Synthesis Of Au Nanorods Research Articles

Exploring the Complex Chemistry and Degradation of Ascorbic Acid in Aqueous Nanoparticle Synthesis

Ascorbic acid (AA) is the most widely used reductant for noble metal nanoparticle (NP) synthesis. Despite the synthetic relevance, its aqueous chemistry remains misunderstood, due in part to various assumptions about its reduction pathway which are insufficiently supported by experimental evidence. This study aims to provide an understanding of the complex chemistry associated with AA under aqueous conditions. We demonstrate that (i) AA undergoes appreciable degradation in alkaline solution on a timescale relevant to NP synthesis, (ii) contrary to popular belief, AA does not degrade into dehydroascorbic acid (DHA), nor is DHA the oxidized product of AA under noble metal NP synthetic conditions, (iii) DHA, which readily degrades under alkaline conditions, can also effectively reduce metal salt precursors to metal NPs, (iv) neither ascorbate nor dehydroascorbate act as surface capping agents post‐synthetically on the NPs (v) AA degradation time greatly affects the morphology and polydispersity of the resultant NP. Results from our mechanistic investigation enabled us to utilize purposefully‐aged reductants to achieve control over shape yield and monodispersity in the seed‐mediated synthesis of Au nanorods. Our findings have important implications for achieving monodispersed products in the many metal NP synthesis reactions that make use of AA as a reducing agent.

NIR-Excitable PEG-Modified Au Nanorods for Photothermal Therapy of Cervical Cancer

Gold (Au) nanorods (NRs) have been extensively used in applications in biomedical fields, thanks to their adjustable optical and stable chemical properties. Especially for the photothermal therapy (PTT) of cancer, Au NRs show an excellent thermal effect in the near-infrared (NIR) light range. However, cetyltrimethylammonium bromide (CTAB), which has been traditionally widely used for the synthesis of Au NRs, is generally cytotoxic. Surface modification is one of the ideal nanotechnologies to reduce toxicity and enhance the biocompatibility of CTAB-stabilized Au NRs. Herein, we designed an NIR-I-excitable photothermal theranostic nanoplatform Au@PEG NRs via modifying Au NRs with [1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethyleneglycol)]] (molecular weight: 2000; DSPE-mPEG2000). The results show that due to the PEG-layer protection, Au@PEG NRs exhibit superior biocompatibility (cell survival rate > 90%) and chemical stability. Meanwhile, Au@PEG NRs exhibited excellent biocompatibility via a hemolysis test, blood routine assays, and histological examinations (H&E). The absorption spectra of Au@PEG NRs showed a negligible change in the different pH (5.5–7.5) solutions, and the in vitro maximum heat dissipation photothermal conversion efficiency was 36.83%. In vivo PTT effects of Au@PEG NRs on a cervical tumor can reach 58.8 °C. The intracellular localization suggests the efficient cellular uptake of Au@PEG NRs. The fact that in vitro Au@PEG NRs assisted PTT under laser irradiation (793 nm) extremely ablates HeLa cells. The ultrastable Au@PEG NRs have significant potential for PTT application of cervical cancer.

Growing AuNRs in a single step with NIR plasmon for superior SERS and plasmonic photothermal performance

Au nanorods (AuNRs) with a variety of aspect ratios (ARs) are used in surface-enhanced Raman spectroscopy (SERS), thermal therapy, and similar applications. In this study, we report a seed-mediated synthesis of AuNRs to cover different ARs in a single step with diluted HCl. This process ensures the realization of longitudinal surface plasmon resonance (LSPR) from the visible to the near-infrared region (∼650–1150 nm), improved uniformity with negligible by-products, and a low amount of surfactant. With no extra organic component, a halide (Cl−) that allows for the unique control of the LSPR is highlighted along with other growth-defining reagents. A comparative SERS of rhodamine and photothermal examinations were performed for halide- and non-halide-grown AuNRs with an AR of ∼7. An excellent enhancement factor (3 × 108) was achieved for the halide-grown AuNRs and was verified by a finite-difference time-domain (FDTD) simulation. Halide-grown AuNRs with a width of ∼5 nm demonstrated a highly improved plasmonic photothermal temperature of 45 °C, compared with that of 35 °C obtained from non-halide-grown AuNRs at a laser power density of only 1.1 W/cm2. The superior efficiency and photothermal effects of halide-grown samples in SERS are among the best reported, and significantly expand the scope of their application.

Synthesis of Au nanorods via autocatalytic growth of Au seeds formed by sonochemical reduction of Au(I): Relation between formation rate and characteristic of Au nanorods

The sonochemical formation of Au seeds and their autocatalytic growth to Au nanorods were investigated in a one-pot as a function of concentration of HAuCl4, AgNO3, and ascorbic acid (AA). The effects of ultrasonic power and irradiation time were also investigated. In addition, the formation rate of Au nanorods was analyzed by monitoring the extinction at 400 nm by UV–Vis spectroscopy and compared with the growth behavior of Au seeds to nanorods. Most of the reaction conditions affected the yield, size, and shape of Au nanorods formed. It was confirmed that the concentration balance between HAuCl4 and AA was important to proceed the formation of Au seeds and nanorods effectively. The formation rate became faster with increasing AA concentration and dog-bone shaped nanorods were formed at high AA concentration. It was also confirmed a unique phenomenon that the shape of Au nanorods changed even after the completion of the reduction of Au(I) in the case of short-time ultrasonic irradiation for Au seed formation.

The role of trace Ag in the synthesis of Au nanorods

One of the more useful syntheses of single crystalline, uniform Au nanorods from Au spherical seeds relies on the addition of trace Ag ions, yet the role that Ag+ plays has remained both elusive and controversial, due in part to lack of knowledge of how the Ag distribution in the nanorod evolves over time. In this work, we fill in this knowledge gap by correlating the spatial distribution of Ag within Au nanorods with nanorod anisotropic growth through time-course X-ray absorption spectroscopy (XAFS)-derived atomic-level elemental coordination paired with electron microscopy for nanoscale morphological analysis. Using this method, a plausible pathway for the conversion of spherical seeds into Au nanorods is proposed. Evidence shows that the nanorod anisotropic growth is directly related to the Ag surface coverage. Anisotropy is induced early in the reaction when Ag first deposits onto the nanoparticle surface, but growth occurs more isotropically as the reaction progresses and Ag diffuses into the nanorod bulk. The results of this investigation and methods employed should be extendable to many anisotropic nanoparticle syntheses that make use of trace elemental species as shape-control additives.

Synthesis of Au Nanorods through Prereduction with Curcumin: Preferential Enhancement of Au Nanorod Formation Prepared from CTAB-Capped over Citrate-Capped Au Seeds

Despite seed-mediated growth approach being the state of art method for synthesis of gold nanorods, the mechanism of gold nanorod formation to control final aspect ratio of the nanorods is not yet well understood. In this work, formation of Au nanorods has been investigated using curcumin, a hydrophobic molecule, as secondary reducing agent, instead of popularly used hydrophilic ascorbic acid. It is found that the nature of the secondary reducing agent plays a crucial role in preferential enhancement of Au nanorod formation. Although in the seed-mediated growth approach the concentration of curcumin has similar effect on Au nanorod formation as observed for ascorbic acid earlier, the combination of the nature of the capping agent of seed particles as well as that of the secondary reducing agent determines the final aspect ratio of the nanorods. Hydrophobic (secondary) reducing agent, curcumin, prefers CTAB capped seed solution compared with citrate capped seed solution. The mechanism of nanorod formation ...

Synthesis of Au nanorods in a low pH solution via seed-media method

The gold (Au) nanorods with various aspect ratios are obtained by a seed-media method in low pH growth solution. Transmission electron microscopy (TEM) and UV-visible spectrophotometry are utilized to characterize the Au nanorods, and the longitudinal absorption peak positions of Au nanorods show different shifting trends of the growth evolutions in various low pH (1∼3) solutions. Other influential factors on the shape of Au nanorod are also systematically studied under low pH reaction condition. The positions of longitudinal peak shift between 600 nm and 900 nm, with the aspect ratios of Au nanorods varying from 2 to 5 both in the simulation and experimental results. The simulation results are in agreement with experimental ones.

Synthesis of Au Nanorods by Using Gamma-ray Irradiation

To synthesize Au nanorods without the use of seed crystals, we applied gamma-ray irradiation from 60Co radioactive source to reduce aqueous solution containing Au ions. The total dose was fixed to 6.0 kGy with various dose rates from 0.4 to 13.6 kGy/h. After the irradiation, two surface plasmon bands in the range around 530 nm and above 700 nm were observed in UV/vis spectra. We observed by a transmission electron microscopy (TEM) that rod-shaped Au nanoparticles with the length of 20–25 nm, and the width of about 7–15 nm were produced as well as a small amount of spherical particles. The size and the aspect ratio of the Au nanorods are well correlated with gamma-ray irradiation dose rate; with increasing the dose rate, their size decreases and the aspect ratio increases.

Synthesis of Au Nanorods by Using Gamma-ray Irradiation

Synthesis of Au Nanorods by Using Gamma-ray Irradiation

Selective Shortening of Single-Crystalline Gold Nanorods by Mild Oxidation

Gold nanorods (NRs) have received much attention due to their size-dependent surface plasmon-related optical properties. A seed-mediated approach has recently been developed for the synthesis of Au NRs with varying length-to-diameter aspect ratios. With the introduction of silver ions in the growth solution, Au NRs of narrow size distributions can be produced in high yields. Herein we describe an approach for the continuous and selective shortening of Au NRs synthesized by the silver ion-assisted seed-mediated method through oxidation with environmentally benign oxygen at slightly elevated temperatures. UV-visible extinction measurements indicate that the longitudinal surface plasmon band of Au NRs decreases in intensity and blue-shifts as a function of the oxidation time. Transmission electron microscopy (TEM) imaging shows that the length of Au NRs decreases with oxidation and their diameter stays almost constant, which suggests that oxidation starts at the ends of Au NRs. The size distributions of shortened Au NRs are similar to those of starting NRs. Further oxidation transforms Au NRs into nanospheres, which become smaller in diameter and finally completely disappear. It has been found that the oxidation rate of Au NRs can be controlled by temperature and acid concentration. Furthermore, high-resolution TEM studies reveal that Au NRs synthesized by the silver ion-assisted seed-mediated method are single crystalline and they stay single crystalline during oxidation. It is expected that Au NRs of any aspect ratio with narrow size distributions within the limit of that possessed by starting NRs can be produced by this mild oxidation approach.