Cooperative Effects in Aligned and Opposed Multicomponent Charge Gradients Containing Strongly Acidic, Weakly Acidic, and Basic Functional Groups

Abstract

Bifunctionalized surface charge gradients in which the individual component gradients either align with or oppose each other have been prepared. The multicomponent gradients contain strongly acidic, weakly acidic, and basic functionalities that cooperatively interact to define surface wettability, nanoparticle binding, and surface charge. The two-step process for gradient formation begins by modifying a siloxane coated silicon wafer in a spatially dependent fashion first with an aminoalkoxysilane and then with a mercapto-functionalized alkoxysilane. Immersion in hydrogen peroxide leads to oxidation of the surface immobilized sulfhydryl groups and subsequent protonation of the surface immobilized amines. Very different surface chemistries were obtained from gradients that either align with or oppose each other. X-ray photoelectron spectroscopy (XPS) data show that the degree of amine group protonation depends on the local concentration of sulfonate groups, which form ion pairs with the resulting ammonium ions. Contact angle measurements show that these ion pairs greatly enhance the wettability of the gradient surface. Finally, studies of colloidal gold binding show that the presence of both amine and thiol moieties enhance colloid binding, which is also influenced by surface charge. Cooperativity is also revealed in the distribution of charges on uniform samples used as models of the gradient surfaces, as evaluated via zeta potential measurements. Most significantly, the net surface charge and how it changes with distance and solution pH strongly depend on whether the gradients in amine and thiol align or oppose each other. The aligned multicomponent gradients show the most interesting behavior in that there appears to be a point at pH ∼ 6.5 where surface charge remains constant with distance. Setting the pH above or below this transition point leads to changes in the direction of charge variation along the length of the substrate.

Applicability of electro-osmotic flow for the analysis of the surface zeta potential

“…3a), and was suggested as a reference material for the non-destructive SZP analysis by the SP method. [45][46][47] The SP data for the PP foil show minimum measurement uncertainty (less than 10%), good repeatability (each point is the mean value of three repetitions) and the IEP at pH 3.8, which is typical for polymers. 48 Oppositely, larger deviations were observed for the indirect EOF method for all three types of tracer particles.…”

On the Importance of Silane Infusion Order on the Microscopic and Macroscopic Properties of Multifunctional Charge Gradients

“… 13 , 14 A second gradient can be added by repeating the infusion process. 15 , 16 An aligned, multicomponent gradient is produced when the two individual gradients are formed in the same direction, whereas an opposed multicomponent gradient is obtained when they are created in opposing directions. 15 , 16 Cooperativity, both positive and negative, can only take place when the different functional groups are adjacent to each other such as that obtained along the entire length of an aligned gradient.…”

“… 15 , 16 An aligned, multicomponent gradient is produced when the two individual gradients are formed in the same direction, whereas an opposed multicomponent gradient is obtained when they are created in opposing directions. 15 , 16 Cooperativity, both positive and negative, can only take place when the different functional groups are adjacent to each other such as that obtained along the entire length of an aligned gradient. In contrast, in an opposed gradient, the mutual mixing of the individual functionalities only takes place near the middle of the substrate where the two gradients intersect.…”

In situ silanization for continuous stationary phase gradients on particle packed LC columns

“…Constructive CRI (i.e., adding ligands to a substrate) is a costefficient and straightforward method to produce multicomponent chemical gradients on silica supports, by exposing one end of a substrate longer than another to a hydrolyzed organosilane solution. [17][18][19][20][21]31,32 Adaption of constructive CRI to a particle packed column that is suitable for LC, however, has not been attainable until now. Compared to planar substrates and monolithic columns, the time-dependent modication of a traditional LC packed column in situ is much more challenging, not only because of the increased back pressure required for infusion into a packed column, but also because of the high surface area and microporosity of the silica powder.…”