Beryllium ion sensing through the ion pair formation between the electrochemically reduced ferrocenyl naphthoquinone radicals and ${\text{Be}}^{2+}$ ions

We explored the mechanistic aspects of ion pair formation between electrochemically reduced radicals (Fc–cnq–1a${}^{\bullet {}_{-}}$/Fc–cnq–1b${}^{\bullet {}_{-}})$ and dianions (Fc–cnq–1a${}^{2-}$/Fc–cnq–1b${}^{2-})$ of ferrocenyl naphthoquinones (Fc–cnq–1a and Fc–cnq–1b) and several metal ions by cyclic voltammetry (CV), square wave voltammetry (SWV) and spectroelectrochemistry, for the first time. The experiments demonstrated that Fc–cnq–1a${}^{2-}$/Fc–cnq–1b${}^{2-}$ were moderately affected with Na${}^{+}$, K${}^{+}$ and Cs${}^{+}$ by slightly shifting to the anodic side, but were strongly influenced with Li${}^{+}$ ion. Fc–cnq–1a${}^{\bullet {}_{-}}$/Fc–cnq–1b${}^{\bullet {}_{-}}$ were not affected by alkali metal ions, indicating no ion pair formation between the radicals and these ions. Fc–cnq–1a${}^{2-}$/Fc–cnq–1b${}^{2-}$ was not evolved in the presence of Be${}^{2+}$, Mg${}^{2+}$ and Ca${}^{2+}$, but Fc–cnq–1a${}^{\bullet {}_{-}}$/Fc–cnq–1b${}^{\bullet {}_{-}}$ appeared with their cathodic waves, and participated with intermediates, [(Fc–cnq–1b)${}_2{}^{\bullet {}_{-}}$–Be${}^{2+}$ and [(Fc–cnq–1a)${}^{\bullet {}_{-}}$–Cl]. The most pronounced effect on the ion-pair formation of the Fc–cnq–1a${}^{\bullet {}_{-}}$/Fc–cnq–1b${}^{\bullet {}_{-}}$ was observed in Be${}^{2+}$, indicating that Fc–cnq–1a or Fc–cnq–1b can selectively sense ultra-trace amount of Be${}^{2+}$ (LOD = 3.6 ppb) among the other metal ions with SWV titration, for the first time, based on the strong ion pair formation reaction between the radicals and Be${}^{2+}$.

Graphical abstract

Synopsis Ultra-trace amount of beryllium ion (${\text{Be}}^{2+}$) sensing (LOD = 3.6 ppb) through the ion pair formation between the electrochemically reduced ferrocenyl naphthoquinone radicals (Fc–cnq–1a${}^{\bullet {}_{-}}$/Fc–cnq–1b${}^{\bullet {}_{-}})$ and ${\text{Be}}^{2+}$ ions by cyclic voltammetry (CV), square wave voltammetry (SWV), and for the first time spectroelectrochemistry is reported in this study. .