(292d) Demonstration of Nanoparticle-Bound Polymer Biodegradation and Resulting Nanoparticle Destabilization

Many engineered nanoparticles are modified by adsorption or grafting of polymers onto their surfaces in order to impart colloidal stability in suspension, to promote adhesion with matrix materials, to increase biocompatibility, or to impart some particular biomolecular targeting capability. By controlling nanoparticle flocculation and surface deposition, surface-bound polymer layers strongly influence nanoparticle fate, transport, and subsequent biological impact in environmental systems. Biodegradation is one potential route to the breakdown of surface-bound polymers, but it is unknown whether such nanoparticle-bound polymers are bioavailable or if the surface confinement restricts microbial access to the chains so that they become recalcitrant to biodegradation. Most engineered nanoparticles are in a size range that cannot be internalized by bacteria, so the bound chains must be accessed extracellularly. Here we demonstrate, for the first time, that a polymer brush bound to nanomaterials can be bioavailable to microbes. Model core/shell nanomaterials with covalently attached polyethylene oxide (PEO) chains were prepared for this study. These were synthesized to obtain a nanomaterial in which biodegradation was the only available coating breakdown mechanism.  Taking the form of a densely crosslinked bottlebrush copolymer with a hydrophobic core, these nanomaterials consisted of poly(ethylene oxide) methacrylate macromonomers crosslinked by hydrophobic divinyl benzene. PEO is a common constituent of consumer products, and PEO-based polymers are common, effective steric stabilizers for nanoparticles and other colloids. Accordingly, this an appropriate model system to represent a class of nanoparticle coatings that is likely to be introduced to the environment.  The nanoparticles were approximately 30 nm in hydrodynamic radius and were colloidally stable in the culture media used in the biodegradation experiments. Microbial samples isolated from river water were enriched by supplying PEO homopolymer as the sole carbon source. The PEO-homopolymer degrading microbial enrichment cultures were transferred to fresh media, then supplied with PEO brush-bound nanoparticles as the sole carbon source. Biomass and CO₂ production measurements indicated that the particle-bound PEO brush was indeed bioavailable to these microbes. DNA sequencing indicated that the mixed culture responsible for degradation included Novosphingobium sp., Pseudomonas sp., and  Hydrogenophaga sp. Brush biodegradation resulted in nanoparticle flocculation. Control experiments demonstrated that byproducts of microbial metabolism or cell debris did not cause the observed nanoparticle flocculation. This points to the need to understand biologically mediated transformations of nanoparticle coatings in order to understand the fate and transport of nanoparticles in the environment.