Highly Rectifying Conical Nanopores in Amorphous SiO2 Membranes for Nanofluidic Osmotic Power Generation and Electroosmotic Pumps

Nanopore membranes are a versatile platform for a wide range of applications ranging from medical sensing to filtration and clean energy generation. To attain high-flux rectifying ionic flow, it is required to produce short channels exhibiting asymmetric surface charge distributions. This work reports on a system of track etched conical nanopores in amorphous SiO2 membranes, fabricated using the scalable track etch technique. Pores are fabricated by irradiation of 920 +/- 5 nm thick SiO2 windows with 2.2 GeV 197Au ions and subsequent chemical etching. Structural characterization is performed using atomic force microscopy, scanning electron microscopy, small-angle X-ray scattering, ellipsometry, and surface profiling. Conducto-metric characterization of the pore surface is performed using a membrane containing 16 pores, including an in-depth analysis of ionic transport characteristics. The pores have a tip radius of 5.7 +/- 0.1 nm, a half-cone angle of 12.6 +/- 0.1 degrees, and a length of 710 +/- 5 nm. The pKa, pKb, and pI are determined to 7.6 +/- 0.1, 1.5 +/- 0.2, and 4.5 +/- 0.1, respectively, enabling the fine-tuning of the surface charge density between +100 and -300 mC m-2 and allowing to achieve an ionic current rectification ratio of up to 10. This highly versatile technology addresses some of the challenges that contemporary nanopore systems face and offers a platform to improve the performance of existing applications, including nanofluidic osmotic power generation and electroosmotic pumps.