Fig. 2.

Citric acid-modified magnetic nanoparticles (CA-MNPs) were observed a with TEM to be ~17 nm in diameter and b with cryo-TEM to be similarly sized but dispersed. After the addition of the luciferase, c cryo-TEM shows that the Luc + MNP are larger (~ 40–50 nm) with a core–shell structure where a lighter luciferase shell surrounds the darker CA-MNP core; the inset shows a single magnified core–shell Luc + MNP structure showing the luciferase shell thickness is ~5 nm