(537f) Bioavailability and Microbial Degradation of Nanomaterial Coatings

Coatings are an integral part of nanoparticle design, imparting changes in particle reactivity, stability and toxicity. These coatings frequently consist of polymers adsorbed or grafted to particle surfaces. The fate of these polymeric coatings is expected to dictate the long term fate, partitioning and ecological impact of engineered nanoparticles in the environment. Depending on the polymer composition and method of surface attachment, some coatings may be removed from particle surfaces by desorption or non-biological hydrolysis processes. Direct microbiological removal or degradation of nanoparticle coatings has not been demonstrated. In this study, we synthesized 30 nm diameter star copolymers consisting of 2000 molecular weight polyethylene oxide (PEO) arms emanating from dense, cross-linked polystyrene-like cores via atom transfer radical polymerization (ATRP). These serve as model nanomaterials with a covalently grafted polymer brush coating for a study of direct coating degradation by microbes. Since the arms are covalently linked to the cores, the possibility of PEO desorption and subsequent microbial degradation of free polymers in solution is eliminated. Strains of PEO-degrading bacteria were isolated from wastewater treatment sludge. A Chryseobacterium sp. culture was grown on either a 2000 molecular weight PEO homopolymer solution or a PEO star polymer solution as the sole carbon source. Cultures grew on both carbon sources. In fact, growth was faster on PEO stars than the homopolymers, despite equivalent PEO concentrations in both systems, thus indicating that the covalently attached PEO coatings on these nanoparticles are bioavailable. PEO star copolymers aggregated after microbial degradation, demonstrating the loss of colloidal stability caused by PEO arm degradation. Such microbiological processing of nanoparticle coatings would have significant implications for the long term mobility of engineered nanoparticles in the environment. Furthermore, since a growing body of evidence shows that toxicity of engineered nanoparticles to cells and organisms is decreased by aggregation, microbial processing may also impact the long-term ecotoxicity of engineered nanoparticles.