Visual monitoring of silver ions and cysteine using bi-ligand Eu-based metal organic framework as a reference signal: Color tonality

• A novel red emitter bi-ligand Eu-MOF was prepared and used as a reference signal in a fluorescence ratiometry probe. • A novel fluorescent ratiometric probe was designed by encapsulating blue fluorescent carbon dots into bi-ligand Eu-MOFs. • This ratiometric fluorescent probes showed high sensitivity and selectivity for detection of Ag + and Cys. • Smartphone-based visual detection was developed depending on color tonality for detection of Ag + and cysteine using RGB analysis. Multicolor visual sensing approach significantly improves accuracy, reproducibility and detection rate of targets. Herein, a novel red emitter bi-ligand Eu-based metal organic framework (BiL-Eu-MOF) was successfully synthesized using a one step solvothermal method and was evaluated as a reference signal for multicolor visual-based ratiometry detection. The synergistic effects offered by the double ligand and Eu significantly showed red shift excitation towards many cations and anions and fluorescence stability at least for 6 months. As a fluorescent multicolor-visual probe, selective blue emitter carbon dots to Ag + ions were encapsulated into the pores of the red emissive of BiL-Eu-MOF to form blue-purple color. Addition of Ag + ions changed emission color from blue-purple to red, while addition of cysteine (Cys) restored the original blue-purple color. Employing RGB analysis using smartphone, the detection mode showed good linearity of Ag + and Cys in the range of 0–660 and 0–405 μM with limit of detection of 2.1 and 0.15 μM, respectively. Ultimately, our probe was applied for the quantification of silver ions in water samples and Cys in human serum samples showing excellent accuracy and precision.