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

Abstract

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 SiO$_2$ membranes, fabricated using the scalable track etch technique. Pores are fabricated by irradiation of 1 $\mu$m thick SiO$_2$ windows with 2.2 GeV $^{197}$Au ions and subsequent chemical etching. Structural characterisation is performed using atomic force microscopy (AFM), scanning electron microscopy (SEM), small angle X-ray scattering (SAXS), ellipsometry, and surface profiling. Conductometric characterisation 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 $\pm$ 0.1) nm, a half-cone angle of (12.6 $\pm$ 0.1)$^{\circ}$, and a length of (710 $\pm$ 5) nm. The $pK_a$, $pK_b$, and $pI$ are determined to 7.6 $\pm$ 0.1, 1.5 $\pm$ 0.2, and 4.5 $\pm$ 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 challenges that contemporary nanopore systems face, and offers a platform to improve the performance of existing applications.