L. Bouffier, S. Reculusa, V. Ravaine, A. Kuhn.
A new simple and versatile method for the preparation of surface wetting gradients is proposed. It is based on the combination of electrode surface structuration introduced by a sacrificial template approach and the formation of a tuneable molecular gradient by bipolar electrochemistry. The gradient involves the formation of a self-assembled monolayer on gold surface by selecting an appropriate thiol molecule and the subsequent reductive desorption by means of bipolar electrochemistry. Under these conditions, the completion of the reductive desorption process evolves along the bipolar surface with a maximum strength localized at the cathodic edge and a decreasing driving force toward the middle of the surface. The remaining quantity of surface-immobilized thiol does therefore vary in function of the axial position, resulting in the formation of a molecular gradient. The surface of the bipolar electrode is characterized at each step of the modification by recording heterogeneous electron transfer. Also, the evolution of static contact angles measured with a water droplet deposited on the surface directly reveals the presence of the wetting gradient which can be modulated by changing the properties of the thiol. This is exemplified with a long hydrophobic alkane-thiol and a short hydrophilic mercaptan.