S. Shiv Shankar

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.

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.

Anisotropic metal nanoparticles have distinct optical behavior when compared with their spherical counterparts. In this report, we demonstrate a simple method involving the reduction of aqueous gold ions by the extract of the lemongrass plant leading to the formation of gold nanotriangles with interesting absorption in the near-infrared (NIR) region of the electromagnetic spectrum. We show that, by simple variation of the experimental conditions, it is possible to vary the size of the gold nanotriangles and, thus, to tune the absorbance of flat gold nanoparticles in the NIR region. The NIR absorption of the gold nanotriangles is expected to be of application in hyperthermia of cancer cells and in IR-absorbing optical coatings. We show that the lemongrass extract synthesized gold nanotriangles can be easily cast in the form of films on glass substrates and that these films are highly efficient in absorbing IR radiation for potential architectural applications.