To the Editor

In general, transmission electron microscopy (TEM) and quantitative methods such as inductively coupled plasma-mass spectrometry (ICP-MS) are used to study the biodistribution of nanomaterials. For example, ICP-MS can detect the elements of nanomaterials and evaluate their biodistribution quantitatively. However, ICP-MS cannot distinguish between elements that are inherent to the nanomaterials and those that are cleaved or released from them. But, unlike ICP-MS, TEM can detect the presence of nanomaterials and identify their location within tissues and cells. Even though the TEM images in our study1 provide only qualitative information, TEM is invaluable for identifying the biodistribution of the silica and TiO2 nanoparticles.

He et al.2 correctly point out the detection limit of TEM and that only some silica and TiO2 nanoparticles could be detected in the small section of the placental and fetal tissue in our study. However, through analysis of several hundreds of TEM sections, we confirmed that silica and TiO2 nanoparticles did accumulate in both the placental and fetal tissues. The observations were not coincidental.

To study the biodistribution of nanomaterials quantitatively, methods such as ICP-MS should be used and, indeed quantitative biodistribution studies of silica and TiO2 nanoparticles in the mouse placenta and fetus are currently under way in our laboratory.