A ternary dual-emission Eu(III)-functionalized HOF/COF hybrid: Tryptophan, 5-hydroxyindoleacetic acid ratiometric fluorescence detection and latent fingerprint identification

Fluorescent covalent organic framework (COFs) and lanthanide functionalized hydrogen-bonded organic framework have been widely studied in the field of fluorescence sensing due to their excellent properties. To our knowledge, there is no report about HOF/COF composites by combining HOF and COF. In our work, we propose a simple in-situ growth method to synthesize HOF/COF by adding pre-synthesized fluorescent COF into HOF preparation system. Based on the coordination post-synthetic modification, the ternary Eu(III) functional HOF/COF hybrid material (Eu@HOF/COF) is obtained, in which partial organic linkers of HOF are transformed to a new structure induced by Eu(III). At the single excitation wavelength, Eu@HOF/COF has two emission peaks corresponding to COF and Eu(III), respectively, and its luminescence is little affected by water molecules, which is relatively rare in previous work. Eu@HOF/COF, as a ratiometric fluorescence sensor can detect tryptophan and its metabolite 5-hydroxyindoleacetic acid with high selectivity, fast response, good sensitivity and recovery. Eu@HOF/COF has good fluorescence emission of Eu(III) and COF, and a large number of free carboxyl groups of HOF react with residues in latent fingerprint (LFP), which is beneficial in LFP identification. In summary, this study not only provides a useful reference for the construction of hybrid materials with Eu(III) and COF dual-emission centers, but also reveals that the new structure formed after the coordination of Eu(III) and HOF to sensitize Eu(III) luminescence, and also has the potential for the early detection of depression and diagnosis of carcinoid tumors.