Nanoscience is a young and growing field of science. It encompasses a diversity of sub-fields such as nanotechnology and nano-medicine, all of them seeking to realize the promises of nanoscale physics. Nano means: “billionth” and conceptually all nano-like terminology implicitly refers to the nano-meter (nm) scale (10-9m). Therefore the size range covered by nanoscience is from 1 to 100 nm, which lays at the boundary of two distinct worlds of physics: the bulk material and the atomic structure. In that particular region, laws of physics transition and while the bulk material exhibits constant physics independently of its size, nanomaterials see their properties and characteristics change as a function of size. That very specific property makes nanomaterials extremely appealing for a variety of applications. These applications cover areas such as electronics, photonics, catalysts, photography, material coatings, but also biotechnology, medicine, pharmacology, textile embedding, paints, household goods, cosmetics, foods and children goods. The current dissertation covers the field of metallic nanoparticles, within which, two types have been considered: selenium nanoparticles (SeNPs) and silver nanoparticles (AgNPs). SeNPs are interesting in inorganic semiconductors and crystal respectively used in electronics and photonics, whereas the interest for AgNPs is due to their strong antimicrobial properties. NPs are not only anthropogenic, but can be produced by a variety of organisms (e.g., bacteria, fungi, yeast or plants). However their biological synthesis remains partially unknown. They can be closely related to chemically produced NPs, but can also exhibit very specific characteristics unobtainable by conventional chemistry. An understanding of the underlying mechanisms of biological synthesis if extended to the industrial level could help achieve better NPs at a lower energetic and environmental cost. The use of nanomaterials such as AgNPs to protect drinking water from pathogens or prevent microbially derived bad odors, present the risk of their release into the environment. A gap of knowledge remains as to the hazards caused by an increase in AgNPs load in freshwater and sediments on the various biotas. This thesis addressed these two fundamental questions in Chapter 1 for SeNPs and Chapters 2, 3 and 4 for AgNPs