Mesoflowers: a new class of highly efficient surface-enhanced Raman active and infrared-absorbing materials

A method for the synthesis of a new class of anisotropic mesostructured gold material, which we call "mesoflowers", is demonstrated. The mesoflowers, unsymmetrical at the single particle level, resemble several natural objects and are made up of a large number of stems with unusual pentagonal symmetry. The mesostructured material has a high degree of structural purity with star-shaped, nano-structured stems. The mesoflowers were obtained in high yield, without any contaminating structures and their size could be tuned from nano- to meso-dimensions. The dependence of various properties of the mesoflowers on their conditions of formation was studied. The near-infrared-infrared (NIR-IR) absorption exhibited by the mesoflowers has been used for the development of infrared filters. Using a prototypical device, we demonstrated the utility of the gold mesoflowers in reducing the temperature rise in an enclosure exposed to daylight in peak summer. These structures showed a high degree of surface-enhanced Raman scattering (SERS) activity compared to spherical analogues. SERS-based imaging of a single mesoflower is demonstrated. The high SERS activity and NIR-IR absorption property open up a number of exciting applications in diverse areas

Aqueous to organic phase transfer of Au<SUB>25</SUB> clusters

Aqueous to organic phase transfer of water soluble sub-nanocluster, Au25SG18 (-SG, glutathione thiolate) is demonstrated using the phase transfer reagent, tetraoctylammonium bromide. The phase transfer occurred by the electrostatic attraction between the hydrophilic carboxylate anion of the glutathione ligand on the cluster surface in the aqueous phase and the hydrophobic tetraoctylammonium cation in the toluene phase. Detailed spectroscopic characterization of the phase transferred cluster using optical absorption, photoluminescence and X-ray photoelectron spectroscopy showed that the cluster retains its integrity during the phase transfer. The interaction of the cluster with the phase transfer reagent can be studied with infrared spectroscopy. The phase transferred cluster can be dried and redissolved in an organic medium, just as the original cluster. This is the first report of the phase transfer of a sub-nanocluster, keeping the cluster core intact. The effect of dilution and pH on phase transfer of this cluster is studied in detail. This method promises several possibilities to explore the properties, reactivity and applications of sub-nanoclusters both in the aqueous and organic phases

Molecular precursor-mediated tuning of gold mesostructures: synthesis and SERRS studies

This article describes the high yield synthesis of a range of anisotropic gold mesostructures such as flowers, cubes, plates, and quasispherical mesostructures using a seed-mediated approach. These structures were formed from precursor seed nanoparticles of gold stabilized by the template, 1,2-phenylenediamine (1,2-PDA). We demonstrated that control of the morphologies from mesoflowers to quasispherical structures is possible with the molecular precursors used in the synthesis of seeds. It was found that concentration of the template, 1,2-PDA added during seed preparation played an important role in the conversion of mesoflowers to quasispherical and cube-like structures. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV-vis spectroscopy and energy dispersive analysis of X-rays (EDAX) were used for the determination of physical and chemical composition of the nano/mesostructures formed. The seed nanoparticles responsible for the formation of these various anisotropic structures were further characterized and analyzed using laser desorption ionization mass spectrometry (LDI MS) and TEM. We demonstrated high surface-enhanced resonance Raman scattering (SERRS) activity of the mesoflowers using crystal violet (CV) as the analyte molecule. The shape-dependent SERRS activity of various meso/nanostructures was also studied. A ~0.8&#215;102 decrease in the SERRS intensity was observed in quasispherical structures compared to mesoflowers. The increased SERRS activity is attributed to the unique shape and nanofeatures present on the mesoflowers, which were absent in the quasispherical mesostructures. We believe that the high SERRS activity exhibited by the mesoflowers may be utilized for developing novel sensors

Supported quantum clusters of silver as enhanced catalysts for reduction

Quantum clusters (QCs) of silver such as Ag7(H2MSA)7, Ag8(H2MSA)8 (H2MSA, mercaptosuccinic acid) were synthesized by the interfacial etching of Ag nanoparticle precursors and were loaded on metal oxide supports to prepare active catalysts. The supported clusters were characterized using high resolution transmission electron microscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, and laser desorption ionization mass spectrometry. We used the conversion of nitro group to amino group as a model reaction to study the catalytic reduction activity of the QCs. Various aromatic nitro compounds, namely, 3-nitrophenol (3-np), 4-nitrophenol (4-np), 3-nitroaniline (3-na), and 4-nitroaniline (4-na) were used as substrates. Products were confirmed using UV-visible spectroscopy and electrospray ionization mass spectrometry. The supported QCs remained active and were reused several times after separation. The rate constant suggested that the reaction followed pseudo-first-order kinetics. The turn-over frequency was 1.87 s-1 per cluster for the reduction of 4-np at 35°C. Among the substrates investigated, the kinetics followed the order, SiO2 > TiO2 > Fe2O3 > Al2O3

Protein-protected luminescent noble metal quantum clusters: an emerging trend in atomic cluster nanoscience

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

Two distinct fluorescent quantum clusters of gold starting from metallic nanoparticles by pH-dependent ligand etching

Two fluorescent quantum clusters of gold, namely Au25 and Au8, have been synthesized from mercaptosuccinic acid-protected gold nanoparticles of 4-5 nm core diameter by etching with excess glutathione. While etching at pH ~3 yielded Au25, that at pH 7-8 yielded Au8. This is the first report of the synthesis of two quantum clusters starting from a single precursor. This simple method makes it possible to synthesize well-defined clusters in gram quantities. Since these clusters are highly fluorescent and are highly biocompatible due to their low metallic content, they can be used for diagnostic applications

Molecular-receptor-specific, non-toxic, near-infrared-emitting Au cluster-protein nanoconjugates for targeted cancer imaging

Molecular-receptor-targeted imaging of folate receptor positive oral carcinoma cells using folic-acid-conjugated fluorescent Au25 nanoclusters (Au NCs) is reported. Highly fluorescent Au25 clusters were synthesized by controlled reduction of Au+ ions, stabilized in bovine serum albumin (BSA), using a green-chemical reducing agent, ascorbic acid (vitamin-C). For targeted-imaging-based detection of cancer cells, the clusters were conjugated with folic acid (FA) through amide linkage with the BSA shell. The bioconjugated clusters show excellent stability over a wide range of pH from 4 to 14 and fluorescence efficiency of ~5.7% at pH 7.4 in phosphate buffer saline (PBS), indicating effective protection of nanoclusters by serum albumin during the bioconjugation reaction and cell-cluster interaction. The nanoclusters were characterized for their physico-chemical properties, toxicity and cancer targeting efficacy in vitro. X-ray photoelectron spectroscopy (XPS) suggests binding energies correlating to metal Au 4f7/2&#732;83.97 eV and Au 4f5/2~87.768 eV. Transmission electron microscopy and atomic force microscopy revealed the formation of individual nanoclusters of size ~1 nm and protein cluster aggregates of size ~8 nm. Photoluminescence studies show bright fluorescence with peak maximum at ~674 nm with the spectral profile covering the near-infrared (NIR) region, making it possible to image clusters at the 700-800 nm emission window where the tissue absorption of light is minimum. The cell viability and reactive oxygen toxicity studies indicate the non-toxic nature of the Au clusters up to relatively higher concentrations of 500 &#181;g ml-1. Receptor-targeted cancer detection using Au clusters is demonstrated on FR+ve oral squamous cell carcinoma (KB) and breast adenocarcinoma cell MCF-7, where the FA-conjugated Au25 clusters were found internalized in significantly higher concentrations compared to the negative control cell lines. This study demonstrates the potential of using non-toxic fluorescent Au nanoclusters for the targeted imaging of cancer

Shell-Isolated Assembly of Atomically Precise Nanoclusters on Gold Nanorods for Integrated Plasmonic-Luminescent Nanocomposites

In this work, we integrate atomically precise noble metal nanoclusters (NCs) on gold nanorods (AuNRs) to create hybrid plasmonic-luminescent nanomaterials. Initially, we assemble luminescent Ag29(LA)12 NC (LA = lipoic acid) to silica shell-encapsulated AuNRs. The resulting nanostructure shows plasmon-enhanced luminescence in aqueous medium as well as in the solid state. Atomic precision of the fluorophores used in this case allows detailed characterization of individual nanocomposites by diverse techniques, including transmission electron microscopy (TEM) and 3D electron tomographic reconstruction. We extend this strategy to prepare similar structures with gold NC protected with bovine serum albumin (Au30BSA). These two examples demonstrate the generic nature of the present strategy in preparing plasmonic-luminescent hybrid nanostructures using atomically precise NCs.acceptedVersionPeer reviewe