Mass Spectrometry Imaging of Untreated Wet Cell Membranes in Solution Using Single-Layer Graphene

We reported on how atomic and molecular secondary ions can be sputtered through single-layer graphene so that untreated wet cell membranes in a solution can be imaged by secondary ion mass spectrometry (SIMS) at subcellular spatial resolution. We observed intrinsic molecular distributions of lipids, such as cholesterol, phosphoethanolamine, and various fatty acids, in untreated wet cell membranes without any labeling. Cell viability assay, optical imaging, and time-lapse SIMS imaging showed that graphene-covered cells on a wet substrate with a cell culture media reservoir were not dead and at least their cellular membranes were not disintegrated during SIMS imaging in an ultra-high vacuum environment. Ion dose dependence studies and Ab-initio molecular dynamic (AIMD) calculations suggest that sputtering through single-layer graphene occurs through a transient hole generated in the graphene layer. Cholesterol imaging of cell membranes showed that methyl-β-cyclodextrin (MβCD) preferentially extracts cholesterol molecules from the cholesterol-enriched regions.