The synthesis of nanoparticles with desired size and shape is an important area of research in nanotechnology. Use of biological system is an alternative approach to chemical and physical procedures for the synthesis of metal nanoparticles. An efficient environment-friendly approach for the biosynthesis of rapid and stable Gold nanoparticles (AuNPs) using whole cells of Geotrichum candidum is discussed in this paper. The enzymes/proteins present in the microorganism might be responsible for the reduction of metal salts to nanoparticles. Various reaction parameters such as culture age, temperature, pH, metal salt, and cell mass concentrations were optimized. The AuNPs were characterized by UV-visible spectroscopy, dynamic light scattering (DLS), energy dispersive spectroscopy (EDS), scanning electron microscope (SEM), and Fourier transform infrared spectroscopy (FTIR). Nanoparticles were isolated by sonicating the whole cells after treatment with Tween 80. The whole cell mediated process showed the simplistic, feasible, easy to scale up, and low-cost approach for the synthesis of AuNPs. 1. Introduction Metal nanoparticles have been an extensive area of research because of their unique chemical, physical, and optical properties [1]. These make them potential candidates in the field of catalysis, labeling, biosensing, drug delivery, antimicrobial, and so forth [2, 3]. AuNPs have wider ranges of applications in the biomedical field for biosensor development, drug delivery, imaging, photo diagnostics, and so forth [4]. Development of reliable processes for the synthesis of metal nanomaterials with excellent dispersity and stability with minimum harmful effects is the need of the day [5]. Traditional chemical and physical methods reported in the literature involve the use of hazardous chemicals and extreme reaction conditions [6–8]. The current research is directed towards the development of eco-friendly protocols for the synthesis of nanomaterials/nanostructures of desirable sizes and shapes [9, 10]. Considering applications of AuNPs in the fields of biology and medicine, environment, and technology, there is a growing need for the development of cost-effective method for the synthesis of new nanoparticles [11]. The biological methods fulfil all the requirements of a process to be green [12]. Synthesis of nanoparticles utilizing biological system such as bacteria, fungi, and several plant extracts have been widely reported in the literature [6, 13–15]. Microorganisms are able to produce metal nanomaterials either intra- or extracellularly. Intracellular
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