Potential-induced switching of electrical contact by controlling droplet shapes at hydrophilic/hydrophobic interfaces Xuemei Wang, Eugenii Katz, Itamar Willner * Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel Received 8 July 2003; received in revised form 17 July 2003; accepted 17 July 2003 Published online: 26 August 2003 Abstract Asymmetrical deformation of an aqueous electrolyte droplet bridging two bipyridinium-modified Au-electrodes is achieved upon the application of voltages resulting in the selective reduction and oxidation of the bipyridinium units on the two electrodes. The different contact angles at the right and left sides of the droplet originate from the electrochemically controlled changes of the wetting properties of the modified electrodes. The reversible and repeatable formation of an electrolytic contact between a side Pt- wire electrode and the respective bottom Au-electrode is controlled by the voltage applied between the two bottom Au-electrodes. Ó 2003 Elsevier B.V. All rights reserved. Keywords: Contact angle; Bipyridinium electrochemistry; Electrochemical switch; Monolayer; Modified electrode 1. Introduction Electrochemical [1–4] and photochemical [5–7] con- trol of the hydrophilic and hydrophobic properties of chemically modified surfaces has attracted research ef- forts in the past decade. Such signal-triggered control of the wettability properties of modified interfaces was applied for the patterning of structures on surfaces [8– 10] and the mechanical pumping of solvents [11]. The electrochemical control of molecular machinery func- tions of supramolecular assemblies in solution and on surfaces was a subject of extensive recent research efforts [12–15]. For example, the potential-induced attraction or repulsion of a carboxylate monolayer associated with an electrode was reported to switch the properties of the interface between hydrophobic and hydrophilic states, respectively [16]. The features of this interface were probed by contact angle measurements and sum-fre- quency generation (SFG) spectroscopy. Recently, we reported [17] on the electrochemical switching of the hydrophilic/hydrophobic properties of a Au-electrode functionalized with a monolayer consisting of bipyridinium units tethered to the electrode surface via a long chain thiol. This monolayer acts as an elec- troactivated ‘‘molecular arm’’ where in the oxidized state the bipyridinium dication units are repelled from the positively charged electrode surface and the interface is hydrophilic, whereas the reduced bipyridinum radical- cation units are attracted to the negatively charged electrode surface and thus, the exposure of the hydro- carbon spacer chains to the solution yields a hydro- phobic interface. Here, we wish to demonstrate a method to utilize the electrochemically triggered hy- drophobic/hydrophilic molecular properties of the bi- pyridinium monolayer for tailoring a macroscopic droplet exhibiting electroswitchable electrical contact. 2. Experimental N -methyl-N 0 -carboxydecyl-4,4 0 -bipyridinium (1) was synthesized as described elsewhere [18]. All other chemicals, including mercaptoethanol, 4-(2-hydroxy- ethyl)piperazine-1-ethanesulfonic acid sodium salt (HEPES) and 1-ethyl-3-(3-dimethylaminopropyl)carbo- diimide (EDC) were purchased from Aldrich and used as supplied. Ultrapure water from Seralpur Pro90CN Electrochemistry Communications 5 (2003) 814–818 www.elsevier.com/locate/elecom * Corresponding author. Tel.: +972-2-6585272; fax: +972-2-6527715. E-mail address: willnea@vms.huji.ac.il (I. Willner). 1388-2481/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2003.07.002