Propagation of Concentration Polarization Affecting Ions Transport in Branching Nanochannel Array

Abstract

A new ionic current rectification device responsive to a broad range of pH stimuli has been fabricated using porous anodic alumina membrane with tailor-made branching nanochannel array. The asymmetric geometry is achieved by changing oxidation voltage using a simple two-step anodization process. Due to the protonation/deprotonation of the intrinsic hydroxyl groups upon solution pH variation, the nanochannels array-based device is able to regulate ionic current rectification properties. The ion rectification ratio of the device is mainly determined by the branching size and surface charges, which is also confirmed by theoretical simulations. In addition, theoretical simulations show that the slight difference in ion rectification ratio for the nanochannel devices with different branching numbers is due to the propagation of concentration polarization. Three dimensional simulations clearly show the nonuniform concentration profiles in stem nanochannel near the junction interface due to the presence of branching nanochannels. The present ionic device is promising for biosensing, molecular transport and separation, and drug delivery in confined environments.

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

“…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 funnel‐shaped 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.…”

“…[42] and Li et al. [43] is another type of asymmetric AAO nanochannel having a nonuniform cross‐section.…”

A review: applications of ion transport in micro‐nanofluidic systems based on ion concentration polarization

“…With the development of nanofluidic technology, nanopores similar to rectifier diodes for rectifying ion currents have been fabricated . The main conditions for nanofluidic ion current rectification (ICR) are the asymmetry of the cross‐sectional area of the nanochannel, the asymmetric concentration of the salt solution and the uneven distribution of the space charge density …”

Construction of biomimetic multi-story nanochannel systems and their ionic current behaviors

“…阳极氧化法是以铝、钛等金属为阳极置于电解质 溶液中, 利用电解作用, 使其表面形成氧化薄膜. Li 等 [18] 通过简单的两步阳极氧化法制备了具有多级结构 的氧化铝纳米通道. Chen等 [19] 通过阳极氧化法制备了 具有"纳通道-离子通道"的两级结构的带有障碍层的 阳极氧化铝.…”

“…由于结构或电荷分布 不对称, 多级化纳米通道表现出离子整流性 [30] . 离子 整流 [18] 是指在纳米通道两端施加大小相同、方向相反 的偏压时, 电流的绝对值不相等, 是一种偏离线性I-V 曲线的非欧姆行为, 类似于半导体二极管的单向导通 特性 [31~33] . 通过改变通道结构或在孔道内表面修饰具 有特殊功能的分子可以调控多级化纳米通道的离子输 运性质.…”