We have investigated the structural properties and formation mechanisms of zinc sulfide nanocrystals by comparing biological and abiotic systems. Using high-resolution transmission electron microscopy and high-angle annular dark-field scanning transmission electron microscopy (HRTEM and HAADF-STEM), we demonstrate that the ZnS nanocrystals formed via dissimilatory bacterial sulfate reduction (examples shown in Fig. 1) are highly distinctive from their abiogenic counterparts in terms of morphology, crystal size, structural defects, dissolution behavior, and the presence of the wurtzite phase. Based on detailed crystal-structure analyses, we have also revealed the underlying mechanisms involved in the biological and abiotic formation of ZnS nanoparticles. Overall, mineralization of ZnS from aqueous solutions follows a non-classical pathway via dominantly forming Zn3S3(H2O)6 nanoclusters, and the subsequent assembly of these clusters. The exclusive ultrafine size of the abiogenic ZnS nanoparticles (i.e., 1-3 nm) regardless of the sulfide-addition rates discloses an intrinsic difficulty in the assembly step however, which prevents the crystal growth of Zn-S nanoclusters. We attribute this difficulty mainly to the polarity-induced instability of the Zn4S6(H2O)44- or Zn6S4(H2O)44- nanostructures, which are formed via preferential cubic stacking of Zn3S3(H2O)6 in aqueous conditions. In the biological system, the presence of polyanionic metabolites likely behaves as effective screening for the dipole moments in Zn4S6(H2O)44- or Zn6S4(H2O)44- structures, and thus, significantly enhances their subsequent crystallographic assembly. The occurrence of structural defects in the larger, biogenic ZnS nanocrystals (highlighted in Fig. 1) provides consistent evidence for the fore-mentioned mineralization pathway of ZnS under aqueous conditions. These findings (Xu et al., 2016) are of fundamental importance to understand the origins and fate of nanoparticles in the environment, and have implications for a range of process including biomineralization, bioremediation of metal-contaminated sites, and the bacterial production of functional nanomaterials.

Crystal structure and growth of ZnS nanoparticles formed via comparative biological and abiotic pathways

253rd National Meeting (2017)

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