Extinction coefficient of gold nanoparticles with different sizes and different capping ligands

Extinction coefficients of gold nanoparticles with core size ranging from approximately 4 to 40 nm were determined by high resolution transmission electron microscopy analysis and UV-vis absorption spectroscopic measurement. Three different types of gold nanoparticles were prepared and studied: citrate-stabilized nanoparticles in five different sizes; oleylamide-protected gold nanoparticles with a core diameter of 8 nm, and a decanethiol-protected nanoparticle with a diameter of around 4 nm. A linear relationship between the logarithms of extinction coefficients and core diameters of gold particles was found independent of the capping ligands on the particle surface and the solvents used to dissolve the nanoparticles. This linear relation may be used as a calibration curve to determine the concentration or average size of an unknown nanoparticle or nanoparticle-biomolecule conjugate sample.     

From homoligand- to mixed-ligand- monolayer-protected metal nanoparticles: a scanning tunneling microscopy investigation

The ligand shell that coats, protects, and imparts a large number of properties to gold nanoparticles is a 2-D self-assembled monolayer wrapped around a 3-D metallic core. Here we present a study of the molecular packing of ligand shells on gold nanoparticles based on the analysis of scanning tunneling microscopy (STM) images. We discuss methods for optimal nanoparticle sample preparation in relation to STM imaging conditions. We show that the packing of a self-assembled monolayer composed solely of octanethiols on gold nanoparticles depends on the particle's diameter with an average headgroup spacing of 5.4 A, which is different from that of similar monolayers formed on flat Au(111) surfaces (5.0 A). In the case of nanoparticles coated with mixtures of ligands-known to phase separate into randomly shaped and ordered domains on flat surfaces-we find that phase separation leads to the formation of concentric, ribbonlike domains of alternating composition. The spacing of these domains depends on the ligand shell composition. We find that, for a given composition, the spacing increases with diameter in a manner characterized by discontinuous transitions at "critical" particle sizes. We discuss possible interpretations for the observed trends in our data.     

Uniformly sized gold nanoparticles derived from PS-b-P2VP block copolymer templates for the controllable synthesis of Si nanowires

A monolayer of gold-containing surface micelles has been produced by spin-coating solution micelles formed by the self-assembly of the gold-modified polystyrene-b-poly(2-vinylpyridine) block copolymer in toluene. After oxygen plasma removed the block copolymer template, highly ordered and uniformly sized nanoparticles have been generated. Unlike other published methods that require reduction treatments to form gold nanoparticles in the zero-valent state, these as-synthesized nanoparticles are in form of metallic gold. These gold nanoparticles have been demonstrated to be an excellent catalyst system for growing small-diameter silicon nanowires. The uniformly sized gold nanoparticles have promoted the controllable synthesis of silicon nanowires with a narrow diameter distribution. Because of the ability to form a monolayer of surface micelles with a high degree of order, evenly distributed gold nanoparticles have been produced on a surface. As a result, uniformly distributed, high-density silicon nanowires have been generated. The process described herein is fully compatible with existing semiconductor processing techniques and can be readily integrated into device fabrication.     

Hydrogel-templated growth of large gold nanoparticles: synthesis of thermally responsive hydrogel-nanoparticle composites

In this paper, we describe a unique strategy for preparing discrete composite nanoparticles consisting of a large gold core (60-150 nm in diameter) surrounded by a thermally responsive nontoxic hydrogel polymer derived from the polymerization of N-isopropylacrylamide (NIPAM) or a mixture of NIPAM and acrylic acid. We synthesize these composite nanoparticles at room temperature by inducing the growth of gold nanoparticles in the presence of preformed spherical hydrogel particles. This new method allows precise control of the size of the encapsulated gold cores (tunable between 60 and 150 nm) and affords composite nanoparticles possessing diameters ranging from as small as 200 nm to as large as 550 nm. Variable-temperature studies show that the hydrodynamic diameter of these composite nanoparticles shrinks dramatically when the temperature is increased above the lower critical solution temperature (LCST); correspondingly, when the temperature is lowered below the LCST, the hydrodynamic diameter expands to its original size. These composite nanoparticles are being targeted for use as optically modulated drug-delivery vehicles that undergo volume changes upon exposure to light absorbed by the gold nanoparticle core.     

Directed self-assembly of two kinds of nanoparticles utilizing monolayer films of diblock copolymer micelles

We demonstrated a self-assembly of two different kinds of nanoparticles simultaneously directed on a monolayer film of diblock copolymer micelles via physical and chemical arrangements. We first incorporated gold nanoparticles physically around the micelles of a monolayer film of PS-PVP micelles having a short-range hexagonal order. Iron oxide nanoparticles were then synthesized chemically in the PVP core area of the ordered micelles, resulting in a mosaic nanopattern of magnetic iron oxide nanoparticles surrounded by metallic gold nanoparticles. Thus, we were able to direct two kinds of nanoparticles to self-assemble in the specific positions as an example of controlled fabrication of nanometer-sized building blocks.     

Chemometric and microscopic analyses for the size growth of monolayer-protected gold nanoparticles during their superlattice formation

Superlattices of gold nanoparticles have been produced at an air/solution interface under a highly acidic condition. The nanoparticle surface is protected by N-acetylglutathione (NAG). During the course of the superlattice formation, size growth of nanoparticles was observed: The superlattices were composed of nanoparticles of 6.6 nm in core diameter, whereas the as-prepared nanoparticles had the core diameter of 1.4 nm. The growth kinetics was pursued by the time evolution of the UV-vis absorption spectra for the sample solution. The change in the absorption spectral profiles was so small that we conducted principal-component analysis (PCA), which is known as a chemometric technique to resolve (or extract) spectra of minute chemical species submerged in the original spectra. Scanning transmission electron microscopy (STEM) corroborated the PCA results, yielding a successful explanation of the growth scheme of the NAG-protected gold nanoparticles.     

Janus particles with a functional gold surface for control of surface plasmon resonance

We report in this study the presence of Janus particles, which are candidates for use with electronic color papers. We used negatively charged polystyrene particles (370 nm) as the core particles, and gold was then sputtered onto their packed monolayer under several conditions. The sputtered particles were next redispersed into the aqueous medium by gentle sonication. Gold nanoparticles localized on one side of the cores could also serve as seeds for subsequent shell growth by electroless gold plating. Through these treatments, a series of well-dispersed Janus particles were obtained with gold nanostructures of different size and shape only on one side. Their dispersions showed different colors originating from the surface plasmon resonance absorption of gold nanoparticles localized on the hemisphere. The particles obtained by this approach have potential applications such as in sensors and electronic color paper.     

Interfacial surface properties of thiol-protected gold nanoparticles: a molecular probe EPR approach

We present a molecular probe technique for accessing interfacial surface electrostatics of ligand-protected gold nanoparticles. A series of ligands with variable length of the hydrocarbon bridge between the anchoring sulfur and the reporting pH-sensitive nitroxide is described. The protonation state of this probe is directly observed by EPR spectroscopy. For tiopronin-protected Au nanoparticles, we observed an increase in pKa of up to ca. 1.1 pH units that was affected by the position of the reporter moiety with respect to the monolayer interface.     

Facile synthesis and functionalization of water-soluble gold nanoparticles for a bioprobe

In this work, we report the size tunable synthesis of water-dispersed gold nanoparticles by using octadecylamine (ODA) as the reducing agent, that electrostatically complexes with the chloroaurate ions, reduces them, and subsequently caps the gold nanoparticles. Amine-capped gold nanoparticles, thus formed, were subsequently coordinated with a secondary monolayer of an anionic surfactant, sodium bis(2-ethylhexyl)-sulfosuccinate (AOT) which helps in providing sufficient hydrophilicity to the gold nanoparticles. Functionalized gold nanoparticles were characterized by UV-vis, IR spectrophotometric, dynamic light scattering, zeta-potential and transmission electron microscopic techniques, which demonstrated high stability of gold nanoparticles in aqueous media, indicating stabilization via bilayers of ODA and AOT. The gold nanoparticles were further conjugated with a protein (bovine serum albumin) and the interaction was investigated by circular dichroism studies as well as by measuring the fluorescence quenching of the tryptophan residues of protein molecules after the binding of nanoparticles to specific sites of the protein. The binding constant and the stoichiometry values indicated that the particles with larger core size are less site-specific but show higher binding affinity with protein molecules. The use of a bio-compatible synthetic process and the stabilization of the gold nanoparticles by ODA and AOT are interesting from the point of view of making bioprobes for life science applications.     

Size dependent catalysis with CTAB-stabilized gold nanoparticles

CTAB-stabilized gold nanoparticles were synthesized by applying the seeding-growth approach in order to gain information about the size dependence of the catalytic reduction of p-nitrophenol to p-aminophenol with sodium borohydride. Five different colloidal solutions of stabilized gold nanoparticles have been characterized by TEM, AFM, UV-Vis, SAXS, and DLS for their particle size distributions. Gold nanoparticles (mean sizes: 3.5, 10, 13, 28, 56 nm diameter) were tested for their catalytic efficiency. Kinetic data were acquired by UV-Vis spectroscopy at different temperatures between 25 and 45 °C. By studying the p-nitrophenol to p-aminophenol reaction kinetics we determined the nanoparticle size which is needed to gain the fastest conversion under ambient conditions in the liquid phase. Unexpectedly, CTAB-stabilized gold nanoparticles with a diameter of 13 nm are most efficient.     

湿化学镀SPR金基底及其性能表征

建立一种由单一水相操作的表面等离子体共振法(SPR)制备金基底.即:在3-氨基丙基三甲氧基硅烷(APTMS)修饰的玻璃片上自组装一层细小的金胶纳米粒子,以此为催化模板,利用化学镀技术在纳米尺度范围内控制金膜的均匀增长,获得优异SPR信号响应所需要的金膜形貌和厚度.紫外光谱(UV-vis),透射电镜(TEM)观测表明,纳米金膜催化模板粒径约为2.5 nm.扫描电镜(SEM)观察湿化学镀SPR金基底均匀分布,粒径约为40 nm.与商品化真空镀金基底相比,湿化学镀金基底对乙醇的SPR响应强度相当,且可调控性更高.     

Expeditious synthesis of water-soluble, monolayer-protected gold nanoparticles of controlled size and monolayer composition

A protocol is reported for the preparation of water-soluble, thiol-protected Au nanoparticles (Au-MPC) where dioctylamine is used as a stabilizing agent when the gold cluster is formed using the two-phase Brust and Schiffrin procedure. The amount of amine controls the size of the nanoparticles in the 1.9-8.9 nm diameter range. The final stabilization of the gold clusters by addition of functionalized thiols is performed under very mild conditions compatible with most biomolecules. The procedure is suitable for a wide variety of functional groups present in the thiol and allows one to use thiol mixtures with a precise control of their composition in the monolayer. As a proof of principle, examples of nanoparticles protected with thiols comprising functional groups ranging from polyethers, saccharides, polyamines and ammonium ions are reported.     

Effects of the surface pressure on the formation of Langmuir-Blodgett monolayer of nanoparticles

The effects of the surface pressure on the particle arrangement of Langmuir-Blodgett (LB) monolayers of alkanethiol-capped gold nanoparticles were studied. The LB monolayers were prepared from a highly concentrated particle solution, which increases film fabrication efficiency but readily causes small particle voids in the particle array. Overcompressing the LB monolayer to a high surface pressure restructured the particles and eliminated the voids. When the gold particles capped by dodecanethiol were 8.5 nm in diameter, the particle arrangement was vastly improved and a wafer-scale LB monolayer was transferred onto a substrate at the surface pressure of 20 mN/m.     

Monolayer/bilayer transition in Langmuir films of derivatized gold nanoparticles at the gas/water interface: an x-ray scattering study

The microscopic structure of Langmuir films of derivatized gold nanoparticles has been studied as a function of area/particle on the water surface. The molecules (AuSHDA) consist of gold particles of mean core diameter D approximately 22 angstroms that have been stabilized by attachment of carboxylic acid terminated alkylthiols, HS-(CH2)15-COOH. Compression of the film results in a broad plateau of finite pressure in the surface pressure versus area/particle isotherm that is consistent with a first-order monolayer/bilayer transition. X-ray specular reflectivity (XR) and grazing incidence diffraction show that when first spread at large area/particle, AuSHDA particles aggregate two dimensionally to form hexagonally packed monolayer domains at a nearest-neighbor distance of a = 34 angstroms. The lateral positional correlations associated with the two-dimensional (2D) hexagonal order are of short range and extend over only a few interparticle distances; this appears to be a result of the polydispersity in particle size. Subsequent compression of the film increases the surface coverage by the monolayer but has little effect on the interparticle distance in the close-packed domains. The XR and off-specular diffuse scattering (XOSDS) results near the onset of the monolayer/bilayer coexistence plateau are consistent with complete surface coverage by a laterally homogeneous monolayer of AuSHDA particles. On the high-density side of the plateau, the electron-density profile extracted from XR clearly shows the formation of a bilayer in which the newly formed second layer on top is slightly less dense than the first layer. In contrast to the case of the homogeneous monolayer, the XOSDS intensities observed from the bilayer are higher than the prediction based on the capillary wave model and the assumption of homogeneity, indicating the presence of lateral density inhomogeneities in the bilayer. According to the results of Bragg rod measurements, the 2D hexagonal order in the two layers of the bilayer are only partially correlated.     

Molecular‐Dynamics‐Simulation‐Directed Rational Design of Nanoreceptors with Targeted Affinity

Here, we demonstrate the possibility of rationally designing nanoparticle receptors with targeted affinity and selectivity for specific small molecules. We used atomistic molecular‐dynamics (MD) simulations to gradually mutate and optimize the chemical structure of the molecules forming the coating monolayer of gold nanoparticles (1.7 nm gold‐core size). The MD‐directed design resulted in nanoreceptors with a 10‐fold improvement in affinity for the target analyte (salicylate) and a 100‐fold decrease of the detection limit by NMR‐chemosensing from the millimolar to the micromolar range. We could define the exact binding mode, which features prolonged contacts and deep penetration of the guest into the monolayer, as well as a distinct shape of the effective binding pockets characterized by exposed interacting points.     

电化学沉积法制备金（核）-铜（壳）纳米粒子阵列

以组装在有机分子自组装膜/金基底电极上的Au纳米粒子阵列为电化学沉积模板,制备了金(核)-铜 (壳)纳米粒子阵列.选用巯基十一胺（AUDT）和巯基癸烷（DT）混合自组装膜作为基底电极与Au纳米粒子的耦联层,可以在一定的电位下实现金属Cu在Au纳米粒子上的选择性沉积.将沉积电位控制在－0.03 V（vs SCE）时,沉积初期（t ≤ 15 s,沉积粒子粒径 ≤ 20 nm ）金(核)-铜 (壳)粒子具有良好的单分散性和近似球形,而且粒径实验值同计算值非常吻合.     

Photopolymerization of diacetylene-capped gold nanoparticles

Gold nanoparticles (AuNPs) coated with the diacetylene henicosa-10,12-diyn-1-yl (DS9) disulfide were successfully prepared by direct synthesis in toluene solutions. The average size of the nanohybrid metal core was finely adjusted by manipulation of the preparative conditions in the diameter range from 1.6 to 7.5 nm, as determined by TEM characterization. The topochemical polymerization of DS9 chemisorbed onto the gold nanoclusters of different size was carried out in colloidal suspensions by exposure to UV radiation and the process was monitored by UV-Vis and Raman spectroscopies. The results showed that in these assemblies the monomer undergoes an intra-particle polymerization and that the dominant polydiacetylene phase present is ruled by the core size. The deposition of the photoirradiated colloids onto different substrates was found to leave the polydiacetylene conjugation unaltered.     

Double hydrophilic block copolymer monolayer protected hybrid gold nanoparticles and their shell cross-linking

This paper describes the syntheses of core/shell gold nanoparticles stabilized with a monolayer of double hydrophilic block copolymer and their stimuli responsiveness before and after shell cross-linking. The hybrid nanoparticles consist of gold core, cross-linkable poly(2-(dimethylamino)ethyl methacrylate) (PDMA) inner shell, and poly(ethylene oxide) (PEO) corona. First, diblock copolymer PEO-b-PDMA was prepared via the reversible addition-fragmentation chain transfer (RAFT) technique using a PEO-based macroRAFT agent. The dithioester end group of PEO-b-PDMA diblock copolymer was reduced to a thiol end group. The obtained PEO-b-PDMA-SH was then used to prepare diblock copolymer stabilized gold nanoparticles by the "grafting-to" approach. 1,2-Bis(2-iodoethoxy)ethane (BIEE) was utilized to selectively cross-link the PDMA residues in the inner shell. The stimuli responsiveness and colloidal stability of core/shell gold nanoparticles before and after shell cross-linking were characterized by laser light scattering (LLS), UV-vis transmittance, and transmission electron microscopy (TEM). At pH 9, the average hydrodynamic radius Rh of non-cross-linked hybrid gold nanoparticles starts to increase above 35 degrees C due to the lower critical solution temperature (LCST) phase behavior of the PDMA blocks in the inner shell. In contrast, Rh of the shell cross-linked gold nanoparticles were essentially independent of temperature. Core/shell gold nanoparticles before and after shell cross-linking exhibit reversible swelling on varying the solution pH. Compared to non-cross-linked core/shell gold nanoparticles, shell cross-linking of the hybrid gold nanoparticles leads to permanent core/shell nanostructures with much higher colloidal stability and physically isolates the gold core from the external environment.     

Tuning the size,concaveness, and aspect ratio of concave cubic gold nanoparticles produced with high reproducibility

Reproducible fabrication of concave cubic gold nanoparticles with precise control over size, concaveness, and aspect ratio is important because the nanoscale structural characteristics can influence their plasmonic and catalytic properties. However, this is particularly challenging because the number of synthetic parameters involved in the fabrication strategy adds complexity to the reaction mechanism. Here, we introduce a simplified seed-mediated method and uncover the unknown conceptual insights on how the different halides and their concentration influence the surface structure and stability of underpotential silver monolayer deposited on the high energy facets of nanoparticles. The results reveal that adding Br^? and I^? ions to growth solution offers a predominant way to control the reaction kinetics and engineering nanoparticles with a predefined size, morphology, concaveness, aspect ratio, and plasmonic properties. Using spectroscopy and microscopy techniques, we shed new light on the reaction kinetics of concave cubic gold nanoparticles using the combined influence of silver underpotential deposition and halides. The strategy developed here can be expanded to fabricate gold nanoparticles of complex geometries. The results from our electromagnetic calculations suggest that the self-assembled superstructure of concave cubic gold nanoparticles can be more appealing for developing an ultra-sensitive sensing platform than to self-assembled superstructures of truncated cubic gold nanoparticles.     

纳米金汞核-壳粒子的电化学制备及其光谱性质

在聚乙烯吡啶修饰导电玻璃电极表面进行了金纳米粒子的二维单层结构组装,通过电沉积方法在金粒子表面制备了纳米汞壳层.研究结果表明,汞壳层的形成导致了内部金粒子表面等离子体共振的谱峰红移和强度衰减.吸附于汞壳表面的结晶紫分子因可承受被金核增强的电磁场,而使其拉曼散射得到极大的增强.