Technological tools that assist in the prevention and detection of ecological disasters are a current priority. Particularly, the detection of contaminants such as cobalt (Co) is important due to their harmful impact on humans and the ecosystem. When cobalt is discharged into aquifers due to anthropogenic activities, it contaminates the marine fauna and flora. In consequence, the potential risk of intoxication in humans and the environment is evident through the food chain. In this paper, we report the preliminary results regarding the fluorescent dye calcein (C₃₀H₂₆N₂O₁₃) used as a sensor for the detection of cobalt levels in aqueous solutions. The sensor cell based on Calcein is built by fixed in layers by means of thermoplastic polyurethane (TPU) and adjusted to pH=7. The layer shows a fluorescence emission in the range of λ=545 nm to 570 nm when it is excited by optical fields at a wavelength centered at 465 nm. By the contact of different Cobalt concentrations to the calcein layer structure, quenching of the fluorescence intensity is observed. The results indicate that the sensor exhibits a linear response of the fluorescence quenching related to the cobalt concentration level in the range of 10-5 to 10-3 mol/L. Additionally, the proposed sensor has a simple experimental set-up, low cost, and does not require additional complex instrumentation.