Bioaccumulation and in-vivo dissolution of CdSe/ZnS with three different surface coatings by Daphnia magna.

Daphnia magna were exposed to nano-sized CdSe/ZnS quantum dots (QDs) having three different surface coatings. QDs were investigated for their aqueous stability in the test media (hard reconstituted laboratory water) and for their uptake, elimination, and in-vivo dissolution. Positively charged QDs (QEI) and negatively charged QDs (QSH) were electrostatically stable, whereas neutrally charged QDs (QSA) showed aggregation and sedimentation over 48-h. After 24h of exposure to QDs (100µg/L as total Cd), the D. magna whole body Cd concentration significantly increased with no mortality for all QDs. Uptake patterns differed among the three coatings and Cd concentration reached 1460±50, 1014±99, and 584±81µg Cd/g dry wt for QEI, QSH, and QSA, respectively. Significant amounts of QEI and QSA (40% and 43%, respectively) remained in the D. magna after 24h of depuration, while 89% QSH were readily excreted within the initial 1h of the depuration stage. Soluble Cd was released from QDs during both the uptake and depuration. Release of Cd was higher in QEI and QSA than QSH, possibly resulting from the longer retention of QEI and QSA in the D. magna than QSH. These results imply that the surface charge of QDs plays a significant role in both the exposure to organisms and the in-vivo dissolution of nanoparticles.