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Organisationseinheit der BAM
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.
Experimental N K-edge NEXAFS data of surface immobilized azide, 1,2,3-triazole and terpyridyl groups are interpreted with the help of DFT spectrum simulations. Assignments of π* resonances in experimental N K-edge NEXAFS spectra to nitrogen atoms within these functional groups have been made. The azide was immobilized on gold as the head group of a thiol SAM, 1,2,3-triazole was formed on this SAM by click reaction and terpyridyl groups were introduced as substituents of the acetylene used for the click reaction. For azide-terminated molecules, DFT spectrum simulations are found to be useful to find measurement conditions delivering experimental N K-edge NEXAFS data with negligible X-ray damage. The 1,2,3-triazole group is found to be rather stable under X-ray irradiation.
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.
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.
We investigated the ageing of amine-terminated
self-assembled monolayers (amine-SAMs) on different silica
substrates due to exposure to different ambient gases,
pressures, and/or temperatures using time-of-flight secondary
ion mass spectrometry (ToF-SIMS) with principal component
analysis and complementary methods of surface
analysis as X-ray photoelectron spectroscopy (XPS) and
near edge X-ray absorption fine structure (NEXAFS). The
goal of this study is to examine the durability of primary
amine groups of amine-SAMs stored in a user laboratory
prior to being used as supports for biomolecule immobilization
and other applications. We prepared amine-SAMs on
the native oxides of silicon wafers and glass slides using 3-
aminopropyl triethoxysilane, by using optimized conditions
such as anhydrous organic solvent and reaction time scale of
hours to avoid multilayer growth. Selected commercial
amine-SAM slides have been investigated, too. When the
amine-SAMs are exposed to air, oxygen incorporation
occurs, followed by formation of amide groups. The formation
of oxygen species due to ageing was proved by ToFSIMS,
XPS, and NEXAFS findings such as CNO- secondary
ion emission at m/z 42, observation of the N 1s HNC=O
component peak at 400.2400.3 eV in XPS, and, last but not
least, by formation of a π*(HNC=O) resonance at 401 eV in
the N K-edge X-ray absorption spectrum. It is concluded
that the used multi-method approach comprising complementary
ToF-SIMS, XPS, and NEXAFS analyses is well
suited for a thorough study of chemical aspects of ageing
phenomena of amine-SAM surfaces.
Research on carbohydrate based interactions with proteins, nucleic acids or antibodies has gained increased interest in the last years especially in clinical diagnosis or drug development. The efficiency of diagnostic interfaces depends upon the number of probe molecules, e.g. carbohydrates. The control of surface parameters as density and distribution of immobilized carbohydrates is essential for a reliable interaction with protein analytes. A controlled production of biomolecular interfaces can be reached by a stepwise quality control during buildup of these biointerfaces.
Here, ß-amino-cyclodextrin molecules were attached to amine-reactive silicon oxide surfaces via click chemistry to construct a model biosensor surface. The amount of surface bound carbohydrates was determined indirectly after chemical derivatization with 4-(trifluoromethyl)-benzylamine (TFMBA). Moreover, these surfaces were used to form host-guest complexes of ferrocene (guest) and β-cyclodextrin (host) moieties to mimic the target binding (sensing) of the model biosensor. Surface chemical analysis of all steps during biosensor construction was performed using X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure spectroscopy (NEXAFS). Our approach widens the possibilities to generate switchable surfaces based on ß-Cyclodextrin surfaces for biosensor applications.
In this study, a new direct functionalization method of silicon nitride (Si3N4) using azidation and click chemistry is presented. First, amino groups (NHx) were created on a Si3N4 substrate by fluoride etching. These NHx-terminated Si3N4 surfaces were analyzed by chemical derivatization X-ray photoelectron spectroscopy (CD-XPS) with 4-trifluoromethylbenzaldehyde (TFBA) and a derivatization yield of 20% was concluded. In the second step freshly prepared NHx surfaces were transformed into azides which were used immediately in a click reaction with halogenated alkynes. The presented combination of amination, azidation and click reaction is a promising alternative for common silane-based Si3N4 functionalization methods.
We report a surface analytical study of poly(methyl methacrylate) (PMMA) microparticles (beads) with a grafted shell of poly(acrylic acid) (PAA) with thicknesses up to 4 nm using scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), and near-edge X-ray adsorption fine structure (NEXAFS) spectroscopy. These polymer microparticles were analyzed before and after reaction of the surface carboxyl (CO2H) groups with 2,2,2-trifluoroethylamine (TFEA) to gain a better understanding of methods with use of covalently bound probe molecules for surface group analysis. The results obtained with chemical derivatization XPS using TFEA are discussed in terms of surface quantification of reactive CO2H groups on these PAA-coated microparticles. A labeling yield of about 50% was found for TFEA-derivatized particles with amounts of surface-grafted CO2H groups of 99 µmol/g or more, which is consistent with predicted reaction yields for homogeneously dispersed PAA hydrogels.
In order to investigate molecular recognition on surfaces, an azide-functionalized monolayer was deposited on gold. The monolayer was characterized by X-ray photoelectron spectroscopy (XPS) and angle-resolved near-edge X-ray absorption fine structure (NEXAFS) experiments and the decomposition of the azide upon irradiation with X-ray beams was investigated. Subsequently, various alkyne-functionalized host and guest molecules were attached to the azide by 1,3-dipolar cycloaddition. These modified surfaces and their host–guest chemistry were analysed by XPS and angle-resolved NEXAFS. The reversibility of guest binding was shown for one example as a proof of principle.
In this study, epoxy-terminated silicon oxide surfaces were chemically derivatized with trifluoroacetic anhydride (TFAA) and 4-(trifluoromethyl)-benzylamine (TFMBA) and analyzed by X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy and water contact angle measurements. TFAA was used for quantitative derivatization to determine the amount of reactive epoxy groups on these surfaces. Furthermore, epoxy-terminated surfaces were derivatized with TFMBA (a model compound for biomolecules with an amino linker) yielding secondary amines because of formation of covalent C–N bonds between TFMBA and the epoxy films. Fluorine being part of TFAA and TFMBA composition was used to follow the progress of the chemical derivatization reaction. Both derivatization agents – TFAA and TFMBA – gave comparable reaction yields of ~60% on epoxy silicon surfaces, whereas on epoxy glass slides, the derivatization yields were considerably lower (30–40%).The protocol for attachment of TFMBA (model compound) on epoxy-modified surfaces was adapted to (bio)molecules with an amino linker, e.g. carbohydrates to prepare carbohydrate-functionalized biointerfaces.