We acquired electron tomograms of plastic sections of E. coli BL21 cells expressing FtsZ-cSeNP, expecting to observe individual cSeNPs organized by FtsZ filaments. See M3 for electron tomography collection methods. A complication we encountered is that the solvents used in typical plastic embedding of cells for EM, such as acetone, dissolve SeNPs (see solvent-screening SI). (Figure SeNP-dissolve, SI). We implemented SeNP crystallization by adding Cu(OAc)2 to particle rich cells as a postfixation step to stabilize the NPs. The Cu 2+ ions react with cSeNPs to form CuSe metal selenides (Figure , panel E). [cite preprint] The resulting CuSe nanoparticles resist solvent dissolution.