Extracellular biosynthesis of silver nanoparticles using the fungus Fusarium semitectum

Abstract

Development of environmental friendly procedures for the synthesis of metal nanoparticles through biological processes is evolving into an important branch of nanobiotechnology. In this paper, we report on the use of fungus “Fusarium semitectum” for the extracellular synthesis of silver nanoparticles from silver nitrate solution (i.e. through the reduction of Ag⁺ to Ag⁰). Highly stable and crystalline silver nanoparticles are produced in solution by treating the filtrate of the fungus F. semitectum with the aqueous silver nitrate solution. The formations of nanoparticles are understood from the UV–vis and X-ray diffraction studies. Transmission electron microscopy of the silver particles indicated that they ranged in size from 10 to 60 nm and are mostly spherical in shape. Interestingly the colloidal suspensions of silver nanoparticles are stable for many weeks. Possible medicinal applications of these silver nanoparticles are envisaged.

Introduction

The nanoparticles of noble metals are found to have potential applications in various fields like microelectronics [1], optical devices [2], catalysis [3] and drug delivery system [4], etc. The noble metal nanoparticles exhibit new physico-chemical properties which are not observed either in the individual molecules, or in the bulk metals [3], [5], [6]. For example, gold and silver nanoparticles exhibit strong absorption of electromagnetic waves in the visible range due to surface plasmon resonance (SPR). SPR is caused due to collective oscillations of the conduction electrons of nanoparticles upon irradiation with visible light [7]. The SPR is highly influenced by shape and size of the nanoparticles. Recently, the absorption spectra of individual silver nanoparticles were correlated with their size and shape determined by transmission electron microscopy (TEM) [8]. The results indicate that spherical and roughly spherical nanoparticles absorb in the blue region of the spectrum, while decahedral nanoparticles and particles with triangular cross-sections absorb in the green and red part of the spectrum, respectively. The width and position of the SPR not only depends on the particle size as suggested earlier, but also on the chemical properties of the nanocrystalline surface, referred to as chemical interface damping [9]. Thus, an exquisite control of size, composition, morphology, stability and environmental friendly synthesis are the features which are highly desirable. Though there are several physical and chemical methods for synthesis of metallic nanoparticles, to achieve these objectives, researchers turned to biological systems. Many organisms, both unicellular and multicellular are known to produce inorganic nanomaterials either intracellularly [10] or extracellularly [10], even though the actual mechanisms are not fully understood because of the complexity of most biological reactions. Varieties of inorganic nanomaterials are synthesised by biological processes using bacteria, yeast and fungi [11], [12], [13]. While intracellular synthesis in principle may accomplish a better control over the size and shape distributions of the nanoparticles, product harvesting, and recovery are more cumbersome and expensive. The extracellular synthesis by comparison is more adaptable to the synthesis of a wider range of nanoparticles systems. Among various metal nanoparticles, silver nanoparticles have several important applications in the field of biolabelling [14], sensors, antimicrobial agents and filters [15] and hence are being intensively studied employing Fusarium oxysporum [10], Pseudomonas stutzeri [11], Rhodococcus sp. [16], Thermomonospora [10], Phaenero chaete chrysosporium [17], etc. In the present investigation we report the extracellular biosynthesis of silver nanoparticles employing the fungus Fusarium semitectum. F. semitectum is commonly available fungus found in marshland regions. In literature we have not come across using this fungus for formation and stabilization of silver nanoparticles in aqueous system. The local environment suits for this fungus, and also, this fungus is related to F. oxysporum in many aspects. Hence we have undertaken this fungus in the present study. The present study includes time dependent formation of silver nanoparticles employing UV–vis spectrophotometer, size and morphology by employing TEM, structure from powder X-ray diffraction (XRD) technique and understanding of protein–silver nanoparticles interaction from Fourier transform infrared (FT-IR) spectroscopy.