Negative incremental resistance induced by calcium in asymmetric nanopores.

Single polymer nanopores with permanent surface charges act as rectifiers of ionic current. The pores are tapered cones with narrow openings of several nanometers and wide openings of approximately 1 mum. The pores are cation-selective, and in symmetric solutions of potassium chloride they rectify the flow of potassium ions from the small opening toward the wide opening. Millimolar concentrations of calcium reverse the rectification, and a negative incremental resistance is observed. These observations can be interpreted by a model containing flashing ratchets.