Controlled interparticle spacing for surface-modified gold nanoparticle aggregates

Aggregation of gold nanoparticles of increasing size has been studied as a consequence of adsorption of 2-aminothiophenol (ATP) on gold nanoparticle surfaces. The capping property of ATP in the acidic pH range has been accounted from UV-vis absorption spectroscopy and surface-enhanced Raman scattering (SERS) studies. The effect of nanoparticle size (8-55 nm) on the nature of aggregation as well as the variation in the optical response due to variable degree of interparticle coupling effects among the gold particles have been critically examined. Various techniques such as transmission electron microscopy, X-ray diffraction, zeta-potential, and average particle size measurement were undertaken to characterize the nanoparticle aggregates. The aggregate size, interparticle distances, and absorption band wavelengths were found to be highly dependent on the pH of the medium and the concentration of the capping agent, ATP. The acquired SERS spectra of ATP relate the interparticle spacing. It has been observed that the SERS signal intensities are different for different sized gold nanoparticles.     

Construction of simple gold nanoparticle aggregates with controlled plasmon–plasmon interactions

We have developed a colloidal assembly for the study of plasmon–plasmon interactions between gold nanoparticles. Colloidal aggregates of controlled size and interparticle spacing were synthesized on silica nanoparticle substrates. Following the immobilization of isolated gold nanoparticles onto silica nanoparticles, the surfaces of the adsorbed gold nanoparticles were functionalized with 4-aminobenzenethiol. This molecular linker attached additional gold nanoparticles to the ‘parent' gold nanoparticle, forming small nanoparticle aggregates. The optical absorption spectrum of these clusters differed from that of gold colloid in a manner consistent with plasmon–plasmon interactions between the gold nanoparticles.     

Molecular-level control of feature separation in one-dimensional nanostructure assemblies formed by biomolecular nanolithography

In this paper, we present a convenient and reliable method to organize small gold nanoparticles (d(CORE) = 1.5 nm) into linear chains with precisely controlled interparticle spacing over a range of 1.5-2.8 nm through biomolecular nanolithography. Controlling the feature separations of 1 to a few nanometers with angstrom-level precision is a key requirement in electronic and optical applications of nanostructures to tune the properties of the nanostructures and manipulate the interactions between neighboring structures. Here, chains are formed in solution by utilizing functional-group-directed self-assembly to organize ligand-stabilized gold nanoparticles onto DNA templates. The spacing between neighboring nanoparticles can be controlled chemically and tuned at the molecular level by utilizing nanoparticles possessing ligand shells of varying thickness to achieve angstrom-level resolution at spacings of 1.5, 2.1, and 2.8 nm. The small standard deviation (< or = 20%) in the values for the interparticle spacing illustrates the reproducibility of the approach. Because the interparticle spacing is enforced by the ligand shell rather than the scaffold, the spacing is uniform even in nonlinear sections of the chain. We further show that the assembly process is robust and produces extended linear nanoparticle chains of up to 1 microm in length and a total coverage of > 90%. All structures and interparticle spacings were analyzed using transmission electron microscopy. Our results demonstrate the potential of scaffold-assisted assembly approaches for patterning features with tunable dimensions on a length scale that is important for future applications of these materials in nanoscale electronics and optics.     

Optical switching of coupled plasmons of Ag-nanoparticles by photoisomerisation of an azobenzene ligand

The optical switching of coupled plasmons of silver nanoparticles derivatised with a photoisomerisable azobenzene ligand is presented. It is shown that nanoparticle clusters, linked with an azobenzene dithiol molecule, display switchable optical properties. The photoisomerisation of the linker molecule was used to vary the separation between nanoparticles, which was monitored by changes in the UV-Vis-spectra of the plasmon band of adjacent nanoparticles. A red-shift due to the appearance of a coupled longitudinal plasmon band was observed resulting from the formation of nanoparticle clusters. The maximum absorbance wavelength of this secondary plasmon band was altered by isomerisation of the linker and the spectral changes observed were in good agreement with theory and earlier measurements for gold. Evidence of energy transfer between a nanoparticle and an azobenzene terminated monothiol attached to it was also observed in the UV-Vis spectra.     

Control of gold nanoparticle aggregates by manipulation of interparticle interaction

The size of gold nanoparticle aggregates was controlled by manipulating the interparticle interaction. To manipulate the interparticle interaction of gold nanoparticles prepared by citrate reduction, we applied the substitutive adsorption of benzyl mercaptan on the particle surface in the absence of the cross-linking effect. Various experimental techniques such as UV-vis absorption spectroscopy, surface-enhanced Raman scattering, quasi-elastic light scattering, and zeta-potential measurement were used to characterize the nanoparticle aggregates. Our results suggest that the replacement of the trivalent citrate ions adsorbed on the nanoparticle surface with monovalent benzyl mercaptan ions should destabilize the particles, causing aggregation and hence the increase in the size of nanoparticle aggregates. These experimental results were successfully rationalized by the classical DLVO (Derjaguin-Landau-Vervey-Overbeek) theory that describes the interparticle interaction and colloidal stability in solution. Our findings suggest that the control of surface potential is crucial in the design of stable gold nanoparticle aggregates.     

Controlled Self-Assembly of Metallacycle-Bridged Gold Nanoparticles for Surface-Enhanced Raman Scattering

In this work, well-defined two-dimensional metallacycles have been successfully employed for the well-controlled self-assembly of gold nanoparticles (AuNPs) into discrete clusters such as dimers, trimers, tetramers, pentamers and even hexamers at the water–oil interface for the first time. Furthermore, the modular construction of metallacycle molecules allows precise control of spacing between the gold nanoparticles. Interestingly, it was found that interparticle spacing below 5 nm created by molecular metallacycles in the resultant discrete gold nanoparticle clusters led to a strong plasmon coupling, thus inducing great field enhancement inside the gap between the NPs. More importantly, different discrete clusters with precise interparticle spacing provide a well-defined system for studying the hot-spot phenomenon in surface-enhanced Raman scattering (SERS); this revealed that the SERS effects were closely related to the interparticle spacing.     

偶氮苯衍生物自组装单分子膜中的分子取响

利用反射红外光谱研究了金表面一系列具有不同碳链长度的偶氮苯巯基衍生物的自组装单分子膜.通过对比各向同性样品的透射谱和单分子膜的反射谱中各个吸收峰强度,定量地研究了分子中各部分的取向与分子结构的关系.我们分别提出了烷基链和偶氮基团取向计算的方法,利用该方法成功地求得了分子中各部分在膜的倾角.结果显示,当分子中烷基链长度增大时,碳链和偶氮苯基团相对于法线的倾斜逐渐加剧.这种倾角的变化归因于分子中碳链间范德华引力增大时,引起分子逐渐倾斜以达到最佳的范德华接触.同时研究发现,烷基链和偶氮基团受碳长度变化的影响并不相同.当分子中亚甲基数目增多时,烷基链的倾角迅速增大而偶氮苯倾角的增大则相对缓慢,这反映了它们在空间需求和本身刚性上的不同。     

Collective and individual plasmon resonances in nanoparticle films obtained by spin-assisted layer-by-layer assembly

Nanoscale uniform films containing gold nanoparticle and polyelectrolyte multilayer structures were fabricated by the using spin-assembly or spin-assisted layer-by-layer (SA-LbL) deposition technique. These SA-LbL films with a general formula [Au/(PAH-PSS)nPAH]m possessed a well-organized microstructure with uniform surface morphology and high surface quality at a large scale (tens of micrometers across). Plasmon resonance peaks from isolated nanoparticles and interparticle interactions were revealed in the UV-visible extinction spectra of the SA-LbL films. All films showed the strong extinction peak in the region of 510-550 nm, which is due to the plasmon resonance of the individual gold nanoparticles redshifted because of a local dielectric environment. For films with sufficient density of gold nanoparticles within the layers, the second strong peak was consistently observed between 620 and 660 nm, which is the collective plasmon resonance from intralayer interparticle coupling. Finally, we suggested that, for certain film designs, interlayer interparticle resonance might be revealed as an independent contribution at 800 nm in UV-visible spectra. The observation of independent and concurrent individual, intralayer, and interlayer plasmon resonances can be critical for sensing applications, which involve monitoring of optomechanical properties of ultrathin optically active compliant membranes.     

Tailoring the refractive indices of thin film polymer metallic nanoparticle nanocomposites

We demonstrate how to tailor the spatial distribution of gold nanoparticles (Au-NPs) of different sizes within polystyrene (PS) thin, supported, film hosts, thereby enabling the connection between the spatial distribution of Au-NPs within the polymer film and the optical properties to be determined. The real, n, and imaginary parts, k, of the complex refractive indices N = n(λ)+ik(λ) of the nanocomposite films were measured as a function of wavelength, λ, using multivariable angle spectroscopic ellipsometry. The surface plasmon response of films containing nearly homogeneous Au-NP distributions were well described by predictions based on classical Mie theory and the Drude model. The optical spectra of samples containing inhomogeneous nanoparticle distributions manifest features associated with differences in the size and interparticle spacings as well as the proximity and organization of nanoparticles at the substrate and free surface.     

Spectroscopic characterizations of molecularly linked gold nanoparticle assemblies upon thermal treatment

The understanding of surface properties of core-shell type nanoparticles is important for exploiting the unique nanostructured catalytic properties. We report herein findings of a spectroscopic investigation of the thermal treatment of such nanoparticle assemblies. We have studied assemblies of gold nanocrystals of approximately 2 nm core sizes that are capped by alkanethiolate shells and are assembled by covalent or hydrogen-bonding linkages on a substrate as a model system. The structural evolution of the nanoparticle assemblies treated at different temperatures was probed by several spectroscopic techniques, including UV-visible, Fourier transform infrared (FTIR), and X-ray photoelectron spectroscopy (XPS). The results show that the capping/linking shell molecules can be effectively removed to produce controllable surface and optical properties. The data further revealed that the thermally induced evolution of the surface plasmon resonance property of gold nanoparticles is dependent on the chemical nature of the linker molecule. The spectral evolution is discussed in terms of changes in particle size, interparticle distance, and dielectric medium properties, which has important implications for controlled preparation and thermal processing of core-shell nanostructured metal catalysts.     

Monodispersed core-shell Fe3O4@Au nanoparticles

The ability to synthesize and assemble monodispersed core-shell nanoparticles is important for exploring the unique properties of nanoscale core, shell, or their combinations in technological applications. This paper describes findings of an investigation of the synthesis and assembly of core (Fe(3)O(4))-shell (Au) nanoparticles with high monodispersity. Fe(3)O(4) nanoparticles of selected sizes were used as seeding materials for the reduction of gold precursors to produce gold-coated Fe(3)O(4) nanoparticles (Fe(3)O(4)@Au). Experimental data from both physical and chemical determinations of the changes in particle size, surface plasmon resonance optical band, core-shell composition, surface reactivity, and magnetic properties have confirmed the formation of the core-shell nanostructure. The interfacial reactivity of a combination of ligand-exchanging and interparticle cross-linking was exploited for molecularly mediated thin film assembly of the core-shell nanoparticles. The SQUID data reveal a decrease in magnetization and blocking temperature and an increase in coercivity for Fe(3)O(4)@Au, reflecting the decreased coupling of the magnetic moments as a result of the increased interparticle spacing by both gold and capping shells. Implications of the findings to the design of interfacial reactivities via core-shell nanocomposites for magnetic, catalytic, and biological applications are also briefly discussed.     

Array of molecularly mediated thin film assemblies of nanoparticles: correlation of vapor sensing with interparticle spatial properties

The ability to tune interparticle spatial properties of nanoparticle assemblies is essential for the design of sensing materials toward desired sensitivity and selectivity. This paper reports findings of an investigation of molecularly mediated thin film assemblies of metal nanoparticles with controllable interparticle spatial properties as a sensing array. The interparticle spatial properties are controlled by a combination of alpha,omega-difunctional alkyl mediators (X-(CH(2))(n)-X) such as alkyl dithiols, dicarboxylate acids, and alkanethiol shells capped on nanoparticles. Alkanethiolate-capped gold and gold-silver alloy nanoparticles (2-3 nm) were studied as model building blocks toward the thin film assemblies, whereas the variation of alkyl chain length manipulates the interparticle spacing. The thin films assembled on an interdigitated microelectrode array platform are characterized for determining their responses to the sorption of volatile organic compounds (VOCs). The correlation between the response sensitivity and the interparticle spacing properties revealed not only a clear dependence of the sensitivity on alkyl chain length but also the occurrence of a dramatic change of the sensitivity in a region of chain length for the alkyl mediator comparable with that of the capping alkyl chains. This finding reflects a balance between the interparticle chain-chain cohesive interdigitation and the nanostructure-vapor interaction which determines the relative change of the electrical conductivity of the inked nanoparticle thin film in response to vapor sorption. The results, along with statistical analysis of the sensor array data in terms of sensitivity and selectivity, have provided important insights into the detailed delineation between the interparticle spacing and the nanostructured sensing properties.     

Interactions of Schiff-base ligands with gold nanoparticles: structural, optical and electrocatalytic studies

A study on optical and electrochemical properties resulting upon interaction of Schiff base ligands with gold nanoparticles is presented. The measurements of the optical absorption and fluorescence properties have provided important information about structure-properties dependence. We show that in function of the isomer structure and its attachment orientation with respect to the metal nanoparticle, their optical properties can be modulated. Nanoparticle assemblies mediated by 3,4-DHS were also obtained based on a control of the interparticle interactions and their electrocatalytic activity toward NADH oxidation was investigated.     

The optically detected coherent lattice oscillations in silver and gold monolayer periodic nanoprism arrays: the effect of interparticle coupling

Using femtosecond transient spectroscopy, we studied the optically detected laser-induced coherent phonon oscillation of monolayers of periodic arrays of prismatic-shaped silver and gold nanoparticles, assembled by using the technique of nanosphere lithography. In this method, the same size of polystyrene sphere and the same vacuum conditions are used. Under these circumstances, the gold nanoprisms formed are found to have sharper tips than the corresponding silver nanoprisms. For both gold and silver nanoparticles, the surface plasmon absorption maximum is found to depend linearly on size. The coherent lattice oscillation periods are also found to depend linearly on size. However, although the observed dependence for the silver nanoparticle is found to follow the calculated dependence of a single particle on size (based on a one-dimensional standing wave model), the gold nanoparticle deviates from this model, and the deviation is found to increase with the size of the nanoparticles. This deviation can be explained by considering interparticle coupling. A simple interparticle lattice oscillating dipolar coupling model of the dimer is found to qualitatively account for both the sign and the size dependence of the deviation. The absence of this deviation in the silver nanoparticle arrays is blamed on the weak interparticle coupling due to their rounded tips and the possibility of oxidation of their surfaces.     

Organo-soluble chiral thiol-monolayer-protected gold nanorods

Here, we report the synthesis and characterization of organo-soluble chiral thiol-monolayer-protected gold nanorods. The resulting gold nanorods respectively covered with two opposite enantiomers via the strong covalent Au-S linkage were found to not only be stable in both organic media and solid state, but also show optical activity. Their circular dichroism (CD) spectra exhibited a mirror image relationship, indicating that enantiomeric thiol surfactant on gold surface can produce the corresponding enantiomeric gold nanorods. The densely packed azobenzene thiol monolayer on gold surface exhibited a photoresponsive behavior upon irradiation with 254 nm light instead of 365 nm light, which was found to have an effect on plasmonic absorption of gold nanorods.     

Plasmon interactions between gold nanoparticles in aqueous solution with controlled spatial separation

The effects of interparticle distance on the UV-visible absorption spectrum of gold nanocrystals aggregates in aqueous solution have been investigated. The aggregates were produced by ion-templated chelation of omega-mercaptocarboxylic acid ligands covalently attached to the nanoparticles surface. Variation of the ligand chain length provides control over the interparticle separation in the aggregates. The UV-visible spectra consist typically of a single particle band and a secondary band at higher wavelengths associated with the formation of aggregates in solution. The position of the latter depends on interparticle separation up to distances of approximately 8 nm, in accordance with existing models. Potential applications therefore include distance sensitive labels or proximity probes. Conversely, variation of the ligand length allows the preparation of nanostuctured materials with tuned optical properties.     

Preparation of controlled gold nanoparticle aggregates using a dendronization strategy

In this work, a dendronization strategy was used to control interparticle spacing and the optical properties of gold nanoparticle (NP) aggregates in aqueous media. To achieve this goal, two dendritic disulfides bearing different functionalities on their periphery were synthesized and used as ligands to dendronize gold NPs. The dendronized NPs then undergo aggregation; this process was followed by UV-vis spectroscopy, dynamic light scattering (DLS), and transmission electronic microscopy (TEM) measurements and correlated with Generalized Mie Theory electrodynamics calculations. For comparison, NP functionalization was also studied using a nondendritic ligand. It was found that the use of dendritic disulfides allows for the preparation of controlled NP aggregates. This study demonstrates how different dendronization parameters, such as disulfide concentration, temperature, time and nature of the ligand (dendritic vs nondendritic), determine the control exerted over the size and stability of the NP aggregates.     

Interparticle spacing control in the superlattices of carboxylic acid-capped gold nanoparticles by hydrogen-bonding mediation

We have demonstrated that carboxylic acid-capped gold nanoparticles were self-assembled to form two-dimensional (2D) and/or three-dimensional (3D) superlattices at an air/water interface in the presence of a bifunctional hydrogen-bonding mediator such as 4-pyridinecarboxylic acid (PyC) or trans-3-(3-pyridyl)acrylic acid (PyA). Transmission electron microscopy revealed a hexagonal close-packed arrangement of nanoparticles in the superlattice with an extension of interparticle spacing. In the 2D superlattices, larger particles produced a higher-quality assembly having long-range translational ordering. Attenuated total reflectance IR (ATR-IR) spectroscopy revealed the presence of hydrogen bonds between the mediator used and the capping agents of carboxylic acid on nanoparticle surfaces. Since the experimentally obtained interparticle separation distance agreed approximately with that obtained by the geometrical model calculations, we conclude that the hydrogen-bonding mediation controlled the interparticle spacing or structure by monomolecular incorporation between adjacent nanoparticles in the superlattices.     

Optical limiting of a covalently bonded gold nanoparticle/polylysine hybrid material

The optical limiting performance of a covalently bonded gold nanoparticle (approximately 2 nm)/polylysine hybrid material (AuNP-PLL) was investigated using 4.1 ns laser pulses at 532 nm. The hybrid material exhibits enhanced optical limiting in comparison to individual nanoparticles, presumably due to the interparticle electromagnetic interactions between particles in close proximity. Reverse saturable absorption and/or free carrier absorption were found to be the dominant contributor(s) to the optical limiting of the hybrid material.     

Mediator-template assembly of nanoparticles

The ability to construct size- and shape-controllable architectures using nanoparticles as building blocks is essential for the exploration of nanoparticle-structured properties. This paper reports findings of an investigation of a mediator-template strategy for the size-controllable assembly of nanoparticles. This strategy explores multidentate thioether ligands as molecular mediators and tetraalkylammonium-capped gold nanoparticles (5 nm) as templates toward the preparation of size-controllable and monodispersed spherical assemblies ( approximately 20-300-nm diameters). The combination of the mediation force of the multidentate thioether and the hydrophobic force of the tetraalkylammonium template establishes the interparticle linkage and stability. The morphological properties of the spherical assemblies have been characterized using TEM, AFM, and SAXS techniques. The finding of the soft-hard nature of the nanoparticle assemblies and their interactions with contacting substrates could form the basis of a new strategy for manipulating nanoscale linkages between nanoparticle assemblies, soldering nanoelectronics, and constructing nanosensor devices. The intriguing light scattering and optical absorption properties in response to assembly, disassembly, sizing, and interparticle spacing parameters have been characterized by dynamic light scattering and spectrophotometric measurements. The discovery of the controlled disassembly into individual nanoparticles and the size regulation by a third capping component could form the basis for applications in controlled drug delivery. The fundamental basis for the mediator-template strategy as a versatile assembly technique is further discussed in terms of experimental and theoretical correlations of the morphological and optical properties.