Calixarene-encapsulated nanoparticles: self-assembly into functional nanomaterials

Calixarenes are excellent surfactants for enhancing the dispersion and self-assembly of metal nanoparticles into well-defined structures, particularly those with unit length scales in the 10-100 nm size range. Particles within these ensembles are strongly coupled, giving rise to unique collective optical or magnetic properties. The self-assembled nanostructures described in this feature article include 2D arrays of colloidal Au nanoparticles with size-dependent plasmonic responses, and sub-100 nm Co nanoparticle rings with chiral magnetic states. These nanoparticle assemblies may be further developed for applications in chemical sensing based on surface-enhanced Raman scattering (SERS) and as binary elements for nonvolatile memory, respectively.     

Simple reductant concentration-dependent shape control of polyhedral gold nanoparticles and their plasmonic properties

We report a facile seed-mediated method for the synthesis of monodisperse polyhedral gold nanoparticles, with systematic shape evolution from octahedral to trisoctahedral structures. The control over the particle growth process was achieved simply by changing the concentration of the reductant in the growth solution, in the presence of small spherical seed nanoparticles. By progressively increasing the concentration of the reductant used in the growth solution (ascorbic acid), while keeping the amount and type of added surfactant constant, the morphology of the gold nanoparticles was varied from octahedral to truncated octahedral, cuboctahedral, truncated cubic, cubic, and finally trisoctahedral structures. These nanoparticles were monodisperse in size, possessed similar volumes, and were naturally oriented so that their larger crystal planes were face down on quartz substrates when deposited from the solution. By adjusting the volume of gold seed nanoparticle solution added to a growth solution, the size of the simplest gold nanoparticles (with a highly symmetric cubic morphology) could be tuned from 50 ± 2.1 to 112 ± 11 nm. When other seed nanoparticles were used, the size of the cubic Au nanoparticles reached 169 ± 7.0 nm. The nanoparticle growth mechanism and the plasmonic properties of the resulting polyhedral nanoparticles are discussed in this paper.     

Preparation of crystalline gold nanoparticles at the surface of mixed phosphatidylcholine-ionic surfactant vesicles

The formation of gold nanoparticles and the crystal growth at the surface of mixed phosphatidylcholine (PC)-ionic surfactant vesicles was investigated. The PC-bilayer surface was negatively charged by incorporating sodium dodecyl sulfate (SDS) and positively charged by adding hexadecyltrimethylammonium chloride (CTAB). The mass ratio phosphatidylcholine:surfactant was fixed in both cases at 1:1. The gold nanoparticle formation was studied by using transmission electron microscopy (TEM) combined with dynamic light scattering (DLS) and UV-vis absorption spectroscopy. TEM micrographs confirm that the particle formation occurs on the vesicle surface. However, the reduction process depends on the ionic surfactant incorporated into the vesicles, the vesicle size distribution, as well as the temperature used for the reduction process. Thereby, it becomes possible to control the crystal growth of the individual spherical gold nanoparticles in a characteristic way. Red colored colloidal dispersions consisting of monodisperse spherical nanoparticles with an average particle size between 2 and 8 nm (determined by dynamic light scattering) can be obtained by using a monodisperse SDS-modified vesicle phase. When the temperature is increased to 45 degrees C, a crystallization in rod-like or triangular structures is observed. In the CTAB-based template phase in general larger gold particles of about 35 nm are formed. In similarity to the anionic vesicles a temperature increase leads to the crystallization in triangular structures.     

Surface-enhanced Raman nanoparticle beacons based on bioconjugated gold nanocrystals and long range plasmonic coupling

We have developed a new class of surface-enhanced Raman scattering beacons (SERS beacons) that can be turned on and off by long-range plasmonic coupling, induced by biomolecular recognition and binding events. The beacons are based on colloidal gold nanocrystals in two sizes (40 and 60 nm) and are prepared by spectral encoding with a Raman reporter molecule, functionalized with thiolated DNA probes, and stabilized and protected by low molecular weight poly(ethylene glycol)s (PEGs). The results show the SERS signal intensities increase by 40-200-fold when the nanoparticle beacons are activated by plasmonic coupling, much higher than the bright-to-dark intensity ratios reported for traditional molecular beacons. Multivalent gold nanoparticles also have exquisite specificity and are able to recognize single-base mismatches or mutations. This class of SERS nanoparticle beacons has novel mechanisms for molecular detection and signal amplification, and its long-range coupling nature raises new opportunities in developing plasmonic probes to detect proteins, cells, and intact viruses.     

Poly(ethyleneimine) as reducing and stabilizing agent for the formation of gold nanoparticles in w/o microemulsions

This paper is focused on the use of branched poly(ethyleneimine) (PEI) as reducing as well as stabilizing agent for the formation of gold nanoparticles in different media. The process of nanoparticle formation was investigated, in the absence of any other reducing agents, in microemulsion template phase in comparison to the nucleation process in aqueous polymer solution.

On the one hand, it was shown that the polyelectrolyte can be used for the controlled single-step synthesis and stabilization of gold nanoparticles via a nucleation reaction and particles with an average diameter of 7.1 nm can be produced.

On the other hand, it was demonstrated that the polymer can also act as reducing and stabilizing agent in much more complex systems, i.e. in water-in-oil (w/o) microemulsion droplets. The reverse microemulsion droplets of the quaternary system sodium dodecylsulfate (SDS)/toluene–pentanol (1:1)/water were successfully used for the synthesis of gold nanoparticles. The polymer, incorporated in the droplets, exhibits reducing properties, adsorbs on the surface of the nanoparticles and prevents their aggregation. Consequently, nanoparticles of 8.6 nm can be redispersed after solvent evaporation without a change of their size.

Nevertheless, the polymer acts already as a “template” during the formation of the nanoparticles in water and in microemulsion, so that an additional template effect of the microemulsion is not observed.

The particle formation for both methods is checked by means of UV–vis spectroscopy and the particle size and size distribution are investigated via dynamic light scattering and transmission electron microscopy (TEM).     

One-pot tuning of Au nucleation and growth: from nanoclusters to nanoparticles

We describe a simple and effective method to obtain colloidal surface-functionalized Au nanoparticles. The method is primarily based on irradiation of a gold solution with high-flux X-rays from a synchrotron source in the presence of 11-mercaptoundecanoic acid (MUA). Extensive tests of the products demonstrated high colloidal density as well as excellent stability, shelf life, and biocompatibility. Specific tests with X-ray diffraction, UV-visible spectrometry, visible microscopy, Fourier transform infrared spectroscopy, dark-field visible-light scattering microscopy, and transmission electron microscopy demonstrated that MUA, being an effective surfactant, not only allows tunable size control of the nanoparticles, but also facilitates functionalization. The nanoparticle sizes were 6.45 ± 1.58, 1.83 ± 1.21, 1.52 ± 0.37 and 1.18 ± 0.26 nm with no MUA and with MUA-to-Au ratios of 1:2, 1:1, and 3:1. The MUA additionally enabled functionalization with l-glycine. We thus demonstrated flexibility in controlling the nanoparticle size over a large range with narrow size distribution.     

Synthesis of radioactive gold nanoparticle in surfactant medium

The present study describes the synthesis of radioactive gold nanoparticle in surfactant medium. Proton irradiated stable ¹⁹⁷Au and radioactive ¹⁹⁸Au were simultaneously used for production of radioactive gold nanoparticle. Face centered cubic gold nanoparticles with size of 4–50 nm were found in proton irradiated gold foil. However, the size of nanoparticle varies with pH using both stable and radioactive gold.     

A small-angle X-ray scattering study of nanoparticle assembly in an aligned nematic liquid crystal

The assembly of colloidal particles in a nematic liquid crystal has been investigated using small-angle X-ray scattering. The structure and orientation of nanoparticle assemblies in bulk samples of aligned nematic liquid crystal have been determined. The method offers some advantages over optical microscopy, which is usually restricted to investigations of thin cells and micron-sized particles. The scattering from chains of particles has been calculated, and comparison with experimental results has shown that suspensions of 48 and 105 nm diameter silica nanoparticles formed highly ordered structures perpendicular to the liquid crystal director, consistent with quadrupolar defect-induced assembly.     

Dense aqueous colloidal gold nanoparticles prepared from highly concentrated precursor solution

Gold nanoparticles were fabricated by reduction of highly concentrated Au(III) ions (200 mM) with casein proteins from milk. The gold nanoparticles were converted to nanoparticle-powders after washing and subsequent vacuum drying without aggregation. The nanoparticle-powders completely re-dispersed in aqueous solution, and stable colloidal gold nanoparticles were obtained. UV-vis extinction spectra and dynamic light scattering (DLS) measurements revealed that large assemblies (size, ca. 3 μm) and subaggregates (size, <0.5 μm) composed of gold nanoparticle-casein protein chain-Au(III) ion were dynamically formed and disintegrated over the course of the growth of the gold nanoparticles. Fourier transform infrared (FT-IR) spectra indicated conformational changes of casein proteins induced by the interaction of casein protein-Au(III) ion and -gold nanoparticle. Finally, rapid, one-pot, and highly concentrated synthetic procedures of gold and silver nanoparticle powders protected by casein (mean diameters below 10 nm) were successfully developed using 3-amino-1-propanol aqueous solutions as reaction media. Dense colloidal gold (40 g L(-1)) and silver (22 g L(-1)) nanoparticle aqueous solutions were obtained by re-dispersing the metal nanoparticle powders.     

Influence of intense pulsed laser irradiation on optical and morphological properties of gold nanoparticle aggregates produced by surface acid-base reactions

Gold nanoparticles were surface modified with an ionizable and pH-sensitive monolayer of thiobarbituric acid (TBA). By variation of the pH value of the solution, nanoparticle aggregates can be produced in a controlled way. The aggregates thus prepared were irradiated with an intense pulsed laser at 532 nm. The products in solution were examined by transmission electron microscopy (TEM) and optical absorption spectroscopy. The TEM images of the products revealed that the nanoparticle aggregates dissociate upon laser irradiation and form much smaller gold nanoparticles. The optical absorption spectra measured simultaneously show the gradual disappearance of the absorbance band of the aggregates at around 680 nm. Additionally, a blue shift (from 534 to 524 nm) of the resonance absorbance corresponding to isolated nanoparticles has been observed. All the observations suggest that the colloidal solution becomes more stable after laser irradiation. Both the reduced nanoparticle size and the stabilizing TBA ligands present on the particle surface contribute to the acquired stability of the colloidal solutions.     

Foam films in the presence of functionalized gold nanoparticles

Dry aqueous foams made of anionic surfactant (SDS) and spherical gold nanoparticles are studied by small angle X-ray scattering and by optical techniques. To obtain stable foams, the surfactant concentration is well above the critical micelle concentration. The specular reflectivity signal obtained on a very thin film (thickness 20 nm) shows that functionalized nanoparticles (17 nm typical size) are trapped within the film in the form of a single monolayer. In order to isolate the film behavior, investigations are made on a single film confined in a tube. The film thinning according to the ratio of functionalized nanoparticle and SDS micelles (1:1, 1:10, 1:100) is mainly governed by the structural arrangement of SDS micelles. In thick films, nanoparticles tend to form aggregates that disappear during drainage. In particular self-organization of nanoparticles (with different surface charge) inside the film is not detected.     

Change Color Effect and Spectral Properties of Gold Nanoparticle-cationic Surfactants System

The change color effect of gold nanoparticle solutions was studied by means of resonance scattering and absorption spectrometry and scan electron microscopy. The red Au nanoparticles with a size of 10 nm exhibit a resonance absorption peak and a resonance scattering peak all at 525 nm. After some inorganic electrolyte was added to a red Au nanoparticles solution, the color of the solution became blue and the absorbance at 600-700 nm was significantly increased. The ratio of the concentration of rnonovalent cations, at which the resonance scattering of the system at 525 nm is maximal to that of divalent cations, is in the range of 100 : 1--100 ; 1. 8. It is in good agreement with the Schulze-Hardy rule of the coagulation value of electrolyte. After adding some cationic surfactants to the above solution, the color of the solution is in deep blue, with two resonance absorption peaks at 550 and 680 nm, and a greatly enhanced resonance scattering peak at 525 nm. The experiments demonstrate that the stronger the hydrophobicity of the cationic surfactant is, the stronger the change color effect of the Au nanoparticle solution promoted by cationic surfactant is. The change color effect of Au nanoparticle solution is resulted from the increased diameter of Au nanoparticles, and the changes of resonance absorotion DeaR and resonance scattering.     

The feasibility of inert colloidal processing of silicon nanoparticles

Silicon nanoparticles have important applications, including nonvolatile floating-gate memory devices. To prevent device performance variations, particle size and oxide thicknesses need to be controlled with a high degree of precision. Additionally, producing well-ordered, two-dimensional arrays of nanoparticles may require the exploitation of self-assembly techniques and colloidal forces, which in turn requires that silicon nanoparticles first come into contact with liquids. Until recently, aerosol silicon nanoparticle collection into liquid was assumed to be an inert process. Once formed, the silicon nanoparticle colloid was assumed to be inert. In fact, silicon nanoparticles produced in the aerosol phase by dilute silane pyrolysis and size classified with a differential mobility analyzer undergo a size reduction upon collection in ethylene glycol, water, and ethanol. Unclassified polydisperse silicon aerosol nanoparticles with an average diameter of 11 nm become monodisperse when collected in a colloid and have a final particle diameter of 2-5 nm. Further evidence suggests that silicon nanoparticles collected in ethanol react with the ethanol to produce tetraalkylorthosilicate-like species. Collections of aerosol silicon nanoparticles in degassed water do not show measurable differences between the aerosol and colloidal size distributions. This reduced reactivity to the solvent indicates that the presence of dissolved oxygen in the solvent may be responsible for the reactivity between the silicon nanoparticles and the solvent.     

Adding sodium dodecylsulfate to the running electrolyte enhances the separation of gold nanoparticles by capillary electrophoresis

This paper describes employing capillary electrophoresis (CE) for the separation of gold colloids in nanometer-size regimes. Adding sodium dodecylsulfate (SDS) surfactant to the running buffer enhances the capability of CE to separate gold nanoparticles. We found that the optimized separation conditions involved SDS (70 mM), 3-cyclohexylamoniuopropanesulfonic acid (CAPS) buffer (10 mM), pH 10.0, and an applied voltage of 20 kV. We propose that the charged surfactants associate onto the surface of the gold nanoparticles and cause a change in the charge-to-size ratio of gold nanoparticle, which is a function of the surface area of nanoparticle and the surfactant concentration of running electrolyte. At high concentrations of the surfactant in the running electrolyte—i.e., when the surface of the gold nanoparticles is fully occupied with SDS—a linear relationship exists between the electrophoretic mobility and nanoparticles having diameters ranging from 5.3 to 38 nm. Based on the results of separating the 5.3 and 19 nm nanoparticles, we estimate that the size resolution (R_s=1.0) is 5.0 nm. The relative standard deviations of the electrophoretic mobilities of the 5.3 and 19 nm gold nanoparticles are 0.97 and 0.54%, respectively.     

Resorcinarene-Encapsulated Nanoparticles: Building Blocks for Self-Assembled Nanostructures

Calix[4]resorcinarene-derived surfactants are highly effectiveat stabilizing metal nanoparticles of different sizes, creating opportunities tofabricate well-defined nanostructures with size-tunable materials properties. Theresorcinarenes have a critical role in the dispersion of nanoparticles under varioussolvent conditions and in the robustness of the protective surfactant layer.Magnetic cobalt particles stabilized by resorcinarenes self-assemble intonanostructured ``bracelets' in toluene. Resorcinarene surfactants can also promote theself-organization of gold nanoparticles as large as 170 nm into two-dimensional arrays. Thesenanostructured films possess novel optoelectronic properties such as surface-enhancedRaman scattering (SERS), and are expected to have useful applications for chemical sensing.     

Charged silsesquioxane used as a vehicle for gold nanoparticles to perform the synthesis of catalyst xerogels

The water soluble charged silsesquioxane that contains the bridged 1,4-diazoniabicyclo[2.2.2]octane chloride group, was used as stabilizing agent and size controller in the synthesis of gold nanoparticles smaller than 15?nm in aqueous medium. The gold nanoparticle dispersion was converted in solid powder form by evaporation. This powder presented organized structure imposed by the presence of charged organic group, similar to organized structure already observed for pure silsesquioxane. The gold nanoparticles in solid powder form presented high storage stability for several months, at ambient conditions, and can be completely redispersed in water again. After redispersion, the optical properties of gold nanoparticles, observed by ultra-violet and visible spectroscopy, and their morphological characteristics, investigated by transmission electron microscopy, are preserved. The gold nanoparticle aqueous dispersion was used as a vehicle of nanoparticles in the synthesis of sol?Cgel silica based hybrid material. This xerogel was characterized by N₂ adsorption?Cdesorption isotherms, showing 260?m²g^?1, and it was applied in a satisfactory way as catalyst for p-nitrophenol reduction to p-aminephenol.     

Rapid Cationization of Gold Nanoparticles by Two‐Step Phase Transfer

Cationic gold nanoparticles offer intriguing opportunities as drug carriers and building blocks for self‐assembled systems. Despite major progress on gold nanoparticle research in general, the synthesis of cationic gold particles larger than 5 nm remains a major challenge, although these species would give a significantly larger plasmonic response compared to smaller cationic gold nanoparticles. Herein we present the first reported synthesis of cationic gold nanoparticles with tunable sizes between 8–20 nm, prepared by a rapid two‐step phase‐transfer protocol starting from simple citrate‐capped particles. These cationic particles form ordered self‐assembled structures with negatively charged biological components through electrostatic interactions.     

金纳米粒子表面自组装巯基十一烷醇单分子层体系的制备及其光散射特性研究

采用柠檬酸钠还原法制备了水相金纳米粒子, 通过巯基的自组装, 成功获得了巯基十一烷醇(MUN)单分子层保护的金纳米粒子. 用紫外可见光谱、透射电子显微镜、激光散射粒度分析、同步散射光谱和发射光谱等手段对组装前后的金纳米粒子的性质进行了研究. 结果表明: 制备的金纳米粒子最大吸收波长518 nm, 形状规则, 粒度均匀, 平均粒径为14.6 nm, 每个粒子含有约9.64×10⁴原子; 组装之后的金纳米粒子表面等离子体共振吸收峰红移17.0 nm, 平均粒径增大为20.2 nm, 组装层的平均厚度2.8 nm, 与MUN分子长度相当, 结合量实验证明每一个金纳米粒子可以结合约7.52×10³个MUN, 表面覆盖率为83.6%, 粒子分散均匀, 稳定性增强可长期保存; 同步散射光谱变化和发射光谱中分频、差频和倍频峰的存在证明, 金纳米粒子组装前后均具有非线性光学特性.     

Formation of gold@polymer core-shell particles and gold particle clusters on a template of thermoresponsive and pH-responsive coordination triblock copolymer

Template synthesis of various morphological gold colloidal nanoparticles using a thermoresponsive and pH-responsive coordination triblock copolymer of poly(ethylene glycol)-b-poly(4-vinylpyridine)-b-poly(N-isopropylacrylamide) is studied. The template morphology of the thermoresponsive and pH-responsive coordination triblock copolymer, which can be tuned by simply changing the pH or temperature of the triblock copolymer aqueous solution, ranges from single chains to core-corona micelles and further to micellar clusters. Various morphological gold colloidal nanoparticles such as discrete gold nanoparticles, gold@polymer core-shell nanoparticles, and gold nanoparticle clusters are synthesized on the corresponding template of the triblock copolymer by first coordination with gold ions and then reduction by NaBH4. All three resultant gold colloidal nanoparticles are stable in aqueous solution, and their sizes are 2, 10, and 7 nm, respectively. The gold@polymer core-shell nanoparticles are thermoresponsive. The gold nanoparticle cluster has a novel structure, and each one holds about 40 single gold nanoparticles.     

<Emphasis Type="Italic">In situ</Emphasis> analysis of the formation steps of gold nanoparticles by oleylamine reduction

A colloidal solution of gold nanoparticles is synthesized with the use of sodium tetrachloroaurate(III) as a precursor, oleylamine as a reducer and surfactant, and 1-octadecene as a solvent. Reaction stages are analyzed in situ by optical (UV-vis) absorption spectroscopy with a simultaneous analysis of particle sizes by dynamic light scattering and X-ray absorption near edge spectroscopy for the analysis of the gold oxidation state. After the synthesis the size of obtained nanoparticles is determined by transmission electron microscopy. The analysis of the obtained experimental data reveals the presence of three main steps in the reduction reaction mechanism, corresponding to Au³⁺, Au⁺, Au⁰, which enables the construction of the reaction model. The reaction mechanism involves the formation of gold(I) complexes with oleylamine, followed by polymerization and the formation of gold nanoclusters coated with oleylamine.