Matter of age: growing anisotropic gold nanocrystals in organic media

We investigated the influence of the reduction state of gold ions on the growth of gold nanocrystals in N,N-dimethyl formamide (DMF). While freshly prepared solutions of AuCl3 produce spherical nanocrystals, aged precursor solutions containing mainly Au+ ions and Au(0) atoms lead to various branched nanoparticles. Furthermore, we show that also the amount of the reducing and stabilisation agent tetra-n-octylammonium formate (TOAF) plays a decisive role on the shape of the nanocrystals, allowing us to grow triangular and cubic nanoparticles.     

Polyhedral gold nanocrystals with O h symmetry: from octahedra to cubes

We report the shape and size control of polyhedral gold nanocrystals by a modified polyol process. The rapid reduction of gold precursors in refluxing 1,5-pentanediol has successfully provided a series of gold nanocrystals in the shape of octahedra, truncated octahedra, cuboctahedra, cubes, and higher polygons by incremental changes of silver nitrate concentration. All nanocrystals were obtained quantitatively and were uniform in shape and size in the range of approximately 100 nm. Smaller octahedra and cubes were also prepared by using large amounts of PVP. Silver species generated from AgNO3 seemed to determine the final nanocrystal morphology by the selective growth of {111} and/or the restriction of {100}. The shape evolution of the particles was addressed by quenching the reactions at different time intervals. The approximately 60 nm seeds were generated rapidly and grown slowly with simultaneous edge sharpening. Aging the reaction mixture focused the size and shape of the nanocrystals by Ostwald ripening. We believe that our selective growth conditions can be applied to other shapes and compositions of face-centered cubic metals.     

Electrochemical synthesis of gold nanocrystals and their 1D and 2D organization

Size-controlled gold nanocrystals were conveniently synthesized through direct electroreduction of bulk AuCl(4)(-) ions in the presence of poly(N-vinylpyrrolidone) (PVP). PVP greatly enhanced the gold particle formation process and also significantly retarded the gold electrodeposition process, allowing the electrochemical synthesis of gold nanocrystals to be carried out in the form of simple electroreduction. This novel electrochemical method may be extended to synthesis of other noble metal nanoparticles with controllable size on a large scale. The PVPK90-protected gold nanocrystals spontaneously self-assembled into nearly ordered 2D close-packed arrays and interesting 1D nanostructures. The aggregation of unstable PVPK17-protected gold nanocrystals resulted in the formation of ultrathin single-crystalline films. PVP plays multifunctional roles in controlling the size and shape of gold nanocrystals and in inducing individual gold nanocrystals to construct 1D nanostructures. The nanoparticle self-assembling technique based on PVP offers a simple, but effective, path to organize individual gold nanoparticles into various 1D and 2D nanostructured materials.     

Fabrication of Hierarchical Fe3O4@SiO2@P(4VP‐DVB)@Au Nanostructures and Their Enhanced Catalytic Properties

Hierarchical Fe₃O₄@SiO₂@P(4VP‐DVB)@Au nanostructures were prepared in which the slightly cross‐linked, thin poly(4‐vinylpyridine‐co‐divinylbenzene) (P(4VP‐DVB)) shells were constructed onto Fe₃O₄@SiO₂ nanospheres, followed by in situ embedding of gold nanocrystals homogeneously into the P4VP chains. These slightly cross‐linked chains, easily swollen by the reactants, make the gold nanocrystals accessible to the reactants, and the thin shell (about 15 nm) reduces the diffusion distance of the reactants to the active gold nanocrystals (about 5 nm), thereby enhancing their catalytic activity and utility. At the same time, confinement of gold nanocrystals within the P4VP shells prevents their migration and coagulation during catalytic transformations. Hence the nanocomposites exhibit high activity (up to 4369.5 h^?1 of turnover frequency (TOF)) and controllable magnetic recyclability without any significant loss of gold species after ten runs of catalysis in the reduction of 4‐nitrophenol.     

用含偕胺肟基螯合纤维吸附、还原Au^3＋的研究（Ⅱ）：—氧化还原过程的初步研究

讨论了含偕胺肟基螯合纤维对Au^3 的还原过程及螯合与还原反应间的关系。结果表明，含偕胺肟基螯合纤维在吸附Au^3 的过程中，功能基螯合Au^3 之后再将其部分还原成单质金，而偕胺肟基则先被氧化成酰氨基进而变成羧基，吸附了Au^3 的纤维经径高温灼烧后合理使可得到海绵金。     

Seed‐Mediated and Iodide‐Assisted Synthesis of Gold Nanocrystals with Systematic Shape Evolution from Rhombic Dodecahedral to Octahedral Structures

We report the development of a seed‐mediated and iodide‐assisted method for the synthesis of monodisperse gold nanocrystals with systematic shape evolution from rhombic dodecahedral to octahedral structures. Particle growth is complete in 15 min at room temperature, so the process is fast and energy‐efficient. By progressively increasing the volume of KI used in a growth solution while keeping the amount of ascorbic acid added constant, nanocrystals with morphologies that vary from rhombic dodecahedral to rhombicuboctahedral, edge‐ and corner‐truncated octahedral, corner‐truncated octahedral, and octahedral structures were synthesized. The nanocrystals are monodisperse in size and readily form self‐assembled structures on substrates. By simply adjusting the volume of gold seed solution added to a growth solution, particle sizes of the octahedral gold nanocrystals can be tuned with average opposite corner‐to‐corner distances of 42, 48, 54, 60, 68, 93, 107, and 125 nm. In the presence of HAuCl₄, iodide may act as a reducing agent. Variation of its volume in the solution may slightly modulate the reduction rate and affect the final crystal morphology. Intermediate structures collected during crystal growth reveal the presence of many twisted structures that surround a developing nanocrystal core. This nanocrystal growth mechanism and the less important role of surfactant in directing the polyhedral nanocrystal morphology is discussed.     

Synthesis of icosahedral gold nanocrystals: a thermal process strategy

We demonstrate a one-step thermal process route to the synthesis of icosahedral gold nanocrystals. By regulating the concentrations of poly(vinyl pyrrolidone) (PVP) and HAuCl4 or changing the temperature, we can readily access the shapes of icosahedral nanocrystals with good uniformity. These gold nanostructures, with unique geometrical shapes, might find use in areas that include photonics, optoelectronics, and optical sensing. We also observed that these gold nanocrystals have a strong tendency to be immobilized spontaneously on the glass substrate.     

Enhancement of the stability and reaction rate by thenoyl and CF3 groups in the formation of gold nanocrystals using β-diketones

We describe a synthetic route for the high yield production of Au nanomaterials via a simple one step reduction process. Thenoyltrifluoroacetone was used as a reducing and stabilizing agent as well for the synthesis of gold nanoparticles. The reaction rate for the formation of Au nanoparticles using thenoyltrifluoroacetone was much faster than that of any other β-diketones such as acetylacetone. By simply varying the reaction temperature and the concentration, the shape and size of the resulting Au nanocrystals were easily controlled. The colloidal state of the Au nanocrystals in water lasts several weeks without any spectral changes.     

High-yield Synthesis of Branched Gold Nanocrystals by a Sodium Diphenylamine-4-Sulfonate Reduction Process in Polyethylene Glycol Aqueous Solution

For developing new excellent electrocatalysts toward methanol and oxygen, branched Au nanocrystals have been prepared in high yield by the reaction of HAuCl₄ and sodium diphenylamine‐4‐sulfonate in the presence of PEG (polyethylene glycol) at room temperature. When the amount of PEG was in the range of 1–3 mL, branched Au nanocrystals were all obtained, and the amounts of sodium diphenylamine‐4‐sulfonate and HAuCl₄ had no obvious effect on the morphology of gold nanocrystals. PEG molecules play an important role in the formation of branched gold nanocrystals. The nanocrystals were characterized by transmission electron microscopy (TEM), selected area electron diffraction, high‐resolution transmission electron microscopy (HRTEM) and UV‐Vis absorption spectrum. HRTEM research suggests that the tips of gold nanocrystals grow selectively in the [111] directions. The UV‐Vis absorption spectrum displays two‐separated surface plasmon resonance peaks.     

A distinct platinum growth mode on shaped gold nanocrystals

An ultralow amount of platinum can be deposited on the gold surface using copper underpotential deposition and galvanic exchange. The platinum tended to deposit as layers on the octahedral gold nanocrystals with an Au(111) surface, while it aggregated and formed small particles on the cubic gold nanocrystals with an Au(100) surface.     

金纳米晶制备和表面修饰中的分子配体

配体在纳米晶的制备和表面功能化过程中起着至关重要的作用。本文对金纳米晶制备和表面修饰中常见的分子配体,如柠檬酸根、巯基化合物、表面活性剂、树枝状分子、生物分子等的研究进展进行了概述。重点介绍了不同分子配体在金纳米晶尺寸形貌控制及表面功能化等方面的特点和作用,并对相关研究领域未来的发展趋势进行了展望。     

Direct synthesis of branched gold nanocrystals and their transformation into spherical nanoparticles

We report a facile synthesis of branched gold nanocrystals by the addition of a suitable amount of NaOH to an aqueous solution of cetyltrimethylammonium bromide (CTAB), HAuCl(4), and ascorbic acid. The branched nanocrystals were formed within minutes of reaction and showed monopod, bipod, tripod, and tetrapod structures. They are crystalline and have smooth surfaces. These gold multipods are kinetically controlled products and are thermodynamically unstable. The branched nanocrystals quickly transformed into spherical nanoparticles within 1 h of reaction, and the process was essentially complete after 2 days. The morphological transformation has been monitored by both UV-vis absorption spectroscopy and electron microscopy. The appearance of two major absorption bands for the branched gold nanocrystals eventually became only a single band at 529 nm for the spherical nanoparticles. The resulting nanoparticles are single crystals with diameters of 20-50 nm and do not show a faceted structure. When the freshly prepared branched nanocrystals are kept in a refrigerator at 4 degrees C, their multipod structure can be preserved for over a month without significant spectral shifts.     

Sorption and binding of nanocrystalline gold by Merino wool fibres--an XPS study

Uniform, monodisperse gold nanocrystals have been adsorbed and chemically bound to Merino wool fibres, providing a permanent colouration through the interaction of visible light with the plasmon resonant modes of the nanocrystals. Surface analysis by X-ray photoelectron spectroscopy confirmed that the nanocrystalline gold was bound through the nitrogen of the amino groups on the surface of the gold to the keratin of the fibres. No shift in the absorptions attributed to the plasmon resonance modes of the nanocrystals were observed.     

Study of nucleation and growth mechanism of the metallic nanodumbbells

We propose a general nucleation and growth model that can explain the mechanism of the formation of CoPt(3)/Au, FePt/Au, and Pt/Au nanodumbbells. Thus, we found that the nucleation event occurs as a result of reduction of Au(+) ions by partially oxidized surface Pt atoms. In cases when Au(3+) is used as a gold precursor, the surface of seeds should be terminated by ions (e.g., Co(2+), Pb(2+)) that can reduce Au(3+) to Au(+) ions, which can further participate in the nucleation of gold domain. Further growth of gold domain is a result of reduction of both Au(3+) and Au(+) by HDA at the surface of gold nuclei. We explain the different ability of CoPt(3), Pt, and FePt seeds to serve as a nucleation center for the reduction of gold and further growth of dumbbells. We report that the efficiency and reproducibility of the formation of CoPt(3)/Au, FePt/Au, and Pt/Au dumbbells can be optimized by the concentration and oxidation states of the surface ions on metallic nanocrystals used as seeds as well as by the type of the gold precursor.     

Synthesis of monodisperse spherical nanocrystals

Much progress has been made over the past ten years on the synthesis of monodisperse spherical nanocrystals. Mechanistic studies have shown that monodisperse nanocrystals are produced when the burst of nucleation that enables separation of the nucleation and growth processes is combined with the subsequent diffusion-controlled growth process through which the crystal size is determined. Several chemical methods have been used to synthesize uniform nanocrystals of metals, metal oxides, and metal chalcogenides. Monodisperse nanocrystals of CdSe, Co, and other materials have been generated in surfactant solution by nucleation induced at high temperature, and subsequent aging and size selection. Monodisperse nanocrystals of many metals and metal oxides, including magnetic ferrites, have been synthesized directly by thermal decomposition of metal-surfactant complexes prepared from the metal precursors and surfactants. Nonhydrolytic sol-gel reactions have been used to synthesize various transition-metal-oxide nanocrystals. Monodisperse gold nanocrystals have been obtained from polydisperse samples by digestive-ripening processes. Uniform-sized nanocrystals of gold, silver, platinum, and palladium have been synthesized by polyol processes in which metal salts are reduced by alcohols in the presence of appropriate surfactants.     

Polypyrrole–gold nanostructured composite,active and durable electrocatalytic material

The consequences of treatment of gold nanocrystals present in polypyrrole–gold composites and electrodeposited gold nanocrystals (PPY–Au and Au NPs) with OH radicals generated in Fenton’s reaction were investigated. Particularly, the changes in the morphology and the electrochemical properties of those materials are shown. For both materials, the etching effect was noticed. Contrary to significantly reduced catalytic activity, the changes observed in size and shape of gold nanostructures were less pronounced. In the case of PPY–Au composite material, the etching effects were less intense. Even after a 60-min radical treatment of the PPY–Au composite material, the gold–nanocrystal catalytic activity remained high. The limited dissolution of the gold nanocrystals in the PPY–Au composites, compared to bare Au NPs, can be explained by the presence of the polymer which served as a kind of protective barrier against the oxidizing agent. A decrease in the electrocatalytic properties vs. the electrooxidation of ethanol of both forms of gold nanocrystals were observed after the treatment with hydroxyl radicals.     

Multifunctional particles: Magnetic nanocrystals and gold nanorods coated with fluorescent dye-doped silica shells

Multifunctional colloidal core-shell nanoparticles of magnetic nanocrystals (of iron oxide or FePt) or gold nanorods encapsulated in silica shells doped with the fluorescent dye, Tris(2,2′-bipyridyl)dichlororuthenium(II) hexahydrate (Rubpy) were synthesized. The as-prepared magnetic nanocrystals are initially hydrophobic and were coated with silica using a microemulsion approach, while the as-prepared gold nanorods are hydrophilic and were coated with silica using a Stöber type of process. Each approach yielded monodisperse nanoparticles with uniform fluorescent dye-doped silica shells. These colloidal heterostructures have the potential to be used as dual-purpose tags—exhibiting a fluorescent signal that could be combined with either dark-field optical contrast (in the case of the gold nanorods), or enhanced contrast in magnetic resonance images (in the case of magnetic nanocrystal cores). The optical and magnetic properties of the fluorescent silica-coated gold nanorods and magnetic nanocrystals are reported.     

Self-assembly patterns formed upon solvent evaporation of aqueous cetyltrimethylammonium bromide-coated gold nanoparticles of various shapes

Gold nanocrystals of various shapes, which were produced in high yield in the presence of cetyltrimethylammonium bromide (CTAB), showed a range of two-dimensional self-assembly patterns upon drying from aqueous solution. The interparticle spacings were independent of the size and shape of the gold nanocrystals. Energy-dispersive X-ray analysis (EDAX) and Fourier-transform infrared (FTIR) spectroscopic studies revealed that the CTAB molecules adsorb onto surfaces of the gold nanocrystals in a bilayer or multilayer fashion, consistent with other groups' results. Zeta potential measurements showed that CTAB-coated nanocrystals were positively charged and the zeta potential remained almost the same upon two centrifugations and redispersion of the nanocrystals in deionized water, confirming the high stability of the surfactant-nanoparticle interaction. The nanocrystal shape strongly influenced the nature of the self-assembly patterns, in some cases in accord with theoretical predictions. CTAB is proposed as the medium for self-assembly, via interdigitation of its hydrophobic chains from adjacent nanocrystals for close contact, or via sharing a layer of counterions for larger inter-nanocrystal spacings.     

Effects of changes in the interparticle separation induced by alkanethiols on the surface plasmon band and other properties of nanocrystalline gold films

Effects of changing the interparticle separation on the surface plasmon bands of ultrathin films of gold nanoparticles have been investigated by examining the interaction of alkanethiols of varying chain length on nanocrystalline gold films generated at the organic-aqueous interface. Adsorption of alkanethiols causes blue-shifts of the surface plasmon adsorption band, the magnitude of the shift being proportional to the chain length. The disordered nanocrystals thus created (lambdamax, 530 m) are in equilibrium with the ordered nanocrystals in the film (lambdamax, 700 m) as indicated by an isosbestic point around 600 nm. Long chain thiols disintegrate or disorder the gold films more effectively, as demonstrated by the increased population of the thiol-capped gold nanocrystals in solution. The rate of interaction of the thiols with the film decreases with the decreasing chain length. The effect of an alkanethiol on the spectrum of the gold film is specific, in that the effects with long and short chains are reversible. The changes in the plasmon band of gold due to interparticle separation can be satisfactorily modeled on the basis of the Maxwell-Garnett formalism. Spectroscopic studies, augmented by calorimetric measurements, suggest that the interaction of alkanethiols involves two steps, the first step being the exothermic gold film-thiol interaction and the second step includes the endothermic disordering process followed by further thiol capping of isolated gold particles.     

Plasmonic vesicles of amphiphilic gold nanocrystals: self-assembly and external-stimuli-triggered destruction

We have developed a new class of plasmonic vesicular nanostructures assembled from amphiphilic gold nanocrystals with mixed polymer brush coatings. One major finding is that the integration of gold nanocrystals (nanoparticles and nanorods) with two types of chemically distinct polymer grafts, which are analogous to block copolymers as a whole, creates a new type of hybrid building block inheriting the amphiphilicity-driven self-assembly of block copolymers to form vesicular structures and the plasmonic properties of the nanocrystals. In contrast to other vesicular structures, the disruption of the plasmonic vesicles can be triggered by stimulus mechanisms inherent to either the polymer or the nanocrystal. Recent advances in nanocrystal synthesis and controlled surface-initiated polymerization have opened a wealth of possibilities for expanding this concept to other types of nanocrystals and integrating different types of nanocrystals into multifunctional vesicles. The development of multifunctional vesicles containing stimuli-responsive polymers could enable their broader applications in biosensing, multimodality imaging, and theragnostic nanomedicine.