Self–assembled monolayers of organosulphur molecules bearing calix[4]arene moieties O. Cavalleri a,b , M. Vignolo a , G. Strano b , C. Natale b , R. Rolandi a,b , S. Thea c , M. Prato a,b , G. Gonella a,b , M. Canepa a,b , A. Gliozzi a,b, * a INFM, Unita ` di Genova, Italy b Dipartimento di Fisica, Universita ` di Genova, Genoa, Italy c Dipartimento di Chimica e Chimica Industriale, Universita ` di Genova, Genoa, Italy Received 10 July 2003; received in revised form 18 September 2003; accepted 22 September 2003 Abstract We investigate the self-assembly of modified calix[4]arene on gold surfaces. Calix[4]arene was modified through a reaction sequence which led to assembling of the crown-5 moiety and to the insertion of two thioether groups into the starting molecule. The so-obtained calix[4]arene-crown-5 bis(7-thiatridecyloxy) (hereafter called calix[4]arene) was in the stable 1,3-alternate conformation. The calix[4]arene/ gold interface was investigated by means of spectroscopic ellipsometry (SE), scanning tunneling microscopy (STM) and cyclic voltammetry (CV). SE data indicate a layer thickness compatible with the formation of a monomolecular layer. This result is confirmed by STM imaging which shows the formation of a high density of small pits, one gold layer deep, a typical feature of self-assembled organosulphur monolayers on gold. CV measurements performed in presence of the [Ru(NH 3 ) 6 2+/3 + ] redox couple indicate a passivation of the metal electrode, resulting in a reduction of the redox current, after the layer deposition. CV has also been used to investigate the selectivity properties of calix[4]arene-covered gold electrodes by measuring the redox current decrease in the presence of different salt solutions. It is found that calix[4]arene-covered electrodes are able to complex K + and Ba 2+ , while no complexation is observed in the case of Li + , Na + , Cs + , Mg 2+ and Ca 2+ . D 2004 Elsevier B.V. All rights reserved. Keywords: Modified calix[4]arene; Self-assembled monolayers; Spectroellipsometry; Scanning tunneling microscopy; Cyclic voltammetry 1. Introduction In the 1980s, new technology requirements for chemi- cally active materials led to the development of new techniques to bind organic molecules to the surface of inorganic substrates. The possibility to assemble organic layers exposing different functionalities allows one to tailor the physicochemical properties of the surface and prompts their use for wettability, lubrication and corrosion control as well as for the development of biosensors able to perform molecular/ionic recognition [1]. Organic mole- cules that covalently bind to either conductor or insulator surfaces can form thin ordered films [self-assembled monolayers (SAMs)]. Among SAMs, large attention has been recently paid to layers formed by organosulphur molecules which bind to transition metals, like gold or silver [2]. Here we focus on the preparation and characterization of functionalized gold surfaces obtained by the self-assem- bly of calix[4]arene-crown-5-bis-sulfide. This molecule was synthesized as a first step to obtain selective recogni- tion of ions. Calixarenes are ditopic receptor molecules, which have been widely studied in recent years for the selective recognition of ions and neutral molecules [3]. Synthesis was carried out starting from calix[4]arene through a reaction sequence which led to insertion into the molecule of proper chemical functions, i.e. two thio- ether groups, which serve as anchoring groups to enable the self-assembly of the molecule to the metal substrate, and a crown-5 moiety (Fig. 1). The stiffening of calix[4]- arene by introducing a polyether chain bridging two opposite phenolic groups, resulting in a stable 1,3-alternate 1567-5394/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.bioelechem.2003.09.010 * Corresponding author. Dipartimento di Fisica, Universita ` di Genova, Via Dodecaneso 33, I-16146 Genoa, Italy. Tel.: +39-10-3536221; fax: +39- 10-314218. E-mail address: gliozzi@fisica.unige.it (A. Gliozzi). www.elsevier.com/locate/bioelechem Bioelectrochemistry 63 (2004) 3 – 7