Rakhi Majumdar

Acacia nilotica (Babool) leaf extract mediated size-controlled rapid synthesis of gold nanoparticles and study of its catalytic activity

The leaf extract of Acacia nilotica (Babool) is rich in different types of plant secondary metabolites such as flavanoids, tannins, triterpenoids, saponines, etc. We have demonstrated the use of the leaf extract for the synthesis of gold nanoparticles in water at room temperature under very mild conditions. The synthesis of the gold nanoparticles was complete in several minutes, and no extra stabilizing or capping agents were necessary. The size of the nanoparticles could be controlled by varying the concentration of the leaf extract. The gold nanoparticles were characterized by HRTEM, surface plasmon resonance spectroscopy, and X-ray diffraction studies. The synthesized gold nanoparticles have been used as an efficient catalyst for the reduction of 4-nitrophenol to 4-aminophenol in water at room temperature.

Self-assembly of a renewable nano-sized triterpenoid 18β-glycyrrhetinic acid

The nano-sized triterpenoid 18β-glycyrrhetinic acid extractable from Glycyrrhiza glabra self assembled in different liquids affording mostly nano to microsized spherical and flower like objects consisting of fibrillar networks yielding thermoreversible gels. The self-assemblies have been utilized for the templated growth of CdS nanoparticles.

Natural triterpenoids as renewable nanos

Plant metabolites are the most significant source of renewable chemical feedstocks for a sustainable future. Among various plant secondary metabolites, triterpenoids are a large and structurally diverse 30-carbons subset of the major component terpenoids. Whereas the acyclic triterpene squalene can exist in many different conformations, the cyclic triterpenoids are more rigidified and largely chiral. The tetra and pentacyclic triterpenoids are more abundant than other triterpenoids and molecules having ten chiral centers are common. We show that acyclic and mono-cyclic to fused pentacyclic triterpenes are all nano-sized molecules having varied rigid and flexible lengths. Monte-Carlo investigation of their conformational space revealed that the nanometric lengths are maintained even in the folded conformers rendering all the triterpenoids useful as renewablenanos.

Self-assembly of ketals of arjunolic acid into vesicles and fibers yielding gel-like dispersions.

Ten aliphatic and aromatic ketals of arjunolic acid, a renewable, nanosized triterpenic acid which is obtainable from Terminalia arjuna, have been synthesized upon condensation with aldehydes. Self-assembly properties of the ketals have been studied in a wide range of organic liquids. With the exception of the p-nitrobenzylidene derivative, low concentrations of the ketals self-assemble and form gel-like dispersions in many of the organic liquids examined. The morphologies of the assemblies, studied at different distance scales by optical, electron, and atomic-force microscopies, consisted of fibrillar networks and vesicles which were able to entrap 5(6)-carboxyfluorescein as a guest molecule. X-ray diffractograms indicate that the fibrillar objects are crystalline. A charge-transfer complex was formed from a 1:1 mixture of ketal derivatives with electron-donating and electron-accepting groups, and the 9-anthrylidene derivative in its fibrillar network dimerized upon irradiation. Results demonstrate that subtle changes in the ketal structures can lead to very different aggregation pathways.

Vesicular self-assembly of a natural triterpenoid arjunolic acid in aqueous medium: study of entrapment properties and in situ generation of gel–gold nanoparticle hybrid material

A natural pentacyclic triterpenoid arjunolic acid self-assembled hierarchically in aqueous solvents yielding vesicular structures of nano to micrometer diameters affording gels. This self-assembly has been utilized for the entrapment and controlled release of anticancer drugs such as doxorubicin at physiological pH. A gel–gold nanoparticle hybrid material has also been prepared by in situ generated gold nanoparticles at room temperature.

Mimusops elengi bark extract mediated green synthesis of gold nanoparticles and study of its catalytic activity

The bark extract of Mimusops elengi is rich in different types of plant secondary metabolites such as flavonoids, tannins, triterpenoids and saponins. The present study shows the usefulness of the bark extract of Mimusops elengi for the green synthesis of gold nanoparticles in water at room temperature under very mild conditions. The synthesis of the gold nanoparticles was complete within a few minutes without any extra stabilizing or capping agents and the polyphenols present in the bark extract acted as both reducing as well as stabilizing agents. The synthesized colloidal gold nanoparticles were characterized by HRTEM, surface plasmon resonance spectroscopy and X-ray diffraction studies. The synthesized gold nanoparticles have been used as an efficient catalyst for the reduction of 3-nitrophenol and 4-nitrophenol to their corresponding aminophenols in water at room temperature.

Self-assembly of Renewable Nano-sized Triterpenoids

Studies on plant metabolites have gained renewed interest in recent years because these can serve as renewable chemicals for the development of a sustainable society. Among various plant secondary metabolites, terpenoids constitute the major component and triterpenoids are the 30C subset of it. In recent years, triterpenoids have drawn the attention of scientific community due to many of its potential and realized applications in medicine, drug delivery, thermochromic materials, pollutant capture, catalysis, liquid crystals, etc. In this personal review, we have discussed our computational results carried out on sixty representative naturally occurring triterpenoids demonstrating that all the triterpenoids are renewable functional nano-entities. Study of the self-assembly of several triterpenoids such as betulin, betulinic acid, oleanolic acid, glycyrrhetinic acid and arjunolic acid and their derivatives in different liquids have also been discussed. Moreover, the utilization of the resulting supramolecular architectures such as vesicles, spheres, flowers and fibrillar networks of nano- to micrometer dimensions and gels have also been discussed in the perspective of green, renewable and nanos.

A charge transfer complex nematic liquid crystalline gel with high electrical conductivity

We describe the rheological, dielectric and elastic properties of a nematic liquid crystal gel created using an anthrylidene derivative of arjunolic acid, a chiral triterpenoid, obtained from the extracts of the wood of Terminalia arjuna. In this novel gel, having the electron-donor and acceptor components as minority constituents, the gelation and strengthening of charge-transfer complex (CTC) formation are seen to be occurring concomitantly. In addition to being mechanically strong with a large storage modulus, the gel with the maximized CTC exhibits Frank bend elastic constant values that approach nanonewton levels. The highlight of the study is the observation of 4–5 orders of magnitude increase in electrical conductivity for this gel, a value that is higher than even in the CT complexes of 2-d ordered columnar structures. A further important advantage of the present system over the columnar complex is that the high conductivity is seen for ac probing also, and owing to the nematic nature can be switched between its anisotropic limits. Some of these features are ascribed to a specific molecular packing architecture, which reduces the trapping of the charge carriers.

A Novel Trihybrid Material Based on Renewables: An Efficient Recyclable Heterogeneous Catalyst for C−C Coupling and Reduction Reactions

The generation of organic-inorganic hybrid materials from renewable resources and their utilization in basic and applied areas has been at the forefront of research in recent years for sustainable development. Herein, a novel organic-inorganic trihybrid material was synthesized by in situ generation of palladium nanoparticles (PdNPs) in a hybrid gel matrix based on renewable chemicals. Constituents of the hybrid gel included a pentacyclic triterpenoid arjunolic acid extractable from Terminalia arjuna and the leaf extract of Chrysophyllum cainito rich in flavonoids. We took advantage of the presence of flavonoid molecules in this hybrid gel to generate an advanced trihybrid gel through in situ reduction of doped Pd(II) salts to stable PdNPs. The xerogel of this trihybrid material was used as a recyclable heterogeneous catalyst for C-C coupling and reduction reactions in aqueous media. We also demonstrated that the in situ generated PdNPs containing trihybrid material was a more efficient catalyst than the trihybrid material generated with presynthesized PdNPs.