Time-of-flight secondary ion mass spectrometry is utilized to characterize the response of LangmuirBlodgett (LB) multilayers under the bombardment by buckminsterfullerene primary ions. The LB multilayers are formed by barium arachidate and barium dimyristoyl phosphatidate on a Si substrate. The unique sputtering properties of the C 60 ion beam result in successful molecular depth profiling of both the single component and multilayers of alternating chemical composition. At cryogenic (liquid nitrogen) temperatures, the high mass signals of both molecules remain stable under sputtering, while at room temperature, they gradually decrease with primary ion dose. The low temperature also leads to a higher average sputter yield of molecules. Depth resolution varies from 20 to 50 nm and can be reduced further by lowering the primary ion energy or by using glancing angles of incidence of the primary ion beam.The development of polyatomic projectiles for cluster-based secondary ion mass spectrometry (SIMS) is opening new opportunities for materials characterization. Of special interest is the emergence of molecular depth profiling whereby the projectile removes molecules in nearly a layer-by-layer fashion without the accumulation of chemical damage. [1][2][3][4][5][6][7] This problem has plagued atomic projectiles for many years 8 and has limited sensitivity. When the molecular samples are bombarded with cluster ion sources, the energy is deposited close to the surface and the chemical damage is then removed as fast as it accumulates, leaving subsurface layers relatively intact. [9][10][11][12][13][14][15] The quality of the depth profile has recently been characterized by a cleanup efficiency parameter derived from a simple erosion model for molecular solids. 16 Among all the cluster projectiles, buckminsterfullerene (C 60 + ) generally exhibits the highest cleanup efficiency. 17,18 New fundamental studies of the sputtering process are now required to optimize the experimental parameters for molecular depth profiling. The literature concerning the interactions between energetic cluster ions and molecular solids has grown rapidly, including experimental approaches 16,[19][20][21][22][23][24][25][26] and molecular dynamic (MD) simulations. [11][12][13][27][28][29] While MD simulations have provided insightful understanding, much of the experimental work lacks a quantitative understanding for comparison to the simulation results. Moreover, most of the molecular depth profiling experiments are performed on organic systems either with uniform chemical content or with unknown composition. 3,4,30,31 The analysis of buried organic layers under cluster bombardment has been shown to be feasible, but the degree of beam-induced * To whom correspondence should be addressed. nxw@psu.edu.Supporting Information Available: Representative AFM images of sample roughness ( Figure S1). This is material is available free of charge via the Internet at http://pubs.acs.org. mixing between organic layers is not fully understood. This info...

Langmuir-Blodgett multilayers of alternating barium arachidate and barium dimyristoyl phosphatidate are characterized by secondary ion mass spectrometry employing a 40 keV buckminsterfullerene (C 60 ) ion source. These films exhibit well-defined structures with minimal chemical mixing between layers, making them an intriguing platform to study fundamental issues associated with molecular depth profiling. The experiments were performed using three different substrates of 306 nm, 177 nm, and 90 nm in thickness, each containing six subunits with alternating chemistry. The molecular subunits are successfully resolved for the 306 nm and 177 nm films by cluster ion depth profiling at cryogenic temperatures. In the depth profile, very little degradation was found for the molecular ion signal of the underneath layers compared with that of the top layer, indicating that the formation of chemical damage is removed as rapidly as it is formed. The resolving power decreases as the thickness of the alternating subunits decrease, allowing a depth resolution of 20 to 25 nm to be achieved. The results show the potential of LB films as an experimental model system for studying fundamental features of molecular depth profiling. (J Am Soc Mass Spectrom 2008, 19, 96 -102)

The location of each lipid in a palmitoyloleoylphosphatidylcholine/18:0 sphingomyelin/cholesterol monolayer system is laterally resolved using imaging time-of-flight secondary ion mass spectrometry (TOF-SIMS) without the necessity of adding fluorescent labels. This system of coexisting immiscible liquid phases shows cholesterol domains with sizes and shapes comparable to those in the fluorescence microscopy literature. The results show that SM localizes with cholesterol and that palmitoyloleoylphosphatidylcholine is excluded. Moreover, the segregation is not complete, and there is a small amount of both phospholipids distributed throughout.

To better understand the influence of cholesterol (CH) on dipalmitoylphosphatidylethanolamine (DPPE), Langmuir-Blodgett (LB) model membranes of DPPE with varying amounts of cholesterol were imaged by time-of-flight secondary ion mass spectrometry (ToF-SIMS) and atomic force microscopy (AFM). Cholesterol has a condensing effect on DPPE that at low cholesterol concentrations results in lateral heterogeneity of the LB monolayer. At 4:1 DPPE/CH, islands of DPPE/CH phase exist with a connected DPPE phase. As the concentration of cholesterol is increased, the percolation threshold is crossed and the DPPE/CH phase islands connect to separate the DPPE phase (2:1 DPPE/CH). Finally, at 50 mol % cholesterol a single homogeneous DPPE/CH phase LB monolayer exists. ToF-SIMS of the DPPE/CH phase provides a lower ion signal for the characteristic lipid fragments and substrate apparently owing to the higher molecular density induced by cholesterol. AFM data indicate that the DPPE/CH phase is lower in height than the DPPE phase. As phosphatidylethanolamine is predominant in the inner lipid leaflet of cellular membranes, this work has implications for the understanding of cholesterol domains in the inner leaflet of cells.