Site-directed engineering of alkyne-linked covalent organic frameworks defines divergent pathways for uranyl
Yingfei Hui1, Hao Guo1, Yinsheng Liu2
1Key Lab of Eco-Environments Related Polymer Materials of MOE, Key Lab of Bioelectrochemistry and Environmental Analysis of Gansu Province, College of Chemistry and Chemical Engineering, Northwest Normal University, Gansu International Scientific and Technological Cooperation Base of Water-Retention Chemical Functional Materials, Lanzhou 730070, PR China.
Abstract:
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.
