Surface reactions on demand: electrochemical control of SAM-based reactions.

Self-assembled monolayers (SAMs) are well-ordered, single-molecule-thick structures in two dimensions, which form spontaneously at interfaces (usually at solid surfaces). SAMs have been used for controlling physical properties of interfaces, such as wetting, adhesion, lubrication, and corrosion, as well as understanding fundamental aspects of interfacial phenomena. In addition, SAMs comprise an excellent platform for generating two-dimensional microand nanostructures. For example, SAMs are widely utilized for the spatially resolved immobilization of biomolecules (i.e., DNAs, peptides, and polysaccharides) and cells onto surfaces. Although useful in these various domains, these applications are based on the “static” property of SAMs. In other words, SAMs (especially the head groups of SAMs) are designed to meet the criteria of the applications because the head groups intimately, but “statically”, interact with the outside environment. SAMs execute their predetermined roles once they are formed on surfaces, these roles are exemplified by corrosion barriers, etching masks, and recognition of biological entities. One of the next challenges in the study of SAMs was how to induce surface reactions only when needed (“surface reactions on demand”). These surface reactions on demand have several implications in surface science: 1) we can generate dynamic surfaces (in other words, stimuli-responsive surfaces) in which the physical, chemical, and biological properties of surfaces are reversibly tuned at our disposal as we dynamically tailor the functional groups that interact with the environments, and 2) we can site-selectively localize chemical reactions at surfaces, leading to independent addressability of surface reactions. Two reaction methods would be obvious candidates for surface reactions on demand: photochemically induced and electrochemically induced reactions. Photochemical induction can be combined with photolithographic techniques. The “static” property of the SAMs manifests in the fabrication of well-known DNA microarrays in which photolabile protecting groups are site-selectively removed by photolithography. Recently, a light-induced Wolff rearrangement of diazomethylcarbonyl groups was combined with photolithography to generate micropatterns. In addition to the static surface, dynamic changes in the water wettability of surfaces were demonstrated by the use of a light-induced, reversible cis–trans transition of azo groups. On the other hand, electrochemical induction has some advantages over other methods for surface reactions on demand. It is easily incorporated into electronic devices because the electrochemically induced surface reaction involves an electron transfer between a surface (e.g., gold and silicon) and a reaction site. The electrochemical control of surface reactions at electrodes could generate independently addressable electrodes because a reaction can be induced electrochemically on a designated electrode. Furthermore, dynamic control of surface properties could be achieved easily by electrical potentials and reversible oxidation–reduction reactions. Initial attempts to use electrochemistry for SAM-based reactions were at the early stage in the development of surface reactions on demand. These attempts were based on simple desorp-