Exploring Capacitance and Non-Linear Electrokinetics in Nanopores Yields Insight into Single Molecule Recordings
The pervasive model for a solvated, ion-filled nanopore is often a resistor in parallel with a capacitor. However, for conical nanopore geometries, we propose the inclusion of a Warburg-like element which is necessary to explain otherwise anomalous observations such as negative capacitance and lowpass filtering of translocation events (i.e., a phenomenon we term Warburg filtering). The negative capacitance observed here is characterized as having long equilibration times and memory (i.e., mem-capacitance) at negative voltages. Next, we used the transient occlusion of the pore using {lambda}-DNA and 10-kbp DNA to test whether events are being attenuated by purely ionic phenomena even when there is sufficient amplifier bandwidth. The inclusion of the Warburg-like element is mechanistically linked to concentration polarization and the activation energy to generate and maintain localized concentration gradients. We conclude the study with a new interpretation of molecular translocations which is not simply based on the pulse-like resistance changes but rather a complex and non-linear storage of ions that changes during molecular transit.