Allotropy of selenium nanoparticles: Colourful transition, synthesis, and biotechnological applications

Abstract

Abstract Elemental selenium (Se0) nanomaterials undergo allotropic transition from thermodynamically-unstable to more stable phases. This process is significantly different when Se0 nanoparticles (NPs) are produced via physico-chemical and biological pathways. While the allotropic transition of physico-chemically synthesized Se0 is fast (minutes to hours), the biogenic Se0 takes months to complete. The biopolymer layer covering biogenic Se0 NPs might be the main factor controlling this retardation, but this still remains an open question. Phylogenetically-diverse bacteria reduce selenium oxyanions to red amorphous Se0 allotrope, which has low market value. Then, red Se0 undergoes allotropic transition to trigonal (metallic grey) allotrope, the end product having important industrial applications (e.g. semiconductors, alloys). Is it not yet clear whether biogenic Se0 presents any biological function, or it is mainly a detoxification and respiratory by-product. The better understanding of this transition would benefit the recovery of Se0 NPs from secondary resources and its targeted utilization with respect to each allotropic stage. This review article presents and critically discusses the main physico-chemical methods and biosynthetic pathways of Se0 (bio)mineralization. In addition, the article proposes a conceptual model for the resource recovery potential of trigonal selenium nanomaterials in the context of circular economy.