In one aspect, the invention provides mesoporous silica nanoparticles (MSNPs), monodisperse populations of MSNPs and related protocells which exhibit single cell binding specificity to the substantial exclusion of non-targeted cells. For example, MSNPs and protocells of the invention may be used to target specific delivery of therapeutic agents to cancer cells or to specific blood vessel types (e.g. in the arterial, venous and/or capillary vessels or any combination of vessels). Related protocells, pharmaceutical compositions and therapeutic and diagnostic methods are also provided.
STATEMENT REGARDING FEDERAL FUNDING
 This invention was made with government support under DE-AC04-94AL85000 awarded by the U.S. Department of Energy and U01 CA15179201 by the National Institutes of Health. The government has certain rights in the invention.