pH-responsive zwitterionic polypeptide as a platform for anti-tumor drug delivery.

•Nanoparticles based on poly(amino acid)s.•Modified with zwitterionic lysine groups as the hydrophilic segments.•DOX loaded via cleavable hydrazone bonds, hydrophobic interaction and π-π stacking.•A pH-dependent drug release profile.•Particles with surface charge conversion at different pH.