Snigdhamayee Praharaj

Anisotropic growth of gold clusters to gold nanocubes under UV irradiation

A chiral reagent, 2-naphthol, has been introduced under alkaline solution as a reductant for HAuCl(4) in CTAB micelle to produce exclusively cubic gold nanoparticles under UV photoactivation. Prolonged irradiation helped the digestion of the primarily evolved spherical particles into smaller gold nanocubes, which then act as tiny cubic seeds, leading to the formation of larger nanocubes. The smaller cubes take the assistance of CTAB under alkaline condition to serve as the seed in directing the transformation of all the spherical colloids into cubic shapes under continuous irradiation via Ostwald ripening. The shape transformation of the nanoparticles has been monitored by repetitive TEM imaging and absorption spectral analysis. The FTIR analysis proves that the gold nanocubes are capped by CTAB. The XRD pattern authenticates the formation of the fcc gold nanocubes. GCMS studies in turn confirmed the presence of hydroxylation of 2-naphthol in the course of the reaction, leaving exclusively cubic gold nanoparticles at the final stage of the photoactivation reaction.

Cysteine functionalized copper organosol: synthesis, characterization and catalytic application

We herein report a facile one-pot synthesis, stabilization, redispersion and Cu–S interaction of L-cysteine and dodecanethiol (DDT) protected copper organosol in toluene from precursor copper stearate using sodium borohydride in toluene under a nitrogen atmosphere. Surface modification of the synthesized copper organosol with an amino acid L-cysteine and an alkanethiol (dodecanethiol, DDT) is accomplished by a thiolate bond between the used ligands and nanoparticle surface. The cysteine molecule binds the copper surface via a thiolate and amine linkage but not through electrostatic interaction with the carboxylate group due to the solvent polarity and dielectric medium. Fourier transform infrared (FTIR) analysis was performed to confirm the surface functionalization of the amino acid and DDT to the copper surface. Copper organosol has been characterized by optical spectroscopy (UV/vis), transmission electron microscopy (TEM), x-ray photoelectron spectroscopy (XPS) and x-ray diffraction (XRD). The as-synthesized particles are spherical in shape and exhibit a Mie scattering profile with an absorption maxima in the visible range. Copper nanoparticles capped by cysteine and/or DDT in non-aqueous media are found to represent an interesting catalytic approach for the synthesis of octylphenyl ether.

Dipole-dipole plasmon interactions in self-assembly of gold organosol induced by glutathione

Assemblies of gold nanoparticles in an organic medium have been synthesized to study the plasmon–plasmon interactions amongst the gold nanoparticles. A pH-sensitive biomolecule, glutathione (GSH), has been introduced as a molecular linker of the ‘parent’ gold nanoparticles to obtain small nanoparticle aggregates. The optical spectra of gold nanoparticles shifted to the red region indicate dipole–dipole interactions in the gold particle assembly. The aggregates have been characterized by UV-Vis, FTIR, HRTEM and XRD techniques. A controlled method of aggregating gold nanoparticles in organic solvents has been achieved successfully under controlled pH conditions with different concentrations of the molecular linker, GSH. The pH dependent anchoring of GSH onto gold surfaces has been proved beyond doubt to bring about nanoparticle aggregation.

Silver organosol: synthesis, characterization and localized surface plasmon resonance study

In this article a simple and reproducible technique for the synthesis of a silver organosol is reported from a specific silver precursor, solid silver acetate. Molten hexadecylamine acts as a solvent for silver acetate and imparts stability to the evolved nanoparticles. The amine-capped organosol shows unique stability as neither agglomeration nor oxidation takes place over one year. The synthesised silver particles have been characterised by UV-visible, TEM, XRD, XPS, FTIR and thermogravimetric studies. The hexadecylamine-stabilised silver organosol was employed to examine the altered optical properties in different solvent systems and with different ligands by accounting for the changes in the localised surface plasmon resonance (LSPR) spectrum. It was observed that the position of the surface plasmon band of silver nanoparticles is greatly affected by the solvents and ligands under consideration. The quantitative alteration of the LSPR spectrum involving encapsulated nanoparticles in a dielectric ligand shell has been rationalised from Mie theory. It has also been shown that cationic and anionic surfactants of different chain lengths induce changes in the optical properties of silver nanoparticles whereas, zwitterionic amino acid molecules reflect insignificant changes in the LSPR spectrum. The max of the LSPR gradually shifts to red with the increase in chain length of both the cationic and anionic surfactants, indicating specific binding of the surfactant molecules around silver nanoparticles. Finally the affinity of the synthesised silver nanoparticles for amine molecules has been accounted for by taking the HSAB principle into consideration.

Effect of concentration of methanol for the control of particle size and size-dependent SERS studies.

In this paper the effect of concentration of cosolvent (methanol) for the formulation of particles size has been discussed briefly. The binary solvent system has been used which is prepared by simple mixing of two solvents. The morphology of the particles was controlled by varying the amount of cosolvent, keeping the concentration of the stabilizer and reducing agent constant. The polarity of the solvent, transport of the Au(III) ions, and coordinating ability of the solvent play vital roles for nucleus formation and the growth process, which subsequently form different size particles. The particles formed in methanol at lower composition are angular. At higher concentrations of methanol they are spherical. In all the cases the particle size increases as the concentration of the cosolvent decreases. In the methanol system, particles form multiple twined structures and the twining of the particles increase with the decrease of methanol content. The particles have been characterized by XRD analysis, UV-visible spectroscopy, and transmission electron microscopic (TEM) studies. The variable size of the particles, obtained in a water methanol system, was employed for SERS measurement. A Raman probe, Rhodamine 6G (Rh 6G), has been found to be suited for the surface modification of the gold particles and it has also been demonstrated that the larger particles show better SERS signal than the corresponding smaller ones.