Murali Sastry

Synthesis of gold nanotriangles and silver nanoparticles using Aloe vera plant extract.

Biogenic gold nanotriangles and spherical silver nanoparticles were synthesized by a simple procedure using Aloe veraleaf extract as the reducing agent. This procedure offers control over the size of the gold nanotriangle and thereby a handle to tune their optical properties, particularly the position of the longitudinal surface plasmon resonance. The kinetics of gold nanotriangle formation was followed by UV‐vis‐NIR absorption spectroscopy and transmission electron microscopy (TEM). The effect of reducing agent concentration in the reaction mixture on the yield and size of the gold nanotriangles was studied using transmission electron microscopy. Monitoring the formation of gold nanotriangles as a function of time using TEM reveals that multiply twinned particles (MTPs) play an important role in the formation of gold nanotriangles. It is observed that the slow rate of the reaction along with the shape directing effect of the constituents of the extract are responsible for the formation of single crystalline gold nanotriangles. Reduction of silver ions by Aloe veraextract however, led to the formation of spherical silver nanoparticles of 15.2 nm ± 4.2 nm size.

Extracellular biosynthesis of silver nanoparticles using the fungus Fusarium oxysporum

The development of reliable, eco-friendly processes for the synthesis of nanomaterials is an important aspect of nanotechnology today. One approach that shows immense potential is based on the biosynthesis of nanoparticles using biological micro-organisms such as bacteria. In this laboratory, we have concentrated on the use of fungi in the intracellular production of metal nanoparticles. As part of our investigation, we have observed that aqueous silver ions when exposed to the fungus Fusarium oxysporum are reduced in solution, thereby leading to the formation of an extremely stable silver hydrosol. The silver nanoparticles are in the range of 5-15 nm in dimensions and are stabilized in solution by proteins secreted by the fungus. It is believed that the reduction of the metal ions occurs by an enzymatic process, thus creating the possibility of developing a rational, fungal-based method for the synthesis of nanomaterials over a range of chemical compositions, which is currently not possible by other microbe-based methods.

Geranium leaf assisted biosynthesis of silver nanoparticles.

Development of biologically inspired experimental processes for the synthesis of nanoparticles is evolving into an important branch of nanotechnology. In this paper, we report on the use of Geranium (Pelargonium graveolens) leaf extract in the extracellular synthesis of silver nanoparticles. On treating aqueous silver nitrate solution with geranium leaf extract, rapid reduction of the silver ions is observed leading to the formation of highly stable, crystalline silver nanoparticles in solution. Transmission electron microscopy analysis of the silver particles indicated that they ranged in size from 16 to 40 nm and were assembled in solution into quasilinear superstructures. The rate of reduction of the silver ions by the geranium leaf extract is faster than that observed by us in an earlier study using a fungus, Fusarium oxysporum, thus highlighting the possibility that nanoparticle biosynthesis methodologies will achieve rates of synthesis comparable to those of chemical methods. This study also represents an important advance in the use of plants over microorganisms in the biosynthesis of metal nanoparticles.

BIOREDUCTION OF AUCL4− IONS BY THE FUNGUS, VERTICILLIUM SP. AND SURFACE TRAPPING OF THE GOLD NANOPARTICLES FORMED

Fungi make piles of gold! A green-chemistry route, based on the bioreduction of AuCl 4 - ions by the fungus Verticillium sp., for the formation of gold nanoparticles is demonstrated. The TEM micrograph shows a single Verticillium cell after reaction with gold ions and entrapment of gold nanoparticles on the cell wall and cytoplasmic membrane.

Bioreduction of chloroaurate ions by geranium leaves and its endophytic fungus yields gold nanoparticles of different shapes

Development of biologically inspired experimental processes for the synthesis of nanoparticles is an important branch of nanotechnology. In this paper, we report on the use of geranium leaves (Pelargonium graveolens) and its endophytic fungus in the extra-cellular synthesis of gold nanoparticles. Sterilized geranium leaves and an endophytic fungus (Colletotrichum sp.) growing in the leaves were separately exposed to aqueous chloroaurate ions. In both cases, rapid reduction of the metal ions was observed resulting in the formation of stable gold nanoparticles of variable size. In the case of gold nanoparticles synthesized using geranium leaves, the reducing and capping agents appear to be terpenoids while they are identified to be polypeptides/enzymes in the Colletotrichum sp. case. The biogenic gold nanoparticles synthesized using the fungus were essentially spherical in shape while the particles grown using the leaves exhibited a variety of shapes that included rods, flat sheets and triangles. While the exact reasons for shape variability are not clear at this stage, the possibility of achieving nanoparticle shape control in a host leaf–fungus system is potentially exciting.

Nanocrystalline TiO2 studied by optical, FTIR and X-ray photoelectron spectroscopy : correlation to presence of surface states

Nanophase TiO2 has been synthesized in an organic medium by two different routes, yielding two different structural phases viz. rutile and anatase. These differences are reflected in the different characterization techniques used to study the nanoparticles. UV-vis spectroscopy shows absorption-peak related particle sizes of ∼2.5 nm and agrees with transmission electron microscopy (TEM) estimates of 2.5–5.0 nm. Grazing incidence X-ray diffraction shows rutile and anatase phase with an overlay of Ti2O3 for the different routes considered. Differences in the nature of the transition from absorption plots not withstanding, a direct transition is confirmed. Photo-luminescence (PL) spectra for the two particulate structures shows prominent red shifted peaks at 314 nm and 399 nm, respectively (0.81 eV from the excitation), and also reveals vibrational features around the maximum PL signal. In addition a Ti3+ (PL) state is observed at 617.5 nm for both cases, a feature governed by the Auger process. Fourier transform infrared (FTIR) studies reveal weak complex vibrations between the Ti–O oxide species and also additional unsaturated sites (Ti3+) through incorporation of (OH) groups, not otherwise seen in bulk titania. A surface consisting of 6Ti3+–OH for coordinative saturation (octahedral site), along with 4Ti4+–O (tetrahedral) is thus necessary. Grazing incidence X-ray diffraction studies shows the presence of the rutile phase of TiO2 and also a sub-oxide phase of Ti (Ti2O3). X-ray photoemission spectra (XPS) of thin films of TiO2 confirms the oxide phase and also the presence of sub-valence states. The XPS and FTIR spectra confirm the presence of adsorbed sites for coordinative saturation of sub-valence states (Ti2+, Ti3+), through hydroxyl incorporation. These sites are amplified as the particle size is reduced, opening avenues for additional coordination, leading to important applications. In this case, a Ti2O3 overlay saturates the surface of titania. Subtle differences are observed in the data vis-a-vis literature reports.

Extracellular Synthesis of Gold Nanoparticles by the Fungus Fusarium oxysporum

A green chemistry approach to nanoparticle synthesis is the exciting possibility opened up by the fungus Fusarium oxysporum. The fungus, when exposed to aqueous AuCl 4 − ions, reduces the metal ions; this leads to the extracellular formation of gold nanoparticles.

Intracellular synthesis of gold nanoparticles by a novel alkalotolerant actinomycete, Rhodococcus species

The development of reliable, eco-friendly processes for the synthesis of nanoscale materials is an important aspect of nanotechnology. In this paper, we report on the use of an alkalotolerant actinomycete (Rhodococcus sp.) in the intracellular synthesis of gold nanoparticles of the dimension 5–15 nm. Electron microscopy analysis of thin sections of the gold actinomycete cells indicated that gold particles with good monodispersity were formed on the cell wall as well as on the cytospasmic membrane. The particles are more concentrated on the cytoplasmic membrane than on the cell wall, possibly due to reduction of the metal ions by enzymes present in the cell wall and on the cytoplasmic membrane. The metal ions were not toxic to the cells and the cells continued to multiply after biosynthesis of the gold nanoparticles.

Investigation into the interaction between surface-bound alkylamines and gold nanoparticles

In addition to alkanethiols and phosphine derivatives, alkylamines have been investigated as capping agents in the synthesis of organically dispersible gold nanoparticles. However, reports pertaining to gold nanoparticle derivatization with alkylamines are relatively scarce and their interaction with the underlying gold support is poorly understood. In this paper, we attempt a more detailed examination of this problem and present results on the Fourier transform infrared spectroscopy, thermogravimetry, nuclear magnetic resonance, and X-ray photoemission (XPS) characterization of gold nanoparticles capped with the alkylamines laurylamine (LAM) and octadecylamine (ODA). The capping of the gold nanoparticles with the alkylamines was accomplished during phase transfer of aqueous gold nanoparticles to chloroform containing fatty amine molecules. Thermogravimetry and XPS analysis of purified powders of the amine-capped gold nanoparticles indicated the presence of two different modes of binding of the alkylamines with the gold surface. The weakly bound component is attributed to the formation of an electrostatic complex between protonated amine molecules and surface-bound AuCl4-/AuCl2- ions, while the more strongly bound species is tentatively assigned to a complex of the form [AuCl(NH2R)]. The alkylamine monolayer on the gold nanoparticle surface may be place exchanged with other amine derivatives present in solution.

Gold Nanotriangles Biologically Synthesized using Tamarind Leaf Extract and Potential Application in Vapor Sensing

The size and shape dependent electronic and chemical properties of metal nanoparticles has drawn the attention of chemists, physicists, biologists, and engineers who wish to use them for the development of new generation nanodevices. In this article, we report the synthesis of gold nanotriangles using tamarind leaf extract as the reducing agent. On treating aqueous chloroauric acid solution with tamarind leaf extract, rapid reduction of the chloroaurate ions is observed leading to the formation of flat and thin single crystalline gold nanotriangles. Atomic force microscopy analysis of the gold nanotriangles indicated that they ranged in thickness from 20 to 40 nm. These gold nanotriangles with unique and highly anisotropic planer shapes might find application in photonics, optoelectronics, and optical sensing. We investigated the effect of different organic solvent vapors like methanol, benzene and acetone on the conductivity of tamarind leaf extract reduced gold nanotriangles. I‐V characteristics measurement of these nanotriangles was done in presence of these organic solvent vapors. The observed characteristics suggest the application of gold nanotriangles to future chemical sensors. BA thanks the Indian Academy of Sciences, Bangalore for a Summer Fellowship. BA and MC thank the Director, National Chemical Laboratory (NCL), Pune for permission to carry out this research at NCL.