Figure 1.

Passive and active targeting approaches of nanoprobes in cancer diagnosis. Passive tumor targeting is achieved by extravasation of nanoprobes through increased permeability of the tumor vasculature and ineffective lymphatic drainage (EPR effect). Environmentally sensitive nanoprobes (left inset) are another example of passive targeting that takes advantage of the characteristics of the tumor-associated microenvironments (i.e., acidic environment, and overexpressing enzymes). Active tumor targeting (right inset) can be achieved by functionalization of nanoprobes with targeting ligands that promote cell-specific recognition and binding