Printing chemical gradients

We describe a method to exploit the mass-transfer limitations of microcontact printing for the fabrication of surfaces with well-defined, arbitrarily shaped composition variations. An analysis of the transport processes reveals that the printing of hexadecanethiol (HDT) from poly(dimethylsiloxane) is purely diffusion-controlled. Stamps with geometries that enhance surface-normal diffusion paths therefore allow not only the contours, but also the local density of self-assembled monolayers to be controlled. We use stamps with variable thickness and uniform ink concentration to print HDT density gradients on gold, depleting the stamps during the process. In the second step, a perfluorinated thiol fills the vacancies in the partial monolayer to form a two-component gradient that we analyze by means of X-ray photoelectron spectroscopy and spectroscopic ellipsometry. Linear and radial gradients are shown here as examples for a wide range of geometries that can be fabricated with high precision using the method.