Non-viral vectors for delivering agents to a site within the body of an animal comprise a biocompatible nanoparticle conjugated to an avidin/biotin complex and a water soluble linear polymer comprising multiple binding sites. The agents to be delivered are conjugated to each of the multiple binding sites, thereby increasing the loading capacity of the system. Where the agents comprise siRNA, each biotin/avidin complex may carry greater than four siRNA. The biocompatible nanoparticle may also comprise a fluorescent dye for in vivo imaging.