Amorphous structure and crystal stability determine the bioavailability of selenium nanoparticles.

Microorganisms play a critical role in the biogeochemical cycling of selenium, often reducing selenite/selenate to elemental selenium nanoparticles (SeNPs). These SeNPs typically exist in an amorphous structure but can transform into a trigonal allotrope. However, the crystal structural transition process and its impact on selenium bioavailability have not been well studied. To shed light on this, we prepared chemosynthetic and biogenic SeNPs and investigated the stability of their crystal structure. We found that biogenic SeNPs exhibited a highly stable amorphous structure in various conditions, such as lyophilization, washing, and laser irradiation, whereas chemosynthetic SeNPs transformed into a trigonal structure in the same conditions. Additionally, a core-shell structure was observed in biogenic SeNPs after electron beam irradiation. Further analysis revealed that biogenic SeNPs showed a coordination reaction between Se atoms and surface binding biomacromolecules, indicating that the outer layer of Se-biomacromolecules complex prevented the SeNPs from crystallizing. We also investigated the effects of SeNPs crystal structures on the bioavailability in bacteria, yeast, and plants, finding that the amorphous structure of SeNPs determined Se bioavailability.