Biocompatibility of Metal–Phenolic Network-Coated Nanoparticles ^†

Metal–phenolic networks (MPNs) are novel adsorbent materials that have promising applications in the environmental remediation of organic pollutants and heavy metals. For efficient adsorption, the specific surface area of MPN materials can be increased by coating nanoparticle surfaces with MPNs. Such MPN-coated nanoparticles may prove very efficient in various environmental applications; however, their safety needs to be tested at the early stages of material development. Here, free-living freshwater-ciliated protozoa were used as model organisms for testing the biocompatibility of iron–tannic acid network-coated Au nanoparticles (Fe–TA@Au NPs). Viability after 24 h Fe–TA@Au NP exposure was measured using an ATP assay kit, and intracellular reactive oxygen species (ROS) were quantified using 2′,7′-dichlorodihydrofluorescein diacetate (H₂DCFDA) [1]. Microscopy was used to qualitatively characterize the swimming behavior of protozoa as well as the uptake and depuration of Fe–TA@Au NPs in the ciliates. The results showed that Fe–TA@Au NPs were not lethal to the protozoa at the maximum concentration tested (~10¹⁰ particles/mL). Despite MPN-coated NP phagocytosis by protozoa and accumulation in food vacuoles, the MPNs did not affect the swimming behavior or viability during 24-h exposure. To test the performance of the novel materials as heavy metal toxicity-mitigating agents, co-exposures of protozoa to Fe–TA@Au NPs and toxic levels of copper salt (CuSO₄, LC₅₀~3 mg/L) were conducted. Fe–TA@Au NPs completely rescued the protozoa from cell death induced by CuSO₄. The underlying mechanism of toxicity mitigation could have been the removal of Cu ions by MPNs or the tannic acid in the MPNs acting as an antioxidant, as evidenced by reduced levels of ROS in the protozoa. Thus, the study showed that the effective levels of Fe–TA@Au NPs were biocompatible with unicellular freshwater model organisms and have potential for applications in metal contamination remediation in aqueous environments.