Eu-functionalized covalent organic framework for ratiometric fluorescence detection and adsorption of tetracycline and information steganography.

Functional materials with organic/inorganic composites as the main matrix and rare earth ion complexes as the guest have shown a very broad application prospect for antibiotic sensors. However, Eu-complex often relies on a single fluorescence response signal, which is susceptible to changes in the detection environment and cannot simultaneously detect and remove tetracycline (TC). Herein, green fluorescent covalent two-dimensional organic framework (COF-TD) is synthesized, followed by modification of Eu to synthesize COF-TD@Eu. In the ratiometric sensor, Eu serves as the recognition site and specific response probe for TC, while COF-TD is the fluorescence reference and carrier for Eu. Due to the antenna effect, TC enhances the red fluorescence of Eu, while the green fluorescence of COF-TD remains almost stable. Based on the change of fluorescence intensity and fluorescence color from green to red, the efficient ratiometric sensing can be finished in 1 min. The developed method shows high sensitivity with a detection limit of 0.3 μM and high selectivity to TC which makes the method applicable to detect TC in traditional Chinese medicine preparations. In addition, due to the high specific surface area of COFs and specific adsorption sites, COF-TD@Eu also shows good performance for TC removal. The findings show that the maximum adsorption capacity is 137.3 mg g and the adsorption equilibrium is reached in 30 min. Smartphone assisted COF-TD@Eu for both ratiometric fluorescence detection and detecting the absorption of TC is proposed for the first time. The molecular cryptosteganography that transforms the selective response of COF-TD@Eu to binary strings is anticipated to advance utilization of nanomaterials in logic sensing and information safety.