Influence of ionic concentration on transport properties of single asymmetric nanopores in polymer films

It has been demonstrated that cone-shaped nanopores with charged surfaces are cation selective, and, for the same magnitude of applied voltage, show preferential cation flow from the narrow entrance to the wide opening of the cone [1]. Presently, there are two models which describe this nonlinear current-voltage characteristic seen in polymer nanopores. The first model focuses on the presence of an electro-mechanical gate in the narrow end, or “tip” of the pore, which induces changes in pore diameter in response to an applied electric field [2], while the second points to the importance of electrostatic interactions between ions passing through the pore, and surface charges on the pore walls. This non-zero surface charge creates an internal electrostatic potential, with a profile dependent on pore shape. In a conical pore, this profile resembles an asymmetric “ratchet” tooth [3], which gives rise to an electrostatic trap for cations at the pore tip [4]. The depth of this trap differs with the polarity of the applied voltage, resulting in these conical nanopores having an asymmetric, diode-like current-voltage (I-V) characteristic. Also, the depth depends on the pore diameter, surface charge, as well as screening of these fixed surface charges by mobile ions (i.e. the thickness of the electrical double layer at the pore surface [3]). This doublelayer thickness is influenced by electrolyte concentration, so if electrostatic interactions are responsible for the rectification effect, the current-voltage characteristic should depend strongly on this concentration parameter. The single-pore membranes used here are prepared in PET of 12 μm thickness by asymmetric track-etching [1]. The resulting conical pores had narrow openings of 4-20 nm. I-V curves of the conical nanopores were measured in aqueous KCl solutions of various concentrations ranging from 0.01 to 3 M. For lower salt concentrations, the electrical double layer at the pore walls is thicker, i.e. fewer counter-ions are screening the surface charges, whose effect is therefore more pronounced. So, one would expect that lowering the electrolyte concentration will enhance the rectification effect. Surprisingly, however, for PET pores the rectification increases with decreasing concentration only to 0.1 M KCl (Figure 1). As the concentration is decreased further, the degree of rectification drops again for the majority of the pores examined. The origin of this effect is not yet clear, but it seems to be related to the presence of polymer ‘dangling’ ends, since conical, negatively charged, Au nanotubes do not show this effect [4]. These dangling ends could act as an electro-mechanical gate, changing the pore diameter for different applied voltages, somewhat similar to the discrete conductance states between which PET pores fluctuate under certain conditions [2]. Also, our results have shown that the pores exhibit a nonclassical conductance vs. electrolyte concentration