Calixarene-poly(dithiophene)-based chemically modified electrodes

The stepwise synthesis of cone and partial cone 1,3-bridged n-propoxy-calix [4]crown ethers ("monomers" 2 and 3) with an electropolymerizable 2,2 ' -di thiophene-3-yl-hexylene functionality at the lower rim, is described. The p otential of 2 and 3 as sensing agents for alkali metal ions was investigate d by H-1 NMR titration experiments with NaSCN and KSCN. The results obtaine d have confirmed that the presence of the heterocyclic subunit does not aff ect the well-known size-selectivity observed with calix[4]crowns. Monomers 2 and 3 were electropolymerized (Pt as a working electrode, CH2Cl2/CH3CN, B u4NPF6) to produce the title chemically modified electrodes (CMEs). After c oating with a PVC membrane containing a lipophylic cation exchanger, CMEs b ased on calix[4]-crown-5 2b (cone) and 3b (partial cone) were tested for th e potentiometric recognition of alkali metal ions in aqueous solution. In a greement with NMR titration studies, a satisfactory potentiometric response in terms of K+/Na+ selectivity was obtained only with CME 2b (pK(K/Na) 1.5 1). The amperometric responses of PVC-uncoated CMEs were studied by cyclic voltammetry (CV) experiments in CH,CN solution. High Na+ selectivity was fo und with the CME based on partial cone calix[4]crown-4 3a, and frequency re sponse analysis (FRA) measurements support this finding.