Critical review: impacts of macromolecular coatings on critical physicochemical processes controlling environmental fate of nanomaterials†
Attachment of engineered and naturally occurring macromolecules greatly affects the environmental fate and toxicity of engineered nanomaterials (ENMs). A better understanding of macromolecule–ENM interactions at the nanoscale will improve the ability to predict the effects of macromolecular coatings, e.g. natural organic matter (NOM), on ENM fate, reactivity, and toxicity. This review briefly discusses relevant theory from colloid and polymer science for highly idealized polymers on surfaces that can be used to describe ENM environmental behaviors and introduces classes of macromolecules of interest in the field of environmental nanotechnology. Methods to characterize adsorbed macromolecules on ENMs are presented along with their limitations for ENMs in natural systems. Finally, the current state of knowledge regarding the effects of attached organic macromolecules, both engineered and incidental, on the environmental fate and reactivity of ENMs is critically reviewed. These concepts in whole are synthesized to identify the fundamental gaps in understanding and metrology that must be addressed to improve our mechanistic understanding of the effects of organic macromolecules on ENM environmental fate, and approaches to correlate the properties of coated ENMs to their environmental fate are discussed. We postulate that a first principles approach to modeling ENM–macromolecule interactions is not warranted, particularly for complex and heterogeneous natural macromolecules. On the other hand, a mechanistic understanding is needed to inform parameter selection for empirical correlations, which may offer tractable alternatives to predicting the behavior of macromolecule–coated ENMs. Development of these empirical correlations and prediction of the long-term fate of ENMs is currently hampered by incomplete characterization of the adsorbed macromolecule layer properties and their evolution over time in natural systems.