Aspect Ratio Of Gold Nanorods Research Articles

Two-photon luminescence imaging of Bacillus spores using peptide-functionalized gold nanorods

Bacillus subtilis spores (a simulant of Bacillus anthracis) have been imaged by two-photon luminescence (TPL) microscopy, using gold nanorods (GNRs) functionalized with a cysteine-terminated homing peptide. Control experiments using a peptide with a scrambled amino acid sequence confirmed that the GNR targeting was highly selective for the spore surfaces. The high sensitivity of TPL combined with the high affinity of the peptide labels enables spores to be detected with high fidelity using GNRs at femtomolar concentrations. It was also determined that GNRs are capable of significant TPL output even when irradiated at near infrared (NIR) wavelengths far from their longitudinal plasmon resonance (LPR), permitting considerable flexibility in the choice of GNR aspect ratio or excitation wavelength for TPL imaging.

PH Controlled Synthesis of High Aspect-Ratio Gold Nanorods

Here we report the two-step synthesis of high aspect-ratio gold nanorods via combining low pH growth and higher cetyltrimethylammonium bromide concentration. The approach is motivated by the "slow growth principle," which has been demonstrated recently as a effective protocol to prepare high aspect-ratio one-dimensional nanostructures. When the pH value of the growth solution gradually decreased, the growth rate of nanorods was significantly decreased and a continuous morphological changing from uniform monodisperse gold nanospheres to extraordinary long gold nanorods occured. By simultaneously decreasing the growth pH and increasing the concentration of cetyltrimethylammonium bromide, the length of gold nanorods could be further elongated to above 2 μm with large aspect-ratio of above 80. This kind of high aspect-ratio gold nanorods could be excellent building blocks for optical or electrical nanodevices in future.

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.

Size Tunable Gold Nanorods Evenly Distributed in the Channels of Mesoporous Silica

Uniformly distributed gold nanorods in mesoporous silica were synthesized in situ by performing a seed-mediated growth process in the channels of SBA-15 which functions as a hard-template to confine the diameter of gold nanorods. By changing the amount of gold precursor, gold nanorods were prepared with a fixed diameter (6-7 nm) and tunable aspect ratios from 3 to 30. Transmission electron microscope and electron tomography were utilized to visualize the gold nanorods supported on one piece of SBA-15 segment and showed a fairly uniform 3-dimensional distribution of gold nanorods within the SBA-15 channels. The longitudinal plasmon resonances of the gold nanorods/SBA-15 composites analyzed by diffuse reflectance UV-vis spectra were found to be tunable depending on the length of gold nanorods. No significant decrease in surface area and/or pore size of the composite was found after growth, indicating the growth process did not disrupt the open mesoporous structure of SBA-15. The combination of the tunable size of the nanorods and their 3-dimensional distribution within the open supporting matrix makes the gold nanorods/SBA-15 composites interesting candidates to systematically study the influence of the aspect ratio of gold nanorods on their properties and potential applications, i.e., catalyst, optical polarizer, and ultrasensitive medical imaging technique.

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.

Luminescent Silicon Nanoparticles Formed in Solution

Here we report the two-step synthesis of high aspect-ratio gold nanorods via combining low pH growth and higher cetyltrimethylammonium bromide concentration. The approach is motivated by the "slow growth principle", which has been demonstrated recently as a effective protocol to prepare high aspect-ratio one-dimensional nanostructures. When the pH value of the growth solution gradually decreased, the growth rate of nanorods was significantly decreased and a continuous morphological changing from uniform monodisperse gold nanospheres to extraordinary long gold nanorods occured. By simultaneously decreasing the growth pH and increasing the concentration of cetyltrimethylammonium bromide, the length of gold nanorods could be further elongated to above 2 microm with large aspect-ratio of above 80. This kind of high aspect-ratio gold nanorods could be excellent building blocks for optical or electrical nanodevices in future.

Shape, Orientation, and Stability of Twinned Gold Nanorods

Although energetically unfavorable, twinning is often observed in many faceted nanomaterials including nanocrystals, nanorods, and nanowires, even following annealing. Unfortunately, twinning is neglected by most analytical models, which consequentially limits theoretical descriptions of nanostructured materials to more idealized (less realistic) approximations. In the present study, a new general analytical model for the investigation of nanomaterials of arbitrary shape and with any configuration of twin planes is used to examine twinning in unsupported gold nanorods of various orientations and aspect ratios. The results point to a close competition between nonconvex pentagonal nanorods, and 〈011〉 oriented face-centered cubic (fcc) nanorods containing multiple longitudinal twin planes, in excellent agreement with experimental studies reported in the literature.

Direct High-Yield Synthesis of High Aspect Ratio Gold Nanorods

We report a new development for the direct high-yield synthesis of high aspect ratio gold nanorods. By using a modified seed-mediated synthesis approach for the preparation of high aspect ratio gol...

Modeling the preferred shape, orientation and aspect ratio of gold nanorods

It has been widely shown that gold nanorods produced via a variety of methods may have average aspects between ∼2–40, depending on the synthetic conditions, and other thermodynamic considerations. This is significant, since the optical properties are highly dependant on the aspect of the nanorods. Furthermore, the number of different shapes and axial orientations produced depend partially on the aspect ratio, which clearly indicates that the relationship between shape, orientating and aspect is also significant. Presented here are the results of a systematic study of this relationship, using 30 candidate nanorod structures and an analytical shape-dependent thermodynamic model with input from relativistic first-principles calculations. The results show that gold nanorods with principle axes in the ±[011] orientation are energetically preferred over alternative ±[001] and ±[111] orientations (under ambient conditions), but that the stability of the ±[111] orientation increases significantly with both aspect and temperature, in agreement with other experimental and computational studies on selected types of structures reported in the literature.

Spectrally-selective gold nanorod coatings for window glass

The unique optical properties of gold nanorods, which exhibit tuneable absorption as a function of their aspect ratio, suggest that they might have potential applications in coatings for solar control on windows. Here we explore the properties of coatings produced by attaching gold nanorods to the surface of glass. Such coatings can attenuate solar radiation effectively, even at very low gold contents, but the figure-of-merit,T vis/Tsol, of our experimental coatings was close to unity, indicating that they are not spectrally selective, However, calculations are presented to show how coatings comprised of a blend of rods with aspect ratios of greater than 3 can produce coatings withT vis/Tsol of up to at least 1.4. The maximum value possible for perfectly spectrally-selective coating in sunlight is 2.08. Unfortunately, the practical realization of such coatings requires the further development of reliable methods to scale up the production of gold nanorods of longer aspect ratios.

Metallodielectric Hollow Shells: Optical and Catalytic Properties

Metallodielectric composites with tunable optical properties were prepared by layer-by-layer assembly of gold nanorods on polystyrene (PS) spheres and subsequent deposition of SiO2 or TiO2 encapsulating shells through a sol-gel process. The optical properties of the core-shells were tailored in the visible and the near-infrared region through the gold nanorod aspect ratio and the gold nanoparticle density. Removal of the PS core by dissolution in an appropriate solvent, such as THF, yielded metallodielectric hollow shells with optical properties identical to those of the original composites. The presence of gold and the porosity of the SiO2 or TiO2 shells, suitable to allow diffusion of reactants and products, make these materials of interest as catalysts, as demonstrated by the reduction of potassium hexacyanoferrate(III) with NaBH4.

Temperature-induced control of aspect ratio of gold nanorods

Aspect ratio of gold nanorods can be controlled by simply adjusting the reaction temperature in the seed-mediated synthesis of the nanorods. The gold nanorods were synthesized by the injection of gold nanoparticle seeds of around 4nm in diameter into a reaction mixture containing hydrogen tetrachloroaurate, hexadecyltrimethylammonium bromide, and ascorbic acid. Average aspect ratio of the resulting nanorod increases from 1 to about 40 with decreasing the reaction temperature from 315to276K, which can be attributed to the temperature-induced change in the shape of the micellar templates. For further understanding of the growth mechanism, silver nanoparticles were also used as seeds in the preparation of the gold nanorod.

Two-step synthesis of high aspect ratio gold nanorods

AbstractWe describe a very simple, two-step synthetic method to prepare gold nanorods with extremely high aspect ratios (> 20) and average lengths of more than 1000 nm. The method is based on a seed-mediated growth in presence of the surfactant cetyltrimethylammonium bromide. The length and aspect ratios of the nanorods can be manipulated by varying the surfactant concentration.

Seed-Mediated Synthesis of High Aspect Ratio Gold Nanorods with Nitric Acid

We report a new approach for the seed-mediated synthesis of gold nanorods with high aspect ratios by the addition of nitric acid. Various amounts of nitric acid were added during nanorod growth to significantly enhance the production of high aspect ratio nanorods. The resulting nanorods have uniform diameters of 19−20 nm and can reach lengths of 400−500 nm to yield nanorods with average aspect ratios of 21−23. The nanorods represent a large fraction of the gold nanostructures synthesized, with triangular, truncated triangular, and hexagonal nanoplates about 120−200 nm in width making up rest of the gold nanostructures formed. These nanorods spontaneously self-assemble into a high-density three-dimensional packing structure with the nanorods arranged side-by-side and end-to-end. These nanorods possess a 5-fold twinned structure with ten {111} end faces and five {100} side faces. UV−vis absorption spectrum shows a transverse plasmon absorption band at 508 nm. The longitudinal plasmon absorption band should appear beyond 2500 nm due to the extremely long length of these nanorods. The presence of nitrate ions, rather than the slight pH change caused by nitric acid, is believed to have a greater effect on the formation of these nanorods.

Aspect Ratio Dependence of the Enhanced Fluorescence Intensity of Gold Nanorods: Experimental and Simulation Study

Experimental observations and theoretical treatments are carried out for the band shape and relative intensity of the emission from gold nanorods of various aspect ratios in the range between 2.25 (1.5 theory) and 6.0 (9 theory). The calculation of the fluorescence spectra requires knowledge of the nanorod size distribution, the enhancement factors, and the shape of the unenhanced fluorescence spectrum. The size distribution is determined from the fit of the observed absorption spectrum for each value of aspect ratio studied to the theoretical model of Gans. The theory by Boyd and Shen is used for calculating the enhancement of the fluorescence spectrum of the previously observed weak emission of bulk gold, which originates from the interband transition. This is carried out for nanorods of different aspect ratios. To compare theory to the observed nanorod fluorescence spectra, which suffer from self-absorption, the calculated nanorod fluorescence spectra are corrected for this effect using the observed absorption spectra. The comparison between the observed and the calculated fluorescence band shapes is found to be good. The calculated changes in the relative intensities upon changing the aspect ratios are found to be much greater than that observed. This is due to the fact that for the observed emission of all the nanorods studied nonradiative processes dominate the relaxation mechanism of the excited state, a fact that was not included in the theoretical treatments.

Gold Nanorods Grown Directly on Surfaces from Microscale Patterns of Gold Seeds

We report a strategy for patterning surfaces with high aspect ratio gold nanorods (Au NRs) by using a poly(dimethylsiloxane) (PDMS) stamp to microcontact print 3−5 nm diameter gold nanoparticle (Au NP) “seeds” onto mercaptan-functionalized Si/SiOx surfaces and subsequently grow the seeds into Au NRs via seed-mediated reduction of HAuCl4 with ascorbic acid in the presence of cetyltrimethylammonium bromide (CTAB). The patterns also contain a large number of spherical particles, which are largely removed and separated from NRs using adhesive tape. The same PDMS stamp was used more than 30 times to produce well-defined patterns without reinking with Au NP seeds. Controlled patterning of Au NRs on surfaces is important for electronics, sensing, optical, and plasmonic applications.

Seed Mediated Growth of Gold Nanoparticles on Indium Tin Oxide Electrodes: Electrochemical Characterization and Evaluation

AbstractThe seed mediated growth of gold nanoparticles on indium tin oxide (ITO) surfaces is described, which provided a new method for the construction of gold nanoparticles‐modified ITO electrodes even without using conventional linker molecules, such as (3‐mercaptopropyl)trimethoxysilane (MPTMS). In this process, ultra‐small gold seeds were formed and anchored on the ITO surfaces first. When the seeds‐deposited ITO surfaces came into contact with the gold growth solution, the growth of gold nanoparticles occurred. The morphology of the as‐prepared modified surface was found to be strongly dependent upon the substrate. On an MPTMS‐silanized ITO, mostly gold nanospheres were formed (Au/MPTMS/ITO).In contrast, on a bare ITO, some high aspect ratio gold nanorods as well as large amounts of gold nanospheres were grown and directly attached (Au/ITO). The as‐prepared two gold nanoparticles‐modified ITO electrodes exhibited different electrochemical characteristics, attributed to the influence of MPTMS molecules. Compared with Au/MPTMS/ITO, the Au/ITO was more efficient in decreasing the heterogeneous charge transfer resistance and improving the reaction reversibility of [Fe(CN)6]3−/[Fe(CN)6]4− and exhibited more attractive bioelectrocatalytic properties. Due to the block effect of the thiol monolayer, Au/MPTMS/ITO depressed the selectivity for the determination of epinephrine (EP) in the presence of ascorbic acid (AA) while Au/ITO could improve the detection sensitivity and retain the selectivity.

The Aspect Ratio Dependence of the Fluorescence of Gold Nanorods: An Experimental and Theoretical Study

ABSTRACTExperimental observations and theoretical treatments are carried out for the band shape and relative intensity of the emission from gold nanorods of various aspect ratios in the range between 2.6 (1.5 theory) and 6.3 (9 theory). The calculation of the fluorescence spectra requires knowledge of the nanorod size distribution, the enhancement factors and the shape of the un-enhanced fluorescence spectrum. The comparison between the observed and calculated fluorescence band shapes is found to be good. The calculated changes in the relative intensities with aspect ratios are found to be much greater than that observed experimentally. This is due to the fact that for the observed emission of all the rods studied, nonradiative processes dominate the relaxation mechanism of the excited state, a fact that was not included in the theoretical treatments. Experimental results and theoretical treatments will be presented.

Synthesis and Manipulation of High Aspect Ratio Gold Nanorods Grown Directly on Surfaces

Here we describe the synthesis of Au nanorods directly on glass surfaces using seed-mediated deposition of Au from AuCl4- onto surface-attached 3-5 nm diameter Au nanoparticles (AuNPs) in the presence of cetyltrimethylammonium bromide (CTAB). The average length (200 nm to 1.2 microm) and aspect ratio (6-22) of the nanorods increases with increasing AuCl4- concentration. Short, low aspect ratio Au nanorods are manipulated with an atomic force microscopy (AFM) tip, while longer, high aspect ratio nanorods are bent and broken with the AFM tip.

Dependence of the Gold Nanorod Aspect Ratio on the Nature of the Directing Surfactant in Aqueous Solution

A three-step seed-mediated growth method was used to make gold nanoparticles. Different surfactants, alkyltrimethylammonium bromides (CnTAB, n = 10, 12, 14, 16, and 18) and cetylpyridinium chloride (C16PC), were chosen as stabilizers. In general, it was found that as the length of the surfactant chain increased, the resulting gold nanoparticles' aspect ratio increased: the aspect ratio was 1 (for C10TAB), 5 ± 2 (C12TAB), 17 ± 3 (C14TAB), and 23 ± 4 (C16TAB). The plasmon absorption maxima for the gold nanoparticles varied as a function of the shape, from 520 nm (spheres) to beyond 2000 nm (high aspect ratio nanorods). We propose that the surfactant binds as a bilayer to the growing nanoparticle and assists in nanoparticle elongation via a “zipping” mechanism.