Showing papers by "Nicholas Winograd published in 2005"

The Magic of Cluster SIMS

Abstract: Low topography, enhanced high-mass ion yields, and low damage cross sections have researchers thinking about new applications that may lead to the discovery of new biology.

Depth Profiling of Peptide Films with TOF-SIMS and a C60 Probe

Abstract: A buckminsterfullerene ion source is employed to characterize peptide-doped trehalose thin films. The experiments are designed to utilize the unique sputtering properties of cluster ion beams for molecular depth profiling. The results show that trehalose films with high uniformity can be prepared on Si by a spin-coating technique. Bombardment of the film with C60+ results in high quality time-of-flight secondary ion mass spectrometry spectra, even during ion doses of up to 3 × 1014 ions/cm2. This result is in contrast to atomic bombardment experiments in which the dose of incident ions must be kept below 1012 ions/cm2 so as to retain mass spectral information. Moreover, since the films are of uniform thickness, it is possible to depth-profile through the film and into the Si substrate. This experimental protocol allows the yield of trehalose molecular equivalents and the degree of interface mixing to be evaluated in detail. When doped with a variety of small peptides up to a molecular weight of m/z 500, w...

Microscopic insights into the sputtering of thin organic films on Ag{111} induced by C60 and Ga bombardment.

Abstract: Molecular dynamics computer simulations have been employed to model the bombardment of Ag{111} covered with three layers of C6H6 by 15 keV Ga and C60 projectiles. The study is aimed toward examining the mechanism by which molecules are desorbed from surfaces by energetic cluster ion beams and toward elucidating the differences between cluster bombardment and atom bombardment. The results show that the impact of the cluster on the benzene-covered surface leads to molecular desorption during the formation of a mesoscopic scale impact crater via a catapulting mechanism. Because of the high yield of C6H6 with both Ga and C60, the yield enhancement is observed to be consistent with related experimental observations. Specific energy and angle distributions are shown to be associated with the catapult mechanism.

Secondary ion MS imaging of lipids in picoliter vials with a buckminsterfullerene ion source.

Abstract: Investigation of the spatial distribution of lipids in cell membranes can lead to an improved understanding of the role of lipids in biological function and disease Time-of-flight secondary ion mass spectrometry is capable of molecule-specific imaging of biological molecules across single cells and has demonstrated potential for examining the functional segregation of lipids in cell membranes In this paper, standard SIMS spectra are analyzed for phosphatidylethanolamine, phosphatidylglycerol, phosphatidylserine, phosphatidylinositol, cholesterol, and sulfatide Importantly, each of the lipids result in signature mass spectral peaks that allow them to be identified These signature peaks are also useful for imaging experiments and are utilized here to simultaneously image lipids on a micrometer scale in picoliter vials Because the low secondary ion signal achieved for lipids from an atomic primary ion source makes cell-imaging experiments challenging, improving signal with cluster primary ion sources is of interest Here, we compare the secondary ion yield for seven lipids using atomic (Ga+ or In+) ion sources and a buckminsterfullerene (C60+) primary ion source A 40-1000-fold improvement in signal is found with C60+ relative to the other two ion sources, indicating great promise for future cellular imaging applications using the C60+ probe

High-resolution TOF-SIMS imaging of eukaryotic cells preserved in a trehalose matrix.

Abstract: A novel, trehalose-glycerol matrix was utilized to generate high-resolution, TOF-SIMS images of macrophages and glial cells. Viable cells incubated in 50 mM trehalose, then lyophilized in a 50 mM trehalose, 10−15% (w/w) glycerol rinse, are preserved and chemically profiled. These experiments demonstrate the utility of the disaccharide matrix as an efficient, cost-effective alternative to cryogenics for SIMS and other ultrahigh-vacuum (UHV) analyses of biological species. Cellular processes on oligodendrocytes and astrocytes, 1−3 μm in width, were well resolved for cells in the trehalose−glycerol matrix. The viscous cell matrixes were fractured and analyzed at room temperature and maintained their three-dimensional integrity under UHV. Images have been generated with a Au primary ion source near the static limit of 1012 ions/cm2. Though these nucleated cells do not remain viable after desiccation, TOF-SIMS imaging and subsequent rehydration reveals structural and morphological preservation. Eliminating the...

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

Abstract: 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.

Chemical pathways in the interactions of reactive metal atoms with organic surfaces: vapor deposition of Ca and Ti on a methoxy-terminated alkanethiolate monolayer on Au.

Abstract: In situ time-of-flight secondary ion mass spectrometry, infrared spectroscopy, and X-ray photoelectron spectroscopy measurements have been used to characterize the interfacial chemistry that occurs upon physical vapor deposition of Ti and Ca atoms onto a -OCH3 terminated alkanethiolate self-assembled monolayer (SAM) on Au{111}. While the final result for both metals is near-exhaustive degradation of the methoxy terminal group and partial degradation of the alkyl chains to inorganic products such as carbides, hydrides, and oxides, the reaction mechanisms differ significantly. Titanium reacts in parallel with the -OCH3 and -CH2- units, extensively degrading the latter until a metallic overlayer forms preventing further degradation. At this point, there is a cessation of the Ti -SAM reactions. In contrast, Ca is initially consumed by the -OCH3 terminal group via a reaction mechanism involving two -OCH3 groups; subsequent depositions lead to alkyl chain degradation, but at a rate slower than that for Ti deposition. These results demonstrate the subtle differences in chemistry that can arise in the vapor deposition of reactive metals, and have important implications for the behavior of electrical interfaces in organic and molecular devices made with Ti or Ca top

Modification and Stability of Aromatic Self-Assembled Monolayers upon Irradiation with Energetic Particles

Abstract: We have studied ion and electron irradiation of self-assembled monolayers (SAMs) of 2-(4‘-methyl-biphenyl-4yl)-ethanethiol (BP2, CH3−C6H4C6H4CH2CH2−SH), phenyl mercaptan (PEM, C6H5CH2CH2−SH), and 4‘-methyl-biphenyl-4-thiol (BP0, CH3−C6H4C6H4−SH) deposited on Au(111) substrates. Desorption of neutral particles from PEM/Au and BP2/Au was investigated using laser ionization in combination with mass spectrometry. The ion-induced damage of both BP2 and PEM SAMs is very efficient and interaction with a single ion leads to the modification of tens of molecules. This feature is the result of a desorption process caused by a chemical reaction initiated by an ion impact. Both for ions and electrons, experiments indicate that the possibility for scission of the Au−S bond strongly depends on the chemical nature of the SAM system. We attribute the possible origin of this effect to the orientation of the Au−S−C angle or adsorption sites of molecules. The analysis of electron-irradiated PEM/Au and BP2/Au, using ion-init...

Evolution of the interface and metal film morphology in the vapor deposition of Ti on hexadecanethiolate hydrocarbon monolayers on Au.

Abstract: The combination of in situ X-ray photoelectron spectroscopy, infrared reflection spectroscopy, atomic force microscopy, and time-of-flight secondary ion mass spectrometry are used to probe the nature of the evolving interface chemistry and metal morphology arising from Ti vapor deposition onto the surface of a CH3(CH2)15S/Au{111} self-assembled monolayer (SAM) at ambient temperature. The results show that for a deposition rate of ∼0.15 Ti atom·nm-2·s-1 a highly nonuniform Ti overlayer is produced via a process in which a large fraction of impinging Ti atoms do not stick to the bare SAM surface. The adsorbed atoms form isolated Ti clusters and react with CH3 groups to form carbide products at the cluster−SAM interfaces. Further growth of Ti clusters appears to be concentrated at these scattered reaction centers. The SAM molecules in the local vicinity are subsequently degraded to inorganic products, progressing deeper into the monolayer as the deposition proceeds to give an inorganic/organic nanocomposite....