Metal ions in the context of nanoparticles toward biological applications

• Nanoparticles can release toxic ions.
• Ions attached to the nanoparticle surface can provide contrast for imaging.
• Nanoparticles can be used as ion-selective probes.

In this review the relation of inorganic colloidal nanoparticles with metal ions in biological media is discussed. Metal ions in the context of nanoparticles are relevant because of completely different reasons. First, considering unwanted effects, upon corrosion/dissolution some particular inorganic nanoparticles can release metal ions and cause toxicity, such as in the case of Cd2+, Zn2+, and Ag+. Second, intentional modification of nanoparticles with particular metal ions can provide contrast for imaging applications, such as Gd3+ ions for magnetic resonance imaging. Third, nanoparticles themselves can be used for the detection of metal ions, even inside cells. Thus, there are several unwanted/intended scenarios where the interplay between nanoparticles, metal ions, and biological systems is of paramount importance.