Bubble-Guided Foam Nanochannels for Tunable Ionic Transport

Biological nanochannels, as representative soft nanochannels, exhibit remarkably high efficiency, operating at energy levels only slightly above thermal noise. In contrast, solid-state nanochannels have garnered considerable attention across diverse fields due to their unique properties, including ionic current rectification, ionic Coulomb blockade, and ionic concentration polarization. However, their performance is strongly influenced by thermal noise, necessitating relatively high operating thresholds and, consequently, leading to substantial energy consumption. In this study, we report the fabrication of a bubble-regulated foam nanochannel on a nanocapillary platform and systematically investigate its ion-transport characteristics. Current-voltage (I-V) measurements reveal three distinct behaviors─rectification, linear, and voltage-activated─arising from variations in Tween 60 concentration and bubble volume. As the concentration of Tween 60 increases, a self-assembled monolayer (SAM) gradually forms on the surface of the foam nanochannel, leading to a reduction in the surface charge. This transition results in the progressive transformation of the I-V response from rectification to linear behavior. With further increases in Tween 60 concentration, a well-defined SAM layer develops at the gas-liquid interface, producing a confined nanochannel approximately 2 nm in diameter. Ion transport through this ultranarrow channel requires partial dehydration to overcome the energy barrier, likely driven by interactions between the hydrophilic SAM layer and solvated water molecules. The driving voltage is used to compensate for this ion dehydration, which results in voltage-activated transport behavior. The proposed mechanism was further validated using finite element method (FEM) simulations, which incorporate ionic hydration and size effects, offering quantitative insight into the observed phenomena. Overall, this work provides new mechanistic insights into the operation of soft nanofluidic systems and advances their potential for energy-efficient applications.