Seven Models for Outer Surface-Only Functionalized Nanofluidic Systems

Ionic current rectification (i.e., ICR) is a pivotal phenomenon in nanofluidic systems, crucial for various applications such as bioanalysis, iontronics, ion sieving, and energy conversion. Despite its importance, previous studies on the ICR have predominantly concentrated on the inner wall of nanochannels and the functional elements (FEs) distributed within it, often overlooking the outer surface (OS) and its FEs, including steric hindrance, surface charge, or space charge contributions to ICR, not to mention the potential synergistic influence of these factors on ICR. This study addresses the gap by demonstrating the substantial impact of OS functional elements (FEOS) on ICR based on porous polyethylene terephthalate nanochannels featuring only OS functionalization at the tip side. We established seven distinct models and revealed that the synergistic effects of surface charge, space charge, and steric hindrance of the FEOS are identified as pivotal in the ICR process. Using polyethylenimine as FEOS, an ICR of ∼1000 at 0.1 M KCl was achieved, the highest value regulated by only FEOS. Furthermore, we also demonstrated the ability to regulate ICR via the OS and the potential sensing capability of the nanochannels. These findings highlight the role of FEOS in the nanochannel electrokinetics and can inform the design of FEOS-engineered nanofluidic devices for ionic circuit chips, biosensing, energy conversion, and DNA analysis.