Environmental behavior and ecotoxicity of microplastics (MPs) are significantly influenced by the omnipresent self-assembly of microbial extracellular polymeric substances (EPS) on them. However, mechanisms of EPS self-assembly onto MPs at nanoscale resolution and effects of aging are unclear. For the first time, temporospatial nano-heterogeneity of self-assembly of EPS onto fresh and one-year aged polypropylene (PP) MPs were investigated by atomic-force-microscopy-infrared-spectroscopy (AFM-IR). Natural aging caused high degree nanoscale fragmentation of MPs physically and chemically. Self-assembly of EPS on MPs was aging-dependent. Polysaccharides were assembled on MP surface faster than proteins. Initially, regardless of the fresh or aged MPs, polysaccharides and proteins, with the former being predominant, were successively and separately assembled to different nanospaces because of their competition for binding sites. More and more proteins and polysaccharides were superimposed on each other with assembly time due to intermolecular forces. The nanochemical textural analysis showed that the nano-heterogeneity of EPS assembly to MPs was clearly correlated with the aging-induced nanochemical and nanomechanical heterogeneity of MP surface. The spontaneous self-assembly of EPS with temporospatial nano-heterogeneity on MPs have multiple impacts on behavior, ecotoxicity and fate of MPs and their associated pollutants as well as other key ecological processes in aquatic environment.
Keywords: AFM-IR; Extracellular polymeric substances; Microplastics; Nanospectroscopy; Self-assembly.
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