Tunable rectifications in nanofluidic diodes by ion selectivity of charged polystyrene opals for osmotic energy conversion

Nanofluidic diodes mimicking biological ion channels are emerging as desirable materials for controllable ion transport in artificial nanochannels due to their rectification properties. However, the precise control of the chemical composition, geometric structure, surface charges and subsequent rectification of nanofluidic diodes is still challenging. Herein, bottom-up self-assembly is used to fabricate nanofluidic diodes with building blocks of an alumina nanoporous membrane and ion-selective polystyrene opals. Through changing the sphere sizes of opals, the surface charges of opals and the assembly sequence of the building blocks, the chemical composition, geometric structure and surface charges of nanofluidic diodes are tailored on demand, which results in precise control of the rectification. The ion rectification of these assembled diodes is theoretically investigated by a model calculation based on Poisson-Nernst-Planck equations. The bottom-up assembled diodes are applied in conversion of osmotic energy into electricity, which demonstrates a high conversion performance.