A realistic picture of a cell is that of a highly viscous, condensed gel-like substance, crowded with macromolecules that are mostly anchored to membranes and to intricate networks of cytoskeletal elements. Theoretical and experimental approaches to investigating crowding have not considered the role of diffusion through a crowded medium in affecting the selectivity and specificity of reactions. Such diffusion is especially important when one considers interfaces, where at least one reactant must move through the medium and reach the interface. Here, we address this issue by directly investigating how diffusion through a gel medium affects the competition between a single specific reaction and a large number of weak nonspecific interactions, a process that is typical of reactions occurring at interfaces. We present an approach for achieving orientation-controlled interactions based on the configuration-dependent diffusion rate of the reacting molecule through a gel medium. The effectiveness of the method is demonstrated by the high selectivity obtained both in the adsorption of DNA to a surface and in DNA hybridization to preadsorbed single-strand oligomer on a surface.