Engineered nanoparticle transformations: Rethinking toxicity in water

The burgeoning production and utilization of engineered nanoparticles (ENPs) in recent years has precipitated the intentional and inadvertent discharge of ENPs into the environment, where undergo different transformations. Extensive research has investigated the mechanisms underlying the environmental transformations of metal-based ENPs, with a focus on alterations in the properties of their transformation products. It is widely recognized that ENP-biota interactions are influenced by various ENP characteristics, such as size, shape, surface area, chemical composition, surface charge, and chemistry. As a result of transformations, changes in ENP properties are anticipated to affect biotic interactions, including cellular recognition and trafficking, thus impacting organismal responses. This hypothesis has only recently been subjected to experimental scrutiny, mainly within simplified ENP-organism systems. Major studies indicate that the acute toxicity of transformed ENPs is largely driven by the rate and yield of metal ion release, similar to pristine ENPs. However, when transformations reduce ENP dissolution, they may enhance environmental persistence, rendering other toxicity mechanisms more significant. We meticulously examine available data on the toxicity of various transformed ENPs, aiming to systematically assess the actual responses of aquatic biota concerning altered ENP properties and differing environmental factors. In this context, we highlight scenarios involving multiple ENP transformations and specific local environmental modifications. These research directions warrant further exploration, especially under real-world conditions. Such efforts will expand the database, which, through the application of modern machine learning and artificial intelligence tools, can aid in predicting the fate of ENPs released from the increasing array of nano-products. • ENPs transformations alter material properties and fate in the environment. • Environmental conditions greatly influence the type and course of ENP transformations. • Organism microenvironments create specific conditions for ENPs transformations. • Biotransformation of ENPs primarily mitigates their acute toxicity to aquatic biota. • Reduced dissolution, direct contact, and ROS levels govern lower toxicity of transformed ENPs.