**4. Microbial generation of Se- and Te-nanostructures**

It is nowadays recognized the key role played by bacteria not only as tool for bioremediation purposes of highly contaminated Se- and Te-matrices, but also as a mean by which the less toxic and bioavailable elemental form of these chalcogens (i.e., Se0 and Te0 ) are generated and recovered. Indeed, yet Se and Te are elements featured by unique chemical-physical (i.e., semiconductive, photoconductive and catalytic) properties [166–169], which result to be emphasized in the nanosized material containing Se0 and Te0 as building blocks, forming nanoparticles (NPs) and/or nanorods (NRs). Se and Te as nanoscale structures are characterized by a large surface-to-volume ratio and a large surface energy as compared to their bulk counterparts [8], which make them suitable for biotechnological applications, such as: biomedicine, electronics, environmental engineering and agricultural industries [168, 170], to name a few. Since bacteria are considered inexpensive catalysts, their use for the production of Se- and Te-based nanostructures is an attractive choice over the chemical synthesis processes [79]. Thus, microorganisms capable of generating biogenic nanomaterials are seen as *green* and cost-effective exploitable methods to synthesize high-quality nanostructures [10], whose process occurs at standard conditions (i.e., near neutral pH, controlled temperature and pressure), and, more importantly, avoiding the use of harsh reducing agents as well as the production of toxic wastes deriving from the chemical synthesis approaches [171].

Considering the peculiar photoconductive, semiconductive and optical properties of Se, the use of Se-based nanomaterials has been investigated in a wide range of applications, such as in the production of new optical devices, photovoltaic solar cells, photographic exposure meters and rectifiers and photo-assisted fuel cells [172–175]. Moreover, Se-nanostructures resulted to act as good catalyst for both the chelation of mercury ions (Hg2+) present as contaminants in different polluted environments [176], and the degradation of several toxic chemical compounds (e.g., trypan blue dye) [177], as well as an efficient bio-sensor for H<sup>2</sup> O2 in different matrices [178]. Similarly, Te is a narrow band-gap *p*-type semiconductor, which is featured by high photoconductivity, piezoelectricity and thermoelectricity [168, 169]. These versatile properties led to the exploitation of Te-nanomaterials as optoelectronic, piezoelectric and thermoelectric devices, infrared detectors and gas sensors [179, 180], to name a few. Further, since these chalcogen-nanostructures showed great adsorptive ability, biological reactivity and antioxidant functions, their use in biomedicine have been recently explored [8, 170, 181]. Both Se- and Te-nanomaterials resulted efficient tools in protecting living organisms from DNA oxidation [181], as well as promising antimicrobial and anticancer agents [182–187]. In this regard, several Se-nanostructures produced by different microorganisms have been tested for their antimicrobial efficacy, highlighting their ability to prevent the growth of pathogenic bacteria (i.e., *E. coli*, *P. aeruginosa*, *S. aureus*) either in the form of planktonic cells or as biofilms [182, 183, 186, 187]. Particularly, biogenic Se-nanomaterials resulted to be more efficient as compared to those synthesized by mean of chemical processes, showing a strong inhibitory effect of pathogenic bacterial growth at lower concentrations [183]. Moreover, studies carried out to evaluate the cytotoxicity of biogenic Se-nanostructures toward human cell lines (i.e., fibroblasts and dendritic cells) revealed their high biocompatibility [187], which is a fundamental feature for their possible biomedical applications. Although Te-nanostructures produced by microorganisms are less studied for biomedical applications than those containing Se, recently the potential of such nanomaterials as antimicrobials has been assessed [186], showing their good efficacy in inhibiting pathogens growth. Further, a promising technological application of biogenic Te-based nanostructures regards the production of quantum dots (QDs), which are semiconductors nanocrystals featured by unique electronic and optical properties, due to quantum confinement effects [188].
