Carbon dots are a new and important form of zero-dimensional carbonaceous nanomaterials. In our present work, disperse water-soluble fluorescent carbon dots were prepared using coffee grounds as raw material by hydrothermal treatment. The as-prepared carbon dots exhibit an excitation-dependent and high photostable photoluminescence behavior which can be applied to Hela cell imaging. Notably, the fluorescence of the carbon dots can be quenched by Fe3+ ions, which can serve as a useful fluorescent probe for detecting Fe3+ ions.