Design and Fabrication of Wettability Gradients with Tunable Profiles Through Degrafting Organosilane Layers from Silica Surfaces by Tetrabutylammonium Fluoride

Surface-bound gradients allow for a high-throughput approach to evaluate surface interactions for many biological and chemical processes. We fabricate surface-bound gradients using a simple, two-step procedure that permits precise tuning of the gradient profile. We deposit a homogeneous self-assembled monolayer (SAM) of silane and generate a surface coverage gradient by vertically immersing the sample into the solution of tetrabutylammonium fluoride which cleaves the Si-O bonds at the surface. We model the kinetic of degrafting using a series of first order rate equations, based on the number of broken attachment points. Degrafting of mono-functional silanes exhibits an exponential decay in surface coverage as a function of degrafting time. The degrafting of tri-functional silanes is delayed due to the presence of multiple attachment points. We examine the effects of degrafting temperature and time and demonstrate the ability to control reliably the silane gradient profile. We observe a relatively homogenous coverage of silane throughout the process, when compared to additive approaches of gradient formation. We design and form linear gradients in silane coverage to demonstrate the reproducibility and tuneability of this approach.