title: Quantification of variable functional-group densities of mixed-silane monolayers on surfaces via a dual-mode fluorescence and XPS label.
abstract: The preparation of aminated monolayers with a controlled density of functional groups on silica surfaces through a simple vapor deposition process employing different ratios of two suitable monoalkoxysilanes, (3-aminopropyl)diisopropylethoxysilane (APDIPES) and (3-cyanopropyl)dimethylmethoxysilane (CPDMMS), and advances in the reliable quantification of such tailored surfaces are presented here. The one-step codeposition process was carried out with binary silane mixtures, rendering possible the control over a wide range of densities in a single step. In particular, APDIPES constitutes the functional silane and CPDMMS the inert component. The procedure requires only small amounts of silanes, several ratios can be produced in a single batch, the deposition can be carried out within a few hours and a dry atmosphere can easily be employed, limiting self-condensation of the silanes. Characterization of the ratio of silanes actually bound to the surface can then be performed in a facile manner through contact angle measurements using the Cassie equation. The reliable estimation of the number of surface functional groups was approached with a dual-mode BODIPY-type fluorescence label, which allows quantification by fluorescence and XPS on one and the same sample. We found that fluorescence and XPS signals correlate over at least 1 order of magnitude, allowing for a direct linking of quantitative fluorescence analysis to XPS quantification. Employment of synchrotron-based methods (XPS; reference-free total reflection X-ray fluorescence, TXRF) made the traceable quantification of surface functional groups possible, providing an absolute reference for quantitative fluorescence measurements through a traceable measurement chain.
authors.affiliation: Division 1.9 Chemical and Optical Sensing, BAM Federal Institute for Materials Research and Testing , Richard-Willstätter-Strasse 11, 12489 Berlin, Germany.
authors.forename: Tobias
authors.initials: T
authors.lastname: Fischer
authors.forename: Paul M
authors.initials: PM
authors.lastname: Dietrich
authors.forename: Cornelia
authors.initials: C
authors.lastname: Streeck
authors.forename: Santanu
authors.initials: S
authors.lastname: Ray
authors.forename: Andreas
authors.initials: A
authors.lastname: Nutsch
authors.forename: Alex
authors.initials: A
authors.lastname: Shard
authors.forename: Burkhard
authors.initials: B
authors.lastname: Beckhoff
authors.forename: Wolfgang E S
authors.initials: WE
authors.lastname: Unger
authors.forename: Knut
authors.initials: K
authors.lastname: Rurack
mesh_terms: D003432:Cross-Linking Reagents / Q000737:chemistry*; D005453:Fluorescence; D018625:Microscopy, Atomic Force; D056951:Photoelectron Spectroscopy / Q000379:methods*; D012821:Silanes / Q000032:analysis* / Q000737:chemistry*; D013499:Surface Properties
chemical_list: D003432:Cross-Linking Reagents; D012821:Silanes