Cyto- and Nanotoxicity

The only organic fluorophores approved by the Food and Drug Administration (FDA) for use in humans are fluorescein (e.g., for opthalometry), Nile Blue, and 

Toxicity of nanoparticles is a much more complicated issue as compared with organic fluorophores Nanoparticles may be nanotoxic, they may contain cytotoxic elements or compounds, or their surface ligands/coating may contain toxic species. Nanotoxicity refers to the ability of a substance to be intrinsically cytotoxic due to its size (and independent of its constituent materials). The most prominent example of nanotoxicity is asbestos. Even though there are no systematic studies on the nanotoxicity of different nanocrystals available the results from several cytotoxicity studies suggest that nanotoxicity is not dominating for nanoparticular reporters [85, 86]. 

The preparation of both, the particles themselves and the protective surface layer, has direct influence on their cytotoxicity. It is common belief that in the case of core/shell nanoparticles, properly prepared, close shell or multiple shells such as ZnS/Si02-shells prevents the leakage of toxic elements and thus makes cytotoxicity unlikely. Naturally, a better solution is to avoid cytotoxic materials in the first place. QDs, for example, can be synthesized without utilization of any class A or B elements InP/ZnS QDs have photophysical properties comparable to those of CdSe-based systems [43, 93]. Principally, whenever a new approach for QD synthesis or coating is used or if the QDs are applied in an extreme environment that could compromise their integrity, it is recommended to assess their cytotoxicity. 

The work on the toxicity of nanoparticular reporters is still in its infancy. The clear evaluation of cytotoxicity will require verified data using at least two or more independent test systems, standardization in the experimental set-up and exposure conditions in order to be reliable. In addition, the involvement of toxicologists in the systematic assessment of QD toxicity would be beneficial. 

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