Self-Assembled Monolayer of a Redox-Active Calix[4]arene: Voltammetric Recognition of the Ba^(2+) Ion in Aqueous Media

The Redox-active monolayer of a novel calix[4]arene recognizing redox-inactive ionic species by voltammetry is reported. Calix[4]arene-disulfide-diquinone, which is not only redox-active but is also a highly selective ionophore for the Ba^(2+) ion, spontaneously forms a stable and dense monolayer film on gold. The redox-active calixarene monolayer selectively recognizes Ba^(2+) ion in aqueous media, and the voltammetric signals are proportional to the ionic concentration. A new voltammetric peak can be detected by square-wave voltammetry upon adding a dilute solution containing Ba^(2+) ion having a concentration as low as 1.0 x 10^(-6) M. The Langmuir plot (1/i_(p) vs 1/[Ba^(2+)]) shows a linear slope in the range from 1.0 x 10^(-6) M to 1.0 x 10^(-4) M. This modified electrode does not show any significant interference from alkali and alkaline earth metal ions except for Sr^(2+) and Ca^(2+). Only 100- and 500-fold concentrations of Sr^(2+) and Ca^(2+) ions, respectively, can lead to voltammetric responses comparable to that of Ba^(2+).