Retrospective sputter depth profiling using 3D mass spectral imaging.

A molecular multilayer stack composed of alternating Langmuir-Blodgett films was analyzed by ToF-SIMS imaging in combination with intermediate sputter erosion using a focused C cluster ion beam. From the resulting dataset, depth profiles of any desired lateral portion of the analyzed field-of-view can be extracted in retrospect, allowing the influence of the gating area on the apparent depth resolution to be assessed. In a similar way, the observed degradation of depth resolution with increasing depth of the analyzed interface can be analyzed in order to determine the 'intrinsic' depth resolution of the method.

Fundamental studies of molecular depth profiling and 3D imaging using Langmuir-Blodgett films as a model.

Molecular depth profiling and three-dimensional imaging using cluster projectiles and SIMS have become a prominent tool for organic and biological materials characterization. To further explore the fundamental features of cluster bombardment of organic materials, especially depth resolution and differential sputtering, we have developed a reproducible and robust model system consisting of Langmuir-Blodgett (LB) multilayer films. Molecular depth profiles were acquired, using a 40-keV C(60) (+) probe, with LB films chemically alternating between barium arachidate and barium dimyristoyl phosphatidate.

Mass spectral imaging of glycophospholipids, cholesterol, and glycophorin a in model cell membranes.

Time of flight secondary ion mass spectrometry (ToF-SIMS) and the Langmuir-Blodgett (LB) technique have been used to create and analyze reproducible membrane mimics of the inner and outer leaflets of a cellular membrane to investigate lipid-protein and lipid-lipid interactions. Films composed of phospholipids, cholesterol and an integral membrane protein were utilized. The results show the outer membrane leaflet mimic (DPPC/cholesterol/glycophorin A LB film) consisting of a single homogeneous phase whereas the inner membrane leaflet mimic (DPPE/cholesterol/glycophorin A LB film) displays heterogeneity in the form of two separate phases.

Depth resolution during C60+ profiling of multilayer molecular films.

Time-of-flight secondary ion mass spectrometry is utilized to characterize the response of Langmuir-Blodgett (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 C60 ion beam result in successful molecular depth profiling of both the single component and multilayers of alternating chemical composition.

Chemically alternating Langmuir-Blodgett thin films as a model for molecular depth profiling by mass spectrometry.

Langmuir-Blodgett multilayers of alternating barium arachidate and barium dimyristoyl phosphatidate are characterized by secondary ion mass spectrometry employing a 40 keV buckminsterfullerene (C60) 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.

Short-chain alcohols promote accelerated membrane distention in a dynamic liposome model of exocytosis.

We have used amperometric measurements in a model system consisting of two liposomes connected with a membrane nanotube to monitor catechol release during artificial exocytosis and thereby to elucidate the effect of small-chain alcohols on this dynamic membrane process. To determine the rate of membrane shape change, catechol release during membrane distention was monitored amperometrically, and the presence of alcohols in the buffer was shown to accelerate the membrane distention process in a concentration-dependent manner. Compression isotherms for the same lipid composition in the absence and presence of ethanol and 1-propanol were measured to determine how these short-chain alcohols affect the lipid packing in monolayers.

Investigating lipid interactions and the process of raft formation in cellular membranes using ToF-SIMS.

There is an increased interest in how lipids interact with each other, especially in the lateral separation of lipids into coexisting liquid phases as this is believed to be an attribute of raft formation in cell membranes. ToF-SIMS has shown itself to be an excellent tool for investigating cellular and model membrane systems and will be perhaps the most powerful one for investigating raft formation. Results from our laboratory show the capability of ToF-SIMS at identifying unequivocally the content of coexisting liquid lipid phases.

Sphingomyelin/phosphatidylcholine and cholesterol interactions studied by imaging mass spectrometry.

Label-free imaging mass spectrometry is utilized the first time to study lipid-lipid interactions in a model membrane system. Ternary lipid mixtures of cholesterol (CH), sphingomyelin (SM), and phosphatidylcholine (PC) on supported Langmuir-Blodgett films are investigated as a mimic of the cellular membrane. The unique chemical specificity and imaging capability allow identification and localization of each lipid molecule in the membranes.

Localization of sphingomyelin in cholesterol domains by imaging mass spectrometry.

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.

Lateral heterogeneity of dipalmitoylphosphatidylethanolamine-cholesterol Langmuir-Blodgett films investigated with imaging time-of-flight secondary ion mass spectrometry and atomic force microscopy.

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.