Abstract: The present disclosure is directed to a method for purifying a sample containing nucleic acids to obtain isolated nucleic acids of a desired size range, either above a size cut-off, below a cut-off, or within a defined band of sizes, including: a) combining a nucleic acid-containing sample with a binding buffer to provide a binding mixture; b) contacting the binding mixture with a silica nanomembrane, wherein the silica nanomembrane adsorbs nucleic acids from the binding mixture within a desired size-range; and c) separating the bound nucleic acid from the remaining sample. Kits including a silica nanomembrane, a binding buffer and one or wash buffers are also provided herein.