Interactions, Transformations, and Bioavailability of Nano-Copper Exposed to Root Exudates

Due to the potential for interactions between crop plants and engineered nanomaterials (ENMs), there is increasing interest in understanding the bioavailability and effects of ENMs released into soil systems. Here, we investigate the influence of root exudates on the fate of ENMs from a thermodynamic perspective. Nano isothermal titration calorimetry was applied to determine thermodynamic parameters for the interaction between nanocopper (nCu) and synthetic root exudate (SRE) and its components (including sugars, organic acids, amino acids, and phenolic acids), as well as Cu²⁺ and SRE. The measured binding constant (K_d = 5.645 × 10³ M^-1) indicated strong interactions between nCu particles and SRE, as well as with individual organic acids. The interaction between Cu²⁺ and SRE was stronger (K_d = 7.181 × 10⁴ M^-1) but varies for the individual SRE components. nCu dissolution in the presence of SRE was the predominant interaction. In addition, SRE resulted in a complex transformation of nCu, where Cu²⁺, Cu⁺, and Cu⁰ were formed via oxidation and reduction. Plant-nCu exposure experiments indicate that the binding of SRE with nCu and dissolved Cu ions can significantly decrease Cu uptake and bioaccumulation in plants. nITC provides a fundamental thermodynamic understanding of interactions between nCu and plant root exudates, providing an important tool for understanding plant NP-interactions.