Characterization of negative differential resistance in asymmetric nanopores obtained from two soluble electrolytes

Negative differential resistance (NDR) phenomena in nanofluidic diodes are characterized by a decrease in the electrical current with the increase in the transmembrane potential beyond a system-dependent threshold voltage. Here, we describe the NDR due to a nanoscale salt precipitation that occurs in a conical nanopore in contact with two highly soluble salts in the external solutions. The new experimental design permits tunable and reproducible NDR effects that can be characterized from the I–V curves for different pairs of soluble salts reacting at the pore tip to form the salt precipitate. The effects of pH and salt concentration on the precipitation, together with the use of electrolytes with different temperature-dependent solubilities, provide a complete description of the chemical NDR mechanism. In addition, the good reproducibility and stability observed over different voltage cycles suggest that the threshold potential needed for precipitation can be used for Ca2+ sensing in the range 1–1000 mM.