Electrochemical and density functional theory investigation on the differential behaviors of core-ring structured NiCo 2 O 4 nanoplatelets toward heavy metal ions

In order to further improve the electroanalytical performance toward heavy metal ions, core-ring structured NiCo2O4 nanoplatelets were used to modify glass carbon electrode (GCE) for the determination of heavy metal ions in water. Owing to the high surface area of NiCo2O4 nanoplatelets, the Pb(II) sensitivity increased by a factor of 1.70, and the detection limit decreased by a factor of 2.64 as compared to solid NiCo2O4 nanoparticles modified GCE. Interestingly, NiCo2O4 nanoplatelets showed different sensitivities toward heavy metal ions with the same valence states, following the order Pb(II) > Cd(II) > Hg(II) > Cu(II). To better and scientifically understand the difference in sensitivity, adsorption and desorption abilities were integrated into account. Density functional theory calculations verified that the adsorption capability of NiCo2O4 toward Pb(II) was strongest among all heavy metal ions, thereby resulting in the largest sensitivity. Further desorption current measurements indicated the large desorption barrier of Cu(II) was another important factor leading to its lowest sensitivity. Finally, the applicability of the proposed method was demonstrated by the detection of heavy metal ions in real seawater.