Ferritins naturally form nanocages to store and transport iron and are of interest in the fields of nanomedicine and drug delivery. The interior cavity of ferritins can be loaded with a variety of small molecules upon pH-induced disassembly and reassembly and the outer surface functionalised for targeting or soliciting a host response.

 

Here, we show how mass photometry can be used to quantify assembly, load and homogeneity at single particle resolution. Such measurements use little sample, are fast, label-free, and broadly applicable to natural and synthentic nanocages.