Polymer mediated self-assembly of magnetic nanoparticles

We present a simple polymer-mediated process of assembling magnetic FePt nanoparticles on a solid substrate. Alternatively absorbing the PEI molecule and FePt nanoparticles on a HO-terminated solid surface leads to a smooth FePt nanoparticle assembly with controlled assembly thickness and dimension. Magnetic measurements show that the thermally annealed FePt nanoparticle assembly as thin as three nanoparticle layers is ferromagnetic. The magnetization direction of this thin FePt nanoparticle assembly is readily controlled with the laser-assisted magnetic writing. The reported process can be applied to various substrates, nanoparticles, and functional macromolecules and will be useful for future magnetic nanodevice fabrication.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

Size crystallographic effects for silver and gold nanoparticles dispersed in a biopolymer matrix

Crystallographic size effects occurring during the formation of zero-valence silver and gold nanoparticles dispersed in a biopolymer polysaccharide matrix (arabinogalactan) have been studied by means of X-ray diffraction analysis. The average size of the nanoparticles has been found to increase with the increase in the metal content in the nanocomposite. Stabilization of the nanoparticles by the polymer matrix is accompanied by the decrease in the unit cell parameter of the metal correlated with the decrease in the coherent scattering length.     

Assembly of gold nanoparticles mediated by multifunctional fullerenes

The understanding of the interparticle interactions of nanocomposite structures assembled using molecularly capped metal nanoparticles and macromolecular mediators as building blocks is essential for exploring the fine-tunable interparticle spatial and macromolecular properties. This paper reports the results of an investigation of the chemically tunable multifunctional interactions between fullerenes (1-(4-methyl)-piperazinyl fullerene, MPF) and gold nanoparticles. The interparticle spatial properties are defined by the macromolecular and multifunctional electrostatic interactions between the negatively charged nanoparticles and the positively charged fullerenes. In addition to characterization of the morphological properties, the surface plasmon resonance band, dynamic light scattering, and surface-enhanced Raman scattering (SERS) properties of the MPF-mediated assembly and disassembly processes have been determined. The change of the optical properties depends on the pH and electrolyte concentrations. The detection of the Raman-active vibration modes (Ag(2) and Hg(8)) of C60 and the determination of their particle size dependence have demonstrated that the adsorption of MPF on the nanoparticle surface in the MPF-Au nm assembly is responsible for the SERS effect. These findings provide new insights into the delineation between the interparticle interactions and the nanostructural properties for potential applications of the nanocomposite materials in spectroscopic and optical sensors and in controlled releases.     

Enhanced ordering in gold nanoparticles self-assembly through excess free ligands

Self-assembly of nanometer-sized particles is an elegant and economical approach to achieve dense patterns over large areas beyond the resolution and throughput capabilities of electron-beam lithography. In this paper, we present results of self-assembly of oleylamine-capped gold nanoparticles with 8.0 ± 0.3 nm diameter into densely packed and well-ordered monolayers with center-to-center distance of ～11 nm. Self-assembly was done in a Langmuir-Blodgett trough and picked up onto Si substrates. The nanoparticles undesirably assembled within micrometer-sized "droplets" that were organic in nature. However, within these droplets, we observed that the addition of the excess ligand, oleylamine, drastically enhanced the self-assembly of the nanoparticles into monolayers with near-perfect ordering. This approach has the potential use in templated self-assembly of nanoparticles for rearranging poorly ordered assembly into a commensurate prepatterned substrate.     

Water-soluble conjugated polymer-induced self-assembly of gold nanoparticles and its application to SERS

Gold nanochains were prepared by the assembly of citrate-stabilized gold nanospheres induced by cationic conjugated polymers. This assembly method was rapid, and the assembled product was very stable. A longitudinal plasmon resonance band was formed as a result of the coupling of gold nanoparticles and can be tuned from visible to near-infrared by adjusting the polymer/Au molar ratio. The gold nanochains were used as a SERS substrate and gave an enhancement factor of 8.4 x 10 (9), which is approximately 400 times larger than that on the isolated gold nanosphere substrate. The giant SERS enhancement is ascribed to the large electromagnetic fields of coupled gold nanoparticles.     

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.     

Spatially controlled self-assembly of gold nanoparticles encased in alpha-helical polypeptide nanospheres

Gold nanoparticles (Au-NPs) are encased in aqueous nanospheres of alpha-helical poly(gamma-benzyl L-glutamate)s (PBLG, number average degree of polymerization: n = 32), with spatially controlled self-assembly structures of solid core-shell nanospheres or double-layered hollow nanocapsules.     

Fractal-like self-assembly of oligo(p-phenylene vinylene) capped gold nanoparticles

Reversible interparticle self-organization is reported for gold nanoparticles functionalized with an oligo(p-phenylene vinylene) moiety in butanol. The aggregates show a clear melting temperature at 80 degrees C, and atomic force microscopy and transmission electron microscopy indicate a fractal-like organization of the particles.     

Interparticle interactions in glutathione mediated assembly of gold nanoparticles

The understanding of the detailed molecular interactions between (GSH) glutathione molecules in the assembly of metal nanoparticles is important for the exploitation of the biological reactivity. We report herein results of an investigation of the assembly of gold nanoparticles mediated by glutathione and the disassembly under controlled conditions. The interparticle interactions and reactivities were characterized by monitoring the evolution of the surface plasmon resonance band using the spectrophotometric method and the hydrodynamic sizes of the nanoparticle assemblies using the dynamic light scattering technique. The interparticle reactivity of glutathiones adsorbed on gold nanoparticles depends on the particle sizes and the ionic strength of the solution. Larger-sized particles were found to exhibit a higher degree of interparticle assembly than smaller-sized particles. The assembly-disassembly reversibility is shown to be highly dependent on pH and additives in the solution. The interactions of the negatively charged citrates surrounding the GSH monolayer on the particle surface were believed to produce more effective interparticle spatial and electrostatic isolation than the case of OH (-) groups surrounding the GSH monolayer. The results have provided new insights into the hydrogen-bonding character of the interparticle molecular interaction of glutathiones bound on gold nanoparticles. The fact that the interparticle hydrogen-bonding interactions in the assembly and disassembly processes can be finely tuned by pH and chemical means has implications to the exploitation of the glutathione-nanoparticle system in biological detection and biosensors.     

Two strategies for the self-assembly of gold nanoparticles: Photoreaction and radical reaction

Poly(N-isopropylacrylamide)-b-poly(vinylpyridine) (PNIPAAm-b-PVP) and poly(N-isopropylacrylamide-co-hydroxylethyl methacrylate)-b-poly(vinylphenol) (P(NIPAAm-co-HEMA)-b-PVPhol) were synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization. Above two copolymers could form complex in pure water and in DMF/water environment with the DMF content lower than 40% by hydrogen bondings. The morphologies of the complex were investigated by transmission electron microscope (TEM). It was found that the dimension of the complex in strong acid (pH 1.0) or base environment (pH 12.0) was smaller than the one in weak acid or in neutral environment. After the shell cross-linking of the complex, the complex showed a type of "swollen" state in acid or base environment which is similar to the properties of microgel.     

Didodecyldimethylammonium bromide lipid bilayer-protected gold nanoparticles: synthesis, characterization, and self-assembly

Didodecyldimethylammonium bromide (DDAB) lipid bilayer-protected gold nanoparticles (AuNPs), which were stable and hydrophilic, were synthesized by in situ reduction of HAuCl(4) with NaBH(4) in an aqueous medium in the presence of DDAB. As-prepared nanoparticles were characterized by UV-vis spectra, transmission electron microscopy, dynamic light scattering analysis, and X-ray photoelectron spectroscopy. All these data supported the formation of AuNPs. Fourier transform infrared spectroscopy (FTIR) and differential thermal analysis/thermogravimetric analysis data revealed that DDAB existed in a bilayer structure formed on the particle surface, resulting in a positively charged particle surface. The FTIR spectra also indicated that the DDAB bilayer coated on the surface of AuNPs was probably in the ordered gel phase with some end-gauche defects. On the basis of electrostatic interactions between such AuNPs and anionic polyelectrolyte poly(sodium 4-styrenesulfonate) (PSS), we successfully fabricated (PSS/AuNP)(n)() multilayers on a cationic polyelectrolyte poly(ethylenimine) coated indium tin oxide substrate via the layer-by-layer self-assembly technique and characterized as-formed multilayers with UV-vis spectra and atomic force microscopy.