Shape-controlled fabrication of micron-scale surface chemical gradients via electrochemically activated copper(I) click chemistry

We report an electrochemical method for the shape-controlled fabrication of micron-scale surface- bound chemical gradients. The approach is based on employing platinum microelectrode arrays on glass for the establishment of a Cu(I) solution gradient via local electrochemical reduction of Cu(II) (cathodic reaction), and oxidation of the generated Cu(I) back to Cu(II) (anodic reaction), under ambient conditions. The Cu(I) solution gradient, in the presence of an alkyne in solution and an azide monolayer on the glass surface in between the platinum electrodes, is exploited for the surface- confined gradient fabrication via the Huisgen 1,3-dipolar cycloaddition (CuAAC). Owing to the high sensitivity of the CuAAC on the Cu(I) concentration, we demonstrate here the control of the shape of the micron-scale surface gradient, in terms of steepness and surface density, as a function of the reaction conditions. The surface gradients were assessed by fluorescence microscopy and time-of-flight secondary ion mass spectrometry (Tof-SIMS). Moreover, bi-component and biomolecular gradients have been fabricated and a method for the electrochemically mediated patterning of surface chemical gradients on external azide-functionalized substrates has been developed for the implementation of bi-directional 2D surface gradients.