Interface Chemistry and Molecular Bonding of Functional Ethoxysilane-Based Self-Assembled Monolayers on Magnesium Surfaces

Abstract

The modification of magnesium implants with functional organic molecules is important for increasing the biological acceptance and for reducing the corrosion rate of the implant. In this work, we evaluated by a combined experimental and theoretical approach the adsorption strength and geometry of a functional self-assembled monolayer (SAM) of hydrolyzed (3-aminopropyl)triethoxysilane (APTES) molecules on a model magnesium implant surface. In time-of-flight secondary ion mass spectrometry (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS), only a minor amount of reverse attachment was observed. Substrate-O-Si signals could be detected, as well as other characteristic APTES fragments. The stability of the SAM upon heating in UHV was investigated additionally. Density-functional theory (DFT) calculations were used to explore the preferred binding mode and the most favorable binding configuration of the hydrolyzed APTES molecules on the hydroxylated magnesium substrate. Attachment of the molecules via hydrogen bonding or covalent bond formation via single or multiple condensation reactions were considered. The impact of the experimental conditions and the water concentration in the solvent on the thermodynamic stability of possible APTES binding modes is analyzed as a function of the water chemical potential of the environment. Finally, the influence of van der Waals contributions to the adsorption energy will be discussed.

Metal–Phosphate Bilayers for Anatase Surface Modification

“…HT leads to a slight increase in dye loading, which may be related to desorption of contaminants or induction of reactions between previously physisorbed species. 33 Results demonstrated that up to 65% more dye may be incorporated on the surface through the formation of phosphate−metal oxide bilayers on anatase. No adverse effects of the bilayers on the photoelectronic properties of anatase TiO 2 were observed by incident photon to current efficiency (IPCE) analysis 40 (see Figures S4 and S5).…”

“…HT leads to a slight increase in dye loading, which may be related to desorption of contaminants or induction of reactions between previously physisorbed species . Results demonstrated that up to 65% more dye may be incorporated on the surface through the formation of phosphate–metal oxide bilayers on anatase.…”

“…Heat-treating the samples below the anatase to rutile transition temperature (after the bilayer was already formed) was performed to improve the binding of nonreacted, physisorbed material. 33 Samples were heat-treated in air for different periods of time before being immersed in SA solution. According to the contact angle values, comparing the effect of 1, 2, and 3 h HT, 2 h was found to be sufficient for heat-treating the samples, as no further improvement in the amount of adsorbed molecules was observed after long exposure.…”

“…HT had a positive influence on the cell performance. A stronger bound dye, for example, by heat-induced bond formation of physisorbed or weakly bond adsorbates, 33 may facilitate charge injection into the electrode. Efficiency values increased from 0.035% for the nonmodified electrode up to 0.096% for the heat-treated cobalt-modified TiO 2 .…”

“…The metal oxide bilayers lead to a larger number of SA molecules bound to the surface, indicated by an increase in the contact angle from 52.33° for the nonmodified anatase to 100.21° and 109.93° for samples modified with Ni and Co, respectively. Heat-treating the samples below the anatase to rutile transition temperature (after the bilayer was already formed) was performed to improve the binding of nonreacted, physisorbed material . Samples were heat-treated in air for different periods of time before being immersed in SA solution.…”