Magnetic Water Cleaning of Estrogens via Tight yet Dynamic Binding to Nanoparticles

Micropollution is an ever-increasing concern to human health and wildlife caused by population growth, industrialization, and intensive agriculture. An example of such micropollutants are natural and synthetic estrogen hormones. They are found everywhere in the aquatic environment at very low but dangerous concentrations. In this article, we demonstrate a supramolecular design principle for adsorption of such low-concentrated estrogens onto tailored superparamagnetic iron oxide nanoparticles (SPIONs) enabling efficient magnetic water cleaning. We facilitate the adsorption by tuning the SPION surface with a binary self-assembled monolayer (SAM) composed of two phosphonic acid derivatives: one serving as hydrophobic interaction site and the other improving water dispersibility of the system. Next to validating the concept in real river water, we conclude the demonstration of our nanomaterials by unveiling the estrogen-SAM interaction at dilute conditions via synergistic combination of model systems, characterization techniques, and simulation. We experimentally deduce tight binding of estrogens combined with simulations that identify different, dynamic adsorption motifs at molecular space and time scale including a rare intercalation state, in which the hydrophobic pollutants minimize their contact to water. We believe that beyond our materials, these insights on the pollutant-adsorbent interface underline the importance and potential of rational, molecular-scale design of nano-adsorbents.