Given this sensitivity to experimental conditions, it is also important to characterize the NPs for each experiment conducted.
The preparation of stock NP suspensions for characterization before conducting laboratory experiments usually uses external mixing forces, like solvent dispersion, shaking, centrifugation, ultrafiltration, sonication, as well as surface modification and coatings to make NPs disperse evenly. All of these processes and treatments may change the properties of the NPs and may, therefore, be environmentally unrealistic compared with NPs released to the environment. Environmental factors, such as water pH, salinity and temperature, dissolved organic material, and natural competing cations, are likely to play important roles in determining the dispersion, toxic consequences, and compartment in which NPs are retained in the environment. Ideally, characterization of NPs should be performed under conditions as close as possible to the relevant exposure medium. For NPs intentionally or accidentally introduced into the aquatic environment, sediment is likely to act as a potential sink as it does for many chemicals. However, we know little about the state of sediment-associated NPs due to the limitation of techniques and methods for characterizing NPs in such complex and ''dirty'' media. We can start our exposures with nicely dispersed and well characterized NPs in a deionized water stock solution, but as soon as we add the solution to sediment we are working literally with a black box. Although we might expect NPs to become highly aggregated upon contact with sediment and therefore lose their tendency to behave differently than their chemically identical counterparts, our preliminary results suggest that this is not necessarily the case. Because the reactivity and toxicity of NPs are believed to be influenced by such features as their size, shape, surface coating, and other properties, we conclude that both the physical and chemical properties of NPs must be systematically and adequately defined before toxicological studies and risk assessment. The publication of toxicity test results should require that a characterization be performed in stock solutions used for testing. At the same time, there is a pressing need for the development of better methods for effectively characterizing NPs in complex environmental media (e.g., seawater, sediment) and living tissue.