Comprehensive knowledge over the shape of nanomaterials is a critical factor in designing devices with desired functions. Due to this reason, systematic efforts have been made to synthesize materials of diverse shape in the nanoscale regime. Anisotropic nanomaterials are a class of materials in which their properties are direction-dependent and more than one structural parameter is needed to describe them. Their unique and fine-tuned physical and chemical properties make them ideal candidates for devising new applications. In addition, the assembly of ordered one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) arrays of anisotropic nanoparticles brings novel properties into the resulting system, which would be entirely different from the properties of individual nanoparticles. This review presents an overview of current research in the area of anisotropic nanomaterials in general and noble metal nanoparticles in particular. We begin with an introduction to the advancements in this area followed by general aspects of the growth of anisotropic nanoparticles. Then we describe several important synthetic protocols for making anisotropic nanomaterials, followed by a summary of their assemblies, and conclude with major applications.

A novel interfacial route has been developed for the synthesis of a bright-red-emitting new subnanocluster, Au(23), by the core etching of a widely explored and more stable cluster, Au(25)SG(18) (in which SG is glutathione thiolate). A slight modification of this procedure results in the formation of two other known subnanoclusters, Au(22) and Au(33). Whereas Au(22) and Au(23) are water soluble and brightly fluorescent with quantum yields of 2.5 and 1.3 %, respectively, Au(33) is organic soluble and less fluorescent, with a quantum yield of 0.1 %. Au(23) exhibits quenching of fluorescence selectively in the presence of Cu(2+) ions and it can therefore be used as a metal-ion sensor. Aqueous- to organic-phase transfer of Au(23) has been carried out with fluorescence enhancement. Solvent dependency on the fluorescence of Au(23) before and after phase transfer has been studied extensively and the quantum yield of the cluster varies with the solvent used. The temperature response of Au(23) emission has been demonstrated. The inherent fluorescence of Au(23) was used for imaging human hepatoma cells by employing the avidin-biotin interaction.

No abstract

Noble metal quantum clusters (NMQCs) are the missing link between isolated noble metal atoms and nanoparticles. NMQCs are sub-nanometer core sized clusters composed of a group of atoms, most often luminescent in the visible region, and possess intriguing photo-physical and chemical properties. A trend is observed in the use of ligands, ranging from phosphines to functional proteins, for the synthesis of NMQCs in the liquid phase. In this review, we briefly overview recent advancements in the synthesis of protein protected NMQCs with special emphasis on their structural and photo-physical properties. In view of the protein protection, coupled with direct synthesis and easy functionalization, this hybrid QC-protein system is expected to have numerous optical and bioimaging applications in the future, pointers in this direction are visible in the literature.

1,4-Benzenedimethanethiol (HS−CH2−C6H4−CH2−SH, BDMT) adsorbs dissociatively on silver and gold surfaces yielding self-assembled monolayers with the thiolate structure. Whereas the molecule adsorbs flat on silver as a result of the loss of two thiol protons, it adsorbs with the molecular plane perpendicular to the gold surface with the loss of one thiol proton. This is manifested by the presence of the ring C−H and S−H stretches and the S−C−H bend for the Au monolayer and the absence of these for the Ag monolayer in the surface-enhanced Raman spectra. The difference in adsorbate geometry is presumably due to the changes in interaction and not to differences in the lattice constants of the two surfaces, which is rather small. In both cases, metal−adsorbate π bonding is weak, resulting in only small shifts in the ring modes. BDMT monolayers are more stable than alkanethiol monolayers and desorb only at a fairly high temperature of 423 K in air, whereas alkanethiols desorb below 373 K. An increase in temperature leads to structural changes in the Au monolayer and the molecules begin to lie flat on the surface, and desorption occurs from this state. The Ag monolayer is less stable thermally and desorption is eventless. Difference in the desorption temperatures point to the importance of energetics of self-assembly in determining the stability. The thiol proton on gold surfaces can be removed easily by exposing the monolayer to basic solutions. The gold monolayer upon exposure to thiols leads to the formation of disulfides, suggesting the formation of a prototypical bilayer. A completely new S−S stretching frequency is seen upon reaction with 4-methoxybenzenethiol (MOBT) with the complete absence of the S−H stretch. Other spectral features of the bilayer can be attributed to BDMT and MOBT subunits. The respective thiols in solution, however, do not react, leading to disulfide. Reaction with silver monolayer leads to the displacement of BDMT for MOBT and no reaction is observed. With n-alkanethiols, the reactivity decreases with the alkane chain length. The alkanethiol part of the spectrum resembles that of the corresponding self-assembled monolayer. Surface-enhanced reactivity of the type observed here has not been reported hitherto. The MOBT part of the bilayer desorbs first after cleaving the S−S bond and BDMT leaves the surface subsequently. X-ray exposure of the monolayers leads to beam-induced damage which is manifested in the Raman spectra. Whereas the damage is severe for Ag, part of the Au monolayer survives X-ray exposure.