Cation pumping against a concentration gradient in conical nanopores characterized by load capacitors

We study the cation transport against an external concentration gradient (cation pumping) that occurs in conical nanopores when zero-average oscillatory and white noise potentials are externally applied. This pumping, based on the electrically asymmetric nanostructure, is characterized here by a load capacitor arrangement. In the case of white noise signals, the conical nanopore acts as an electrical valve that allows extraction of order from chaos. No molecular carriers, specific ion pumps, and competitive ion-binding phenomena are required. The nanopore conductance on/off states mimic those of the voltage-gated ion channels in the cell membrane. These channels allow modulating membrane potentials and ionic concentration gradients along oscillatory pulses in circadian rhythms and the cell cycle. We show that the combination of asymmetric nanostructures with load capacitors can be useful for the understanding of nanofluidic processes based on bioelectrochemical gradients.

Impact of Surface Charge Directionality on Membrane Potential in Multi-ionic Systems

The membrane potential (V_mem), defined as the electric potential difference across a membrane flanked by two different salt solutions, is central to electrochemical energy harvesting and conversion. Also, V_mem and the ionic concentrations that establish it are important to biophysical chemistry because they regulate crucial cell processes. We study experimentally and theoretically the salt dependence of V_mem in single conical nanopores for the case of multi-ionic systems of different ionic charge numbers. The major advances of this work are (i) to measure V_mem using a series of ions (Na⁺, K⁺, Ca²⁺, Cl^-, and SO₄^2-) that are of interest to both energy conversion and cell biochemistry, (ii) to describe the physicochemical effects resulting from the nanostructure asymmetry, (iii) to develop a theoretical model for multi-ionic systems, and (iv) to quantify the contributions of the liquid junction potentials established in the salt bridges to the total cell membrane potential.

Optimizing Energy Transduction of Fluctuating Signals with Nanofluidic Diodes and Load Capacitors

The design and experimental implementation of hybrid circuits is considered allowing charge transfer and energy conversion between nanofluidic diodes in aqueous ionic solutions and conventional electronic elements such as capacitors. The fundamental concepts involved are reviewed for the case of fluctuating zero-average external potentials acting on single pore and multipore membranes. This problem is relevant to electrochemical energy conversion and storage, the stimulus-response characteristics of nanosensors and actuators, and the estimation of the accumulative effects caused by external signals on biological ion channels. Half-wave and full-wave voltage doublers and quadruplers can scale up the transduction between ionic and electronic signals. The network designs discussed here should be useful to convert the weak signals characteristic of the micro and nanoscale into robust electronic responses by interconnecting iontronics and electronic elements.