Ion current rectification behavior of a nanochannel having nonuniform cross‐section

Abstract

Ion current rectification behavior of a nanochannel having nonuniform cross-sectionDue to its versatile applications in biotechnology, ion current rectification (ICR), which arises from the asymmetric nature of the ion transport in a nanochannel, has drawn much attention, recently. Here, the ICR behavior of a pH-regulated nanochannel comprising two series connected cylindrical nanochannels of different radii is examined theoretically, focusing on the influences of the radii ratio, the length ratio, the bulk concentration, and the solution pH. The results of numerical simulation reveal that the rectification factor exhibits a local maximum with respect to both the radii ratio and the length ratio. The values of the radii ratio and the length ratio at which the local maximum in the rectification factor occur depend upon the level of the bulk salt concentration. The rectification factor also shows a local maximum as the solution pH varies. Among the factors examined, the solution pH influences the ICR behavior of the nanochannel most significantly.Abbreviations: AAO, anodic aluminum oxide; EDL, electric double layer; ICR, ion current rectification; IEP, isoelectric point been concluded that the ICR phenomenon of nanochannels/nanopores can be attributed to their asymmetric characteristics. These include, for instance, geometry [20][21][22][23][24][25][26][27][28], surface charge [29, 30], chemical composition [31, 32], wettability [33], and external applied fields such as pH gradient [34], electrolyte concentration gradient [35], and pressure gradient [36]. Among these, geometry-induced ICR draws much attention because the associated nanochannel fabrication methods are convenient and controllable. These include, for example, track-etching [37], focused ion beam sculpting [38], and electron-beam lithography [39]. Through these methods, various types of nanochannels have been fabricated such as cylindrical [27], conical [22], hourglass-shaped [26], cigar-shaped [20], bullet-shaped [25], dumbbell-shaped [21], and funnelshaped nanochannels [23]. The ICR behavior of an asymmetric nanochannel can be influenced by factors including ionic species [40], ion concentration [35, 41], applied electric potential [40], pH [42, 43], temperature [44], and nanochannel length and its opening radii [24,45,46]. The cone angle [24] and the ratio of cylindrical segment/conical segment [23] are important to conical and funnel-shaped nanochannels, respectively.Previous studies regarding asymmetric nanochannel focused mainly on polymeric material such as polycarbonate Color online: See article online to view Figs. 1-8 in color.

Modulation of Ionic Current Rectification in Ultrashort Conical Nanopores

“…ICR can be induced by asymmetry in nanopore geometry, as done in conically shaped nanopores, 2 as well as nanopores with stepped cross-section. 12 Asymmetric surface properties of nanopores, e.g. asymmetric charge density, 13 wettability 14 and slip length 15 of surfaces along the pore axis can also produce ICR.…”

“…ICR can be induced by asymmetry in the nanopore geometry, as done in conically shaped nanopores, 2 as well as nanopores with a stepped cross section. 12 Asymmetric surface properties of nanopores, for example, asymmetric charge density, 13 wettability, 14 and slip length, 15 of surfaces along the pore axis can also produce ICR. Nanopores in contact with solutions that differed in conductivity achieved by concentration gradients 16 and viscosity gradients 17,18 were reported to exhibit current rectification as well.…”

“…One voltage polarity corresponding to the high-conductance state is caused by the accumulation of ions, while the opposite voltage polarity leads to formation of a depletion zone and low ionic conductance. ICR can be induced by asymmetry in the nanopore geometry, as done in conically shaped nanopores, as well as nanopores with a stepped cross section . Asymmetric surface properties of nanopores, for example, asymmetric charge density, wettability, and slip length, of surfaces along the pore axis can also produce ICR.…”

Nanosensing of Acetylcholine Molecules: Influence of the Association Mechanism

“…Due to their capability of simulating versatile charged conditions and responding to environmental changes, biologically inspired artificial nanopores are applied widely in many areas, including ionic current rectification (ICR), − energy conservation, − desalination, − and sensing. − Among these, ICR has been studied extensively in the past decades. This phenomenon refers to the diodelike behavior of a nanopore, where its ionic current–voltage relationship does not follow the conventional Ohm’s rule when the polarity is switched.…”

Improved Rectification and Osmotic Power in Polyelectrolyte-Filled Mesopores

“…For example, the diode-like ion current rectification (ICR) behavior, which assures ions preferentially transporting in one direction and hence amplifies ionic current, can be observed when the symmetry of ionic concentration profiles along the axis of a nanopore is broken [ 18 , 19 ]. Considerable experimental [ 20 , 21 , 22 ] and theoretical [ 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 ] efforts have been made on ICR in nanofluidics and all these studies concluded that the ICR property only can emerge in case the pore size is comparable to the EDL thickness.…”