical potential. The deposition can be classified into three different potential ranges on the nonseeded and seeded electrodes on the basis of the amount of Au deposited and the morphology of the deposited nanostructures. On the nonseeded glass/ITO/MPTMS electrode, at potentials ranging from -0.30 to -0.20 V, there are a significant number of Au deposits on the surface with mainly branched morphology. At deposition potentials ranging from -0.10 to 0.27 V, there is very little deposition of Au but the few deposits also have a branched morphology. At 0.27 V and higher, there is no Au deposition on the glass/ITO/MPTMS electrode. Because Au seeds catalyze Au deposition, the three potential ranges, the amount of Au, and the morphologies are quite different on the glass/ITO/MPTMS/Au NP seed electrodes compared to those on the nonseeded glass/ITO/MPTMS electrodes. There is a significant amount of Au (more than on the nonseeded electrode) on the surface over a wider range of potentials from -0.30 to 0.27 V, and they have spherical morphology. From 0.30 to 0.35 V, less Au deposits on the electrode and there are 5-15% Au NRs on the surface in addition to spherical NPs. Above 0.35 V, there is no Au deposition on the glass/ITO/MPTMS/Au seed electrode. For depositions within the potential range of 0.30 to 0.35 V on glass/ITO/MPTMS/Au seed electrodes, the size and shape distributions of the Au nanostructures, including NRs, are similar to those previously synthesized by chemical seed-mediated growth in solution and directly on nonconductive surfaces. The yield, length, and aspect ratio of the Au NRs depend on the deposition time; the average length ranges from about 100 to 400 nm for times of 30 to 120 min. The electrochemical seed-mediated growth of Au is optimal from 0.30 to 0.35 V versus Ag/AgCl under our conditions, which could be useful for enhancing the signal in sensing strategies that employ Au NPs as optical or electrochemical tags.

Fabrication, stabilization, and optical properties of gold nanorods with silver shells

We describe experimental results on the synthesis and optical properties of gold nanorods with silver coating. The nanoparticles were fabricated by the seed-mediated growth of gold nanorods in a growth solution with ionic surfactant CTAB (cetyltrimethylammonium bromide); then the particles were separated in the density gradient of glycerol and the silver ions were reduced by ascorbic acid in the presence of polyvinylpirrolidone at alkalic conditions. The formation of a silver nanoshell was controlled by the shift of plasmon resonances (PR) of extinction and differential light scattering, by the appearance of characteristic Ag peaks in the EDX spectra of samples, by TEM data, and by visually inspecting changes in colloid colors. Theoretical calculations with a two-layered spheroid model are in agreement with spectral measurements. To evaluate the silver nanoshell thickness, we calculate a calibration curve for the relative PR shift. It is noted that the technology described gives particles that are instable in water environment, leading to notable drift of PRs and optical spectra in whole. We have found that the optical properties of gold-silver nanorods can be stabilized by transferring in ethanol or by the addition of polyacrilic acid. The basic advantage of the described protocol is the fine controlled tuning of the extinction and light scattering PRs of two-layered nanorods from NIR to 550 nm with an accuracy of about 10 nm. Such particles may find promising applications in biophotonics, bioimaging of cellular structures, contrasting OCT tissue images, etc.

Influence of Iodide Ions on the Growth of Gold Nanorods: Tuning Tip Curvature and Surface Plasmon Resonance

This paper describes morphological and optical changes induced by seed-mediated growth of gold nanorods in the presence of iodide ions. Addition of small amounts of iodide to the growth solution results in the growth of nanoparticles with dumbbell-like structure, meaning that gold salt reduction takes place preferentially at the rod tips. However, when excess iodide is added, homogeneous rod growth is observed, and therefore the original shape is retained. By controlling the experimental conditions, the position of the longitudinal plasmon band of grown nanorods can be shifted up to as much as 250 nm. These optical effects were also simulated by means of the boundary element method (BEM), achieving an excellent agreement with the experimental spectra. X-ray photoelectron spectroscopy (XPS) and surface enhanced Raman spectroscopy (SERS) analysis of the gold nanorods before and after iodide addition revealed the presence of AuI and AgI at the particles surface. A growth mechanism is proposed on the basis of preferential iodide adsorption at the tips {111} facets, leading to the formation of AgI, followed by reduction of gold salt precursor due to a decrease in the surface redox potential.

Direct monitoring of gold nanorod growth

The seed-mediated growth of gold nanorods is shown to be strongly dependent on the reaction time and chemical environment of the reaction solution. The versatile seed-mediated approach in aqueous surfactant solutions has been used in this study for the synthesis of gold nanorods. Changes in the aspect ratio of gold nanorods were reflected in shifts of the plasmon resonance peaks and were monitored using UV-Visible absorption spectroscopy (UV-Vis) to follow the different stages of gold nanorod formation as a function of time and varying amounts of silver ion. Unlike the use of strong reducing agents to make spherical gold nanoparticles, the growth of gold nanorods requires weak reducing conditions, leading to an unknown degree of gold reduction. Therefore, cyclic voltammetry was used to electrochemically interrogate the entire reaction from gold seed to gold nanorod as a function of time. Data obtained revealed that time-dependent gold species are involved in gold nanorod formation.

Seed-Mediated Growth of Gold Nanorods Directly on Surfaces

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