Regulation of cadmium bioaccumulation in zebrafish by the aggregation state of TiO2 nanoparticles

Highlights: • Effects of nanoparticle aggregation on Cd bioaccumulation were investigated. • AnaNPs could inhibit Cd bioaccumulation by reducing its free ion concentration. • AnaNPs could induce Cd bioaccumulation by carrying Cd directly into zebrafish. • Aggregation changed the balance between AnaNPs' inhibitory and inductive effects. The potential effects of engineered nanoparticles (NPs) on metal bioaccumulation in aquatic organisms have been the focus of increasing research attention. However, while NPs typically aggregate, the role of aggregation in NP-mediated metal bioaccumulation is largely unknown. The present study investigated the effects of polyacrylate-coated TiO₂ (anatase) NPs (AnaNPs) on Cd bioaccumulation in zebrafish. The Ca concentration in the experimental medium was manipulated to regulate AnaNP aggregation. At the low Ca concentration, the AnaNPs were well-dispersed and there was little bioaccumulation. Under this condition, Cd bioaccumulation was mainly via the uptake of free ions (Route 1), with only a minor contribution from NP-Cd complexes (Route 2). Therefore, AnaNPs decreased Cd bioaccumulation, as their inductive carrier effect could not offset the inhibition induced by the decrease in the free Cd ion concentration as a result of NP adsorption. At the high Ca concentration, the AnaNPs aggregated and their bioaccumulation increased. Accordingly, Cd bioaccumulation was equally accounted for by Routes 1 and 2 but the overall amount of Cd remained unchanged because the inductive effect of the AnaNPs offset their inhibitory effect. Thus, during risk evaluations of NPs, the contribution of aggregation to metal bioaccumulation should be considered.