Cobalt ions (Co) are among heavy metals ions which cause pollution in environment because of their toxicity and improper degradation. In this work, a new fluorescent approach based on silicon nanoparticles (Si NPs) was designed for Co detection. The fluorescent Si NPs were prepared by mixing 3-aminopropyl trimethoxysilane (APTES) and basic fuchsin, and under the excitation of 400 nm, they emitted green fluorescence at 515 nm. The prepared Si NPs were highly soluble in water, stable to salt and pH, and their fluorescence emission was extremely constant, with the quantum yield of 2.28%. The detailed mechanism studies showed that Co effectively quenched the fluorescence of Si NPs by forming static complex. After optimizing the reaction parameters, a good linear relationship for Co was observed from 0.2 to 60 μM, and the limit of detection was 0.14 μM that is lower than the guideline announced by Department of Environmental Protection for drinking water (1.7 μM). The preparation method of Si NPs was cheap, rapid and simple, and the fluorescent approach was applied to determine Co in Yellow river water, drinking water, and industrial wastewater. Moreover, the Si NPs has good response to exogenous Co in HepG2 cell imaging.