How microbial biofilms impact the interactions of Quantum Dots with mineral surfaces

The increasing use of Quantum Dots (QDs) - nanoparticles exhibiting unique optical properties – and their incorporation in multiple engineering products is likely to result in the release of this new class of contaminants into natural systems. In soils, bacterial biofilms and mineral surfaces form highly reactive interfaces, which may control QDs' environmental fate. However, little is known regarding QDs' stability in, and modes of interactions with, biofilm/mineral interfaces. This study examines the interactions, distributions and stability of thioglycolic acid-capped CdSe/ZnS QDs at the corundum (α-Al2O3)/Shewanella oneidensis MR-1 interface, for exposure times ranging between 1 h to 24 h. Long Period – X-ray Standing Wave – Fluorescence Yield spectroscopy and Grazing Incidence – X-ray Absorption Spectroscopy were used. Results indicate increases in Zn and Se concentrations within the biofilm/crystal system with time, demonstrating its high accumulation capacity over 24 h. In addition, dissolution of a part of the ZnS shell occurs within 1 h, highlighting the potential degradation of QDs when exposed to the biofilm/crystal compartment. Once released, Zn(II) migrates toward the biofilm-crystal interface and interacts preferentially with the crystal surface. In contrast, the remaining CdSe core is mostly preserved, and stays within the biofilm thickness. However, at 24 h, Se and Zn present similar distribution profiles indicating a general reduction in ZnS shell dissolution at this longer exposure time.