An improved nanoparticle for transfecting cells is provided. The nanoparticle includes a core polyplex and a silica coating on the core polyplex and, optionally, a polymer attached to an outer surface of the silica coating, where the polyplex includes an anionic polymer, a cationic polymer, a cationic polypeptide, and a polynucleotide. Also provided is an improved method of modifying intracellular polynucleotides. The method includes contacting a cell with a nanoparticle that includes a core polyplex and a silica coating on the core polyplex and, optionally, a polymer attached to an outer surface of the silica coating, where the polyplex includes an anionic polymer, a cationic polymer, a cationic polypeptide, and a polynucleotide.
