Self-assembled monolayer of a redox-active calix[4]arene: Voltammetric recognition of the Ba2+ ion in aqueous media

The Redox-active monolayer of a novel calix[4]arene recognizing redox-inact ive ionic species by voltammetry is reported. Calix[4]arene-disulfide-diqui none, which is not only redox-active but is also a highly selective ionopho re for the Ba2+ ion, spontaneously forms a stable and dense monolayer film on gold. The redox-active calixarene monolayer selectively recognizes Ba2ion 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 Ba2+ ion having a con centration as low as 1.0 x 10(-6) M. The Langmuir plot (1/i(p) vs 1/[Ba2+]) shows a linear slope in the range from 1.0 x 10(-6) M to 1.0 X 10(-4) M. T his modified electrode does not show any significant interference from alka li and alkaline earth metal ions except for Sr2+ and Ca2+. Only 100- and 50 0-fold concentrations of Sr2+ and Ca2+ ions, respectively, can lead to volt ammetric responses comparable to that of Ba2+.