Nanoparticle crystals or superlattices (SLs) are three dimensional arrangements of nanoparticles in the micrometre regime. In SLs, the particles are periodically arranged in a coherent long range order and hence they show collective properties. Various spectroscopic, scattering and imaging techniques have been used to understand the structure of self-assembled SLs. Extensive interest in particle SLs is due to the collective properties of the building blocks, which help us to understand the evolution in properties of organized structures. Controlling the assembly of such organized solids may open up new opportunities for fundamental studies as well as for engineering advanced materials with useful attributes. This review presents our efforts in creating SLs of noble metal nanoparticles and studies performed with those materials. © the Owner Societies 2011.

Pradeep Thalappil

Department Of Chemistry

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IIT Madras

Physical Chemistry Chemical Physics

Researchers reported about the systematic size evolution of an organic-soluble, atomically precise silver cluster. Evolution of precise silver cluster larger nanoparticles was observed by mass spectrometry, while the formation of nanoparticles and crystals was investigated by electron microscopy. A solid-state route was used to synthesize atomically precise clusters and conduct the investigations. The method involves grinding the precursors in a mortar and pestle to form silver thiolates. Addition of sodium borohydride in the solid state to this mixture with continued grinding produced well-defined clusters that were extracted with suitable solvents. A mixture of AgNO3 and tetraoctyl ammonium bromide (TOABr) was taken in an agate mortar and ground uniformly. The product was extracted in ethanol and centrifuged to remove extra thiol in the supernatant and finally the residue was extracted in toluene, which gave reddish-brown solution.

Particle and Particle Systems Characterization

Evolution of atomically precise silver clusters to superlattice crystals

Mercaptosuccinic acid protected gold nanoparticles (Au@MSA) self assemble to form superlattice (SL) crystals at the air-water interface. These have been used for gas adsorption. The current-voltage (I-V) characteristics of the SL film with embedded SL crystals, obtained by four probe measurements, show Ohmic conduction. The conductance observed was proportional to the polarizability of the adsorbed gases. The current through the SL decreases on adsorption of the gas along with decrease in the SERS intensity of a probe molecule from the crystals. We rationalise our observation of the linear dependence of the conductance on the polarizability of the adsorbed gas using a simple model calculation. Variation of the conductance may be useful in designing electrical switches operating at the nanometre length scales. © 2011 The Royal Society of Chemistry.

Fulltext

Nanoscale

Gold nanoparticle superlattices as functional solids for concomitant conductivity and SERS tuning

chemical equation presentation Interfacial etching of silver nanoparticles and separation of the products by Polyacrylamide gel electrophoresis afforded Ag8 and Ag7 clusters with red and bluegreen fluorescence emission, respectively (see picture). The strongly temperaturedependent emission of the clusters suggests potential applications, whilst their facile phase transfer to organic media facilitates further studies. © 2010 Wiley-VCH Verlag GmbH &amp; Co. KCaA, weinheim.

Angewandte Chemie - International Edition

Luminescent Ag7 and Ag8 clusters by interfacial synthesis

We developed a cheap and rapid method for the fabrication of 3D nanoparticle superlattices (SLs) of Au@SGAN and Au@MSA (N-acetyl glutathione (SGAN) and mercaptosuccinic acid (MSA) protected gold nanoparticles, respectively) in gram scale, at a liquid-liquid interfaces under flowing nitrogen gas. While available methods take several weeks to make crystalline SLs, the present route makes them in a day. Morphology of these crystals was examined with scanning electron microscopy (SEM), and their structures were probed using transmission electron microscopy (TEM). The surface enhanced Raman scattering (SERS) studies of these crystals were done using crystal violet (CV) molecules as the analyte which exhibited a detection limit of 10-8 M. The SERS spectrum was used to map the Raman images of the superlattce crystals. SERS from the edges of the crystal showed more enhancement than from the flat surfaces, which is in good agreement with theoretical reports of such anisotropic structures. The sides of the crystals are not sharp, and they show corrugations at the nanometer scale. This helps to produce more "hot spots" at the edges, which result in larger electric field enhancement from these locations. The enhancement factors (EF) for Au@MSA and Au@SGAN SLs were calculated to be around 1.47 x 106 and 3.60 x 105, respectively. More enhancements from Au@MSA SL compared to that of Au@SGAN could be attributed to the smaller chain length of the MSA molecule, which allows closer analyte approach to the nanoparticle surface. © 2009 American Chemical Society.

Chemistry of Materials

Gold nanoparticle superlattices: Novel surface enhanced raman scattering active substrates

Fluorescent three-dimensional (3-D) superlattices of dansyl glutathione protected gold nanoparticles, with potential applications in molecular detection, have been synthesized at an air/water interface by controlling the pH of the nanoparticle suspension. The number of fluorophores per nanoparticle was calculated to be ∼127. Morphologies of the superlattice crystals were examined using scanning electron microscopy (SEM). Most of the crystals observed were triangular in shape. High-resolution transmission electron microscopy (HRTEM) and small angle X-ray scattering (SAXS) were used to study the packing of nanoparticles in these crystals. Both these studies showed that the nanoparticles were arranged in a face-centered cubic (fcc) pattern with a particle-particle distance (center-center) of ∼10.5 nm. Evolution of the crystal morphologies with time was also examined. The fluorescence properties of these triangles were studied using confocal fluorescence imaging and confocal Raman mapping, which were in good agreement with the morphologies observed by SEM. The superlattice exhibits near-infrared (NIR) absorption in the range 1100-2500 nm. Easy synthesis of such functional nanoparticle-based solids makes it possible to use them in novel applications. We utilized the fluorescence of dansyl glutathione gold superlattice crystals for the selective detection of bovine serum albumin (BSA), the major protein constituent of blood plasma, based on the selective binding of the naphthalene ring of the dansyl moiety with site I of BSA. © 2009 Tsinghua University Press and Springer-Verlag.

Nano Research

Fluorescent superlattices of gold nanoparticles: A new class of functional materials

Fluorescent gold nanoparicles were prepared and fluorescent 3D superlattices were made with them at an air/water interface. Fluorescein-labeled Au@MSA, denoted Au@MSA/SAMSA was prepared by ligand exchange reaction. The emission peak of Au@MSA/SAMSA is found to appear at 675 nm, greatly red-shifted compared to the SAMSA solution sample or an isolated Au@MSA/SAMSA particle. It is also noted that the parent fluorophore, fluorescein isothiocyanate (FITC), shows a red shift of ca. 105 nm in the solid state compared to its emission spectrum in solution. Particles of core diameter 3.43 nm exhibit hexagonal assembly extending over several micrometers. The superlattices are fluorescent even though SAMSA is close to the metallic core and strong quenching is expected.

Journal	Physical Chemistry Chemical Physics
ISSN	14639076
Open Access	No