Post-synthetic modification of COFs are helpful strategies for introducing functionalities into them when the direct synthetic approach cannot be employed for diverse application.

Due to the excellent ion-sensing potential of covalent organic frameworks (COFs), the new imine-linked conjugated COF (IC-COF) is synthesized through a water-based synthesis reaction between 1,5-diaminonaphthalene and 2,4,6-tris(4-formylphenoxy)-1,3,5-triazine to create a luminescence sensor. It is noteworthy that the green synthesized IC-COF shows excellent selectivity to phosphate ions (PO₄^3-) with a detection limit of 0.61 μM. The recyclability performance of IC-COF is high, indicating that it can be reused without a significant reduction in performance (5.2% decline after 5 cycles). Theoretical calculations using the density functional theory are performed on the IC-COF-PO₄^3- and IC-COF-Cu⁺ complexes to explore the sensing mechanism. The fluorescence quenching in the presence of PO₄^3- ions is attributed to the difference between PO₄^3- binding sites to the IC-COF compared to Cu⁺, which leads to the considerable change in the IC-COF absorption spectrum from 400 to 600 nm.