The present invention provides a fluorescent silica-based 
nanoparticle that allows for precise detection, characterization, monitoring and treatment of a 
disease such as 
cancer. The 
nanoparticle has a range of diameters including between about 0.1 nm and about 100 nm, between about 0.5 nm and about 50 nm, between about 1 nm and about 25 nm, between about 1 nm and about 15 nm, or between about 1 nm and about 8 nm. The 
nanoparticle has a fluorescent compound positioned within the nanoparticle, and has greater brightness and fluorescent 
quantum yield than the free fluorescent compound. The nanoparticle also exhibits high biostability and 
biocompatibility. To facilitate efficient urinary 
excretion of the nanoparticle, it may be coated with an 
organic polymer, such as poly(
ethylene glycol) (PEG). The small size of the nanoparticle, the silica base and the 
organic polymer coating minimizes the 
toxicity of the nanoparticle when administered 
in vivo. In order to target a specific 
cell type, the nanoparticle may further be conjugated to a ligand, which is capable of binding to a 
cellular component associated with the specific 
cell type, such as a 
tumor marker. In one embodiment, a therapeutic agent may be attached to the nanoparticle. To permit the nanoparticle to be detectable by not only 
optical fluorescence imaging, but also other imaging techniques, such as 
positron emission 
tomography (PET), 
single photon emission computed tomography (SPECT), computerized 
tomography (CT), 
bioluminescence imaging, and 
magnetic resonance imaging (MRI), radionuclides / radiometals or paramagnetic ions may be conjugated to the nanoparticle.