Verma et al. Nanoscale Res Lett 2011, 6:16 http://www.nanoscalereslett.com/content/6/1/16 high-resolution pole piece with a point–point resolution of 1.9 A° (TEM, Tecnai [FEI]-12v.G-20). Surface topol- ogy was measured by atomic force microscopy (AFM) in the contact mode on a VEECO Digital Instruments multimode scanning probe microscope equipped with a Nanoscope IV controller. Results and Discussion The endophytic Aspergillus clavatus strain was isolated from sterilized stem tissues of Azadirachta indica. This strain is identified using modern taxonomic keys with microscopic observations (Figure 1). One can observe the conidiophore with the naked eye, since this species of Aspergillus possesses largest conidiophores among the Aspergillus spp. (Figure 1a), the club-shaped conidio- phores are 2–4 mm in length, stipes smooth-walled hyaline (inset Figure 1c). Conidial heads radiate, later splitting into several columns. Vesicles are clavate, 40–60 mm diameter conidiogenous cells are uniseriate, conidia smooth walled, pale green, ellipsoidal, 7–8 × 2–3 micrometre (Figure 1b, c). The morphotaxonomic keys of this fungus was so strong that one does not require any molecular identification. When the biomass of fun- gus was challenged with 1 mM HAuCl4 aqueous solution, a rapid change in the colour of the biomass was observed from fresh white to the dark purple (Figure 2), similarly the reaction mixture for fungal extract was also observed (inset, Figure 2). This change in colour was due to the collective coherent oscillation of conduction electrons at the surface of the gold nanoparticles when these particles interact with the oscillating electric field of the incident light, a phenomenon called surface plasmon resonance (SPR). This change in colour indicates that reduction in AuCl4- ions takes place. When this reaction was traced with UV–vis spectroscopy, gold SPR bands were observed at ca. 540 nm, which steadily increases in inten- sity as a function of time of reaction. A 72-h reaction Page 3 of 7 mixture has greater absorption intensity at ca. 540 than 48 h reaction mixture (Figure 3). The surface plasmon bands for the gold nanoparticles usually ranges between 510 and 560 nm in aqueous solution depending upon the function of their morphology, since plasmon bands are very sensitive to the length and sharpness of the tips of nanomaterials. The spherical nanoparticles, however, have strong absorption at about 520 nm with almost no absorption after 600 nm; however, the triangular shape has absorption at 540 which extends well in near infra red region (NIR). At maturation of reaction, the wave- length of surface plasmon bands stabilizes at 555 nm (Figure 3). Thus, the wavelength of peak absorption depends upon several factors such as particle size, dielec- tric constant of surrounding media and the inter-particle distance [19]. The representative bright field TEM images (Figure 4a, b) shows a relatively large population of flat gold nanotriangles along with some spherical and hexa- gonal gold nanoparticles formed by the spontaneous reduction in aqueous chloroaurate ions (AuCl-4) by fer- mentation extract of A. clavatus. The images also con- firm that the gold nanotriangles are single crystalline in nature and could be indexed based on the face-centred cubic (fcc) structure of gold. The purified gold nanotrian- gles showed a particle size distribution ranging from 20– 35 nm with an average particle size of 30 ± 2 nm (inset Figure 4b), this indicates that distribution is monodis- persed. Consequently, we obtained a much higher popu- lation of nanotriangles in comparison with other morphologies, this encourage us to go deep into the opti- mization of parameters to get a control over the shape. All the nanotriangles observed under low-resolution TEM were ostensibly flat (Figure 4c–h), and showing tip truncation and rounding from sharp angle (Figure 4c, f, h) to sniped angle (Figure 4d, g) nanotriangles. When sig- nificant rounding (snipping) occurs, these nanostructures remains no longer as triangular nanoprisms and generally Figure 1 The endophytic Aspergillus clavatus, isolated from surface sterilized stem tissues Azadirachta indica A. Juss. a Bunches of conidiophores as visualized onto petriplate, b the club-shaped conidiophores, c stipes smooth walled and hyaline (Bar represents magnifications ×40 for a and b, while ×100 for c).