Effect of freeze-thawing on lipid bilayer-protected gold nanoparticles

In this study, varieties of lipid bilayer-protected gold nanoparticles (AuNPs) were synthesized through a simple wet chemical method, and then the effect of freeze-thawing on the as-prepared AuNPs was investigated. The freeze-thawing process induced fusion or fission of lipid bilayers tethered on the AuNPs. The UV-vis spectra and transmission electron microscopy experiments revealed that the disruption of lipid bilayer structures on the nanoparticles led to the fusion or aggregation of AuNPs. The role of freeze-thawing in the evolution of lipid bilayer-protected AuNPs was studied. The addition of adequate sucrose, a well-known cryoprotectant, effectively prevented the fusion or aggregation of lipid bilayer-protected AuNPs undergoing the freeze-thawing process. The possible mechanism of sucrose preserving the integrity of the lipid bilayer-protected AuNPs was also discussed.     

Photochemical synthesis and self-assembly of gold nanoparticles

Colloidal gold was prepared by UV light irradiation of the mixture of HAuCl₄ aqueous solution and poly(vinyl pyrrolidone) (PVP) ethanol solution in the presence of silver ions. The resulting sheet-like nanoparticles were found to self-assemble into nanoflowers by a centrifuging process. The results of control experiments reflected that only suitable size sheet-like nanoparticles could assemble into the flower-like structures. The presence of Ag ions and PVP are essential for the formation process of nanoflowers. They perform their function by serving as structure-directing agents to produce the sheet-like particles. The appearance of the flower-like assemblages is attributed to the combination of Van der Waals force and the anisotropic hydrophobic attraction between the nanoparticles. The flower-like assemblages films can be used as surface-enhanced Raman spectroscopy (SERS) substrates with 4-aminothiophenol (4-ATP) molecule as a test probe.     

Synthesis, characterization, and self-assembly of protein lysozyme monolayer-stabilized gold nanoparticles

Lysozyme monolayer-protected gold nanoparticles (Au NPs) which are hydrophilic and biocompatible and show excellent colloidal stability (at low temperature, ca. 4 degrees C), were synthesized in aqueous medium by chemical reduction of HAuCl4 with NaBH4 in the presence of a familiar small enzyme, lysozyme. UV-vis spectra, transmission electron microscopy (TEM), atomic force microscopy, and X-ray photoelectron spectroscopy characterization of the as-prepared nanoparticles revealed the formation of well-dispersed Au NPs of ca. 2 nm diameter. Moreover, the color change of the Au NP solution as well as UV-vis spectroscopy and TEM measurements have also demonstrated the occurrence of Ostwald ripening of the nanoparticles at low temperature. Further characterization with Fourier transform infrared spectroscopy (FTIR) and dynamic light scattering indicated the formation of a monolayer of lysozyme molecules on the particle surface. FTIR data also indicated the intactness of the protein molecules coated on Au NPs. All the characterization results showed that the monodisperse Au NPs are well-coated directly with lysozyme. Driven by the dipole-dipole attraction, the protein-stabilized Au NPs self-assembled into network structures and nanowires upon aging under ambient temperature. On the basis of their excellent colloidal stability, controlled self-assembly ability, and biocompatible surface, the lysozyme monolayer-stabilized Au NPs hold great promise for being used in nanoscience and biomedical applications.     

Size-controlled synthesis and characterization of thiol-stabilized gold nanoparticles

Size-controlled synthesis of nanoparticles of less than a few nanometers in size is a challenge due to the spatial resolution limit of most scattering and imaging techniques used for their structural characterization. We present the self-consistent analysis of the extended x-ray absorption fine-structure (EXAFS) spectroscopy data of ligand-stabilized metal nanoclusters. Our method employs the coordination number truncation and the surface-tension models in order to measure the average diameter and analyze the structure of the nanoparticles. EXAFS analysis was performed on the two series of dodecanethiol-stabilized gold nanoparticles prepared by one-phase and two-phase syntheses where the only control parameter was the gold/thiol ratio xi, varied between 6:1 and 1:6. The two-phase synthesis resulted in the smaller particles whose size decreased monotonically and stabilized at 16 A when xi was lowered below 1:1. This behavior is consistent with the theoretically predicted thermodynamic limit obtained previously in the framework of the spherical drop model of Au nanoparticles.     

One-step synthesis and characterization of dendrimer-protected gold nanoparticles

We proposed an economic, convenient, and mild synthesis of dendrimer-protected gold nanoparticles by exposing a third-generation poly(propyleneimine) dendrimer-HAuCl₄ aqueous solution to sunlight without the additional step of introducing other reducing agents and protective agents. Most importantly, it is found that the size of the gold nanoparticles thus formed can be controlled by the molar ratio of the dendrimer to gold. The text was submitted by the author in English.     

Rapid one-step synthesis, characterization and functionalization of silica coated gold nanoparticles

This paper describes a rapid, simple and one-step method for preparing silica coated gold (Au@SiO₂) nanoparticles with fine tunable silica shell thickness and surface functionalization of the prepared particles with different groups. Monodispersed Au nanoparticles with a mean particle size of 16 nm were prepared by citrate reduction method. Silica coating was carried out by mixing the as prepared Au solution, tetraethoxysilane (TEOS) and ammonia followed by microwave (MW) irradiation. Although there are several ways of coating Au nanoparticles with silica in the literature, each of these needs pre-coating step as well as long reaction duration. The present method is especially useful for giving the opportunity to cover the colloidal Au particles with uniform silica shell within very short time and forgoes the use of a silane coupling agent or pre-coating step before silica coating. Au@SiO₂ nanoparticles with wide range of silica shell thickness (5-105 nm) were prepared within 5 min of MW irradiation by changing the concentration of TEOS only. The size uniformity and monodispersity were found to be better compared to the particles prepared by conventional methods, which were confirmed by dynamic light scattering and transmission electron microscopic techniques. The prepared Au@SiO₂ nanoparticles were further functionalized with amino, carboxylate, alkyl groups to facilitate the rapid translation of the nanoparticles to a wide range of end applications. The functional groups were identified by XPS, and zeta potential measurements.     

Fabrication and electrochemical application of three-dimensional gold nanoparticles: self-assembly

Multilayers film of nanostructured citrate-stabilized gold particles (AuNPs) has been fabricated based on the layer-by-layer (LBL) technique using a self-assembled monolayer of 1,4-benzenedimethanethiol (BDMT). The formation of AuNPs and BDMT self-assemblies as alternative multilayers was confirmed by transmission electron microscopy (TEM), X-ray photoelectron spectroscope (XPS), and quartz crystal microbalance (QCM). The formation of uniform AuNP layers with an average monolayer thickness of 5-6 nm was obvious in the TEM images. The existence of BDMT molecules as cross linkers for the AuNPs' layers was proved by XPS measurements. The greater affinity of AuNPs' layers to bind BDMT molecules in comparison with the bare Au bulk electrode was revealed by QCM measurements. Electrochemically, the AuNPs' layers on the electrode surface did not only catalyze the reduction of oxygen (ca. 100-mV positive shift of the reduction peak potential compared with that at the bare Au bulk electrode) but also showed a fascinating nature of working as a renewed activated-electrode surface; a zigzag response was observed for oxygen reduction during alternative immobilization of BDMT and the AuNP layer. The self-assembly of a new AuNPs layer restored the catalytic activity that was entirely blocked by the preceding BDMT layer.     

Polymer and biopolymer mediated self-assembly of gold nanoparticles

Gold nanoparticle-polymer composites are versatile and diverse functional materials, with applications in optical, electronic and sensing devices. This tutorial review focuses on the use of polymers to control the assembly of gold nanoparticles. Examples of synthetic polymers and biopolymers are provided, as well as applications of the composite materials in sensing and memory devices.     

Polymer-mediated chain-like self-assembly of functionalized gold nanoparticles

We report an easy solution phase template-based method to assemble mercaptoundecanoic acid-functionalized gold nanoparticles (MUA-GNPs) along poly(ethylene oxide) (PEO) chains. Transmission electron microscopy (TEM) images show one-dimensional and two-dimensional chain-like sequences of GNPs resembling PEO chains. The progress of the assembly was monitored by the evaluation of surface plasmon resonance band of MUA-GNPs with time by UV-vis spectroscopy. The assembly process is a result of hydrogen bonding interaction between the ethereal oxygen of PEO and carboxylic acid group of MUA attached to GNPs surface, which was confirmed through FTIR spectroscopy. The interaction between PEO and MUA-GNPs was further confirmed by thermal analysis using differential scanning calorimetry.     

Surfactant-assisted synthesis and characterization of stable silver bromide nanoparticles in aqueous media

Colloidal dispersions of silver bromide (AgBr) in aqueous surfactant medium have been prepared using a surfactant-assisted synthesis approach with hexadecyltrimethylammonium bromide (CTAB). The surfactant acts both as source of bromide ion as well as the stabilizing agent. Upon progressive addition of silver nitrate to aqueous CTAB solution, stable AgBr dispersions were obtained. Formation of surfactant cation (CTA(+)) stabilized AgBr was confirmed by way of XRD, FTIR and NMR studies. Thermal behavior of the isolated nanoparticles was investigated by differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA), where the occurrence of phase transition in the surfactant-stabilized nanoparticles was observed. Kinetics of the particle growth was investigated by dynamic light scattering measurements, which predicted the formation of surfactant bilayered structures associated with the nanoparticles of AgBr. Band gap of the nanoparticles was determined by suitably analyzing the UV-visible spectral data, which concluded that the particles behaved like insulators. Morphology of the particles, studied by TEM measurements, was found to be spherical. Finally, enthalpy of formation of surfactant-stabilized AgBr, determined calorimetrically, was found to be dependent on the concentration of the precursors.     

Synthesis and self-assembly of highly monodispersed quasispherical gold nanoparticles

We report the synthesis of cetyltrimethylammonium bromide (CTAB) assisted seed mediated growth of highly pure and monodispersed quasispherical gold nanoparticles (QAuNPs) and their self-assembly on the silica/glass substrates. The seed-mediated growth approach was modified to prepare size-tunable monodispersed QAuNPs with sizes ranging from 20 to 150 nm. The larger, more uniform seeds and lower CTAB concentration resulted in the formation of relatively large QAuNPs with improved monodispersity (relative standard deviation (RSD) of ～5-8%) and high purity in their shapes. In addition, CATB-capped QAuNPs can be spontaneously assembled into closely packed and highly aligned superstructures with well-defined mutillayers (two to six layers) on silica substrates. Furthermore, CATB-capped QAuNPs can easily construct density-controllable QAuNP chips by electrostatic self-assembly, showing their promising applications for single-nanoparticle plasmonic sensors.     

Design, synthesis, and characterization of mesogenic amine-capped nematic gold nanoparticles with surface-enhanced plasmonic resonances

The use of the liquid-crystalline state to control the assembly of large (>5 nm) gold nanoparticles (NPs) is of considerable interest because of the promise of novel metamaterial properties of such systems. Here we report on a new approach for the preparation of large nematic gold NPs using a bifunctional capping agent that enables control over the particle size and serves as a linkage for subsequent functionalization with mesogenic groups. Properties of the NPs were characterized by HRTEM, NMR, DSC, TGA, UV/vis, OPM, and XRD studies. The results confirmed the formation of a stable nematic mesophase above 37.5 °C for NPs in the 6-11 nm size range.     

Size quantized formation and self-assembly of gold encased silver nanoparticles

Mixing aqueous dispersions of thiocyanate ion coated small (< 3.5 nm diameter) gold nanoparticles and EDTA covered larger (> 22 nm diameter) silver nanoparticles, results in the formation of robust gold encased silver nanoparticles; in contrast to using larger (> 11 nm diameter) gold nanoparticles which forms chained structures.     

Metal oxide nanoparticles: synthesis, characterization and application

The synthesis of metal oxide nanoparticles is described in terms of precursor formation, nucleation, growth, and aging processes. The main parameters governing these processes are the solution properties, including the solvent viscosity, dielectric constant and the presence of adsorbing anions, the solubility of the metal oxide, and the metal oxide surface energy.     

Bioinspired synthesis,characterization and antifungal activity of enzyme-mediated gold nanoparticles using a fungal oxidoreductase

The development of efficient cell-free systems of nanoparticle synthesis using microbial enzymes is a growing field of biological and green chemistry for the supportable improvement in nano-biotechnology. In the present study, we established a cell-free system for producing gold nanoparticles (AuNPs) using a fungal oxidoreductase named sulfite oxidoreductase purified to homogeneity from Fusarium oxysporum. The enzyme was purified by ultrafiltration followed by anion exchange chromatography on DEAE Sephadex A-50 gel, and its molecular weight was determined by gel filtration chromatography on Sephacryl S-300 gel. The purified enzyme had a molecular weight of 346 kDa. It was composed of three subunits of 176, 94 and 76 kDa. Purified enzyme was successfully used for production of gold nanoparticles in a cell-free system. Synthesized gold nanoparticles showed the highest absorbance at 520 nm wavelength as shown by UV–visible spectroscopy. They were spherical in shape with an average size of 20 nm as determined by scanning and transmission electron microscopy and dynamic light scattering. Assessment of the antifungal properties of synthesized nanoparticles by disk diffusion method indicated a potent growth inhibitory activity against all tested human pathogenic yeasts and molds by inhibition zones ranged from 10 to 18 mm. Taken together, our enzymatically established method of nanoparticle synthesis using a purified sulfite oxidoreductase of F. oxysporum can be considered as an efficient tool for generating harmless bioactive gold nanoparticles with potential applications in biology, medicine and industry.     

Control of the self-assembly behaviors of charged gold nanoparticles

In this study, the formation mechanism of the self-assembly structures of aqueous gold nanoparticles (NPs) was investigated through deliberately altering the species of ligands, solvent ratios, and especially solvent evaporation conditions. By analyzing UV–vis spectra, transmission electron microscope (TEM) images, and scanning electron microscope (SEM) images, it was found that the self-assembly process was dependent on both the equilibrium of various interparticle interactions in solution and the evaporation rate of solvents. The various interparticle interactions in solution generated an anisotropic interaction for one-dimension (1D) self-assembly, whereas the evaporation rate of solvents determined the specific of 1D derivative structures. Our results demonstrated an efficient protocol for spatial arrays of charged NPs with controllable morphologies.     

Liposome induced self-assembly of gold nanoparticles into hollow spheres

Gold nanoparticles were prepared by the reduction of [(C7H15)4N]+ [AuCl4]- with 3,4-ethylenedioxythiophene (EDOT) as reductant in toluene solution. The employed stabilizers include 3,3'-thiodipropionic acid (TDPA), 1-dodecanethiol (DDT), (+/-)-10-camphorsulfonic acid (CSA), and 11-mercaptoundecanoic acid (MUA). The reaction processes were tracked by UV-vis and FT-IR spectroscopy, and the as-prepared gold nanoparticles were characterized by scanning electron microscopy, transmission electron microscopy, energy-dispersive spectroscopy, and X-ray photoelectron spectroscopy measurements. When TDPA and MUA, which possess both -S- and -COOH groups, were used as the stabilizer in the preparation, the as-prepared nanoparticles could self-assemble into hollow spheres. While when DDT with a -SH group or CSA with a -SO3H group was used as the protecting agents, only discrete gold nanoparticles were observed. The results show that the groups of both -S- and -COOH in the stabilizer play an important role in forming the hollow nanospheres. It is proposed that the formation mechanism of the hollow spheres is a liposome that formed between -COO- and [(C7H15)4 N]+ could act as a template to induce the self-assembly of the gold nanoparticles into the hollow spheres.     

Single-crystalline gold microplates: synthesis, characterization, and thermal stability

Single-crystalline gold microplates of several 10 microm in lateral size, characterized by hexagonal, truncated triangular, and triangular shapes with (111) planes as two basal surfaces, have been synthesized in large quantities through a solution phase process. Significantly, such anisotropic Au nanostructures exhibit remarkable optical properties, in which the dipole plasmon resonance shifting in the NIR region and the quadrupole plasmon resonance at approximately 820 nm were observed. Fragmentation of Au microplates is found when the temperature is higher than 450 degrees C, indicating they are not thermodynamically stable structure at high temperature. Investigations on the Au microplates upon heating suggest that the melting and collapsing start mainly at the edges that should be Au (110) facets. This work is valuable for Au nanostructures applied at elevated temperatures.