Showing papers by "Thalappil Pradeep published in 2011"

Anisotropic nanomaterials: structure, growth, assembly, and functions

Abstract: 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. (Published: 16 February 2011) Citation: Nano Reviews 2011, 2 : 5883 - DOI: 10.3402/nano.v2i0.5883

Copper quantum clusters in protein matrix: potential sensor of Pb2+ ion.

Abstract: A one-pot synthesis of extremely stable, water-soluble Cu quantum clusters (QCs) capped with a model protein, bovine serum albumin (BSA), is reported. From matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry, we assign the clusters to be composed of Cu5 and Cu13 cores. The QCs also show luminescence properties having excitation and emission maxima at 325 and 410 nm, respectively, with a quantum yield of 0.15, which are found to be different from that of protein alone in similar experimental conditions. The quenching of luminescence of the protein-capped Cu QCs in the presence of very low hydrogen peroxide concentration (approximately nanomolar, or less than part-per-billion) reflects the efficacy of the QCs as a potential sensing material in biological environments. Moreover, as-prepared Cu QCs can detect highly toxic Pb2+ ions in water, even at the part-per-million level, without suffering any interference from other metal ions.

Understanding the Evolution of Luminescent Gold Quantum Clusters in Protein Templates

Abstract: We show that the time-dependent biomineralization of Au3+ by native lactoferrin (NLf) and bovine serum albumin (BSA) resulting in near-infrared (NIR) luminescent gold quantum clusters (QCs) occurs through a protein-bound Au1+ intermediate and subsequent emergence of free protein. The evolution was probed by diverse tools, principally, using matrix-assisted laser desorption ionization mass spectrometry (MALDI MS), X-ray photoelectron spectroscopy (XPS), and photoluminescence spectroscopy (PL). The importance of alkaline pH in the formation of clusters was probed. At neutral pH, a Au1+–protein complex was formed (starting from Au3+) with the binding of 13–14 gold atoms per protein. When the pH was increased above 12, these bound gold ions were further reduced to Au(0) and nucleation and growth of cluster commenced, which was corroborated by the beginning of emission; at this point, the number of gold atoms per protein was ∼25, suggesting the formation of Au25. During the cluster evolution, at certain time i...

Thermal conductivity enhancement of nanofluids containing graphene nanosheets

Abstract: This paper envisages a mechanism of heat conduction behind the thermal conductivity enhancement observed in graphene nanofluids Graphene nanofluids have been prepared, characterized, and their thermal conductivity was measured using the transient hot wire method The enhancements in thermal conductivity are substantial even at lower concentrations and are not predicted by the classical Maxwell model The enhancement also shows strong temperature dependence which is unlike its carbon predecessors, carbon nanotube (CNT) and graphene oxide nanofluids It is also seen that the magnitude of enhancement is in-between CNT and metallic/metal oxide nanofluids This could be an indication that the mechanism of heat conduction is a combination of percolation in CNT and Brownian motion and micro convection effects in metallic/metal oxide nanofluids, leading to a strong proposition of a hybrid model

A fifteen atom silver cluster confined in bovine serum albumin

Abstract: Luminescent Ag15 clusters confined in bovine serum albumin (BSA) have been prepared by a simple wet chemical route. The luminescence, exhibiting a maximum at 685 nm, is observable to the naked eye. The chemical composition of these clusters was analyzed using matrix assisted laser desorption ionization mass spectrometry (MALDI MS), X-ray photoelectron spectroscopy (XPS), and energy dispersive analysis of X-rays (EDAX). Intact Ag15@BSA is observed by MALDI MS. Multiple charge states of the cluster are observed confirming the mass assignment. The clusters showed a quantum yield of 10.71% in water and the luminescence was stable in a pH range of 1–12. Stability of the clusters was enhanced by the addition of polyvinylpyrrolidone (PVP). The clusters showed luminescence in the solid state as well. Evolution of clusters with variation in the amount of reducing agent added shows that the cluster formation goes through an intermediate state of bound silver, formed instantaneously after the addition of Ag+, which transforms to the cluster. High yield synthesis and exciting photophysical properties make our new material interesting for various applications such as biolabeling and imaging.

Quantum Clusters in Cavities: Trapped Au15 in Cyclodextrins

Abstract: We have prepared Au15 quantum clusters anchored to α-, β-, and γ-cyclodextrin (CD) cavities. The synthesis process involves the core etching of larger clusters and the simultaneous trapping of the clusters formed inside the CD cavities. The clusters were characterized by various tools, such as optical absorption and luminescence spectroscopies, electrospray ionization−mass spectrometry (ESI-MS), X-ray photoelectron spectroscopy (XPS), circular dichroism spectroscopy, and two-dimensional nuclear magnetic resonance (2D NMR) spectroscopy. Trapping of the cluster in the CD cavity was proven by circular dichroism and also by rotational Overhauser effect spectroscopy (ROESY), in terms of the distinct cross peak between proton “e” of the glutathione (−SG) ligand and the “H3” proton of CD. Dynamic light scattering (DLS) studies showed a hydrodynamic diameter of ∼3−4 nm, indicating one CD molecule per cluster with an extension of one water of hydration. The clusters are intensely luminescent, with major lifetime c...

Transparent, luminescent, antibacterial and patternable film forming composites of graphene oxide/reduced graphene oxide.

Abstract: Multifunctional graphene oxide/reduced graphene oxide (GO/RGO) composites were prepared through electrostatic interaction using biocompatible ingredients. Different functionalities were added to GO/RGO by anchoring materials such as native lactoferrin (NLf), NLf protected Au clusters (designated as Au@NLf), chitosan (Ch) and combinations thereof. Anchoring of Ch and NLf enhances the antibacterial property of RGO/GO. The addition of Ch to RGO/GO not only helped in forming stable dispersions but also helped in fabricating large (cm2) area films through a simple solvent evaporation technique. Functionalities such as photoluminescence were added to Ch-RGO/GO composites by anchoring Au@NLf on it. The composites thus formed showed stable luminescence in presence of various metal ions in the solid state. The composite showed reasonable stability against pH and temperature variations as well. The as-prepared films were transparent and the transparency could be modulated by controlling the concentration of RGO/GO ...

Single- and few-layer graphene growth on stainless steel substrates by direct thermal chemical vapor deposition

Abstract: Increasing interest in graphene research in basic sciences and applications emphasizes the need for an economical means of synthesizing it. We report a method for the synthesis of graphene on commercially available stainless steel foils using direct thermal chemical vapor deposition. Our method of synthesis and the use of relatively cheap precursors such as ethanol (CH(3)CH(2)OH) as a source of carbon and SS 304 as the substrate proved to be economically viable. The presence of single- and few-layer graphene was confirmed using confocal Raman microscopy/spectroscopy. X-ray photoelectron spectroscopic measurements were further used to establish the influence of various elemental species present in stainless steel on graphene growth. The role of cooling rate on surface migration of certain chemical species (oxides of Fe, Cr and Mn) that promote or hinder the growth of graphene is probed. Such analysis of the chemical species present on the surface can be promising for graphene based catalytic research.

Reversible assembly and disassembly of gold nanorods induced by EDTA and its application in SERS tuning.

Abstract: A facile and reversible method for assembling and disassembling gold nanorods (GNRs) using a common chelating agent, ethylenediaminetetraacetic acid (EDTA), is reported. Assembly was induced by the electrostatic interaction between the cetyltrimethylammonium bromide (CTAB) bilayer present on GNRs and EDTA. At lower concentrations of EDTA, end-to-end assembled chains were formed. At higher concentrations of EDTA, these chains come together to form sheet-like structures. The complex of CTAB and EDTA, being labile, disassembles in the presence of stronger chelating agents. Upon addition of metal ions having higher formation constants, EDTA detaches from the GNRs and forms stronger complexes with metal ions, resulting in disassembly. Characteristic changes were observed in the UV/vis spectra. Addition of EDTA resulted in a red shift of longitudinal surface plasmon (LSP) resonance at lower concentrations, indicating an end-to-end assembly. At higher concentrations, the characteristic of side-by-side assembly was seen in the UV/vis spectra. TEM analysis proved the existence of end-to-end chains at lower concentrations of EDTA and side-by-side assembled sheet-like structures at higher concentrations. The addition of metal ions induced disassembly. Even 2 ppb of metal ion was detected using the spectral changes. Disassembly was studied in detail, taking Pb(II) as the model system. Upon addition of Pb(II), TSP showed a blue shift and decreased in intensity while the LSP showed a red shift and increased in intensity. A new peak at a higher wavelength region emerged, pointing to the existence of both side-by-side and end-to-end assembly in the system. TEM analysis showed that the disassembly involves the formation of bundled chains which may be the reason for the observed spectral changes. Surface-enhanced Raman scattering (SERS) activity of the system could be tuned by controlling the concentration of EDTA and the metal ion, Pb(II).

Supported quantum clusters of silver as enhanced catalysts for reduction.

Abstract: 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.

Investigation into the Reactivity of Unsupported and Supported Ag7 and Ag8 Clusters with Toxic Metal Ions

Abstract: We report the chemical interactions of unsupported and alumina-supported Ag7 and Ag8 clusters protected with MSA (mercaptosuccinic acid) with heavy metal ions Hg(II), Cd(II), and Pb(II) in water at different concentrations. The investigation was carried out to determine the feasibility of this interesting new class of materials called quantum clusters for water purification. These systems were studied using various spectroscopic and microscopic techniques such as ultraviolet–visible spectroscopy, Fourier transform infrared spectroscopy, X-ray diffraction, dynamic light scattering, zeta potential measurements, scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, and photoluminescence spectroscopy and in detail by X-ray photoelectron spectroscopy. We observed that the metal ions interact with both the silver atoms of the clusters and the functional groups of the capping agent (MSA). The mercuric ions were reduced to metallic mercury by both supported and unsupported clusters, ...

Au25@SiO2: quantum clusters of gold embedded in silica.

Abstract: Quantum clusters (QC) or subnanoclusters of noble metals constitute a relatively new class of nanomaterials with mole-cular cores composed of a few atoms. Their electronic struc-ture and physicochemical properties are totally different from metallic nanoparticles of the same element. They possess dis-crete electronic energy levels and show ‘molecule-like’ optical transitions in absorption and emission. Even though they are relatively new, several methods have been developed to syn-thesize both aqueous

Pristine and Hybrid Nickel Nanowires: Template-, Magnetic Field-, and Surfactant-Free Wet Chemical Synthesis and Raman Studies

Abstract: Nickel nanowires of high purity and various aspect ratios were synthesized by a wet chemical reduction method without the assistance of surfactants, templates, and external magnetic field. The observed unique optical absorption features of nickel nanowires are highly dependent on the nanowire dimensions. In order to tailor the physical and chemical properties of the nanowires, we have incorporated functionalities in the form of tellurium and zinc oxide coating on them by overgrowth processes using simple wet chemical reactions. The pristine and functional nanowires were characterized by different microscopic and spectroscopic techniques such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), confocal Raman microscopy, X-ray diffraction spectroscopy (XRD), and UV−visible spectroscopy. Our system provides additional details into the growth of anisotropic Ni nanowires, especially in the absence of surfactants, templates, and external magnetic field. We have also demonstrated surfa...

Tissue imprint imaging by desorption electrospray ionization mass spectrometry

Abstract: Cross-sections of Myristica malabarica (Lam) seed and mouse brain tissue were imprinted on such ordinary surfaces as printer paper and TLC plates, and successfully imaged by desorption electrospray ionization mass spectrometry (DESI-MS) at 250 μm resolution Chemical images representing the distribution of the alkaloid malabaricone C in the seed substructures and individual lipids in the substructures of the brain were obtained Practical implications include analysis of irregular or soft materials, easy recording, transportation and storage of the latent image, and posterior analysis of the samples by different techniques without the requirement of addition of matrices or use of specific types of surfaces

Direct analysis of camptothecin from Nothapodytes nimmoniana by desorption electrospray ionization mass spectrometry (DESI-MS).

Abstract: Desorption electrospray ionization was employed for fast and direct ambient detection of the anti-tumor drug, camptothecin, and its derivative, 9-methoxycamptothecin in Nothapodytes nimmoniana. Different parts of the plant such as leaves, stems and bark were examined. The ion intensities suggest that the concentration in bark is higher than that in the leaves and stems. The method does not require any sample preparation or preseparation. The identity of the alkaloids was further confirmed by tandem mass spectrometry.

First Principles Studies of Two Luminescent Molecular Quantum Clusters of Silver, Ag7(H2MSA)7 and Ag8(H2MSA)8, Based on Experimental Fluorescence Spectra

Abstract: We report a joint experimental and theoretical study of two luminescent molecular quantum clusters of silver, Ag7(H2MSA)7 and Ag8(H2MSA)8 (H2MSA is the dicarboxylic acid of mercaptosuccinic acid in thiolate form). Global optimizations and property calculations are performed in the framework of density-functional theory (DFT-PBE) to search for the leading candidates with the lowest energy. The simulated excitation spectra of these two clusters are in good agreement with the corresponding experimental spectra. The presence of −RS–Ag–RS– as a stable motif has been confirmed in all of the lowest energy structures.

Functional noble metal nanoparticle superlattices grown at interfaces

Abstract: 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.

Hybrid A–B–A type nanowires through cation exchange.

Abstract: Hybrid A–B–A type nanowires (NWs) with Ag5Te3–HgTe–Ag5Te3 composition have been created by the reaction of Hg2+ with Ag2Te NWs. The NW morphology of Ag2Te is preserved upon reaction with minor changes and the two separate phases formed are spatially separated within the same NW. The reaction of Hg2+ with Ag2Te NWs was monitored at different concentrations and the reactivity was attributed to cationic exchange depending on solubility products. Hybrid NWs were formed by partial cation exchange only at low concentrations (below 50 ppm) resulting in Ag5Te3 and HgTe within the same NW. However, at high concentrations (above 100 ppm), the HgTe phase alone was formed. These studies have been extended to other metal ions such as Pb2+, Cd2+, and Zn2+ whose reactivity towards Ag2Te NWs is different from that of Hg2+. These ions form a passivating Te oxide layer upon reaction with other metal ions. The mechanism of reactivity of Hg2+ is explained on the basis of free energy of formation of the ionic solid. Phase transition of Hg2+-reacted NWs occurs at a lower temperature than the parent (Ag2Te NWs) and other metal ions-reacted Ag2Te NWs. Details of the process were elucidated using microscopic and spectroscopic investigations. The physical and chemical properties of the individual components within a NW are expected to provide a novel functionality to the metal chalcogenide systems.

Gold Nanoparticle Superlattices as Functional Solids for Concomitant Conductivity and SERS Tuning

Abstract: 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.

Formation of H2+ by Ultra-Low-Energy Collisions of Protons with Water Ice Surfaces

Abstract: The molecular ion of dihydrogen (H2 + ) is produced by 1 eV collisions of protons (H + ) onamorphous water ice surfaces.The reaction is also observed on crystalline ice surfaces, but with lower efficiency. Collisions of D + on amorphous H2O and D2O ices yield D2 + on the former, subsequent to isotope exchange on the H2O surface. Ultra-low-energy collision-induced dihydrogen ion production is also observed from alkanol surfaces, with decreasing efficiency asthe alkyl chainlength increases.Thereis nocorrespond- ing reaction on solid hexane. This endothermic reaction, with implications for interstellar chemistry and plasma etching processes, is proposed to occur as a resultofstabilizationoftheotherreactionproduct,ahydroxylradical(OH  ),on water surfaces through hydrogen-bonding interactions with the surface. These results point to an interesting chemistry involving ultra-low-energy ions on molecular solids.

Organic templated nanometal oxyhydroxide

Abstract: Disclosed are granular composites comprising a biopolymer and one or more nanometal- oxyhydroxide/hydroxide/oxide particles, along with methods for the preparation and use thereof.

Axial flow filter block for water purification

Abstract: PROBLEM TO BE SOLVED: To provide a dead weight feed type axial flow filtration system for treating water.SOLUTION: This system comprises: a filtration medium; a binder obtained by being mixed with the filtration medium, and in which a porous composite material block is formed by sintering the mixture of the filtration medium and the binder; and a housing tube in which the composite material block is arranged therein.SELECTED DRAWING: None

Ag7 and Ag8 Quantum Clusters Supported on Alumina: New Catalysts for Organic Reactions

Abstract: Silver quantum clusters (QCs) stabilized by mercaptosuccinic acid (MSA) have been synthesized at room temperature from Ag nanoparticles through an interfacial etching process. The as-synthesized red emitting QCs were coated on alumina and the supported clusters have been characterized by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), energy dispersive analysis of X-rays (EDAX) and fluorescence spectroscopy. Catalytic activity of Ag QCs supported on alumina have been exploited for the reduction of 4-nitrophenol in presence of sodium borohydride. The reaction followed pseudo-first order kinetics. The catalyst separated from the product remains active and can be recycled several times. Other nitroaromatic compounds have also been tested. Synthesis of the solid phase catalyst, applications and detailed kinetic studies of the reduction of 4-nitrophenol are reported.

Tubular Nanostructures of Cr2Te4O11 and Mn2TeO6 through Room-Temperature Chemical Transformations of Tellurium Nanowires

Abstract: Tubular ternary nanostructures of tellurium were made through chemical transformations of tellurium nanowires (Te NWs). These transformations occur through reactions with CrO3 and KMnO4, two of the strongest oxidizing agents. In the case of CrO3, the 1D structure of the NWs remained intact and the morphology changed to hollow wires of Cr2Te4O11, but reaction with KMnO4 resulted in the loss of 1D structure, forming a carbon onion-like object composed of Mn2TeO6. As the reaction proceeded, the crystallinity of the NWs decreased, and the final products were amorphous. The reaction products were characterized using different spectroscopic and microscopic techniques. Time-dependent transmission electron microscopic (TEM) analysis of both of the reaction products showed that first a shell is formed around the NWs. Further reaction results in the formation of hollow structures. During the reaction with CrO3, TEM in the intermediate stages showed that the periphery of the material was amorphous, whereas the insid...

Size evolution of luminescent lactoferrin protected gold clusters.

Abstract: Earlier, we have reported the synthesis and thorough physicochemical characterization of Au(QC)@NLf, where NLf refers to native lactoferrin. In this paper, we show how the clusters are formed in the protein cavity as a function of time. MALDI MS of the protein samples measured as a function of time of incubation show the evolution of the clusters. The mechanism of cluster growth suggests metal ion transfer across molecules which require further study.