Controlling the Diffusion Profile in the Vapor Phase Deposition of Silanes for Gradient Fabrication

Surface chemical gradients were prepared on porous supports using a modified vapor deposition approach, and the shape and length of the gradient were controlled by restricting the diffusion layer. The gradients were formed on silica thin-layer chromatography (TLC) plates using organomonochlorosilanes of different sizes (C3, C4, C8, and phenyl) and visualized under UV radiation due to loss of fluorescence from the embedded fluorescent dye. Diffuse reflectance FT-IR chemically confirms the presence of C3 ligands. By constricting the diffusion of the silane vapor, the gradient shapes could be changed from concave to flat, and their lengths controlled. Photographs coupled with analysis using ImageJ were used to measure the saturation length of the gradient and the degree of curvature. C3 lengths covered 17.4-69.4% of the length of the TLC plate, and their curvature ranged from -1.9 to 11.9%, depending on the experimental conditions. Gradients formed by significantly restricting the diffusional profiles of the vapor yielded gradients with flat profiles, while those formed traditionally with no restriction of the vapor yielded gradients with an unusual concave shape due to the depletion of vapor in the center of the plate. By fitting the gradient profiles extracted from the photographs with Fick's second law of diffusion, diffusion coefficients (<i>D</i>) could be obtained. For the C3 silane, <i>D</i> was (0.8-2.2) × 10^-3 cm²/s, depending on the specifics of the configuration used. As expected, smaller values of <i>D</i> were obtained for the larger molecular weight silanes (C3 > C4 > phenyl > C8). The lower molecular weight silanes gave rise to a higher degree of silane modification, whereas the larger ones gave rise to a lesser degree of silane modification. This work shows, for the first time, how to control the size and shape of surface chemical gradients on porous supports using a vapor phase diffusion method.