Intracellular delivery of nanomaterials: How to catch endosomal escape in the act

Successful cytosolic delivery of nanomaterials is becoming more and more important, given the increase in intracellular applications of quantum dots, gold nanoparticles, liposomal drug formulations and polymeric gene delivery vectors. Most nanomaterials are taken up by the cell via endocytosis, yet endosomal escape has long been recognized as a major bottleneck in cytosolic delivery. Although it is essential to detect and reliably quantify endosomal escape, no consensus has been reached so far on the methods to do so. This review will summarize and discuss for the first time the different assays used to investigate this elusive step to date.