While the optical and spectroscopic properties of gold nanoparticles are widely used for chemical, bioanalytical, and biomedical applications, the study of the size correlation with these properties for nanoparticles in solutions is rather limited. This paper describes the results of a systematic study of such a correlation for gold nanoparticles with diameters ranging from 10 to 100 nm in aqueous solutions. The high monodispersity of these nanoparticles permitted a meaningful correlation of the particle size with the surface plasmon (SP) resonance band properties and the surface-enhanced Raman scattering (SERS) spectroscopic properties. This correlation is compared to the results from the simulation based on Mie theory. The close agreement between the experimental and the theoretical results provides insight into the validity of determining the wavelength of the SP resonance band as a measure of the particle size. The size correlation with the SERS intensity from the adsorption of 4-mercaptobenzoic acid on the nanoparticles in aqueous solutions reveals the existence of a critical size of the nanoparticles in the solution beyond which the particle−particle interaction is operative and responsible for the SERS effect. These findings serve as the basis of size correlations for exploiting the optical and spectroscopic properties of gold nanoparticles of different sizes in aqueous solutions in analytical or bioanalytical applications.

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.

Nanoscale catalysis depends on spatial and surface arrangements of atoms. The synthesis of core/shell nanoparticles by synthetic approaches, the characterization of the core/shell nanostructures in terms of the relative core/shell compositions, and the demonstration of the electrocatalytic activities for fuel cell reactions (see figure), as reported in this Communication, have important implications to the design of desired nanoscale catalysts.

This paper describes the results of an investigation of the interparticle interactions and reactivities in the assembly of gold nanoparticles mediated by cyanine dyes. The combination of the positively charged indolenine cyanine dyes and the negatively charged gold nanoparticles is shown to form a J-aggregate bridged assembly of nanoparticles, in addition to hydrophobic interparticle and electrostatic dye-particle interactions. Such interparticle interactions and reactivities are studied by probing the absorption of J-aggregates and fluorescence from the dyes and the surface plasmon resonance absorption from the nanoparticles. The J-aggregation of the dyes adsorbed on the nanoparticles is shown to play an important role in the assembly of nanoparticles. The spectral evolution of the J-band of the dyes and the surface plasmon resonance band of the nanoparticles was found to be sensitive to the nature of the charge and the structure of the dyes. The fluorescence quenching for the dyes was shown to be quantitatively related to the surface coverage of the dyes on the nanocrystal surfaces. These findings have provided important information for assessing a two-step process involving a rapid adsorption of the dyes on the nanoparticles and a subsequent assembly of the nanoparticles involving a combination of interparticle J-aggregation and hydrophobic interactions of the adsorbed dyes. The results are discussed in terms of the structural effects of the dyes, and the interparticle molecular interactions and reactivities, which provide important physical and chemical insights into the design of dye-nanoparticle structured functional nanomaterials.

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.

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The ability to control the size and shape of nanoparticle assemblies is essential for the ultimate applications in sensors, catalysis, medical diagnostics, information storage, and quantum computation. This report demonstrates a novel mediator-template strategy toward this ability by exploring molecular driving forces exerted by a tridentate thioether as a mediator and tetraoctylammonium bromide as a templating agent. A combination of the ligand mediation, the surfactant templating, and their relative concentrations served as the driving forces. This combination leads to unprecedented spherical assemblies of gold nanoparticles in controllable sizes via manipulation of the relative concentrations of mediating and templating components.

The ability to create bio-functional nanoprobes for the detection of biological reactivity is important for developing bioassay and diagnostic methods. This paper describes the findings of an investigation of the surface functionalization of gold (Au) and magnetic nanoparticles coated with gold shells (M/Au) by proteins and spectroscopic labels for the creation of nanoprobes for use in surface enhanced Raman scattering (SERS) assays. Highly monodispersed Au nanoparticles and M/Au nanoparticles with two types of magnetic nanoparticle cores (Fe(2)O(3) and MnZn ferrite) were studied as model systems for the bio-functionalization and Raman labeling. Comparison of the SERS intensities obtained with different particle sizes (30-100 nm) and samples in solution versus on solid substrates have revealed important information about the manipulation of the SERS signals. In contrast to the salt-induced uncontrollable and irreversible aggregation of nanoparticles, the ability to use a centrifugation method to control the formation of stable small clustering sizes of nanoparticles was shown to enhance SERS intensities for samples in solution as compared with samples on solid substrates. A simple method for labeling protein-capped Au nanoparticles with Raman-active molecules was also described. The functionalized Au and M/Au nanoparticles are shown to exhibit the desired functional properties for the detection of SERS signals in the magnetically separated reaction products. These results are discussed in terms of the interparticle distance dependence of 'hot-spot' SERS sites and the delineation of the parameters for controlling the core-shell reactivity of the magnetic functional nanocomposite materials in bio-separation and spectroscopic probing.