Probing Heterogeneity and Bonding at Silica Surfaces through Single-Molecule Investigation of Base-Mediated Linkage Failure

Abstract

The nature of silica surfaces is relevant to many chemical systems, including heterogeneous catalysis and chromatographies utilizing functionalized-silica stationary phases. Surface linkages must be robust to achieve wide and reliable applicability. However, silyl ether-silica support linkages are known to be susceptible to detachment when exposed to basic conditions. We use single-molecule spectroscopy to examine the rate of surface linkage failure upon exposure to base at a variety of deposition conditions. Kinetic analysis elucidates the role of thermal annealing and addition of blocking layers in increasing stability. Critically, it was found that successful surface modification strategies alter the rate at which base molecules approach the silica surface as opposed to reducing surface linkage reactivity. Our results also demonstrate that the innate structural diversity of the silica surface is likely the cause of observed heterogeneity in surface-linkage disruption kinetics.

Surface Preparation for Single-Molecule Chemistry

“…Furthermore, the silanes trimethoxy­(2-phenylethyl)­silane and 1 H ,1 H ,2 H ,2 H -perfluorooctyltriethoxy-silane were chosen to introduce a broader range of physical properties of used silane layers. In the beginning, an immobilization routine was executed, wherein a mixture of a dye and inert silane ,,,− is deposited on a silica support in one step (A). Afterward, we implemented a consecutive immobilization scheme (B).…”

“…In the consecutive routine B, only the inert silane was initially deposited on the silica surface. It has been shown that washing of modified silica with polar solvents provides vacant sites. , These sites offer the possibility to be refilled by new silane molecules. Rinsing with acetone after silane deposition proved to be sufficient for generating vacant sites for subsequent immobilization of reactive dye molecules.…”

“…Well-defined silane layers can be generated via liquid- or gas-phase deposition. , The more common liquid deposition is performed with various solvents and silane concentrations. − Here, the layer formation can be subdivided into two major, subsequent processes: first, the fast binding of silane molecules to the surface until surface coverage is reached, and afterward a rather slow reorientation of the organic residues to align them into the vertical axis takes place . The solvent plays an important role, as it controls the stabilization of the silane molecules diffusing and binding to the solid–liquid interface .…”

“…As a consequence, exclusive formation of an ordered monolayer is hard to achieve without strict control of water concentration. One can easily imagine that the amorphous glass surface in combination with possible silane clusters, formed during the immobilization process, leads to heterogeneous microenvironments, which can affect the reactivity of immobilized chemical species . Therefore, maybe the greatest challenge is to establish a surface preparation method that ensures reaction conditions on the surface are similar to those in solution, although silane layers may influence the reactivity of immobilized substrates as well …”

Optical Microresonators for Sensing and Transduction: A Materials Perspective

“…For example, many of the attachment chemistries described above rely on formation of silyl ether linkages that are somewhat labile in water and highly labile in basic solution. [175] Another challenge is targeted surface functionalization, a particularly important goal for multiplexing multiple on-chip resonators. Armani and co-workers have characterized different approaches to functionalization of toroid microresonators with biotin and shown that vapor deposition after plasma etching is the most successful.…”