Probing the tendency for aggregation of nanoplastics in model extracellular biofilm substances with fluorescence correlation spectroscopy

Biofilms are omnipresent in the natural environment. They consist of microbial communities that encapsulate themselves in extracellular polymeric substances. The increasing micro- and nanoplastic pollution is of immense concern. The transport and fate of nanoplastics (NPs) in the environment is strongly affected by biofilm induced aggregation. We apply fluorescence correlation spectroscopy (FCS) to investigate the pH-dependent aggregation tendency of polystyrene (PS) nanoparticles due to interactions with model extracellular biofilm substances, such as alginate and bovine serum albumin (BSA). We show that certain alginate-BSA mixtures convey a lower tendency to nanoparticle aggregation, as compared to alginate or BSA alone. The positively charged BSA promotes nanoparticle aggregation through bridging due to attractive electrostatic nanoparticle-BSA interactions. In a mixture of BSA and negatively charged alginate this interaction is hampered. In the BSA-alginates mixture and in alginate alone other weaker attractive forces are causing aggregation, possibly due to hydrophobic interactions, van der Waals interactions or depletion forces that are not electrostatic in nature and thus are less influenced by the pH-value. Thus it is crucial to consider correlative effects between multiple biofilm components to better understand the aggregation tendency of NPs. A single component model system based on total organic carbon content of extracellular biofilm substances, would have led to an underestimation of the stability towards aggregation. Nevertheless a simple model that only depends on the polysaccharide concentration might be feasible if the protein content is not exceeding a critical value.