Ion current rectification
Ion current rectification is the property of a pore or channel that passes more ionic current at one polarity of voltage than at the opposite polarity of the same size. The current-voltage curve is then not a straight line through the origin but is bent, in the way a diode's is. The effect appears in conical pores with charged walls, such as the tips of glass nanopipettes, and in channels whose surface charge changes sign along their length. It needs a pore whose width is within a few Debye lengths of the double layer on its walls, so it is a property of nanometer-scale openings in dilute salt.
The measure is the rectification ratio, the magnitude of the current at one polarity divided by the magnitude at the other, taken at the same voltage.
Mechanism
Take a conical pore whose walls carry a negative charge, as quartz, glass and the plastic PET do in neutral solution. Where the pore is wide, the electric double layer on the wall fills a small share of the opening, and the solution is nearly neutral. Where the pore is narrow, and the opening is no more than a few Debye lengths across, the double layers fill it and the pore holds mostly cations. The tip selects cations; the wide end does not.
Inside the tip nearly all of the current is carried by cations. In the open solution cations carry only about half of it, and the anions carry the rest. Where the two regions meet, the supply of ions cannot match the demand. When the field drives cations from the wide opening into the tip, the tip draws in more cations than migration brings to it, the ions in and around the tip are depleted, and the conductance falls. When the field is reversed, the tip delivers cations faster than the open solution carries them away, ions accumulate, and the conductance rises. The preferred direction of a negatively charged conical pore is therefore the one that moves cations from the tip toward the base.[1]
Salt concentration sets the scale. The Debye length of a 1:1 electrolyte at room temperature is 0.304 nm divided by the square root of the concentration in moles per liter, which makes it 0.3 nm in 1 M solution, 3 nm in 10 mM and 9.6 nm in 1 mM. In concentrated salt the double layers are too thin to fill a tip, and the curve is nearly straight. Length matters too: simulations of the Poisson-Nernst-Planck equations found that shortening a conical pore lowers its ability to rectify, which bears on pores in membranes only tens of nanometers thick.[2]
History
In 1997 Wei, Bard and Feldberg described current rectification at quartz nanopipet electrodes.[3] Zuzanna Siwy and colleagues then studied it in single conical pores made by etching tracks in polymer films. In 2002 one such pore in plastic, driven by a fluctuating voltage, pumped potassium ions against a concentration gradient.[4] In 2004 they used conical gold nanotubes, whose wall charge could be changed with the choice of electrolyte or by attaching thiols, to show how surface charge produces the effect, and described the tubes as an abiotic mimic of an ion channel that rectifies.[5]
Two papers of 2007 built diodes by design. One made a nanofluidic channel with a surface charge discontinuity, and its current-voltage curves agreed qualitatively with a one-dimensional model.[6] The other patterned a single asymmetric pore so that a sharp boundary separated a positively charged region from a negatively charged one. Ions accumulate or are depleted at the boundary, as in a bipolar semiconductor diode, and the rectification reached several hundreds between −5 and +5 V.[7] Theory has since gone beyond the diode. In 2021 Robin, Kavokine and Bocquet predicted that ions in slits one molecule thick assemble into clusters whose slow motion makes the conduction depend on its history, the property of a memristor, and in simulation two such slits reproduced the voltage spikes of the Hodgkin-Huxley model of a nerve.[8]
In measurement
A pore with this behavior has no single resistance. A conical pore recorded at one polarity gives a different value than the same pore at the other, which complicates sizing it from its conductance.
Rectification can also be the signal. The surface charge of the tip changes when something binds to it, and in 2009 a group showed that highly charged polymers on the glass surface of a 50 nm nanopipette changed its rectification, and that biotin-streptavidin and antibody-antigen binding changed its current.[9]
High salt gives a symmetric trace. The recordings of DNA barcodes in quartz pores about 15 nm wide were made in 4 M lithium chloride (see DNA barcode (nanopore)), where the ideal Debye length is 0.15 nm.
At Molecular Reality
The pore on the company's live molecular stream is far outside the regime. It is 200 µm wide, which in 1 M potassium chloride is about 660,000 Debye lengths, and its entry on the stream's list of provisions records that after wetting it behaved as a hole of that size should.[10] Its current rises in proportion to the voltage, with no bend in the curve.
See also
References
- ^ Cervera, J.; Schiedt, B.; Neumann, R.; Mafé, S.; Ramírez, P. (2006). "Ionic conduction, rectification, and selectivity in single conical nanopores". The Journal of Chemical Physics 124 (10): 104706. doi:10.1063/1.2179797.
- ^ Pietschmann, J.-F.; et al. (2013). "Rectification properties of conically shaped nanopores: consequences of miniaturization". Physical Chemistry Chemical Physics 15 (39): 16917. doi:10.1039/c3cp53105h.
- ^ Wei, C.; Bard, A. J.; Feldberg, S. W. (1997). "Current rectification at quartz nanopipet electrodes". Analytical Chemistry 69 (22): 4627-4633. doi:10.1021/ac970551g.
- ^ Siwy, Z.; Fuliński, A. (2002). "Fabrication of a synthetic nanopore ion pump". Physical Review Letters 89 (19): 198103. doi:10.1103/PhysRevLett.89.198103.
- ^ Siwy, Z.; Heins, E.; Harrell, C. C.; Kohli, P.; Martin, C. R. (2004). "Conical-nanotube ion-current rectifiers: the role of surface charge". Journal of the American Chemical Society 126 (35): 10850-10851. doi:10.1021/ja047675c.
- ^ Karnik, R.; Duan, C.; Castelino, K.; Daiguji, H.; Majumdar, A. (2007). "Rectification of ionic current in a nanofluidic diode". Nano Letters 7 (3): 547-551. doi:10.1021/nl062806o.
- ^ Vlassiouk, I.; Siwy, Z. S. (2007). "Nanofluidic diode". Nano Letters 7 (3): 552-556. doi:10.1021/nl062924b.
- ^ Robin, P.; Kavokine, N.; Bocquet, L. (2021). "Modeling of emergent memory and voltage spiking in ionic transport through angstrom-scale slits". Science 373 (6555): 687-691. doi:10.1126/science.abf7923.
- ^ Umehara, S.; Karhanek, M.; Davis, R. W.; Pourmand, N. (2009). "Label-free biosensing with functionalized nanopipette probes". Proceedings of the National Academy of Sciences 106 (12): 4611-4616. doi:10.1073/pnas.0900306106.
- ^ "Molecular Reality Corporation: live stream and provisions". Molecular Reality Corporation. Retrieved October 25, 2026.
