Potential hazards of 2D nanomaterials to the environment
Creators: name: Masuda, Seigo
Publisher: name: Imperial College London
Year: 2022

description: 2-Dimensional Nanomaterials (2DNMs) have recently gained much momentum in research and industry. Graphene Family Nanomaterials (GFNs) and Transitional Metal Dichalcogenides (TMDs) are two classes of 2DNMs of particular interest due to their potentially wide application such as electronics, sensors, and even biomedical applications. Of the two classes of 2DNMs, graphene oxide (GO) and molybdenum disulfide (MoS2) were selected as materials of interest due to their volume of research interest and production capacity. Although some environmental assessment has been carried out for GO, the literature has mainly focused on the toxicity and biological mechanisms of model pathogenic bacteria and not soil bacteria. Reports have highlighted that the physiochemical property of lateral size of 2DNMs may impact the inhibitory effect and the mechanism of interaction based on computational and in-vitro experiments with GO and pathogenic bacteria. However, findings are not yet conclusive and have not been explored widely beyond the antibacterial application of GO and have not been studied in the context of environmental conditions such as using model environmental media and environmentally relevant bacteria. For MoS2, reports on their toxicity and interaction with bacteria are even scarcer which emphasises the need to assess their toxicity. A series of toxicity tests modified from a standard ISO protocol (10712) was used, for the first time, to study the effect GO and MoS2 on the viability of soil bacteria. Arthrobacter globiformis and Pseudomonas putida, gram-positive and gram-negative bacteria respectively, were exposed to GO under different exposure conditions (chapter 2.4), to different lateral sizes of GO (chapter 3), and to pristine and aged/oxidised MoS2 (chapter 4) under ISO protocol testing conditions for water/soil contamination. The 2DNMS were extensively characterised using techniques, such as Raman Spectroscopy and X-ray photoelectron spectroscopy (XPS) for chemical properties, and Atomic Force Microscopy (AFM) and Dynamic Light Scattering (DLS) for size and colloidal properties, to understand how the physiochemical properties related to toxicity. The literature review highlighted that one of the major inconsistencies in reports was a lack of materials’ characterisation and understanding of these bacteria-materials’ interactions in different experimental conditions. The bioassays revealed that the choice of media and ageing of the material greatly affected the interaction of 2DNMs with bacteria and also the ability of the bacteria to respond to 2DNMs. Pristine MoS2 showed the greatest inhibitory effects on the soil bacteria which decayed as the nanomaterial oxidised. XPS results showed conversion of Mo (IV) species as MoS¬2 to Mo (VI) as soluble molybdate ions. For GO 2DNMs, various media conditions of nutrient-rich media (Luria Bertani Broth), model environmental media (ISO 10712 test media), and minimal salt media (Milli-Q water) showed that different exposure conditions affected their colloidal stability and inhibitory effects. DLS and zeta potential measurements showed increasing stability in the order of Luria-Bertani (LB) broth&lt; ISO 10712&lt; Milli-Q water with increasing agglomerate size of GO in the order of LB broth&gt; ISO 10172&gt; Milli-Q water. The toxicity assay showed that different media varied the GO’s inhibitory effect and growth enhancement depending on the media composition which was attributed to the level of salts and organic substrates. TEM images revealed a “gap” between the GO and the bacteria under media conditions with nutrients. It was hypothesised that soil bacteria could display a protective mechanism under nutrient-rich conditions in which they could defend themselves against GO. Comparison between all critical determinants of toxicity showed that they can be ranked using their effect on inhibition as follows: level of salt, organic substrate, and other substances in the media &gt; ageing of 2DNMs &gt;&gt; species of bacteria &gt; lateral size. The thesis uncouples which system parameters influence outcomes of toxicity assays surveying the interaction between 2DNMS and bacteria and provide guidance for future more standardised toxicity testing of this class of nanomaterial.