Due to the inherent anisotropy of the system, gold nanorods behave differently in comparison to their spherical counterparts. Reactivity of gold nanorods, in presence of cupric ions, was probed in an attempt to understand the chemistry of anisotropic particles. The reaction progresses through a series of intermediates. It can be arrested at any stage to get nanorods of desired dimension and therefore, can be used for their reshaping. The presence or absence of cetyltrimethylammonium bromide (CTAB) on the nanorod surface was found to be determining the site of initiation of the reaction. When a large concentration of CTAB is present in the system, selective etching of the tips of the nanorod occurs and when the nanorods are purified to reduce the amount of CTAB in the solution, the side faces of the nanorod also get reacted. Gold nanorods are converted to particles by further surface reconstructions in a systematic surface specific chemistry.

Pradeep Thalappil

Department Of Chemistry

AMMONIUM COMPOUNDS

Anisotropy

Chemical reactions

chemistry

Concentration (process)

Crystallography

Gold

Metal refining

Nanostructures

Plasmons

Surface reactions

Anisotropic particles

CETYL TRI METHYL AMMONIUM BROMIDE

CU2+

CUPRIC IONS

GOLD NANORODS

INHERENT ANISOTROPY

Selective etching

nanorods

Fulltext

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IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

Bulletin of Materials Science

We report a simple chemical route for converting the dimorphic CdS nanocrystals into pure hexagonal structure by loading ̃5 %Mn 2+ into CdS in presence of a surfactant rather than under harsh conditions reported earlier. Differential scanning calorimetric measurement was used to quantify the percentages of cubic and hexagonal components at lower manganese ion loading, which correlated well with the Short and Steward procedure using X-ray diffraction analysis. Annealing of all lower (&lt; 5 %) manganese ion-loaded samples transforms the cubic component into its hexagonal form. Systematic variation of manganese ion concentration in the range of 0-5 % helps tune in a new combination of photoluminescence bands, composed of a band edge, surface and manganese d-d emissions. The manganese ions located inside the CdS matrix studied by electron paramagnetic resonance (EPR) reveal progressively changing presence of (i) distorted tetrahedral nature of isolated Mn2? and (ii) cluster formation through dipolar and/or exchange coupling arising from Mn 2+-Mn 2+ interaction. There is a neat correlation between PL and EPR properties on the one hand and between EPR and magnetism on the other. Most important part of this investigation lies in manganese coupled ferromagnetism (FM) as compared to our earlier reported anti-FM-coupled superparamagnetism, the difference originating from the use of different ligands. Pre-annealed (as synthesized) and post-annealed CdS/Mn 2+ nanocrystals exhibit FM of manganese origin, the latter exhibiting increased FM due to annealing promoted migration of manganese ions to form more clusters, as confirmed by magnetization experiments. Electron microscopy reveals the formation of nanorods and nanoparticles. © Springer Science+Business Media B.V. 2012.

Journal of Nanoparticle Research

Manganous ion dictated morphology change and ferromagnetism in CdS nanocrystals

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 Cr 2Te4O11, 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 inside, where unreacted parts of Te NWs remained, was crystalline with a clear lattice structure. X-ray photoelectron spectroscopy (XPS) as well as Raman spectroscopy showed a redox reaction in both cases. Studies suggest that both redox reactions and nanoscale Kirkendall effect might be involved in the formation mechanism. © 2011 American Chemical Society.

Journal of Physical Chemistry C

Tubular nanostructures of Cr2Te4O11 and Mn2TeO6 through room-temperature chemical transformations of tellurium nanowires

We report the shape transformation of gold nanorods to spherical nanoparticles, assisted by cupric ions. The reaction proceeds through a series of structures and could be arrested at any stage to produce particles of desired shape. In the presence of a larger concentration of cetyltrimethylammonium bromide (CTAB), selective etching of the tips of the nanorods occurs to a greater extent. The subsequent transformation is driven by the surface reconstruction of nanorods to generate more stable surfaces. As the stability of various surfaces depends on the protecting agent used, the reactivity is modified by controlling its presence at the surface. We show that the body of the rods is more susceptible for reaction at reduced CTAB concentrations. During the conversion to particles, several anisotropic transient structures were observed and were imaged using high-resolution transmission electron microscopy (HRTEM). The transformation occurs due to the hydroxyl radicals produced by Cu21 in the presence of ascorbic acid (AA). A mechanism has been proposed and several control experiments were conducted to test it. The cupric ion induced shape transformations can be extended to other ions, and knowing the mechanism allows the control of the process to stabilize various anisotropic structures. © 2007 American Chemical Society.

Langmuir

Body- or tip-controlled reactivity of gold nanorods and their conversion to particles through other anisotropic structures

We observe the transverse electrokinetic effect (TEK), namely, the generation of a transverse potential difference due to the flow of liquids over a surface on which self-assembled arrays of metallic nanoparticles are immobilized. This paper covers the recent experimental findings and improvements to the theoretical model described in ref 1 [Subramaniam, C.; Pradeep, T.; Chakrabarti, J. Phys. Rev. Lett. 2005, 95, 0164501], leading to a better understanding of the phenomenon. Issues critical for the comprehension and applicability of this effect as a potential flow sensor such as the need for a critical dipole moment, optimum surface coverage, nature of the flow rate dependence, and generalization of the phenomenon for several metallic nanostructures have been addressed in detail from both experimental and theoretical perspectives. This complete work stresses the uniqueness of this new phenomenon which could find applications in emerging areas of science, like microfluidics. © 2007 American Chemical Society.

Transverse electrokinetic effect: Experiments and theory

Hexagonal patterns of gold nanorods were made by assembling gold nanorod-coated poly(N-isopropyl acrylamide) microgels. The required population of nanorods on the microgels was achieved by attaching nanoparticle seeds on the latter and growing them to nanorods. The various materials prepared were characterized by UV-vis spectroscopy and transmission electron microscopy. Similar experiments with nanoparticle-coated or prefabricated nanorod-coated microgels did not give such hexagonal patterns. We suggest that the interlocking of nanorods leads to these regular structures. This is the first report of a solution phase method for assembling nanorods into a hexagonal pattern. © 2007 American Chemical Society.

Gold nanorods grown on microgels leading to hexagonal nanostructures

Bioconjugates of the hemoproteins, myoglobin, and hemoglobin have been synthesized by their adsorption on spherical gold and silver nanoparticles and gold nanorods. The adsorption of hemoproteins on the nanoparticle surface was confirmed by their molecular ion signatures in matrix assisted laser desorption ionization mass spectrometry and specific Raman features of the prosthetic heme b units. High-resolution transmission electron microscopy (HRTEM) and UV-visible spectroscopy showed that the particles retain their morphology and show aggregation only in the case of silver. The binding of azide ion to the Fe(III) center of the prosthetic heme b moiety caused a red shift of the Soret band, both in the case of the bioconjugates and in free hemoproteins. This was further confirmed by the characteristic signature at 2050 cm-1 in the Fourier-transform infrared spectra, which corresponds to the asymmetric stretching of the Fe(III) bound azide. The retention of the chemical behavior of the prosthetic heme group after adsorption on the nanoparticle is interesting due to its implications in nanoparticle supported enzyme catalysis. The absence of morphology changes after the reaction of bioconjugates with azide ion observed in HRTEM studies implies the stability of nanoparticles under the reaction conditions. All these studies indicate the retention of protein structure after adsorption on the nanoparticle surface. © 2007 American Chemical Society.

Hemoprotein bioconjugates of gold and silver nanoparticles and gold nanorods: Structure-function correlations

We report the generation of a potential difference, of the order of tens of millivolts, induced by the flow of polar liquids over an assembly of gold nanoparticles. The device consisted of two conducting glass plates, one of which contained the gold nanoparticle multilayer assembly. The potential generated is in transverse direction to the flow and is dependent on the nature of the flowing liquid. We propose a simple theoretical model to account qualitatively for the generation of the flow-induced transverse potential. © 2005 The American Physical Society.

Physical Review Letters

Flow-induced transverse electrical potential across an assembly of gold nanoparticles

Reactivity and resizing of gold nanorods in presence of Cu2+

Journal	Bulletin of Materials Science
ISSN	02504707
Open Access	No