Fluorescent semiconductor nanoplatelets (NPLs) are a new generation of fluorescent probes. NPLs are colloidal two-dimensional materials that exhibit several unique optical properties, including high brightness, photostability, and extinction coefficients, as well as broad excitation and narrow emission spectra from the visible to the near-infrared spectrum. All of these exceptional fluorescence properties make NPLs interesting nanomaterials for biological applications. However, NPLs are synthesized in organic solvents and coated with hydrophobic ligands that render them insoluble in water. A current challenge is to stabilize NPLs in aqueous media compatible with biological environments. In this work, we describe a novel method to disperse fluorescent NPLs in water and functionalize them with different biomolecules for biodetection. We demonstrate that ligand exchange enables the dispersion of NPLs in water while maintaining optical properties and long-term colloidal stability in biological environments. Four different colors of NPLs were functionalized with biomolecules by random or oriented conformations. For the first time, we report that our NPLs have a higher brightness than that of standard fluorophores, like phycoerythrin or Brilliant Violet 650 (BV 650), for staining cells in flow cytometry. These results suggest that NPLs are an interesting alternative to common fluorophores for flow cytometry and imaging applications in multiplexed cellular targeting.