阳离子/阴离子与单价/二价选择性之间的相互作用：在带负电纳米孔中的局部电荷反转和阴离子泄漏
The interplay of cation/anion and monovalent/divalent selectivity in negatively charged nanopores: local charge inversion and anion leakage
AI总结：
研究揭示了宽纳米孔中AMFE由电荷反转、阴离子泄漏和离子迁移的相互作用主导，表明单价与二价阳离子选择性由阳离子与阴离子选择性调节。
摘要：
异常摩尔分数效应（AMFE）被广泛认为是钙离子与单价离子选择性在带负电孔道中的标志。尽管AMFE在高度阳离子选择性的狭窄离子通道中已被充分理解，但在宽的合成纳米孔中，其中阴离子也可能参与传输，其微观起源仍不明确。在此，我们使用一种简化的能斯特-平面克+局部平衡分子动力学框架来研究带负电PET纳米孔中的离子传输，特别强调建模表面羧酸（COO⁻）基团如何影响电荷反转、离子电流和AMFE。我们系统地比较了固定点电荷模型和显式粒子表示的表面氧原子，并识别出两个控制参数：离子电荷与孔道电荷之间最近距离（DCA）以及网格间距，该间距调节局部化（同时保持平均表面电荷恒定）。通过将孔道扩散系数拟合到三个实验电导率点，我们重现了整个实验AMFE曲线以及在CaCl₂中观察到的阴离子泄漏现象，该现象在实验和分子动力学模拟中均被观测到。值得注意的是，尽管表面基团的微观模型存在显著差异，只要DCA匹配，不同模型所产生的设备级电导率曲线是无法区分的。我们的结果表明，在宽纳米孔中AMFE由电荷反转、阴离子泄漏和离子迁移之间的微妙相互作用所主导，揭示了在宽孔中单价与二价阳离子选择性由阳离子与阴离子选择性之间的相互作用所调节。
英文摘要：
The anomalous mole fraction effect (AMFE) is widely regarded as a hallmark of calcium versus monovalent ion selectivity in negatively charged pores. While AMFE is well understood in highly cation-selective narrow ion channels, its microscopic origin in wide synthetic nanopores, where anions may also contribute to transport, remains less clear. Here, we use a reduced Nernst-Planck + Local Equilibrium Monte Carlo framework to study ionic transport in a negatively charged PET nanopore, with particular emphasis on how the modeling of surface carboxyl (COO$^{-}$) groups influences charge inversion, ionic currents, and AMFE. We systematically compare fixed point-charge models and explicit-particle representations of surface oxygens and identify two controlling parameters: the distance of closest approach (DCA) between ionic charges and pore charges and grid spacing that modulates localization (while keeping average surface charge constant). By fitting pore diffusion coefficients to three experimental conductance points, we reproduce the entire experimental AMFE curve as well as anion leakage in CaCl$_2$ seen in experiments and molecular dynamics simulations. Remarkably, vastly different microscopic models of the surface groups yield indistinguishable device-level conductance curves when the DCA is matched, despite substantial differences in local Ca$^{2+}$ concentration profiles. Our results demonstrate that AMFE in wide nanopores is governed by a delicate interplay between charge inversion, anion leakage, and ionic mobility, underlying that in wide pores monovalent vs.\ divalent cation selectivity is modulated by cations vs.\ anion selecivity.
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