Characterization of Diffusioosmotic Ion Transport for Enhanced Concentration-Driven Power Generation via Charge Heterogeneity in Nanoporous Membranes.

Nanoscopic mass/ion transport through heterogeneous nanostructures with various physicochemical environments occurs in both natural and artificial systems. Concentration gradient-driven mass/ion transport mechanisms, such as diffusioosmosis (DO), are primarily governed by the structural and electrical features of the nanostructures. However, these phenomena under various electrical and chemical conditions have not been adequately investigated. In this study, we fabricated a pervaporation-based particle-assembled membrane (PAM)-integrated micro-/nanofluidic device that facilitates easy tuning of the surface charge heterogeneity in nanopores/nanochannels. The nanochannels in the device consisted of two heterogeneous and in-series PAMs. The device was used to quantitatively measure electric signals generated by DO within the nanochannels with a single electrolyte or a combination of two electrolytes. Then, we characterized ion transport by changing surface charge heterogeneity and applying various electrolytic conditions, characterizing the concentration-driven power generation under these conditions. We found that not only does the charge heterogeneity provide additional resistance to ion transport but also the manipulation of the heterogeneity enables the effective modulation of ion transport and optimization of concentration-driven power generators regarding ion selectivity. In conjunction with the surface charge heterogeneity, the electrolytic conditions significantly affected the net flux of ion transport by enhancing or even negating the ion selectivity. Hence, we anticipate that both the platform and results will provide a deeper understanding of ion transport in nanostructures within complex environments by optimizing and improving practical concentration-driven applications, such as energy conversion/harvesting, molecular focusing/separation, and ionic diodes and memristors.