Correct Identification of the Core‐Shell Structure of Cell Membrane‐Coated Polymeric Nanoparticles

Transmission electron microscopy (TEM) observations of negatively stained cell membrane (CM)‐coated polymeric nanoparticles (NPs) reveal a characteristic core‐shell structure. However, negative staining agents can create artifacts that complicate the determination of the actual NP structure. Herein, it is demonstrated with various bare polymeric core NPs, such as poly(lactic‐co‐glycolic acid) (PLGA), poly(ethylene glycol) methyl ether‐block‐PLGA, and poly(caprolactone), that certain observed core‐shell structures are actually artifacts caused by the staining process. To address this issue, fluorescence quenching was applied to quantify the proportion of fully coated NPs and statistical TEM analysis was used to identify and differentiate whether the observed core‐shell structures of CM‐coated PLGA (CM−PLGA) NPs are due to artifacts or to the CM coating. Integrated shells in TEM images of negatively stained CM−PLGA NPs are identified as artifacts. The present results challenge current understanding of the structure of CM‐coated polymeric NPs and encourage researchers to use the proposed characterization approach to avoid misinterpretations.