A study of fate and behavior of isotopically labelled quantum dots in aquatic environment at relevant concentrations

The transformations (e.g., dissolution, aggregation, sedimentation) of engineered nanoparticles (ENPs) in surface waters are known to be affected by numerous factors such as pH, ionic strength, organic/inorganic phases, bacteria, algae, exopolysaccharides, as well as other metals and contaminants. However, the ENPs concentration itself is infrequently discussed as another controlling factor, since the difficulty of ENPs detection in complex matrices with high geochemical background (i.e., the occurrence of ENPs constitutive elements in the environment at varying concentrations), remains very challenging. In this study, non-traditional stable isotopes (isotopically labelled ENPs) and HR-ICPMS were used to overcome these barriers. For the first time, 7 nm-sized and multi-spiked 111Cd77Se/68ZnS quantum dots (QDs) coated with thioglycolic acid (TGA) were synthesized and the limits of quantification (QD-LOQ) of the method was first assessed by HR-ICPMS in different aquatic media. Remarkably the problem of detecting ENPs in a complex medium containing high background noise is not specific to aquatic environments, but is also encountered in in vivo studies - hence, the QD-LOQ study was extended to different biological media. The transformation of the multi-spiked QDs in synthetic aquatic systems was studied at environmentally relevant concentrations (ppt level), in absence and presence of other metals (e.g., Zn), organic (fulvic acid) and inorganic (goethite) matter, by following their dissolution and homo- and heteroaggregation with these natural phases. The choice of QDs concentration (ppt level) was fixed according to the QD-LOQ study. The difficulties when studying the behavior of QDs in aquatic systems at ppt level concentrations and the subsequent metal speciation were surmounted by coupling the ENPs isotopic labelling technique with centrifugal ultrafiltration (CU) and HR-ICP-MS, in parallel with scanned stripping chronopotentiometry (SSCP). They firmly provided a thorough comprehension regarding the transformation of QDs in the environment, despite the low concentration used. The QDs behavior in aquatic systems is affected by various factors: i) the physicochemical conditions of the medium including the presence of natural organic and mineral matter, ii) the presence of the manufactured coating of the QDs, and iii) the interaction of the metal ions in the medium with the coating itself. Without the isotopic labelling technique, dissolved ENPs would not be observed using the CU/HR-ICP-MS when working at very low concentration in a complex system containing high geochemical background concentration. The exact speciation of the ENPs under environmentally relevant conditions (i.e., with the presence of other metals and organic/inorganic matter) was successfully determined by the coupling of isotopically labelled ENPs with CU/HR-ICPMS and SSCP. KEYWORDS : engineered nanoparticles; CdSe/ZnS quantum dots; isotopic labeling; detection limits; HR-ICP-MS; SSCP; dissolution; speciation; environment; aquatic media