Site-directed engineering of alkyne-linked covalent organic frameworks defines divergent pathways for uranyl adsorption and fluorescence sensing.

A site-directed functionalization strategy was developed using an alkyne-linked covalent organic frameworks (COFs), TBTP-COF, prepared by 2, 4, 6-triformylphloroglucinol (Tp) and 1, 3, 5-tris(4-aminophenylethynyl) benzene (TAEB), as the parent framework. By selectively introducing phosphonic acid (-PO₃H₂) and sulfonic acid (SO₃H) groups into different site regions, phosphonic acid-functionalized COF (PA-TBTP-COF) and sulfonic acid-functionalized COF (SO₃H-TBTP-COF) were prepared, respectively. Both materials integrate uranyl capture and fluorescence sensing functions, but have different focuses in terms of adsorption and sensing. PA-TBTP-COF not only achieves an ultrahigh uranium adsorption capacity of 859.2 mg·g-1 due to multidentate chelation and high phosphonic acid loading but also completely avoids vanadium interference. Although SO3H-TBTP-COF also has a considerable capacity of 659.2 mg·g-1, its most prominent advantage is a low detection limit down to 0.021 μM, because the electron-withdrawing sulfonic acid groups promote charge delocalization and accelerate excited-state electron transfer to uranyl. After conducting a quantitative analysis of the functional group density using inductively coupled plasma mass spectrometry (ICP-MS) and normalizing it to a single site, it is known that the ultra-high adsorption capacity of PA-TBTP-COF originates from the stable capture ability provided by its dense phosphonic acid sites, whereas the high fluorescence detection sensitivity of SO₃H-TBTP-COF comes from its high signal transduction capability due to the rapid electron transfer kinetics. This verifies the effectiveness of the site engineering strategy from two complementary dimensions: adsorption and sensing. Furthermore, these results suggest that precise site-specific chemical tuning of the COF framework allows the selective enhancement of adsorption capacity or sensing sensitivity on the same material platform, thereby achieving on-demand customization of dual functional performance.