Highly Selective and Visual Detection of Co²⁺ Using CDs/MOF-Eu Dual-Wavelength Fluorescent Hydrogels

Contamination by the heavy metal cobalt ion (Co²⁺) poses a serious threat to the ecological environment and human safety. Consequently, there is an urgent need to develop a new method for detecting Co²⁺ that combines rapid response, high sensitivity and high selectivity. In this study, carbon dots (CDs) were prepared using urea and citric acid as raw materials. These were mixed with MOF-Eu and incorporated into a PA hydrogel constructed from sodium alginate (SA) and acrylamide (PAM), resulting in the successful preparation of a PA-CDs@MOF-Eu composite hydrogel with dual-wavelength independent luminescence properties. The results indicate that the hydrogel exhibits characteristic red fluorescence from MOF-Eu upon excitation at 398 nm and characteristic blue fluorescence from CDs upon excitation at 342 nm; precise control of the dual wavelengths can be achieved by adjusting the excitation wavelength. Furthermore, the hydrogel enables highly sensitive and selective visual detection of Co²⁺, with a detection limit (LOD) of 0.22 µM within a linear response range of 0 ~ 120 µM. The fluorescence quenching mechanism of the hydrogel upon introduction of Co²⁺ is attributed to electronic energy transfer (EET). This study provides a new, highly efficient and stable material for the detection of heavy metal ions, offering significant potential for application in fields such as water quality testing and environmental monitoring.