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N Winograd

Publications and source records attributed to N Winograd.

At least 19 recordsLinked to original sources
PubMed2005

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.

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 -OCH(3) 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 -OCH(3) and -CH(2)- 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 -OCH(3) terminal group via a reaction mechanism involving two -OCH(3) 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 contacts.

Journal Article↗
PubMed2005

Modification and stability of aromatic self-assembled monolayers upon irradiation with energetic particles.

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-initiated laser probing, enabled measurements of the cross section for the electron-induced damage of the intact molecule or specific fragment. Analysis of electron-irradiated BP0/Au by using time-of-flight secondary ion mass spectrometry (TOF-SIMS) provides direct evidence for the quasi-polymerization process induced by electron irradiation.

Journal Article↗
PubMed2000

Quantitative chemical analysis of single cells.

A fundamental perspective can be achieved by targeting single cells for analysis with the goal of deconvoluting complex biological functions. However, single-cell studies have their own difficulties, such as minute volumes and sample amounts. Quantitative chemical analysis of single cells has emerged as a powerful new area in recent years due to several technological advancements. The development of microelectrodes has allowed the measurement of redox-active species as a function of cellular dynamics. This miniaturization trend is also evident in the separation sciences with the application of small column separations to single cells. Desorption ionization methods with mass spectrometric detection have shown single-cell capability owing to numerous technological developments. Finally, fluorescence imaging has also progressed to the point where single-cell dynamics can be probed by native fluorescence utilizing either single or multiple photon excitation. The results of these studies are reviewed with an emphasis on the quantitation of single-cell dynamics.

Animals↗
PubMed1999

Spatially resolved detection of attomole quantities of organic molecules localized in picoliter vials using time-of-flight secondary ion mass spectrometry.

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) has been utilized to detect femtomole and attomole quantities of organic species from within silicon nanovials. By using high-density arrays (10,000 nanovials/cm2) it is possible to chemically characterize diverse sample sets within a single chemical image. Molecular sensitivities, for the compounds investigated, very between 85 attomoles and 25 femtomoles, and typical acquisition times are approximately 100 ms per nanovial. These vials are fabricated using photolithography and KOH etching of Si[001] wafers to create wells, with a pyramidal cross section, ranging in size from 25 to 5625 micron 2. The volume ranges from 30 femtoliters to 100 picoliters, respectively. A drawn glass microinjector and solenoid-driven dispenser are utilized to array picoliter volumes of organic compounds into individual silicon nanovials. Solution concentrations typically range from 1 x 10(-2) to 1 x 10(-4) M allowing femtomole and even attomole quantities of material to be dispensed into each vial.

Microchemistry↗
PubMed1999

Photoionization of gas-phase versus ion-beam-desorbed dopamine with femtosecond laser pulses.

We have investigated the photoionization of gas-phase and ion-beam desorbed dopamine using femtosecond laser pulses at wavelengths of 800, 400, 267, and 200 nm. Photoionization of gas-phase dopamine is found to produce the molecular ion, and three fragment ions at all four wavelengths, with the branching ratios strongly wavelength dependent. Photoionization at 400 and 267 nm yields the highest molecular ion signal, while that at 800 and 200 nm produces very little molecular ion signal. An excited-state lifetime of approximately 10 ps following 267-nm excitation has been measured for dopamine using time-resolved pump-probe techniques. The short-lived excited state suggests that internal conversion, intersystem crossing, and/or dissociation is a concern when ionizing at this wavelength using longer laser pulses. Photoionization of ion-beam-desorbed dopamine exhibits a large degree of fragmentation at all four wavelengths, though 267-nm photoionization produces the highest yield of dopamine fragment ions. Power dependence studies show a high degree of internal excitation. A direct comparison of ion yields obtained for photoionization of ion-beam-desorbed dopamine at 267 nm to that for SIMS shows a 20-fold increase in signal.

Chemical Phenomena↗
PubMed1998

Ejection of neutral molecules from ion-bombarded organic surfaces.

Time-of-flight distributions of neutral molecules ejected from various organic surfaces have been measured subsequent to 8 keV Ar+ and H2+ ion bombardment. The distributions show that depending on the physical and chemical nature of the substrate, the neutral molecules have strikingly different desorption profiles. For C6H6/Ag¿111¿, at low coverage the C6H6 molecules eject with energies in the range 0.25-1 eV while at high coverage most of the molecules desorb with thermal kinetic energies (approximately 0.04 eV). At intermediate coverage two peaks are present in the time-of-flight distribution indicating that two different mechanisms contribute to the desorption process. For self-assembled monolayers of phenylethanethiol on Au, while a minor ejection is observed at higher kinetic energy (approximately 1 eV) most of the molecules desorb with thermal kinetic energies (approximately 0.03 eV). Pyrenebutyric acid molecules ejected from monolayer and multilayer samples have kinetic energies close to 0.2 eV. One ejection mechanism is observed in this case. For tryptophan, most molecules eject with kinetic energies close to 0.1 eV. In addition, a feature unique to this case is the continuous emission of molecules from the surface that extends beyond 200 microseconds after ion impact. For all the multilayer samples investigated, a molecular collision cascade in the solid leads to ejection of molecules with kinetic energies in the range 0.1-0.3 eV.

Energy Transfer↗
PubMed1998

Static time-of-flight secondary ion mass spectrometry imaging of freeze-fractured, frozen-hydrated biological membranes.

The study of cell membrane lipid and steroid composition and distribution is important for the understanding of membrane dynamics and function. Here we present efforts to chemically image phospholipid distributions on a submicron scale on freeze-fractured and frozen-hydrated liposomes and red blood cells using time-of-flight secondary ion mass spectrometry. Sample preparation by freeze fracturing of membranes is described. Fragments representative of phospholipid headgroups are found to be localized on both liposomes and red blood cells. In addition, the cholesterol molecular ion [M + H] is localized on liposome surfaces.

Diagnostic Imaging↗
PubMed1998

Vacuum ultraviolet single photon versus femtosecond multiphoton ionization of sputtered germanium clusters.

Neutral atoms and clusters desorbed from a solid germanium surface by ion bombardment are detected by laser postionization and time-of-flight mass spectrometry. Two different photoionization schemes are compared which are generally believed to be candidates for the 'soft' ionization of polyatomic species without significant photon induced fragmentation. First, a single photon ionization process is employed using an F2 laser as an intense VUV source with a photon energy in excess of all relevant ionization potentials. It is shown that the available laser pulse energy is sufficient to saturate the ionization of Ge atoms and all detected Ge(n) clusters. The resulting mass spectra are compared to those obtained with a non-resonant multiphoton ionization process using a high intensity laser delivering pulses of 250 femtoseconds duration at a wavelength of 267 nm. Also in this case, the ionization process can apparently be driven into saturation. The mass spectra measured under these conditions are found to be almost identical to those obtained using single photon ionization. We take this as an indication that the results obtained with both postionization techniques closely reflect the true cluster sputtering yields and, in particular, are not dominated by photon induced fragmentation.

Absorptiometry, Photon↗
PubMed1998

Performance characteristics of a chemical imaging time-of-flight mass spectrometer.

A chemical imaging time-of-flight secondary ion mass spectrometer is described. It consists of a liquid metal ion gun, medium energy resolution reflectron mass analyzer, liquid nitrogen cooled sample stage, preparation chamber and dual stage entry port. Unique features include compatibility with laser postionization experiments, large field of view, cryogenic sample handling capability and high incident ion beam current. Instrument performance is illustrated by the characterization of scanning electron microscopy grids, silver and functionalized polystyrene beads and the postionization of an organic overlayer on a gold substrate.

Diagnostic Imaging↗
PubMed1998

Postionization of molecules desorbed from surfaces by keV Ion bombardment with femtosecond laser pulses.

We report the use of femtosecond laser photoionization of sputtered neutral molecules to enhance the sensitivity of detection and to improve the prospects for molecule-specific imaging experiments. Results are presented for patterned metal oxides, polycyclic aromatic hydrocarbons and several amino acids. In addition to increased signal levels, we find that is photoionization generally yields simpler mass spectra than the corresponding SIMS spectra, although considerable fragmentation is observed in both cases.

Amino Acids↗
PubMed1997

Atomic and molecular imaging at the single-cell level with TOF-SIMS.

A complete cold chain freeze-fracture methodology has been developed to test the feasibility of using time-of-flight secondary ion mass spectrometry (TOF-SIMS) imaging for the molecular analysis of frozen hydrated biological samples. Because the technique only samples the first few monolayers of a sample, water on the surface of a sample can be a major source of interference. This problem can be minimized by placing a cold trap (fracture knife and housing at -196 degrees C) near the fractured sample that is held at a warmer temperature (-97 to -113 degrees C). This results in removal of surface water and prevents condensation on the surface. Although this approach is effective, it has been found that sample warming needs to be carefully controlled due to the volatility of other matrix molecules and the morphological effects imparted onto the cell surface during drying. By utilizing the above handling technique, it has been possible to demonstrate for the first time that TOF-SIMS imaging technology can be used to obtain images of molecular species across a cell surface with a submicrometer ion probe beam. Images of small hydrocarbons and the deliberately added dopants DMSO and cocaine have been obtained with TOF-SIMS of the single-cell organism Paramecium.

Animals↗
PubMed1996

Evaluation of mass spectrometric methods applicable to the direct analysis of non-peptide bead-bound combinatorial libraries.

Electrospray, matrix-assisted laser desorption, and time-of-flight secondary ion mass spectrometry have been explored as possible methods for the identification of active members of molecular combinatorial libraries. All three methods are found to yield accurate molecular weight information about a target molecule angiotensin II antagonist synthesized on a 40-microns polystyrene bead. Structural identification is also possible by accurate mass measurements to eliminate candidate structures with the same nominal mass and by analysis of the fragmentation patterns. In addition, the secondary ion mass spectrometry measurements yield spatially resolved spectra from a single bead after exposure to a suitable gas which clips the covalent bond at the linking position. All three approaches appear to offer a viable screening strategy of non-peptide libraries without the use of additional molecular tags.

Angiotensin II↗