Adaptive Polyzwitterion Coatings Enable Optimal Ion Rectification in Functionalized Nanochannels

The ion-selective properties of biological channels have inspired efforts to replicate their functions by using synthetic conical nanopores. However, challenges such as clogging and the complexities involved in fabricating conical geometries have limited their practical use as smart nanofluidic diodes. In this study, we overcame these issues by investigating pH-sensitive rectification within a cylindrical nanochannel, utilizing ion current rectification (ICR) induced by the nonuniform grafting of a zwitterionic polyelectrolyte layer. The gradual spatial variation in charge along the nanochannel allows precise control over the overlap of the electrical double layer (EDL) along the axial direction, enabling rectification even within a cylindrical structure. For the first time, we identified an optimal thickness of the polyelectrolyte zwitterionic layer at the tip (t_PZL) for specific bulk electrolyte concentrations and solution pH values that maximizes rectification, a finding that challenges the traditional paradigm of a direct, one-to-one correlation between t_PZL and rectification performance. Additionally, the zwitterionic groups on the grafted layer impart pH-dependent tunability to the rectification characteristics, enabling effective control over the direction of the diode-like response. We also developed a simplified analytical model capable of predicting the distinct rectification patterns as a function of t_PZL and determining the optimal t_PZL for given operating conditions. These findings hold important implications for overcoming the fabrication and operational limitations of conical nanochannels and provide a versatile platform for creating smart ion-based sensors that exploit pH-responsive ion rectification.