Didier Léonard

XPS and ToF-SIMS study of freeze-dried and thermally cured melamine–formaldehyde resins of different molar ratios

The surface chemical characterization of melamine–formaldehyde (MF) resins by x-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) is examined in this study. Melamine–formaldehyde resins with different molar ratios of formaldehyde to melamine are synthesized and thermally cured. From XPS measurements, quantitative information is obtained and atomic chemical concentrations show the effect of the molar ratio for the freeze-dried resins. However, the thermally cured resins display a rather similar surface elemental composition. Moreover, because the binding energy values of the main N–C–N and N–C–O groups are too close, XPS does not help to identify changes in chemical structure after curing. The main ToF-SIMS negative and positive mass fragments of the MF resins are identified. Principal component analysis (PCA) is shown to be useful to determine and explain the main differences between all of the ToF-SIMS spectra. It allows us to distinguish the effect of the bulk chemical composition on the respective surface compositions of not only the uncured but also the cured MF resins. Moreover, extended interpretation leads to the identification of peaks characteristic of methylol groups and methylene ether bridges, but their absolute quantification is not straightforward. However, this result indicates that surface analysis helps to characterize the poorly defined chemical structure of MF resins. This is important for understanding the influence of surface chemistry on macroscopic surface properties such as chemical durability under exterior exposure. Copyright

Part 1. N-(m-(3-(trifluoromethyl)diazirine-3-yl)phenyl)-4-maleimido-butyramide (MAD) on silicon, silicon nitride and diamond†

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) and x-ray photoelectron spectroscopy (XPS) are used to characterize the grafting of the reagent N-(m-(3-(trifluoromethyl)diazirine-3-yl)phenyl)-4-maleimido-butyramide (MAD) to various substrates: silicon, silicon nitride and diamond. MAD carries a maleimide function for thermochemical modification of thiolated molecules and a diazirine function that is lost during light activation (350 nm light). Photoactivation leads to carbene-mediated grafting to solid supports. X-ray photoelectron spectroscopy atomic constituents and chemical shifts, as well as ToF-SIMS molecular peaks and characteristic fragments of the photoimmobilized molecule, are identified. Extended interpretation of surface analysis data suggests that diamond is the substrate with the highest MAD grafting efficiency and that the formation of C-O bonds upon diazirine photoactivation is involved. The difference in grafting extent for the three substrates leads to the conclusion that other reaction sites could be involved but they are not identified.

Part 2. N-[m-(3-(trifluoromethyl)diazirine-3-yl)phenyl]-4- (-3-thio(-1-D-galactopyrannosyl)-maleimidyl)butyramide (MAD-Gal) on diamond†

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) and x-ray photoelectron spectroscopy (XPS) are used to characterize a newly synthesized glycosylated photoactivatable reagent designed for surface glycoengineering. The glycoaryldiazirine reagent MAD-Gal (N-[m-(3-(trifluoromethyl)diazirine-3-yl)phenyl]-4-(-3-thio(-1-D- galactopyrannosyl)-maleimidyl)butyramide) is immobilized with 350 nm light on diamond surfaces. The XPS atomic constituents and chemical shifts, as well as ToF-SIMS-specific characteristic fragments of the photoimmobilized molecule, are identified. The grafting on oxygen-containing sites previously observed for N-(m-(3-(trifluoromethyl)diazirine-3-yl)phenyl)-4-maleimido-butyramide (MAD) is confirmed. The difference in grafting efficiency for fresh and aged substrates indicates that other still unidentified reaction sites could be involved. The ToF-SIMS imaging is used to exhibit the ability to pattern the surface with the glycosylated aryldiazirine.

Engineering and Characterization of Polymer Surfaces for Biomedical Applications

The application of synthetic polymers in the growing field of materials for medical applications is illustrated by examples from recent work at the Materials Institute of the Swiss Federal Institute of Technology in Lausanne. The review highlights the need for functionalization and chemical control of material surfaces at a molecular/functional level. After a brief introduction into the surface chemical analysis tools, i.e., X-ray Photoelectron Spectroscopy (XPS) and Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) combined with contact angle measurements, phosphorylcholine biomimicking polymers as well as immobilization of carbohydrates on polystyrene are presented.

Characterization of titanium hydride films covered by nanoscale evaporated Au layers: ToF-SIMS, XPS and AES depth profile analysis

Thin titanium hydride (TiHy) films, covered by ultrathin gold layers, have been compared with the corresponding titanium films after analysis using a combination of time-of-flight SIMS (ToF-SIMS), XPS and AES. The TiHy layers were prepared under UHV conditions by precisely controlled hydrogen sorption at 298 K on Ti film evaporated onto a glass substrate. Both Ti and TiHy films were then covered in situ by a nanoscale Au layer. Analyses were performed in separate systems after long-term exposure of the films to air. The thin gold layers covering the Ti and TiHy surfaces prevent any extensive air interaction with both films, allowing characterization of the bulk Ti and TiHy layers ex situ, even after a long-term application in air. The chemical nature of the TiHy layers has been analysed after sputtering of the Au top layer. The high-mass-resolution positive-ion ToF-SIMS spectra disclosed only one peak at mass 49 (49Ti+) for the Ti and two peaks at mass 49 (49Ti+ and 48TiH+) for the TiHy film, reflecting a difference in hydrogen concentration. Analysis of the features of the Ti Auger spectra during the sputter profile measurements allows the TiHy to be distinguished and characterized in the bulk region of the Au/TiHy layer. Besides TiHy, TiO and TiOH were detected by XPS to be the main chemical compounds in the interface region of the Au/TiHy film.

Round robin of time-of-flight secondary ion mass spectrometry damage studies of a photoimmobilized reagent on diamond surfaces designed for surface glycoengineering

Abstract Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) has previously been successfully applied to characterize the covalent grafting of N-(m-(3-(trifluoromethyl) diazirine-3-yl)phenyl)-4-maleimido-butyramide (MAD), a reagent used for immobilization of biomolecules on solid surfaces, i.e., diamond substrates. In this study, the molecule was used to compare dose-related damage data obtained on two different ToF-SIMS systems, a PHI Trift and a PHI-7200 Reflectron, after shipping identically prepared samples to two independent laboratories. The ToF-SIMS spectrum in the negative ion mode exhibits characteristic signals over a large mass range, allowing to compare spectral differences in data acquired in the static SIMS regime with the two spectrometers to differences in experimental parameters, such as detector voltages. Variations in ion beam damage were not fully reproduced by both ToF-SIMS systems, indicating limitations in the comparison of ion beam damage data using two different sets of experimental parameters. This discrepancy could be related to different mass-dependent ion transmissions of the two spectrometers. The results suggest that a model developed in the literature for the interpretation of ion dose-induced polymer damage can be successfully applied to the case of organic molecules covalently attached to a diamond surface.

Orthogonal chemical functionalization of patterned gold on silica surfaces.

Summary Single-step orthogonal chemical functionalization procedures have been developed with patterned gold on silica surfaces. Different combinations of a silane and a thiol were simultaneously deposited on a gold/silica heterogeneous substrate. The orthogonality of the functionalization (i.e., selective grafting of the thiol on the gold areas and the silane on the silica) was demonstrated by X-ray photoelectron spectroscopy (XPS) as well as time-of-flight secondary ion mass spectrometry (ToF–SIMS) mapping. The orthogonal functionalization was used to immobilize proteins onto gold nanostructures on a silica substrate, as demonstrated by atomic force microscopy (AFM). These results are especially promising in the development of future biosensors where the selective anchoring of target molecules onto nanostructured transducers (e.g., nanoplasmonic biosensors) is a major challenge.