Ultrathin Nanoporous Silicon Nitride Membranes for Separations and Biosensing

Abstract

Separation processes and biosensing are interesting applications for single- and multiple-nanopore membranes. Pore density, surface chemistry and shape are determining factors of the membrane capabilities in these applications. We present ultrathin freestanding SiN membranes with conical or double-conical nanopores prepared by track-etching. This technique allows the membrane porosity to be tuned from a single pore to billions of pores per cm2, and these pores have diameters as small as several nanometers. These conical and double-conical pores give higher permeant fluxes compared to standard membranes with cylindrical pores, which is a key advantage in separation applications. To show the separation capabilities of these membranes, we discriminate between dye and protein molecules based on their size and charge. This separation process operates in physiological electrolyte conditions and is based on an electrostatic mechanism. We also demonstrate that by chemically modifying the pore surface, we can tune the separation behavior. In addition, the conical pore shape results in a shorter effective length region which dominates the pore's electrical resistance. This is advantageous for single biomolecule detection applications such as nanopore-based DNA analysis because the detected signal corresponds to a smaller region of the analyte molecule. We present initial results with single-pore membranes evaluating these advantages.