Model multilayer structures for three-dimensional cell imaging

The prospects for SIMS three-dimensional analysis of biological materials were explored using model multilayer structures. The samples were analyzed in a ToF-SIMS spectrometer equipped with a 20 keV buckminsterfullerene (C ₆₀ ⁺ ) ion source. Molecular depth information was acquired using a C ₆₀ ⁺ ion beam to etch through the multilayer structures at specified time intervals. Subsequent to each individual erosion cycle, static SIMS spectra were recorded using a pulsed C ₆₀ ⁺ ion probe. Molecular intensities in sequential mass spectra were monitored as a function of primary ion fluence. The resulting depth information was used to characterize C ₆₀ ⁺ bombardment of biological materials. Specifically, molecular depth profile studies involving dehydrated dipalmitoyl-phosphatidylcholine (DPPC) organic films indicate that cell membrane lipid materials do not experience significant chemical damage when bombarded with C ₆₀ ⁺ ion fluences greater than 10 ¹⁵ ions/cm ² . Moreover, depth profile analyses of DPPC-sucrose frozen multilayer structures suggest that biomolecule information can be uncovered after the C ₆₀ ⁺ sputter removal of a 20 nm overlayer with no appreciable loss of underlying molecular signal. The experimental results support the potential for three-dimensional molecular mapping of biological materials using cluster SIMS.