Construction of Stable 2D Cationic Breathing Ni-MOF for Cr(VI) Trapping and Electrochemical Sensing

Pollution of surface water by heavy metal hexavalent chromium ions poses a serious threat to human health; herein, a two-dimensional (2D) cationic breathing Ni-MOF with free nitrate ions between the layers was designed and synthesized according to the characteristics of hexavalent chromium ions, {[Ni(L)₂](NO₃)₂·5H₂O}<i>_n</i> (L = 1,3,5-tris[4-(imidazol-1-yl)phenyl]benzene). The flexible layer spacing of the 2D breathing Ni-MOF allows the exchange of NO₃^- by CrO₄^2- without destroying the original structure. Electrostatic and hydrogen bonding interactions between CrO₄^2- and Ni-MOF facilitate its exchange with NO₃^-. Moreover, CrO₄^2- exhibits a higher binding energy with Ni-MOF compared to NO₃^-, and the hydrophobic channels of Ni-MOF favor CrO₄^2- trapping due to its lower hydration energy. Consequently, Ni-MOF demonstrates both effective sorption and electrochemical sensing of Cr(VI), achieving a sensitivity of 2.091 μA μM^-1 and a detection limit of 0.07 μM.