Verma et al. Nanoscale Res Lett 2011, 6:16 http://www.nanoscalereslett.com/content/6/1/16 Page 4 of 7 Figure 2 The flasks containing the biomass of the Aspergillus clavatus biomass in sterile distilled water (Control) and in aqueous solution of 1 mM HAuCl4 after the reaction of 72 h (the biomass colour changed into dark purple). Inset shows the pictures of fermentation broth of Aspergillus clavatus challenged by 1 mM HAuCl4. transformed into nanodisks or in cases of truncation without rounding, hexagonal nanoprisms. Synthesis of gold nanotriangles are kinetically driven process and is a result of aggregation and rearrangement of smaller size particles, which act as a nuclei for further growth into anisotropic triangular structures. It is also observed that the low rate of reduction in metal ions at normal room temperature possibly facilitate the growth of anisotropic nanoparticles, and with slight modifications in the tem- perature and the reaction medium one can be enabled in fabricating the well-defined triangular gold nanoparticles [20]. We have made efforts for the measurements of comparatively large, single gold nanotriangles (inset Figure 5), the height of the nanotriangles was about 140 nm, however, the facets length varies from 130 to 350 nm (Figure 5). But this may not be the case for all the nanotriangles, some has heights as low as 25–30 nm (Figure 7b, d). The AFM studies showed a maximum height of smallest nanotriangle was within 30 nm and the thickness ranges in between 2–8 nm (Figure 7b, d), and that all the triangles observed are equilateral with flat surfaces. In order to confirm the monocrystalline nature of the gold nanoparticles, XRD analysis was performed. Figure 6 shows the X-ray diffraction pattern obtained from the gold nanotriangles. The Bragg reflections obtained from the gold nanotriangle clearly correspond to the fcc crystalline structure of gold. The XRD pattern exhibits four identical diffraction peaks corresponding to the [111], [200], [220] and [311] appearing at 2θ = 38.2°, 44.5°, 65.6° and 78.6° of metal gold, respectively, (Interna- tional Centre for Diffraction Data, ICDD No. 4–0783), indicating that the precipitate is composed of pure crys- talline gold (Figure 6). As per the XRD pattern, a very intense Bragg reflection for the [111] lattice is observed, suggesting that the [111] oriented gold nanotriangles are lying flat [21] on the planar surface, while the reflections correspond to [220] and [311] with lattice spacing of 1.44 and 1.23 A° is specific for the triangular morphology, respectively. It is also notable that the ratio of intensity between the [200] and [111] diffraction peaks for the pre- pared sample is much lower than the standard (0.042 vs. 0.33), and this rationally decreases as the particles size Figure 3 UV–vis spectra recorded as a function of reaction time of an aqueous solution of 1 mM HAuCl4 with the filtrate of the fungal biomass. The spectra show a sharp absorption lmax 540 at 48 h reaction periods, which intensifying with the 72 h reaction time. The inset shows vials containing the filtrate of A. clavatus collected during the reaction period, rapid change in colour pattern due to SPR are clearly visible.