We show that efficient near-infrared (NIR) cyanine fluorophores ((1) under bar and (2) under bar) can be encapsulated into silica nanoparticles providing a highly versatile and unique platform for in vivo diagnostics. Utilizing this platform, multiple fluorophores can be loaded within a single particle allowing the light absorption and emission properties of the nanoparticle to be controlled independent of particle size. Furthermore, such dyed nanoparticles may have extinction coefficients as high as about 100 x 10(6) L mol(-1) cm(-1) in the NIR (on a per mole of particles basis), with quantum yields from about 8-10%. A simple synthetic method for varying particle size and dye-loading level is presented, and a modified Stober synthesis reduces deleterious exposure of the dye to the highly alkaline conditions used. The cyanine dyes are encapsulated in silica in a non-aggregated state and the fluorescence brightness is largely maintained to nominal dye concentrations approaching 50 mu M. The ability to control light absorption and emission properties independent of particle size, and convenient access to particle sizes in the range of 20-100 nm (a size regime difficult to access with other nanoparticle approaches such as quantum dots), are important features for anatomical targeting in in vivo diagnostics and targeted therapeutic applications. (C) 2007 Elsevier Inc. All rights reserved.