Fig. 1.

Building blocks of various types of polymeric nanoparticles with examples of some commonly used polymers and linkages. The main building blocks of polymeric nanoparticles are usually comprised of core-forming polymer; hydrophobic or charged (a), shell-forming polymer; neutral, hydrophilic and flexible properties are important for stealth nanoparticles (b), targeting ligand for selective cellular uptake and accumulation at target sites (c), and linkages between the shell and core and/or targeting moieties (d). Stimuli-responsiveness (pH, temperature, enzymatic, reductive or oxidative, etc.) can be imparted into the core, shell and/or the linkages. Shell or core-crosslinking can be also utilized to enhance the stability of nanoparticles (e).