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With a view to advancing electrochemical sensor technology, considerable interest is currently being shown in the design and synthesis of redox-active molecules that contain a redox center in close proximity to a host binding site."] These systems are capable of recognizing electrochemically the complexation of a guest through space interactions between the redox center and the receptor site. Perturbation of the electrochemical response of the redox center is observed in the presence of the target host, allowing its amperometric or potentiometric titration. In particular, electrochemical recognition and sensing of Group 1 and 2 metal cations by ferrocene crown ether and cryptand ionophores has been successfully achieved in homogeneous solution.["2JFor the development of novel prototypes of electrochemical sensors, it is a challenge to synthesize molecular electrodes based on these redox receptors, which must retain their complexation and recognition behavior after immobilization onto an electrode surface. For instance, incorporating a redox-active receptor into a polymer matrix such as a functionalized polypyrrole film can provide great convenience in fabricating sensors.[31 In this communication, we present the first example of electropolymerization of a pyrrole-substituted ferrocene crown ether and the remarkable behavior of the resulting polymer film modified electrode towards the amperometric recognition of barium and calcium cations.The new ligand L was synthesized according to Scheme 1. Its electrochemistry was investigated by cyclic voltammetry (CV) in lo-' mol L-' tetra-n-butylammonium perchlorate (TBAP) + CH3CN electrolyte. The CV curve is characterized by a quasi-reversible redox wave at 0.53 V, corresponding to the ferrocene/ferricinium couple. This Scheme 1. Synthesis of L.wave is followed at a higher potential by a large peak (Epa = 0.97 V), which is due to the irreversible oxidation of both the pyrrole group and the benzo-crown ether moiety of L. The oxidation of the latter occurs at EPa = 1.13 V in the parent pyrrole-free ferrocene crown ether.[41 It is now well established that the electrooxidative polymerization of functionalized pyrroles is an effective method for carrying out the coating of electrode surfaces by non-passivating polymer layers containing redox-active centers.['] The growth of polyL films on the Pt electrode surface was accomplished by repeated CV scans over the 0 V to 0.8 V potential range, or by controlled potential electrolysis at 0.8 V in millimolar solutions of L. The resulting modified electrodes transferred to clean CH3CN electrolyte display a stable electrochemical response for immobilized L at Efi2 = 0.56 V (Fig. 1, curve 1). Fig. 1. CV curve of a Pt/polyL modified electrode (5 mm diameter) in CH3CN + lo-' rno1L-l T B A P E vs. Ag/lO-* molL-' AgNO, + lo-' mol L-' TBAP + CHICN; scan rate 0.1 V 8; T L = 2 x lo-* molcm-*; [BaZ0] = 0 (I), 10. ' mo1L-I (2),2 x moILY (3).Adv. Mater. 1997,9, No. 9 0 VCH Verlagsgesellschaft mbH, 0-69469 Weinheim, 1997 0935-9648/97/0907-0711$17.5...