Uptake, retention and internalization of quantum dots in Daphnia is influenced by particle surface functionalization

Nanomaterials are a diverse group of compounds whose inevitable release into the environment warrants study of the fundamental processes that govern the ingestion, uptake and accumulation in aquatic organisms. Nanomaterials have the ability to transfer to higher trophic levels in aquatic ecosystems, and recent evidence suggests that the surface chemistry of both the nanoparticle and biological membrane can influence uptake kinetics. Therefore, our study investigates the effect of surface functionalization on uptake, internalization and depuration in Daphnia spp. Uncharged (polyethylene glycol; PEG), positively charged (amino-terminated: NH2) and negatively charged (carboxyl-modified; COOH) cadmium selenide/zinc sulfide quantum dots were used to monitor ingestion, uptake and depuration of nanometals in Daphnia magna and Ceriodaphnia dubia over 24h of exposure. These studies demonstrated that particles with higher negative charge (COOH quantum dots) were taken up to a greater extent by Daphnia (259.17±17.70 RFU/20 Daphnia) than either the NH2 (150.01±18.91) or PEG quantum dots (95.17±9.78), however this is likely related to the functional groups attached to the nanoparticles as there were no real differences in zeta potential. Whole body fluorescence associates well with fluorescent microscopic images obtained at the 24h timepoint. Confocal and electron microscopic analysis clearly demonstrated that all three types of quantum dots could cross the intestinal epithelial barrier and be translocated to other cells. Upon cessation of exposure, elimination of all three materials was biphasic with rapid initial clearance that likely represents elimination of material remaining in the GI tract followed by a much slower elimination phase that likely represents elimination of internalized material. These studies demonstrate that daphnids can take up intact nanomaterial from the water column and that this uptake is strongly influenced by particle surface functionalization. In addition, the usefulness of using quantum dots as a proxy for other nanometals (no acute toxicity, clear visualization in electron microscopy), in conjunction with several different imaging techniques in assessing uptake and accumulation of nanoparticles in daphnids was demonstrated.