Feature Article Silver Ion-Selective Electrodes Based on p-Coordinating Ionophores Without Heteroatoms Johan Bobacka, a Tanja Lahtinen, b Heli Koskinen, a Kari Rissanen, b Andrzej Lewenstam, a Ari Ivaska* a a äbo Akademi University, Process Chemistry Group, c/o Laboratory of Analytical Chemistry, FIN-20500 äbo-Turku, Finland; e-mail: aivaska@abo.fi b Department of Chemistry, University of Jyv‰skyl‰, FIN-40350 Jyv‰skyl‰, Finland Received: December 21, 2001 Final version: February 22, 2002 Abstract Ion-selective electrodes (ISEs) were constructed by using spherical hydrocarbons (cyclophanes) as p-coordinating ionophores in solvent polymeric membranes. Four structurally similar cyclophanes, i.e., [2.2.2]p,p,p-cyclophane, [2.2.2]m,p,p-cyclophane, [2.2.1]p,p,p-cyclophane and [2.2.1]m,p,p-cyclophane were studied as ionophores for Ag  . The ion-selective membranes were composed of the corresponding ionophore (1%), potassium tetrakis(4-chlorophen- yl)borate (0.5%), 2-nitrophenyl octyl ether (65 ± 66%) and PVC (32 ± 33%). The ion-selective membrane was placed on top of a layer of the conducting polymer, poly(3,4-ethylenedioxythiophene) (PEDOT), working as ion-to-electron transducer. The selectivity pattern of the solid-contact ISEs was found to depend strongly on the structure of the cyclophane. The results show that ionophores that do not contain any heteroatoms such as sulfur, nitrogen or oxygen can be highly selective to ™soft∫ cations like Ag  , based solely on cation-p interactions between Ag  and benzene rings. The lipohilic tetrakis(4-chlorophenyl)borate, which also contains p-coordinating phenyl groups, and which is used as the anionic additive in the ion-selective membrane, shows a remarkable selectivity for Hg 2 . The selectivity is discussed in terms of cation-p interactions between Ag  and the ionophores. Keywords: Ion-selective electrode, Conducting polymer, Spherical hydrocarbons, Cyclophane, Silver, Cation-p interaction Dedicated to Professor Gary Christian on the Occasion of His 65 th Birthday 1. Introduction Neutral carriers (ionophores) for ™soft∫ metal ions like Ag  and Hg 2 normally contain heteroatoms like sulfur or nitrogen [1 ± 3]. However, it has recently been reported that some spherical hydrocarbons without heteroatoms may encapsulate metal ions, like Ag  [4]. Among these concave hydrocarbons, [2.2.2]p,p,p-cyclophane was found to extract Ag  to a much greater extent than alkali metal ions, Tl  and Hg 2 [4]. Consequently, ISEs based on [2.2.2]p,p,p-cyclo- phane as neutral carrier in plasticized PVC membranes showed high selectivity to Ag  [4, 5]. Interestingly, Hg 2 caused severe interference for the Ag  -ISE based on [2.2.2]p,p,p-cyclophane [5] although the extraction study showed that the ionophore itself is more selective to Ag  than Hg 2 [4]. This indicates that other components than the ionophore in the ISE membrane may interact strongly with Hg 2 . In contrast to [2.2.2]p,p,p-cyclophane, the structurally similar [2.2.2]m,p,p-cyclophane, showed very poor selectiv- ity to Ag  [5]. Recently, tert-butylcalix[4]arene tetra(allyl ether) containing p-coordinating allyl substituents was found to be a highly selective neutral carrier for Ag  [6]. In this substituted calixarene, p-coordination of Ag  was found to occur mainly at the allyl substituents rather than within the cavity formed by the aromatic rings in the calixarene skeleton [6]. In view of these earlier results, cyclophanes are extremely interesting ionophores for soft cations due to their exclusive p-coordinating properties without any influence of other donors such as sulfur, nitrogen or oxygen atoms in the ionophore. In the present work, four structurally similar cyclophanes, i.e., [2.2.2]p,p,p-cyclophane, [2.2.2]m,p,p-cyclophane, [2.2.1]p,p,p-cyclophane and [2.2.1]m,p,p-cyclophane will be studied as ionophores for Ag  . The ionophores will be used as the active components in Ag  -ISEs based on plasticized PVC membranes and a conducting polymer, poly(3,4-ethylenedioxythiophene), as solid internal contact [7, 8]. This paper is an extension to our recent report on Ag  - ISEs based on [2.2.2]p,p,p-cyclophane [5]. Here, the selectivity of the ISEs will be discussed in terms of cation- p interactions between Ag  and the ionophores. 2. Experimental 2.1. Reagents [2.2.2]p,p,p-cyclophane is commercially available and was obtained from Merck. The isomeric [2.2.2]m,p,p-cyclophane and the smaller analogues [2.2.1]m,p,p-cyclophane and [2.2.1]p,p,p-cyclophane were synthesized via well-known cyclophane methodology where sulfide cyclization is done 1353 Electroanalysis 2002, 14, No.19±20 ¹ 2002 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheim 1040-0397/02/1910-1353 $ 17.50+.50/0