Since fluorescence was first observed in a solution containing quinine sulfate in 1845, methods employing fluorescent materials have abounded. Today, fluorescent probes are widely used in many fields, including biological imaging and security. For all applications brightness, stability, size control and compatibility are key parameters for probe design. Recent research to address these parameters has been focused on synthesizing nanoparticle materials. A class of those materials, core-shell silica nanoparticles, covalently encapsulating organic dyes within the core, have been found to be particularly effective in meeting the need for brighter, more stable probes, while also providing size and compatibility control. This dissertation will describe how I have built on previous research by focusing on the synthesis, characterization and optical properties of core-shell silica nanoparticles that span the full spectral range, from ultraviolet to near-infrared emission. Through scattering corrected comparisons of coumarin dye-containing nanoparticles and dye in aqueous solution, relative quantum yield enhancements of an order of magnitude were measured. Similar brightness enhancements were observed when large Stokes-shift dyes were incorporated into the particles. By creating a graded series of Stokes-shifted particles, single-excitation functionality was added to the fluorescent core-shell silica nanoparticle platform, to allow for spectral multiplexing. Since both light scattering and autofluorescence backgrounds in biological samples tend to be stronger at shorter wavelengths, new probe design has swiftly moved toward near-infrared (NIR) excitation and emission. To this end, NIR fluorescent silica nanoparticles with emissions up to 800nm were synthesized and dye structure-optical property correlations were investigated. Using a self-built fluorescence correlation spectroscopy (FCS) instrument, cyanine dye containing particles of the NIR series were investigated in further depth. The FCS allows for measurement of the size, brightness and concentration of the particles in solution and, with small modifications to the beam-path also provides information on the fluorescence stability of the particles in solution, which is of particular importance in NIR probes. Photostability in aqueous solution was investigated for the NIR dye Cy5 and compared to the dye incorporated into particles, leading to findings that will impact future probe designs.