The disclosure features novel methods of producing nucleic acid lipid nanoparticle (LNP) compositions employing a modifying agent after formation of a precursor nucleic acid lipid nanoparticle, the produced compositions thereof, and methods involving the nucleic acid lipid nanoparticles useful in the delivery of therapeutics and/or prophylactics, such as a nucleic acid, to mammalian cells or organs to, for example, to regulate polypeptide, protein, or gene expression.