Solid-state luminescence turn-on sensing of triethylamine via confining indicator displacement assays in an engineered covalent organic framework.

In this work, we present a novel strategy for developing solid-state luminescence turn-on sensors by constructing indicator displacement assays (IDAs) within a covalent organic framework (COF) composed of 2,4,6-triformylphloroglucinol (Tp) and 5, 5'-diamino-2, 2'-bipyridine (Bpy) ligands. To illustrate our strategy, a robust electron-deficient COF was synthesized through the post-synthetic N-alkylation of its bipyridyl units. The cationic N-alkylated bipyridyl units served as receptors, while the fluorescent indicator, anionic 8-anilino-1-naphthalenesulfonate (ANS-), was incorporated within the framework via anion exchange. The resulting host-guest hybrid exhibited negligible fluorescence due to the efficient electron donor-receptor charge-transfer (CT) quenching of the ANS- indicators. Upon exposure to the strong electron-donating analyte triethylamine (TEA), the competitive CT interaction between the ANS- indicators and TEA with the cationic bipyridyl receptors initiates the indicator displacement process, leading to the significant fluorescence activation of ANS-. This rapid and sensitive fluorescence turn-on response in dioxane enables efficient detection of TE{Wang, 2020 #79}A with a limit of detection (LOD) of 0.95 nM. Moreover, practical applicability was demonstrated through the fabrication of test strips exhibiting visually observable solid-state fluorescence at TEA concentrations as low as 1 μM. This work presents the first demonstration of confining IDAs within COFs for solid-state luminescence turn-on sensing of TEA. It not only establishes a general approach for developing turn-on TEA sensors but also broadens the utility of COFs as versatile platforms for advanced sensing applications.