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Recently, time-of-flight secondary ion mass spectrometry (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS) instrumentation has been used to address areas of interest within micro-fluidic devices providing full access to the surface chemistry established at the bottom of micro-channels therein. After careful calibration, information on surface chemistry as obtained by ToF-SIMS or XPS can be interpreted in terms of wettability expressed as contact angles which are then characteristic for the inner walls of micro-channels. Standard contact angle measurement is not applicable in micro-channels. The approach has been demonstrated to be successful with two different micro-fluidic devices hot embossed into high-end quality poly(methyl methacrylate) (PMMA) or Polycarbonate wafers. A pre-selected surface chemistry at micro-channel walls can be established by plasma technologies but ageing and rinsing effects have to be under control. A combination of ToF-SIMS, XPS and contact angle measurement techniques has been demonstrated to provide the required information. Finally, it is shown by ToF-SIMS and XPS analysis that in the production of micro-fluidic parts during practical processing using hot embossing technologies, material originating from cover foils will reside on the polymer wafer's surface. Moreover, residues of releasing agents as silicone oil used during processing can be detected by ToF-SIMS. Both cover foil residues and silicones are issues of trouble shooting in micro-fluidics because they will change contact angles efficiently.
A novel coordination oligo/polymer is synthesized by metal-directed self-assembly from equimolar amounts of the (dppp)M(OTf)2 precursor complexes (dppp = bis-(diphenylphosphino)-propane, OTf = triflate; M = PdII or M = PtII) and banana-shaped bidentate dipyridyl ligands. The assemblies were characterized by ESI mass spectrometry and NMR spectroscopy. The analysis of the cloudy suspension prepared by dissolving the coordination polymer in aqueous methanol solutions indicates nanosized spherical objects to form. Evidence for vesicle formation from these metallo-supramolecular oligomers comes from (cryogenic) transmission electron microscopy (TEM, cryo-TEM). Atomic force microscopy revealed stable nanospheres on hydrophilic mica and monolayer formation on hydrophobic highly oriented pyrolitic graphite (HOPG) substrates. On mica, also torus-shaped object were observed, which are rationalized by vesicles that opened during the drying procedure and released the internal solvent. Elemental analysis of the nanoassemblies by X-ray photoelectron spectroscopy (XPS) indicates uncoordinated and coordinated pyridines in the coordination polymers that form the nanospheres. Various control experiments using different metal centers and modified ligands support the conclusions.
The wettability of the surfaces inside the microchannels of a microfluidic device is an important property considering a liquid flows through them. Contact angle measurements usually applied to test the wettability of surfaces cannot be used for an analysis of microchannel walls within microfluidic devices. A workaround is the use of surface analytical methods, which are able to reach points of interest in microchannels and may provide information on the surface chemistry established there. In calibrating these methods by using flat polymer wafers, where the contact angle can be measured as usual, data measured in real microchannels can be evaluated in terms of wetting properties. Reference wafers of bisphenol-A polycarbonate, a polymeric material that is often used in fluidic microdevice fabrication, were treated under different oxygen plasma conditions. The modified surfaces were characterized by using XPS, time of flight (ToF)-SIMS and atomic force microscope (AFM). Surface chemistry and surface topography have been correlated with contact angle measurements. In addition, effects of ageing or rinsing after plasma treatment have also been investigated.
Functionalised surfaces are of interest in many fields, e.g. in biomedicine, materials science and molecular electronics. In this study a series of self-assembled aliphatic and aromatic monolayers on gold substrates with terminal amino groups was investigated. Four different thiol molecules were used: aliphatic 11-aminoundecane-1-thiol (AUDT), aromatic 4-aminobenzenethiol (ABT) and aromatic ω-amino thiols with an alkyl spacer as 4-aminophenylbutane-1-thiol (APBT) and 3-(4''-amino-1,1':4',1''-terphenyl-4-yl)propane-1-thiol (ATPT). Evaluation of N 1s XPS data revealed that on the aromatic self-assembled monolayers (SAMs) amino groups exist preferentially as primary amines, whereas on the aliphatic SAM protonated and/or hydrogen-bonded amines are the major species. This result is crosschecked by N K edge near edge X-ray absorption fine structure (NEXAFS) spectroscopy and can be rationalised by the different basicity of aliphatic and aromatic amines.
Self-assembled monolayers of aromatic omega-aminothiols on gold: surface chemistry and reactivity
(2010)
Amino-terminated self-assembled monolayers on gold substrates were studied by X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS) measurements, and atomic force microscopy (AFM). Two different ω-amino-4,4'-terphenyl substituted alkanethiols of the general structure H2N-(C6H4)3-(CH2)n-SH (ATPn) were used: 2-(4''-amino-1,1':4',1''-terphenyl-4-yl)ethane-1-thiol (n = 2, ATP2) and 3-(4''-amino-1,1':4',1''-terphenyl-4-yl)propane-1-thiol (n = 3, ATP3). Moreover, the addressability of amino groups within the films was investigated by chemical derivatization of ATPn SAMs with 3,5-bis(trifluoromethyl)phenyl isothiocyanate (ITC) forming fluorinated thiourea ATPn-F films. Evaluation of high-resolution C 1s and N 1s XPS data revealed successful derivatization of at least 50% of surface amino species. Furthermore, it could be demonstrated by angle-resolved NEXAFS spectroscopy that chemical derivatization with ITC has no noticeable influence on the preferential upright orientation of the molecules in the SAMs.
A combined XPS, NEXAFS, and ToF-SIMS chemical surface characterization of carbohydrate-functionalized gold and glass surfaces is presented. Spot shape and chemical composition across a spot surface are provided by surface-sensitive methods as ToF-SIMS and XPS, used in their imaging modes. Moreover, the feasibility of this multimethod approach to control relevant production steps of a carbohydrate microarray prototype is demonstrated.
Carbohydrate films on gold based on dimannoside thiols (DMT) were prepared, and a complementary surface chemical analysis was performed in detail by X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), near-edge X-ray absorption fine structure (NEXAFS), FT-IR, and contact angle measurements in order to verify formation of ω-carbohydrate-functionalized alkylthiol films. XPS (C 1s, O 1s, and S 2p) reveals information on carbohydrate specific alkoxy (C–O) and acetal moieties (O–C–O) as well as thiolate species attached to gold. Angle-resolved synchrotron XPS was used for chemical speciation at ultimate surface sensitivity. Angle-resolved XPS analysis suggests the presence of an excess top layer composed of unbound sulfur components combined with alkyl moieties. Further support for DMT attachment on Au is given by ToF-SIMS and FT-IR analysis. Carbon and oxygen K-edge NEXAFS spectra were interpreted by applying the building block model supported by comparison to data of 1-undecanethiol, poly(vinyl alcohol), and polyoxymethylene. No linear dichroism effect was observed in the angle-resolved C K-edge NEXAFS.
Microarrays are a versatile platform for diagnostics and high-throughput analysis. Carbohydrate microarrays are valuable tools to investigate interactions with other molecules since many glycans are involved in fundamental biological processes. A combined X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) surface analysis was used to investigate the basic steps in the production of carbohydrate microarrays. The preparation included coupling of a thiol-terminated mannoside to maleimide-functionalized glass surfaces derived from γ-aminopropyl silane (GAPS) slides. XPS results clearly demonstrate successful chemical modification in each fabrication step, and ToF-SIMS imaging revealed immobilized carbohydrates in the spotted regions of the final microarray.
An electron effective attenuation length (EAL) of 1.68nm for Al Kα excited Pd 3d 5/2 photoelectrons with a kinetic energy of 1.152 keV has been determined experimentally using a sputtered Pd film deposited on an ultra flat fused quartz substrate. The film thickness was reduced by Ar ion sputtering several times in order to obtain different Pd film thicknesses which are used to determine experimental EAL values. These results are compared to data generated by using a Simulation of Electron Spectra for Surface Analysis (SESSA) simulation using an inelastic mean free path (IMFP) calculated with the Tanuma–Powell–Penn (TPP)-2M formula and with ‘elastic scattering on and off’. Contributions to the uncertainty budget related to the experimental approach are discussed in detail. Proposals on how to further improve the approach are suggested.