Super-strong interactions between multivalent anions and graphene

Based on the density functional theory (DFT) calculations, we showed that the interactions between different valence anions (${PO}_4^{3-}$, ${CH}_3{PO}_4^{2-}$, ${\left({CH}_3\right)}_2{PO}_4^{-}$) and graphene significantly increased as the valence of anion increased from negative monovalence to negative trivalence. The adsorption energy of ${\left({CH}_3\right)}_2{PO}_4^{-}$ on the electron-rich graphene flake (C₈₄H₂₄) is −8.3 kcal/mol. The adsorption energy of ${CH}_3{PO}_4^{2-}$ on the electron-rich graphene flake (C₈₄H₂₄) is –48.0 kcal/mol, which is about six times that of ${\left({CH}_3\right)}_2{PO}_4^{-}$ adsorption on electron-rich graphene flake (C₈₄H₂₄) and is even much larger than that of ${CO}_3^{2-}$ adsorption on electron-deficient aromatic ring C₆F₆ (–28.4 kcal/mol). The adsorption energy of ${PO}_4^{3-}$ on the electron-rich graphene flake (C₈₄H₂₄) is –159.2 kcal/mol, which is about 20 times that of ${\left({CH}_3\right)}_2{PO}_4^{-}$ adsorption on the graphene flake (C₈₄H₂₄). The super-strong adsorption energy is mainly attributed to the orbital interactions between multivalent anions and graphene. This work provides new insights for understanding the interaction between multivalent anions and π–electron-rich carbon-based nanomaterials and is helpful for the design of graphene-based DNA biosensor. (paper)