Traceability of fluorescent engineered nanomaterials and their fate in complex liquid waste matrices

The number of products containing engineered nanomaterials (ENMs) has increased due to their high industrial relevance as well as their use in diverse consumer products. At the end of their life cycle ENMs might be released to the environment and therefore concerns arise regarding their environmental impact. In order to track their fate upon disposal, it is crucial to establish methods to trace ENMs in complex environmental samples and to differentiate them from naturally-occurring nanoparticles. The goal of this study was to distinctively trace ENMs by (non-invasive) detection methods. For this, fluorescent ENMs, namely quantum dots (QDs), were distinctively traced in complex aqueous matrices, and were still detectable after a period of two months using fluorescence spectroscopy. In particular, two water-dispersible QD-species, namely CdTe/CdS QDs with N-acetyl-l-cysteine as capping agent (NAC-QDs) and surfactant-stabilized CdSe/ZnS QDs (Brij®58-QDs), were synthesized to examine their environmental fate during disposal as well as their potential interaction with naturally-occurring substances present in landfill leachates. When QDs were spiked into a leachate from an old landfill site, alteration processes, such as sorption, aggregation, agglomeration, and interactions with dissolved organic carbon (DOC), led to modifications of the optical properties of QDs. The spectral signatures of NAC-QDs deteriorated depending on residence time and storage temperature, while Brij®58-QDs retained their photoluminescence fingerprints, indicating their high colloidal stability. The observed change in photoluminescence intensity was mainly caused by DOC-interaction and association with complexing agents, such as fulvic or humic acids, typically present in mature landfill leachates. For both QD-species, the results also indicated that pH of the leachate had no significant impact on their optical properties. As a result, the unique spectroscopic fingerprints of QDs, specifically surfactant-stabilized QDs, allowed distinctive tracing in complex aqueous waste matrices in order to study their long-term behavior and ultimate fate.