Engineered nanomaterials that differ in size, shape, surface area, and chemical composition are currently used in many applications. Toxicological profiles of nanoparticles to form the basis for environmental risk assessments are needed, but challenges exist. Toxicological effects from untested compounds in aquatic organisms are routinely predicted using models derived from structurally similar chemical compositions (QSARS); however surface treatments of nanoparticles may directly affect activities of such particles. Toxicological endpoints were measured to quantum dot concentrations consisting of a cadmium-selenium core with a carboxyl-active surface coating (Quantum Dot™ nanocrystals) and suspended in US EPA moderately hard water (hardness = 90 mg/L as CaCO3). Ceriodaphnia dubia were exposed using 48-h US EPA standard test protocol and concurrent LC50s were measured to cadmium as CdCl. 2.5 H₂O for comparison. Measured Cd LC50 in 48-h laboratory exposures was 28.55 μg/L; while quantum dots in concentrations as high as 110 μg/L with measured Cd concentrations of 7048 μg/L, had no measurable 48-h toxicity. These preliminary results suggest that coatings present on quantum dots offer a protective measure while in aqueous media during 48-h test protocol. Further testing is needed to trace the fate and effect of these particles during their transformation within aqueous ecosystems.