Abstract: Solid-state nanofluidic devices have proven to be ideal systems to elucidate the physics of nanoscale ion transport. When the device confinement approaches the Debye length of ions in the fluid, new phenomena occur such as surface-charge-dominated transport and ion-selectivity. Although these effects have been understood for simple monovalent systems, application-critical divalent systems have not been sufficiently studied, which are essential in biological systems. We explore for the first time divalent nanofluidic devices, and observe striking differences including charge inversion at the solid/fluid interface. This counter-intuitive over-screening effect plays a critical role in DNA condensation and colloidal coagulation. We have performed a systematic study to explore the divalent ion-surface interactions and ionic transport at nanoscale, which lies at the heart of interdisciplinary research such as biophysics, colloidal science and energy storage and conversion.