Showing papers by "Thalappil Pradeep published in 2009"
TL;DR: The most important reason for the use of noble metals is the minimal reactivity at the bulk scale, which can be explained by a number of concepts such as electrochemical potential, relativisitic contraction, molecular orbital theory, etc.
725 citations
TL;DR: A novel interfacial route has been developed for the synthesis of a bright-red-emitting new subnanocluster of Au(23) by the core etching of a widely explored and more stable cluster, Au(25)SG(18) (in which SG is glutathione thiolate).
Abstract: 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.
235 citations
TL;DR: In this paper, the arsenite removal performance of manganese oxide-coated-alumina (MOCA) and its interaction with As(III) in drinking water was described.
140 citations
TL;DR: In this article, a novel adsorbent, gold nanoparticle supported on alumina, was developed for the removal of inorganic mercury from water, and the observed adsorption capacity for mercury is 4.065 gm per gm of gold nanoparticles.
Abstract: We describe an innovative approach based on alloying of metals to remove metal ions from drinking water. A novel adsorbent, gold nanoparticle supported on alumina, was developed for the removal of inorganic mercury from water. The observed adsorption capacity for mercury is 4.065 gm per gm of gold nanoparticles, which is ∼10 times higher metal adsorption capacity than previously reported adsorbents. Gold nanoparticle has been supported on alumina, at a capacity of 738 mg/kg alumina, for use in practical applications. Batch and column studies were done for adsorption analysis and a practical filter has been developed. The interaction between gold and mercury was studied using UV-vis, TEM, SEM, EDAX and XRD. The chemistry of metal alloying can be utilized for sequestration of mercury from drinking water. Established separation techniques for recovery of metals from the alloy can be utilized, making this a complete solution for drinking water applications.
123 citations
TL;DR: In this article, a method for the synthesis of anisotropic mesostructured gold material, called mesoflowers, is demonstrated, which resembles several natural objects and is made up of a large number of stems with unusual pentagonal symmetry.
Abstract: 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.
85 citations
TL;DR: The composition of the cluster was determined to be Au(22)[(-SG)(15)(-SAOPPTH(2))(2)], which was supported by mass spectrometry and elemental analysis, and utilized the fluorescence nature of these water-soluble clusters for the fabrication of fluorescent patterns by soft lithography.
Abstract: We synthesized fluorescent, porphyrin-anchored, Au22 clusters in a single step, starting from well-characterized Au25 clusters protected with glutathione (−SG) by a combined core reduction/ligand exchange protocol, at a liquid−liquid interface. The prepared cluster was characterized by UV/vis, photoluminescence, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy, elemental analysis, and matrix-assisted laser desorption ionization mass spectrometry. The absence of a 672 nm intraband transition of Au25 and the simultaneous emergence of new characteristic peaks at 520 and 635 nm indicate the formation of the Au22 core. An increase in the binding energy of 0.4 eV in Au 4f core-level peaks confirmed the presence of a reduced core size. Quantitative XPS confirmed the Au/S ratio. The presence of a free base, tetraphenylporphyrin (H2TPPOAS−), on the Au22 core was confirmed by fluorimetric titrations with Cu2+ and Zn2+ ions. From all of these, the composition of the cluster was determined to be A...
80 citations
TL;DR: In this article, a room-temperature solution-phase route for the preparation of one-dimensional silver telluride nanowires (Ag2Te NWs) was developed.
Abstract: We have developed a room-temperature solution-phase route for the preparation of one-dimensional silver telluride nanowires (Ag2Te NWs). The Ag2Te NWs of 600 nm length and 20 nm diameter were synth...
76 citations
TL;DR: A simple nanomaterial-based reduction strategy that can form leaf-like graphenic structures from graphite oxide (GO) at room temperature is described in this article, where the mixture of Te nanowires (Te NWs) with GO at roo...
Abstract: A simple nanomaterial-based reduction strategy that can form leaf-like graphenic structures from graphite oxide (GO) at room temperature is described. Mixing of Te nanowires (Te NWs) with GO at roo...
75 citations
04 Sep 2009-Journal of Environmental Science and Health Part B-pesticides Food Contaminants and Agricultural Wastes
TL;DR: Visual detection of the presence of pesticide is possible when the color change occurring by the adsorption of pesticides on gold nanoparticles is enhanced by sodium sulfate and there is no need of sample preparation or preconcentration.
Abstract: The presence of parts per billion (ppb) levels of chlorpyrifos (O,O-Diethyl-O-(3,5,6-trichloro-2-pyridyl) phosphorothioate) and malathion (S-1,2-bis(ethoxycarbonyl) ethyl O,O-dimethyl phosphorodithioate), two common pesticides found in the surface waters of developing countries, have been visually detected using gold nanoparticles. Visual detection of the presence of pesticide is possible when the color change occurring by the adsorption of pesticides on gold nanoparticles is enhanced by sodium sulfate. The method presented here is simple and there is no need of sample preparation or preconcentration. The response occurs within seconds and the color change is very clear. The detection is possible if chlorpyrifos and malathion are present up to a concentration of 20 and 100 ppb, respectively. The method shows great potential for on-site pesticide monitoring. The method is also applicable as a qualitative technique for the performance evaluation of various household water filters, which claim pesticide removal.
68 citations
TL;DR: In this paper, shape-selected synthesis of a large number of zinc oxide (ZnO) nano-and microstructures was achieved by the seed-mediated growth of oligoaniline-coated gold nanoparticle precursors.
65 citations
TL;DR: In this paper, the synthesis of luminescent Ag clusters through the interfacial etching of mercaptosuccinic acid (MSA) protected silver nanoparticles, Ag@MSA, with guanine at the water-toluene interface was reported.
Abstract: We report the synthesis of luminescent Ag clusters through the interfacial etching of mercaptosuccinic acid (MSA) protected silver nanoparticles, Ag@MSA, with guanine at the water-toluene interface. The clusters exhibiting well-defined absorption emit in the near-infrared (NIR) region. Crude clusters were separated using polyacrylamide gel electrophoresis (PAGE). The cluster solid, prepared by freeze drying, is highly hygroscopic. Biomolecular markers were used to identify the approximate mass of the cluster which was found to be 7 kDa, as mass spectrometry did not reveal specific signatures. The clusters were investigated using UV-Vis spectroscopy, transmission electron microscopy (TEM), energy dispersive analysis of X-rays (EDAX), X-ray photoelectron spectroscopy (XPS), infrared spectroscopy (IR), and fluorescence spectroscopy. Elemental analysis and IR studies reveal the protection of the cluster by two types of ligands, namely MSA and guanine. Fluorescence of the cluster is highly temperature dependent, with an increase in intensity with decrease in temperature. Influence of different ratios of reactants, etching capacity of different nucleobases and effect of temperature on the synthesis as well as possible single-phase etching were investigated. Sensitivity of the cluster to certain metal ions has been monitored using fluorescence spectroscopy.
TL;DR: In this paper, Glutathione protected Au25 quantum clusters, exhibiting characteristic fluorescence, have been uniformly coated inside and outside of β-Ala-L-Ile dipeptide nanotubes.
Abstract: Glutathione protected Au25 quantum clusters, exhibiting characteristic fluorescence, have been uniformly coated inside and outside of β-Ala-L-Ile dipeptide nanotubes. These coated structures have been imaged using the inherent fluorescence of Au25. Upon exposure to an electron beam, in a transmission electron microscope, the quantum clusters gradually transform to gold nanoparticles, of the metallic size regime. The nanoparticles grow to a size of 4.5 nm and thereafter the particle size is unaffected by electron beam exposure. The nanotubes are intact and this template is shown to control the uniformity of the size of the nanoparticles grown. The quantum clusters can be loaded selectively inside the tubes using capillarity of the nanotubes. The sizes of the nanoparticles grown are tuned using electron beam exposure.
TL;DR: A hierarchical model for the self-assembly of OPV molecules is proposed, which does not alter the morphology or electronic properties of the OPV structures, as revealed by microscopic and spectroscopic studies.
Abstract: Thiol-protected gold nanoparticles (GNPs) have been used to probe the initial stages of the molecular organization of oligo(p-phenylenevinylene) (OPV) gelators. The hybrid materials prepared by the self-assembly of OPVs and GNPs are characterized by optical microscopy, fluorescence microscopy, scanning electron microscopy, transmission electron microscopy, and atomic force microscopy. GNPs are located preferentially on the sides of the OPV structures, which implies the presence of alkyl chains at the edges, which makes the assemblies hydrophobic. TEM analyses at the early stages of self-assembly show tapes that have a width of 4 nm, which upon further self-assembly, form fibrils through hydrogen bonding. The experiment was performed with GNPs protected with dodecane and octadecane thiols. The existence of tapes, ribbons, fibrils, and fibers were confirmed by nanoparticle marking. Based on the experimental data, we have proposed a hierarchical model for the self-assembly of OPV molecules. The presence of nanoparticles does not alter the morphology or electronic properties of the OPV structures, as revealed by microscopic and spectroscopic studies.
TL;DR: In this article, a 3D superlattice of dansyl glutathione protected gold nanoparticles, with potential applications in molecular detection, has been synthesized at an air/water interface by controlling the pH of the nanoparticle suspension.
Abstract: 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.
TL;DR: In this paper, the authors developed a cheap and rapid method for the fabrication of 3D nanoparticle superlattices (SLs) of Au@SGAN and Au@MSA (Nacetyl glutathione (SGAN) and mercaptosuccinic acid (MSA) protected gold nanoparticles, respectively) in gram scale, at a liquid−liquid interfaces under flowing nitrogen gas.
Abstract: 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...
TL;DR: A one-pot synthesis of silver nanoparticle-polymer composites (Ag-PNCs) in water by a novel finding involving the polycondensation of methoxybenzyl chlorides directly on Ag nanoparticle surfaces at room temperature, leading to highly soluble antimicrobial nanocomposites.
Abstract: We report a one-pot synthesis of silver nanoparticle-polymer composites (Ag-PNCs) in water by a novel finding involving the polycondensation of methoxybenzyl chlorides (MeO-BzCl) directly on Ag nanoparticle surfaces at room temperature, leading to highly soluble antimicrobial nanocomposites. The composites, which are soluble in a range of organic solvents, precipitate in the reaction vessel, making their separation simple. Solutions of the composites can be casted directly on substrates or made into freestanding films. The material was found to be stable for nearly 2 years. A range of substrates have been shown to become antibacterial by direct application of this material. The experiments were conducted with Ag-PNC-loaded filter paper strips and glass substrates. The samples were found to be antimicrobial (against Escerichia coli and Aspergillus niger). The simple one-pot approach of this kind to make organic-soluble antibacterial coatings could have wide implications.
TL;DR: Aqueous to organic phase transfer of water soluble sub-nanocluster, Au 25 SG 18 (-SG, glutathione thiolate) is demonstrated using the phase transfer reagent, tetraoctylammonium bromide as mentioned in this paper.
Abstract: Aqueous to organic phase transfer of water soluble sub-nanocluster, Au 25 SG 18 (-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.
TL;DR: The reactivity of tellurium nanowires (Te NWs) with thiol-containing molecules was probed in this paper, where it was understood that there is a redox reaction between TEO2, present on the NW surface, and thiols added into the solution that resulted in the reduction of TeO2 to Te(0) with simultaneous oxidation of thiolS to disulfides.
Abstract: The reactivity of tellurium nanowires (Te NWs) with thiol-containing molecules was probed. Depending upon the presence or absence of the stabilizing surfactant in the solution, the product differs drastically. When excess surfactant was present, the reaction resulted in a core-shell-like structure. Through various spectroscopic and microscopic techniques, it was understood that there is a redox reaction between TeO2, present on the NW surface, and thiols added into the solution that resulted in the reduction of TeO2 to Te(0) with simultaneous oxidation of thiols to disulfides. Energy-dispersive analysis of X-rays (EDAX) confirmed that the shell has large sulfur and oxygen content. Raman spectra as well as the spectral image showed that the shell contains sulfur in the form of disulfides. Time-dependent Raman measurements established the redox nature of the conversion. The redox reaction was confirmed by X-ray photoelectron spectroscopy (XPS) also. When the excess surfactant was removed by repeated centrif...
TL;DR: In this paper, the effect of ligand exchange on the optical and photoluminescence properties of Au25SG18 (SG-glutathione thiolate) clusters was investigated.
Abstract: Effect of ligand exchange on the optical and photoluminescence properties of Au25SG18 (SG-glutathione thiolate) clusters was investigated. A fluorescein-based dye, 5-((2-(and-3)-S-(acetylmercapto)succinoyl)amino)-fluorescein (SAMSA) was anchored on water soluble Au25SG18 clusters by the place exchange reaction and the solid-state emission of the exchange product was studied. Inherent fluorescence image of these clusters was mapped. Organic soluble Au25(SC2H4Ph18 clusters were synthesized as a model system which also showed temperature-dependent solid-state emission, in good agreement with the results from water soluble Au25 clusters.
Patent•
11 Dec 2009TL;DR: An embodiment relates to a man-made object comprising a distinctive structure with a feature to identify the object, wherein the object has a size such that the object is observable under visible light as discussed by the authors.
Abstract: An embodiment relates to a man-made object comprising a distinctive structure with a feature to identify the object, wherein the object has a size such that the object is observable under visible light, wherein the feature is embedded in or on the object and a size of the feature is such that the feature is not observable under visible light, wherein the feature comprises an attribute originating from the feature, and wherein the attribute defines the feature.
TL;DR: In this paper, a metal composite consisting of Ag nanoparticles embedded in patterned polystyrene, silver triphenylphosphine nitrate, Ag(PPh 3 ) 3 NO 3, was chosen as a source of Ag metal as it is soluble in toluene to form a blend with poly styrene, a well-known negative tone e-beam resist.
Abstract: With the intention of producing a metal composite consisting of Ag nanoparticles embedded in patterned polystyrene, silver triphenylphosphine nitrate, Ag(PPh 3 ) 3 NO 3 , was chosen as a source of Ag metal as it is soluble in toluene to form a blend with polystyrene, a well-known negative tone e-beam resist. At a constant e-beam dosage of 150 μC cm -2 , the patterns were produced at 5, 10, and 30 keV beam energies while varying the dwell time at a location (pixel) and the number of passes. The presence of Ag in the patterned regions was confirmed by the EDS analysis. TEM showed the formation of small Ag nanoparticles in the embedded matrix of the polymer. The reduction of Ag precursor following exposure to the e beam was monitored by XPS. UV-vis absorption studies showed a broad peak at 464 nm, typical of surface plasmon absorption by Ag nanoparticles. Confocal fluorescence studies showed the embedded Ag nanoparticles in the carbon matrix to have fluorescence property as well. C-AFM measurements have shown that the patterned blend is well conducting due to the interacting Ag nanoparticles, the resistance being two orders less compared to that of polystyrene itself. The higher the beam energy, the lower was the dwell time required to attain a conducting pattern.
TL;DR: In this paper, the authors investigated the interaction of n-butanol (NBA) with thin layers of water ice prepared in ultra high vacuum in the temperature range of 110-150 K.
Abstract: We have investigated the interaction of n-butanol (NBA) with thin layers of water ice prepared in ultra high vacuum in the temperature range of 110-150 K. From the mass spectra of the chemically sputtered species, created upon the collision of low energy (g30 eV) Ar + ions, we study the process of diffusive mixing of NBA with water ice, at the molecular level. The results show that NBA undergoes diffusive mixing with H2O. Even after depositing 1000 monolayers (MLs) of amorphous solid water (ASW) over NBA, both the species are observed on the surface. However, when NBA is deposited over ASW, no water is seen on the surface above 3-5 MLs of NBA. This could be interpreted as the absence of diffusive mixing in this system or surface segregation of NBA, in view of its lower surface energy just as in the case of liquid alcohols. An isomeric alcohol, namely, tert-butyl alcohol (TBA), also behaves similarly. Although the presence of NBA and TBA is detected, in the presence of ASW, they undergo selective ionization, giving specific peaks in the mass spectrum. D2O behaves in a manner similar to that of H2O. Preliminary experiments with other alcohols; namely, methanol, ethanol, and propanol were also done, and the results suggest that incomplete diffusion or surface segregation begins with propanol.
01 Jan 2009
TL;DR: Intensive farming, rapid industrialization, and increasingly sophisticated lifestyles have added artificial chemicals into many water bodies, and purification technologies have to be efficient for them to be removed at affordable cost.
Abstract: Publisher Summary Intensive farming, rapid industrialization, and increasingly sophisticated lifestyles have added artificial chemicals into many water bodies. Although pesticide residues in groundwater were unexpected years ago as soil was thought to act as a filter, it is an established fact that water derived from groundwater sources is contaminated with them in many parts of the world. Even though these levels are significant vis-a-vis the permissible limits, the concentrations are low in comparison to those of other commonly encountered chemicals, and purification technologies have to be efficient for them to be removed at affordable cost. In addition, the process kinetics has to be reasonably fast so that the amount of adsorbent required is minimal. For such a solution to be useful for all strata of society, the solution should be economically attractive, requiring zero electricity and minimum maintenance. It is imperative to understand that any novel technology should solve drinking water contamination problems in their entirety and not result in toxic by-products or residuals. These offer numerous challenges to chemistry and engineering, some of which are discussed in this chapter with selected examples.
TL;DR: A comparative study of the surface-enhanced Raman scattering (SERS) effect of the spherical gold nanoparticles (NPs) and highly uniform and oriented gold nanotriangles (NTs) and indium tin oxide (ITO) coated conducting glass surfaces is reported.
Abstract: A comparative study of the surface-enhanced Raman scattering (SERS) effect of the spherical gold nanoparticles (NPs) and highly uniform and oriented gold nanotriangles (NTs), anchored on indium tin oxide (ITO) coated conducting glass surfaces is reported. Contact and non-contact mode atomic force microscopic (AFM) analyses confirmed that all the triangles observed are equilateral and uniformly stacked. The NTs-coated ITO exhibited intense near-infrared (NIR) absorption and showed an extremely strong SERS activity than the spherical NPs. The NTs showed a very good SERS enhancement factor on the order of 4 x 10(8). The results are interpreted in terms of the increased density of hot spots on the NTs-coated surface.
TL;DR: In this paper, Diao et al. used optical absorption spectroscopy, atomic force microscopy (AFM), and X-ray diffraction (XRD) to detect the formation of gold nanotriangles (NTs).
Abstract: 2009 WILEY-VCH Verlag Gmb We appreciate the comments of Diao et al. on our recently published paper, which describes the characterization of gold nanotriangles (NTs) using optical absorption spectroscopy, atomic force microscopy (AFM), and X-ray diffraction (XRD). AFM images of the NTs, which confirmed their morphology, were provided, but scanning electron microscopy (SEM) images were not. Here SEM, AFM, and optical images are presented, which suggest that NTs are indeed formed. Each comment has been addressed below. It is well known that artifacts––caused particularly by blunt tips––can appear in AFM measurements. However, after considering the following facts, we are convinced that NTs were genuinely observed. In order to confirm the formation of the NTs, samples were imaged using dynamic force microscopy (DFM, a non-contact AFM technique) at the facilities of another research group (Seiko SPA-400); the results were confirmed for both the parent and the NT surfaces. Various control measurements were performed. Figure 1a–c show AFM images recorded at various stages of growth. NTs of varying size as well as spherical particles can be seen. The particle size increases with the evolution of reaction time. Note that a blunt tip could not have produced images of spherical particles and uniform NTs with sizes that varied systematically as a function of reaction time. Non-contact-mode imaging (using an AFM-CRM 200 microscope, WiTec GmbH) of the NTs was also conducted using a fresh cantilever. Since the tip interacted with the sample for only a short time, degradation of the tip or sample was unlikely. The non-contact-mode topographic and phase AFM images of NTs that were synthesized under optimized conditions are shown in Figure 1d,e. Most of the NTs observed had an edge length of 400–450 nm. There were small variations in the NTdimensions, even with synthesis under optimized conditions. Some areas, which appear as channels void of NTs in between two layers of NTs, are also observed (Fig. 1f). Such an image would not be likely if artifacts were indeed responsible for the results obtained. Other morphologies, such as spheres, are also observed. In addition, it is clear that a large number of lower-lying NTs are stacked and appear to be fused. This is further confirmed by the SEM images. If the tip was worn out or contaminated, the image would have shown separate NTs. However, in this case, the NTs are not well separated, but stacked on top of each other; some of the NTs even lie in different planes. The SEM images taken from different areas of the indium tin oxide (ITO) using a FEI QUANTA-200 SEMmicroscope show the presence of stacked NTs of the same dimensions observed in the AFM images (Fig. 2a,b). Because of the large edge-length/ thickness ratio, the NTs often appear fused and stacked. Corresponding energy dispersive analysis of X-rays (EDAX) data support the formation of Au NTs. The NTs were deformed by the electron beam, as is evident in Figure 2c. In the SEM images, underlying spherical nanoparticles can be clearly seen. The presence of such nanoparticles on different areas of the substrate may explain the increased ratio of the intensities of the (200) and the (111) peaks in the XRD spectra, since diffraction data were taken from very large areas compared to the areas used for the AFM scans. It was possible to increase the edge lengths of the NTs by slow reduction of Au ions on the surfaces of the growing NTs. The sizes of the NTs synthesized according to the reported procedure could be increased by treatment with excess Au3þ and ascorbic acid, allowing imaging of large NTs via optical microscopy (Fig. 2d; WiTec GmbH, CRM 200). Even though several equilateral NTs are observable in such images, their orientations are different from those of the parent NTs. In the UV-vis absorption spectra, the feature at 520 nm can be attributed to the transverse plasmon absorption, as is the case for other anisotropic structures. In addition, possible spherical particles below the surface may contribute to this feature. Only well-structured gold NTs can produce a UV-vis-near-IR absorption spectrum, as indicated in our original paper. Therefore, being able to obtain such a spectrum with characteristic features of gold NTs is in itself a clear indication that NTs are formed. For large numbers of truncated triangles, hexagons, and microplates, the spectra are different. The red-shift in the near-IR absorption spectra with respect to the spectra presented by Shankar et al. may be due to the coupling of the surface plasmon resonances of the triangles, which is caused by the uniform stacking of the individual NTs and their close proximities (as can be seen in the microscopy images). Synthesis of uniform NTs requires optimization and even small changes affect the results significantly. A 0.1mM cetyl trimethylammonium bromide (CTAB) solution of NTs appears turbid at 0 8C. However, upon warming to room temperature by keeping the solution for some time under ambient laboratory conditions, a clear solution was obtained. The sample was cleaned by washing repeatedly with water to remove