Facile gold nanorod purification by fractionated precipitation

Investigation of selective SERS enhancement mechanism of Au nanospheres and Au nanorods based on 2T2D-SERS correlation spectroscopy

The selective enhancement mechanism in surface-enhanced Raman scattering (SERS) is demonstrated. Two different types of single nanoparticles (Au nanosphere and Au nanorod) were used to investigate the role of the localized surface plasmon resonance (LSPR) in SERS spectra by using the two-trace two-dimensional (2T2D) correlation spectroscopy. The SERS intensities of three probe molecules, 4-mercaptobenzoic acid (4-MBA), 4-aminothiophenol (4-ATP), and 4-bromobenzenethiol (4-BBT), respectively, were enhanced but slightly different when adsorbed on Au nanospheres and Au nanorods. 2T2D correlation SERS spectra clearly showed that even with the same shape of Au nanoparticles, the main factors influencing the SERS enhancement can vary depending on the specific type of SERS tags used. Such subtle difference could not be clearly identified by the conventional spectral analysis. This result sheds light on potential applications of 2T2D correlation spectroscopy. For 4-MBA molecules, the a1 and b2 modes are mainly affected by the Au nanospheres and Au nanorods. For 4-ATP molecules, the a1 and b2 modes related to C-S stretching combined with C-C stretching band are mainly affected by Au nanorods and Au nanospheres. For 4-BBT molecules, the a1 and b2 modes of C-C (aromatic ring) stretching band are mainly affected by Au nanorods and Au nanospheres. This study offers valuable insights into the relationship between nanoparticle shape and SERS enhancement.

Temperature-Derived Purification of Gold Nano-Bipyramids for Colorimetric Detection of Tannic Acid

Gold nanostructures have attracted broad attention. Among various nanostructures, gold nanobipyramids have shown great potential in sensing, biomedicine, environmental protection, chemical catalysis, and optics due to their unique physical and optical properties and ease of chemical functionalization. Compared with other plasmonic nanostructures, gold nanobipyramids possess narrow optical resonances, stronger plasmonic local field enhancement, and size- and shape-dependent surface plasmon resonance. However, the synthesis and purification of homogeneous gold nanobipyramids are very challenging. The gold nanobipyramids synthesized via the commonly used seed-mediated growth method have low yields and are often coproduced with spherical nanoparticles. In this study, we reported a temperature-derived purification method for the isolation of gold bipyramids. In the presence of salt, by altering the temperature of the solution, large gold bipyramids can be separated from small spherical nanoparticles. As a result, a yield of as high as 97% gold nanobipyramids can be achieved through a single round of purification, and correspondingly, the ratio between the longitudinal surface plasmon resonance (LSPR) and transverse SPR intensity significantly increases to as high as 6.7. The purified gold nanobipyramids can be used as a colorimetric probe in the detection of tannic acid with a detection limit of 0.86 μM and a linear detection range from 1.25 to 37.5 μM.

Self-assembly of spherical and rod-shaped nanoparticles with full positional control

The controlled positioning of spherical gold nanoparticles and gold nanorods upon self-assembly on a substrate is of great interest for the fabrication of tailored plasmonic devices. Here, an electrostatic approach with a sequential two-step assembly protocol is presented as a cost-effective and high-yield alternative to previously presented, more complex proof of concepts. Three different geometries can be separately produced in large quantities relying on electrostatic attraction and repulsion of the charge-carrying building blocks: a single gold nanoparticle at the tip, the side or on top of a gold nanorod. DLVO theory is used to explain the electrostatic assembly strategy. The process is highly efficient and assembly yields between 79% (at the tip) and 94% (for the nanoparticle at the long side of the nanorod) are achieved.

Enhanced DC-Operated Electroluminescence of Forwardly Aligned p/MQW/n InGaN Nanorod LEDs via DC Offset-AC Dielectrophoresis

We introduce an orientation-controlled alignment process of p-GaN/InGaN multiquantum-well/n-GaN (p/MQW/n InGaN) nanorod light-emitting diodes (LEDs) by applying the direct current (DC) offset-alternating current (AC) or pulsed DC electric fields across interdigitated metal electrodes. The as-forwardly aligned p/MQW/n InGaN nanorod LEDs by a pulsed DC dielectrophoresis (DEP) assembly process improve the electroluminescence (EL) intensities by 1.8 times compared to the conventional AC DEP assembly process under DC electric field operation and exhibit an enhanced applied current and EL brightness in the current-voltage and EL intensity-voltage curves, which can be directly used as the fundamental data to construct DC-operated nanorod LED devices, such as LED areal surface lightings, scalable lightings (micrometers to inches) and formable surface lightings. The enhancement in the applied current, the improved EL intensity, and the increased number of forwardly aligned p/MQW/n InGaN nanorods in panchromatic cathodoluminescence images confirm the considerable enhancement of forwardly aligned one-dimensional nanorod LEDs between two opposite electrodes using DC offset-AC or a pulsed DC electric field DEP assembly process. These DC offset-AC or pulsed DC electric field DEP assembly processes suggest that designing for these types of interactions could yield new ways to control the orientation of asymmetric p/MQW/n InGaN diode-type LED nanorods with a relatively low aspect ratio.

Nanorattles with tailored electric field enhancement

Nanorattles are metallic core-shell particles with core and shell separated by a dielectric spacer. These nanorattles have been identified as a promising class of nanoparticles, due to their extraordinary high electric-field enhancement inside the cavity. Limiting factors are reproducibility and loss of axial symmetry owing to the movable metal core; movement of the core results in fluctuation of the nanocavity dimensions and commensurate variations in enhancement factor. We present a novel synthetic approach for the robust fixation of the central gold rod within a well-defined box, which results in an axisymmetric nanorattle. We determine the structure of the resulting axisymmetric nanorattles by advanced transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS). Optical absorption and scattering cross-sections obtained from UV-vis-NIR spectroscopy quantitatively agree with finite-difference time-domain (FDTD) simulations based on the structural model derived from SAXS. The predictions of high and homogenous field enhancement are evidenced by scanning TEM electron energy loss spectroscopy (STEM-EELS) measurement on single-particle level. Thus, comprehensive understanding of structural and optical properties is achieved for this class of nanoparticles, paving the way for photonic applications where a defined and robust unit cell is crucial.

Recent progress in theranostic applications of hybrid gold nanoparticles

A significant area of research is theranostic applications of nanoparticles, which involves efforts to improve delivery and reduce side effects. Accordingly, the introduction of a safe, effective, and, most importantly, renewable strategy to target, deliver and image disease cells is important. This state-of-the-art review focuses on studies done from 2013 to 2016 regarding the development of hybrid gold nanoparticles as theranostic agents in the diagnosis and treatment of cancer and infectious disease. Several syntheses (chemical and green) methods of gold nanoparticles and their applications in imaging, targeting, and delivery are reviewed; their photothermal efficiency is discussed as is the toxicity of gold nanoparticles. Owing to the unique characterizations of hybrid gold nanoparticles and their potential to be developed as multifunctional, we predict they will present an undeniable role in clinical studies and provide treatment platforms for various diseases. Thus, their clearance and interactions with extra- and intra-cellular molecules need to be considered in future projects.

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.

Engineered gold nanoparticles for photothermal cancer therapy and bacteria killing

Gold nanoparticle mediated photothermal therapy in future medicine.

Single nanoparticle plasmonic sensors

The adoption of plasmonic nanomaterials in optical sensors, coupled with the advances in detection techniques, has opened the way for biosensing with single plasmonic particles. Single nanoparticle sensors offer the potential to analyse biochemical interactions at a single-molecule level, thereby allowing us to capture even more information than ensemble measurements. We introduce the concepts behind single nanoparticle sensing and how the localised surface plasmon resonances of these nanoparticles are dependent upon their materials, shape and size. Then we outline the different synthetic approaches, like citrate reduction, seed-mediated and seedless growth, that enable the synthesis of gold and silver nanospheres, nanorods, nanostars, nanoprisms and other nanostructures with tunable sizes. Further, we go into the aspects related to purification and functionalisation of nanoparticles, prior to the fabrication of sensing surfaces. Finally, the recent developments in single nanoparticle detection, spectroscopy and sensing applications are discussed.