State of the art in gold nanoparticle synthesis

Structurally similar triphenylphosphine-stabilized undecagolds, Au11(PPh3)7Cl3 and [Au11(PPh3)8Cl2]Cl, exhibit distinct ligand exchange pathways with glutathione

Ligand exchange is frequently used to introduce new functional groups on the surface of inorganic nanoparticles or clusters while preserving the core size. For one of the smallest clusters, triphenylphosphine (TPP)-stabilized undecagold, there are conflicting reports in the literature regarding whether core size is retained or significant growth occurs during exchange with thiol ligands. During an investigation of these differences in reactivity, two distinct forms of undecagold were isolated. The X-ray structures of the two forms, Au₁₁(PPh₃)₇Cl₃ and [Au₁₁(PPh₃)₈Cl₂]Cl, differ only in the number of TPP ligands bound to the core. Syntheses were developed to produce each of the two forms, and their spectroscopic features correlated with the structures. Ligand exchange on [Au₁₁(PPh₃)₈Cl₂]Cl yields only small clusters, whereas exchange on Au₁₁(PPh₃)₇Cl₃ (or mixtures of the two forms) yields the larger Au₂₅ cluster. The distinctive features in the optical spectra of the two forms made it possible to evaluate which of the cluster forms were used in the previously published papers and clarify the origin of the differences in reactivity that had been reported. The results confirm that reactions of clusters and nanoparticles may be influenced by small variations in the arrangement of ligands and suggest that the role of the ligand shell in stabilizing intermediates during ligand exchange may be essential to preventing particle growth or coalescence.

Little adjustments significantly improve the Turkevich synthesis of gold nanoparticles

In this report, we show how the classical and widely used Turkevich synthesis can be improved significantly by simple adjustments. The gold nanoparticles (AuNPs) produced with the optimized protocol have a much narrower size distribution (5-8% standard deviation), and their diameters can be reproduced with unrivaled little variation (<3%). Moreover, large volumes of these particles can be produced in one synthesis; we routinely synthesize 1000 mL of ∼3.5 nM AuNPs. The key features of the improved protocol are the control of the pH by using a citrate buffer instead of a citrate solution as the reducing agent or stabilizer and optimized mixing of reagents. Further, the shape uniformity of the particles can be improved by addition of 0.02 mM EDTA. While the proposed protocol is as straightforward as the original Turkevich protocol, it is more tolerant against variations in precursor concentration.

Simple control of surface topography of gold nanoshells by a surfactant-less seeded-growth method

We report the synthesis of branched gold nanoshells (BGNS) through a seeded-growth surfactant-less method. This was achieved by decorating chitosan-Pluronic F127 stabilized poly(lactic-co-gycolic) acid nanoparticles (NPs) with Au seeds (NP-seed), using chitosan as an electrostatic self-assembling agent. Branched shells with different degrees of anisotropy and optical response were obtained by modulating the ratios of HAuCl4/K2CO3 growth solution, ascorbic acid (AA) and NP-seed precursor. Chitosan and AA were crucial in determining the BGNS size and structure, acting both as coreductants and structure directing growth agents. Preliminary cytotoxicity experiments point to the biocompatibility of the obtained BGNS, allowing their potential use in biomedical applications. In particular, these nanostructures with "hybrid" compositions, which combine the features of gold nanoshells and nanostars showed a better performance as surface enhanced Raman spectroscopy probes in detecting intracellular cell components than classical smoother nanoshells.

Sonosynthesis of gold nanoparticles from a geranium leaf extract

A rapid in situ biosynthesis of gold nanoparticles (AuNPs) is proposed in which a geranium (Pelargonium zonale) leaf extract was used as a non-toxic reducing and stabilizing agent in a sonocatalysis process based on high-power ultrasound. The synthesis process took only 3.5 min in aqueous solution under ambient conditions. The stability of the nanoparticles was studied by UV-Vis absorption spectroscopy with reference to the surface plasmon resonance (SPR) band. AuNPs have an average lifetime of about 8 weeks at 4 °C in the absence of light. The morphology and crystalline phase of the gold nanoparticles were characterized by transmission electron microscopy (TEM). The composition of the nanoparticles was evaluated by electron diffraction and X-ray energy dispersive spectroscopy (EDS). A total of 80% of the gold nanoparticles obtained in this way have a diameter in the range 8-20 nm, with an average size of 12±3 nm. Fourier transform infrared spectroscopy (FTIR) indicated the presence of biomolecules that could be responsible for reducing and capping the biosynthesized gold nanoparticles. A hypothesis concerning the type of organic molecules involved in this process is also given. Experimental design linked to the simplex method was used to optimize the experimental conditions for this green synthesis route. To the best of our knowledge, this is the first time that a high-power ultrasound-based sonocatalytic process and experimental design coupled to a simplex optimization process has been used in the biosynthesis of AuNPs.

The Impact of Surface Ligands and Synthesis Method on the Toxicity of Glutathione-Coated Gold Nanoparticles

Gold nanoparticles (AuNPs) are increasingly used in biomedical applications, hence understanding the processes that affect their biocompatibility and stability are of significant interest. In this study, we assessed the stability of peptide-capped AuNPs and used the embryonic zebrafish (Danio rerio) as a vertebrate system to investigate the impact of synthesis method and purity on their biocompatibility. Using glutathione (GSH) as a stabilizer, Au-GSH nanoparticles with identical core sizes were terminally modified with Tryptophan (Trp), Histidine (His) or Methionine (Met) amino acids and purified by either dialysis or ultracentrifugation. Au-GSH-(Trp)2 purified by dialysis elicited significant morbidity and mortality at 200 μg/mL, Au-GSH-(His)2 induced morbidity and mortality after purification by either method at 20 and 200 μg/mL, and Au-GSH-(Met)2 caused only sublethal responses at 200 μg/mL. Overall, toxicity was significantly reduced and ligand structure was improved by implementing ultracentrifugation purifications at several stages during the multi-step synthesis and surface modification of Au-GSH nanoparticles. When carefully synthesized at high purity, peptide-functionalized AuNPs showed high biocompatibility in biological systems.

Silver nanoparticles linked by a Pt-containing organometallic dithiol bridge: study of local structure and interface by XAFS and SR-XPS

Silver nanoparticles (AgNPs) functionalized with an organometallic bifunctional thiol containing Pt(ii) centers, generated in situ from trans-trans-[thioacetyl-bistributylphosphine-diethynylbiphenyl-diplatinum(ii)], were synthesized with different sulphur/metal molar ratios (i.e. AgNPs-1 and AgNPs-2) with the aim to obtain nanosystems of different mean size and self-organization behaviour. AgNPs spontaneously self-assemble, giving rise to 2D networks, as previously assessed. In this work a deeper insight into the chemico-physical properties of these AgNPs is proposed by means of synchrotron radiation induced X-ray photoelectron spectroscopy (SR-XPS) and X-ray absorption fine structure spectroscopy (XAFS) techniques. The results are discussed in order to probe the interaction at the interface between a noble metal and a thiol ligand at the atomic level and the aim of this study is to shed light on the chemical structure and self-organization details of nanosystems. The nature of the chemical interaction between the dithiol ligand and the Ag atoms on the nanoparticle surface was investigated by combining SR-XPS (S2p, Ag3d core levels) and XAS (S and Ag K-edges) analysis. UV-visible absorption and emission measurements were also carried out on all samples and compared with TD-DFT calculations so as to get a better understanding of their optical behavior and establish the nature of the excitation and emission processes.

A biocompatible synthesis of gold nanoparticles by Tris(hydroxymethyl)aminomethane

Gold nanoparticles' novel properties are widely realized in catalysis, plasmonics, electronics, and biomedical applications. For biomedical application, one challenge is to find a non-toxic chemical and/or physical method of functionalizing gold nanoparticles with biomolecular compounds that can promote efficient binding, clearance, and biocompatibility and to assess their safety to other biological systems and their long-term effects on human health and reproduction. In the present study, we describe a new method by using Tris(hydroxymethyl)aminomethane (Tris), a widely used buffer solvent of nucleic acid and proteins, as the reducing agent for synthesizing gold nanoparticles by one step. It is found that Tris carries out the reduction reactions in relatively mild conditions for biomacromolecules. Particularly, it can be used to modify the DNA during the process of preparation of gold nanoparticles. The morphology and size distribution of gold nanoparticles are consistent and were confirmed by many different approaches including dynamic light scattering (DLS), UV-visible (UV-vis) spectrophotometry, atomic force microscopy (AFM), and transmission electron microscopy (TEM).

Hydrogen-assisted fabrication of spherical gold nanoparticles through sonochemical reduction of tetrachloride gold(III) ions in water

Spherical gold nanoparticles (AuNPs) were selectively synthesized through sonochemical reduction of tetrachloride gold(III) ions ([AuCl4](-)) in an aqueous solution of hydrogen tetrachloroaurate(III) tetrahydrate (HAuCl4 · 4H2O) with the aid of hydrogen (H2) gas in the absence of any additional capping agents. On the other hand, various shaped-AuNPs such as spherical nanoparticles, triangular and hexagonal plates were formed from sonochemical reduction of [AuCl4](-) in argon (Ar)-, nitrogen (N2)- or oxygen (O2)-purged aqueous [AuCl4](-) solutions. The selective fabrication of spherical AuNPs assisted by H2 gas is most likely attributed to the generation of hydrogen radicals (H) promoted by the reaction of H2 introduced and hydrogen oxide radicals (OH) produced by sonolysis of water.

Aptamer-conjugated nanomaterials for specific cancer cell recognition and targeted cancer therapy

Based on their unique advantages, increasing interest has been shown in the use of aptamers as target ligands for specific cancer cell recognition and targeted cancer therapy. Recently, the development of aptamer-conjugated nanomaterials has offered new therapeutic opportunities for cancer treatment with better efficacy and lower toxicity. We highlight some of the promising classes of aptamer-conjugated nanomaterials for the specific recognition of cancer cells and targeted cancer therapy. Recent developments in the use of novel strategies that enable sensitive and selective cancer cell recognition are introduced. In addition to targeted drug delivery for chemotherapy, we also review how aptamer-conjugated nanomaterials are being incorporated into emerging technologies with significant improvement in efficiency and selectivity in cancer treatment.

Gold colloids: from quasi-homogeneous to heterogeneous catalytic systems

Ruby red colloids of gold have been used for thousands of years and in the past have attracted much attention due to their optical properties. Surface plasmon resonance (SPR) bands are responsible for gold colloid colors and typically appear for nanometer-sized gold nanoparticles (GNPs). These lie in the visible range and their position (and intensity) depends on the size, distribution of size, and shape of GNPs but also on their interaction with other materials (i.e., support). Scientists consider colloids as quasi-homogeneous systems, but because of their intrinsic thermodynamic instability, they need different capping agents providing sufficient stability. The strength and the nature of the interaction between the protective (or functionalizing) molecule and the GNP surface is of utmost importance. It can determine the catalytic properties of the nanoparticles, as they mainly interact with the active sites, thus interfering with reactant. Therefore, the protective layer should contribute to the colloid stability, but at the same time, it should not be irreversibly adsorbed on the active site of the GNP surface providing convenient accessibility to reactant. From a catalytic point of view, the milder the interaction is between the particle surface and the capping agent, the more the activity increases. Unfortunately, the reaction conditions often do not allow the required stability of GNPs, which constitutes a fundamental prerequisite for stable catalytic activity. Anchoring GNPs on suitable supports can circumvent the problem, and this technique is now considered a valuable alternative to classical methods to produce highly dispersed gold catalysts. In this Account, we describe the advantages in using this technique to produce gold heterogeneous catalysts of high metal dispersion on a large variety of supports with the possibility of tuning to a large extent the size and (even partially) the shape of GNPs. We also review our recent progress on the sol-immobilization technique. Specifically, we highlight how, depending on its nature, the protective agent not only mediates the activity of GNPs in alcohol oxidation process but also actively participates in the anchoring process and to the stability of GNPs depending on the support surface. We can also use the modification of the metal surface operated by the capping agent to prepare bimetallic species and influence the surface potential, which modifies the intrinsic activity of the GNP. In conclusion, this technique implies many contributions (sometimes not yet clarified factors) that are not simply concerning dimension and dispersion of GNPs or type of support. Chemists should make careful selection of the protective agent and reaction parameters depending on which support is used in which reaction.

Nanomaterials enhanced surface plasmon resonance for biological and chemical sensing applications

The main challenge for all electrical, mechanical and optical sensors is to detect low molecular weight (less than 400 Da) chemical and biological analytes under extremely dilute conditions. Surface plasmon resonance sensors are the most commonly used optical sensors due to their unique ability for real-time monitoring the molecular binding events. However, their sensitivities are insufficient to detect trace amounts of small molecular weight molecules such as cancer biomarkers, hormones, antibiotics, insecticides, and explosive materials which are respectively important for early-stage disease diagnosis, food quality control, environmental monitoring, and homeland security protection. With the rapid development of nanotechnology in the past few years, nanomaterials-enhanced surface plasmon resonance sensors have been developed and used as effective tools to sense hard-to-detect molecules within the concentration range between pmol and amol. In this review article, we reviewed and discussed the latest trend and challenges in engineering and applications of nanomaterials-enhanced surface plasmon resonance sensors (e.g., metallic nanoparticles, magnetic nanoparticles, carbon-based nanomaterials, latex nanoparticles and liposome nanoparticles) for detecting "hard-to-identify" biological and chemical analytes. Such information will be viable in terms of providing a useful platform for designing future ultrasensitive plasmonic nanosensors.

Template synthesis of gold nanoparticles with an organic molecular cage

We report a novel strategy for the controlled synthesis of gold nanoparticles (AuNPs) with narrow size distribution (1.9 ± 0.4 nm) through NP nucleation and growth inside the cavity of a well-defined three-dimensional, shape-persistent organic molecular cage. Our results show that both a well-defined cage structure and pendant thioether groups pointing inside the cavity are essential for the AuNP synthesis.

On the mechanism of phase transfer catalysis in Brust-schiffrin synthesis of metal nanoparticles

The two-phase Brust-Schiffrin method (BSM) is used to synthesize highly stable nanoparticles of noble metals. A phase transfer catalyst (PTC) is used to bring in aqueous phase soluble precursors into the organic phase to enable particle synthesis there. Two different mechanisms for phase transfer are advanced in the literature. The first mechanism considers PTC to bring in an aqueous phase soluble precursor by complexing with it. The second mechanism considers the ionic species to be contained in inverse micelles of PTC, with a water core inside. A comprehensive experimental study involving measurement of interfacial tension, viscosity, water content by Karl-Fischer titration, static light scattering, (1)H NMR, and small-angle X-ray scattering is reported in this work to establish that the phase transfer catalyst tetraoctylammonium bromide transfers ions by complexing with them, instead of encapsulating them in inverse micelles. The findings have implications for particle synthesis in two-phase methods such as BSM and their modification to produce more monodispersed particles.

On the mechanism of metal nanoparticle synthesis in the Brust-Schiffrin method

Brust-Schiffrin synthesis (BSS) of metal nanoparticles has emerged as a major breakthrough in the field for its ability to produce highly stable thiol functionalized nanoparticles. In this work, we use a detailed population balance model to conclude that particle formation in BSS is controlled by a new synthesis route: continuous nucleation, growth, and capping of particles throughout the synthesis process. The new mechanism, quite different from the others known in the literature (classical LaMer mechanism, sequential nucleation-growth-capping, and thermodynamic mechanism), successfully explains key features of BSS, including size tuning by varying the amount of capping agent instead of the widely used approach of varying the amount of reducing agent. The new mechanism captures a large body of experimental observations quantitatively, including size tuning and only a marginal effect of the parameters otherwise known to affect particle synthesis sensitively. The new mechanism predicts that, in a constant synthesis environment, continuous nucleation-growth-capping mechanism leads to complete capping of particles (no more growth) at the same size, while the new ones are born continuously, in principle leading to synthesis of more monodisperse particles. This prediction is validated through new experimental measurements.

Packing and self-assembly of truncated triangular bipyramids

Motivated by breakthroughs in the synthesis of faceted nano- and colloidal particles, as well as theoretical and computational studies of their packings, we investigate a family of truncated triangular bipyramids. We report dense periodic packings with small unit cells that were obtained via numerical and analytical optimization. The maximal packing fraction φ(max) changes continuously with the truncation parameter t. Eight distinct packings are identified based on discontinuities in the first and second derivatives of φ(max)(t). These packings differ in the number of particles in the fundamental domain (unit cell) and the type of contacts between the particles. In particular, we report two packings with four particles in the unit cell for which both φ(max)(t) and φ(max)'(t) are continuous and the discontinuity occurs in the second derivative only. In the self-assembly simulations that we perform for larger boxes with 2048 particles, only one out of eight packings is found to assemble. In addition, the degenerate quasicrystal reported previously for triangular bipyramids without truncation [Haji-Akbari et al., Phys. Rev. Lett. 107, 215702 (2011)] assembles for truncations as high as 0.45. The self-assembly propensities for the structures formed in the thermodynamic limit are explained using the isoperimetric quotient of the particles and the coordination number in the disordered fluid and in the assembled structure.

SERS substrates formed by gold nanorods deposited on colloidal silica films

We describe a new approach to the fabrication of surface-enhanced Raman scattering (SERS) substrates using gold nanorod (GNR) nanopowders to prepare concentrated GNR sols, followed by their deposition on an opal-like photonic crystal (OPC) film formed on a silicon wafer. For comparative experiments, we also prepared GNR assemblies on plain silicon wafers. GNR-OPC substrates combine the increased specific surface, owing to the multilayer silicon nanosphere structure, and various spatial GNR configurations, including those with possible plasmonic hot spots. We demonstrate here the existence of the optimal OPC thickness and GNR deposition density for the maximal SERS effect. All other things being equal, the analytical integral SERS enhancement of the GNR-OPC substrates is higher than that of the thick, randomly oriented GNR assemblies on plain silicon wafers. Several ways to further optimize the strategy suggested are discussed.

Synthesis and in vitro studies of gold nanoparticles loaded with docetaxel

The aim of these studies was to synthesize, characterize and evaluate the efficacy of pegylated gold nanoparticles (AuNPs) that differed in their PEG molecular weight, using PEG 550 and PEG 2000. The synthesis of the gold nanoparticles was carried out by modified Brust method with a diameter of 4-15 nm. The targeting agent folic acid was introduced by the covalent linkage. Finally, the anti-cancer drug docetaxel was encapsulated by the AuNPs by non covalent adsorption. The nanoparticles were characterized by transmission electron microscopy and used for in vitro studies against a hormone-responsive prostate cancer cell line, LnCaP. The loaded nanoparticles reduced the cell viability in more than 50% at concentrations of 6 nM and above after 144 h of treatment. Moreover, observation of prostate cancer cells by optical microscopy showed damage to the cells after exposure to drug-loaded AuNPs while unloaded AuNPs had much less effect.

Efficient general procedure to access a diversity of gold(0) particles and gold(I) phosphine complexes from a simple HAuCl4 source. Localization of homogeneous/heterogeneous system's interface and field-emission scanning electron microscopy study

Soluble gold precatalysts, aimed for homogeneous catalysis, under certain conditions may form nanoparticles, which dramatically change the mechanism and initiate different chemistry. The present study addresses the question of designing gold catalysts, taking into account possible interconversions and contamination at the homogeneous/heterogeneous system's interface. It was revealed that accurate localization of boundary experimental conditions for formation of molecular gold complexes in solution versus nucleation and growth of gold particles opens new opportunities for well-known gold chemistry. Within the developed concept, a series of practical procedures was created for efficient synthesis of soluble gold complexes with various phosphine ligands (R3P)AuCl (90-99% yield) and for preparation of different types of gold materials. The effect of the ligand on the particles growth in solution has been observed and characterized with high-resolution field-emission scanning electron microscopy (FE-SEM) study. Two unique types of nanostructured gold materials were prepared: hierarchical agglomerates and gold mirror composed of ultrafine smoothly shaped particles.

DNAzyme-functionalized gold nanoparticles for biosensing

Recent progress in using DNAzyme-functionalized gold nanoparticles (AuNPs) for biosensing is summarized in this chapter. A variety of methods, including those for attaching DNA on AuNPs, detecting metal ions and small molecules by DNAzyme-functionalized AuNPs, and intracellular applications of DNAzyme-functionalized AuNPs are discussed. DNAzyme-functionalized AuNPs will increasingly play more important roles in biosensing and many other multidisciplinary applications. This chapter covers the recent advancement in biosensing applications of DNAzyme-functionalized gold nanoparticles, including the detection of metal ions, small molecules, and intracellular imaging.