Surface Properties of Nanoparticles Dictate Their Toxicity by Regulating Adsorption of Humic Acid Molecules

Abstract

In an aquatic environment, nanoparticles (NPs) acquire coatings of natural organic matter (NOM) that alter the fate and toxicity of NPs. However, the essential relationship between engineered NP properties and NOM assemblage that determines how NPs are modified and behave at biological interfaces remains unclear. Herein, we use a complementary experimental and molecular dynamics approach to elucidate the mechanisms underlying adsorption of humic acid (HA) on NPs of different surface properties and effects on NP interactions with bacteria. Our results demonstrate that hydrophobic and positively charged NPs are more capable of adsorbing HAs in unique standing or lying conformations that expose particular groups to modify NP properties. In particular, HA adsorption increases effective NP size, reduces NP’s hydrophobicity, and reverses NP’s surface charge. As a consequence of HA adsorption, the binding of NPs to bacteria is sterically hindered and suppressed by electrostatic repulsion between negatively charged groups in adsorbed HAs and lipopolysaccharide on the bacterial membrane. The toxicity of NPs is mitigated as seen by reductions of NP association with bacteria and NP-induced membrane damage. These findings provide compelling explanations on the cascading events from dictation of HA adsorption by engineered NP properties to how the modified NP surfaces change their toxicity.