Ion-Selective Permeability of an Ultrathin Nanoporous Silicon Membrane as Probed by Scanning Electrochemical Microscopy Using Micropipet-Supported ITIES Tips

We report on the application of scanning electrochemical microscopy (SECM) to the measurement of the ion-selective permeability of porous nanocrystalline silicon membrane as a new type of nanoporous material with potential applications in analytical, biomedical, and biotechnology device development. The reliable measurement of high permeability in the molecularly thin nanoporous membrane to various ions is important for greater understanding of its structure permeability relationship and also for its successful applications. In this work, this challenging measurement is enabled by introducing two novel features into amperometric SECM tips based on the micropipet-supported interface between two immiscible electrolyte solutions (ITIES) to reveal the important ion-transport properties of the ultrathin nanopore membrane. The tip of a conventional heat-pulled micropipet is milled using the focused ion beam (FIB) technique to be smoother, better aligned, and subsequently, approach closer to the membrane surface, which allows for more precise and accurate permeability measurement. The high membrane permeability to small monovalent ions is determined using FIB-milled micropipet tips to establish a theoretical formula for the membrane permeability that is controlled by free ion diffusion across water-filled nanopores. Moreover, the ITIES tips are rendered selective for larger polyions with biomedical importance, i.e., polyanionic pentasaccharide Arixtra and polycationic peptide protamine, to yield the membrane permeability that is lower than the corresponding diffusion-limited permeability. The hindered transport of the respective polyions is unequivocally ascribed to electrostatic and steric repulsions from the wall of the nanopores, i.e., the charge and size effects.