Debabrata Mandal

Biogenic synthesis of Au and Ag nanoparticles using aqueous solutions of Black Tea leaf extracts

We explored the application of three different aqueous solutions derived from Black Tea leaf extracts in the synthesis of Au and Ag nanoparticles. The plain tea leaf broth, as well as that containing the ethyl acetate extract of tea leaves, were found to be extremely efficient, leading to rapid formation of stable nanoparticles of various shapes: spheres, trapezoids, prisms and rods. For a given metal ion precursor, the kinetics of particle synthesis were remarkably similar in these two solutions, as evidenced from their absorption spectroscopy monitored over time. Moreover, they exhibited similar redox behavior. In contrast, with the other solution, containing the dichloromethane (CH(2)Cl(2)) extract of tea leaves, we failed to detect any nanoparticle generation under similar reaction conditions. Our results suggest that the reduction of metal ions and stabilization of the resultant particles in the first two solutions involved the same class of biomolecules. We identified these biomolecules as the tea polyphenols, including flavonoids, which were present in comparable amounts in both the tea leaf broth and ethyl acetate extract, but are absent in the CH(2)Cl(2) extract of tea leaves. The efficiency of the tea leaf extracts towards Au and Ag nanoparticle synthesis were compared with that of a naturally occurring hydroxyflavonoid, quercetin.

Biogenic synthesis of Ag, Au and bimetallic Au/Ag alloy nanoparticles using aqueous extract of mahogany (Swietenia mahogani JACQ.) leaves

In this paper, we have demonstrated for the first time, the superb efficiency of aqueous extract of dried leaves of mahogany (Swietenia mahogani JACQ.) in the rapid synthesis of stable monometallic Au and Ag nanoparticles and also Au/Ag bimetallic alloy nanoparticles having spectacular morphologies. Our method was clean, nontoxic and environment friendly. When exposed to aqueous mahogany leaf extract, competitive reduction of Au(III) and Ag(I) ions present simultaneously in same solution leads to the production of bimetallic Au/Ag alloy nanoparticles. UV-visible spectroscopy was used to monitor the kinetics of nanoparticles formation. UV-visible spectroscopic data and TEM images revealed the formation of bimetallic Au/Ag alloy nanoparticles. Mahogany leaf extract contains various polyhydroxy limonoids which are responsible for the reduction of Au(III) and Ag(I) ions leading to the formation and stabilization of Au and Ag nanopaticles.

Biogenic synthesis of Au and Ag nanoparticles by Indian propolis and its constituents.

In an attempt to find natural, environmentally benign, green-chemical agents for the synthesis of metal nanoparticles, we have demonstrated for the first time the excellent efficiency of ethanol and water extracts of a natural, non-toxic material, Indian propolis and two of its chemical constituents, pinocembrin and galangin in the rapid synthesis of stable Ag and Au nanoparticles having wide spectrum of fascinating morphologies. Both of these two extracts were found to be extremely efficient in the synthesis of Ag and Au nanoparticles under alkaline condition. For a given metal ion precursor, the kinetics of particle synthesis were remarkably similar in all the cases, as it is evident from the absorption spectra monitored over time. Moreover they exhibited similar redox behavior under alkaline condition (pH approximately 10.62). The efficiency of the ethanol and water extracts of Indian propolis towards Ag and Au nanoparticles synthesis was compared with that of naturally occurring hydroxyflavonoids, pinocembrin and galangin isolated from Indian propolis; which are equally efficient in the rapid synthesis of Ag and Au nanoparticles and stabilization of the resultant particles.

Green chemistry for nanochemistry: exploring medicinal plants for the biogenic synthesis of metal NPs with fine-tuned properties

The quest for energy efficient, cost-effective and eco-friendly synthetic routes for metal NPs with highly specified properties and biocompatibility is an extremely important and challenging job. Over the past few years, a plethora of medicinal plants have been put to trial as the source of multifunctional green chemical agents which might be able to facilitate such synthetic processes. In this review, we have attempted to acquaint the reader with the tremendous flurry of activities going on in the field of biogenic synthesis of metal NPs, and also to highlight its immense growth potential.

Synthesis and spectroscopy of CdS nanoparticles in amphiphilic diblock copolymer micelles.

Amphiphilic diblock copolymers with the same hydrophilic but different hydrophobic blocks were used as stabilizing agents to prepare cadmium sulfide nanoparticles in aqueous solutions containing 5% of different nonaqueous solvents: methanol, THF, and acetone. Nearly spherical nanoparticles with a fair degree of monodispersity and quantum yields of 1.5%-2% were obtained. Optical absorption band edge of the CdS nanoparticles shows a >0.5 eV blueshift compared to that of bulk CdS, indicating a high degree of quantum confinement. The absorption spectra, while insensitive to the nature of the hydrophobic blocks, exhibited a clear dependence on the nature of the minor, nonaqueous solvents. The photoluminescence in all cases was broad and redshifted, indicating a predominance of surface trap-state emission. Time-resolved photoluminescence demonstrates that the trap states are populated within the first 500 fs, followed by decay with a broad range of time constants from 0.1 to >10 ns, low energy traps decaying at a slower rate than high-energy ones. Time-resolved photoluminescence anisotropy revealed that the nanoparticles experience a local microviscosity very similar to that of bulk water. The experimental observations suggest that nanoparticle formation takes place predominantly in the hydrophilic corona region of the micelles, around specific points with high local concentration of the Cd+2-coordinating basic amine groups of hydrophilic block and/or the minor, nonaqueous solvent component.

Proton transfer reactions in nanoscopic polar domains: 3-hydroxyflavone in AOT reverse micelles

A dramatic reduction in the excited-state intramolecular proton transfer (ESIPT) rate is observed for 3-hydroxyflavone (3-HF) within the nanoscopic polar domains of Aerosol-OT (AOT)/n-heptane reverse micelle solutions. It is attributed to the formation of intermolecularly hydrogen-bonded 3-HF:AOT complexes, which cause a significant disruption of intramolecular hydrogen bonding within the complex-bound 3-HF molecules, thereby limiting the overall rate of the ESIPT process. Introduction of strong hydrogen-bonding polar solvents like water or methanol into the reverse micelles causes extensive solvation of the AOT head groups, leading to the collapse of the 3-HF:AOT complex and eventual release of intramolecularly hydrogen-bonded 3-HF molecules which are then able to undergo ultrafast ESIPT. With increasing W-value (W=[polar solvent]:[AOT]), a larger number of 3-HF:AOT complexes are decimated, thus accelerating the overall ESIPT process. In contrast, in presence of solvents like acetonitrile, whose hydrogen-bonding power is inherently weak, the AOT head groups are poorly solvated, so that the 3-HF:AOT complexes are hardly affected at any W-value. Consequently the ESIPT dynamics of 3-HF in acetonitrile-containing AOT reverse micelles is nearly independent of the W-value, and always slower compared to that in water- or methanol-containing AOT reverse micelles. The results highlight the importance of hydrogen-bonding property of the polar solvent on the ESIPT of 3-HF within a nanoscopic domain.

Burmese grape fruit juice can trigger the “logic gate”-like colorimetric sensing behavior of Ag nanoparticles towards toxic metal ions

We report here a very effective, economically viable and environment-friendly protocol for the synthesis of stable, crystalline, spherical Ag nanoparticles (NPs) with appreciable monodispersity using the Burmese grape (Baccaurea ramiflora Lour.; family: Phyllanthaceae) fruit juice as a green multifunctional agent (GMA) under sonochemical condition. These nanoparticles showed strong and selective Hg2+ ion-sensing activity (in μM level), analogous to the molecular logic gate function, in aqueous medium over a wide range of pH (3.73–11.18), which is based on their visual color change, and hence the change in their surface plasmon resonance (SPR) peak position and intensity. Phytochemical screenings of the GMA were also done to identify its active constituents responsible for the rapid synthesis, stabilization and selective sensing activity of the Ag NPs. The Ag NP surface-adsorbed functionalities, which are from these active biomolecules, induced a change in aggregation behavior of these NPs, which ultimately enhanced the ion-sensing activity of these NPs compared to citrate-, CTAB-, SDS- and PVP-stabilized Ag NPs synthesized by chemical method. Moreover, the use of this non-toxic, green multifunctional agent may confer biocompatibility to the Ag NPs, opening up the possibility for their use in the in vivo detection of toxic heavy metal ions.

Direct observation of time-dependent photoluminescence spectral shift in CdS nanoparticles synthesized in polymer solutions

Direct observation of time-resolved emission spectra (TRESs) of cadmium sulfide nanoparticles in polymer solutions was carried out with picosecond resolution using a streak camera. The TRESs were found to undergo a pronounced time-dependent Stokes shift, eventually coinciding with the steady-state photoluminescence spectra within an approximately 40 ns delay time. Moreover, approximately 90% of the shift was complete within the first 1 ns after excitation, in contrast to the fact that overall photoluminescence involves very long time constants of 10-100 ns. The observed Stokes shift dynamics was very similar in CdS nanoparticles stabilized in two very different types of polymer solutions. Thus the solvent and/or polymeric stabilizer appeared to have a minimal effect on the shift. We propose that the relaxation proceeds through an internal mechanism involving the fast decay of high-energy traps into relatively slow-decaying low-energy traps. Time-dependent photoluminescence anisotropy experiments also revealed an approximately 1 ns decay component appearing only in the higher-energy end of the photoluminescence spectrum. Because this time constant is too short to represent rotational diffusion of the nanometer-sized particles, it was associated with the rapid relaxation of the high-energy trap states.

Understanding the efficacy of N,N-dimethylformamide and oxalyl chloride combination as chemoselective O-formylating agent: An unified experimental and theoretical study

ABSTRACT We have developed a simple but efficient synthetic protocol for the O-formylation of a wide range of aromatic hydroxyl/phenolic substrates using an N,N-dimethylformamide (DMF) and oxalyl chloride [(COCl)2] combination in dichloromethane (DCM) as solvent at ambient temperature. The DMF/(COCl)2 combination was found to be highly chemoselective for the aromatic/phenolic hydroxyl group over aliphatic hydroxyl or aromatic amine/thiol groups. This chemoselectivity of DMF/(COCl)2 combination towards O-formylation of aromatic alcohols was explained on the basis of outcomes of both experimental and density functional theory–based theoretical studies. GRAPHICAL ABSTRACT

Harnessing carbazole based small molecules for the synthesis of the fluorescent gold nanoparticles: A unified experimental and theoretical approach to understand the mechanism of synthesis

Six structurally different carbazoles (1-6) were explored as the green reducing agents for the synthesis of the fluorescent Au nanoparticles with tailor-made morphology in anionic (sodium dodecyl sulphate, SDS), cationic (cetyltrimethylammonium bromide, CTAB) and neutral (polyvinylpyrrolidone, PVP) micelle medium. Structure of the carbazoles played an important role in controlling the morphology, rate of formation and fluorescent activity of the Au nanoparticles. The Au nanoparticles formed in-situ also simultaneously catalyzed the intermolecular CC and NN couplings between the carbazoles, leading to the corresponding bis-carbazole derivatives. The free and bis-carbazole derivatives functionalized the surface of the synthesized Au nanoparticles and thereby controlling their morphology and fluorescence activity. A computational study was also made to determine the origin of the absorption and emission bands of the synthesized nanoparticles. The combined experimental and theoretical studies unraveled the nanoparticle formation process and mechanistic pathway of this green and easily implementable synthetic protocol of Au nanoparticles.