A unimolecular nanoparticle, a composition thereof, and methods of use thereof that includes 1) a dendritic polymer having a molecular weight of about 500-120,000 Da and terminating in hydroxyl, amino or carboxylic acid groups 2) cationic polymers attached to at least a majority of the terminating groups of the dendritic polymer via a pH-sensitive linker, wherein each cationic polymer comprises a polymeric backbone attached to cationic functional groups and to weakly basic groups by disulfide bonds, wherein the molar ratio of cationic functional groups to weakly basic groups ranges from 1:1-5:1, and has a molecular weight from about 1,000-5,000 Da and 3) poly(ethylene glycol) attached to a plurality of cationic polymers and having a terminal group selected from a targeting ligand, OH, O-alkyl, NH2, biotin, or a dye, wherein the terminal group of at least one poly(ethylene glycol) is having a molecular weight of about 1,000-15,000 Da.
