Regenerable and stable sp² carbon-conjugated covalent organic frameworks for selective detection and extraction of uranium

The widespread use of nuclear power, uranium mining, accidents, and improper waste disposal have introduced significant amounts of uranyl (UO₂²⁺) into the environment, demanding materials that can both detect and extract UO₂²⁺ in real time and under harsh conditions. Existing porous materials face limitations: amorphous porous organic polymers (POPs) suffer from irregular pores and slow mass transfer; many metal–organic frameworks (MOFs) lack stability in acid/base, high temperature, or radiation; and commonly used COFs based on boron–oxygen or imine linkages can degrade under such conditions, hindering regeneration and practical use. COFs offer tunable porosity, high surface area, and post-synthetic functionalization for targeted binding, but robust sp² carbon-conjugated COFs have been synthetically challenging and largely unexplored for UO₂²⁺ sensing/extraction. The study aims to design a chemically, thermally, and radiationally stable sp² carbon-conjugated fluorescent COF incorporating amidoxime ligands in open 1D channels to enable selective, rapid, and regenerable UO₂²⁺ detection and extraction.

Prior UO₂²⁺ sorbent platforms include POPs, MOFs, hydrogels, and imine- or boron-linked COFs; however, many show poor structural stability under acids/bases or radiation, slow kinetics, and/or limited regenerability. Schiff-base COFs (e.g., COF-TpAb-AO and o-TDCOF) incorporate amidoxime for uranium capture but suffer from hydrolytic/radiation instability of their linkages. Recent progress in sp² carbon-conjugated COFs (e.g., sp²c-COF, TP-COF, Por-sp²c-COF, g-C₃N₄ COF) demonstrates enhanced stability and conjugation, yet their application to UO₂²⁺ detection/extraction has been untested. Fluorescent UO₂²⁺ sensing with various materials often exhibits poor selectivity and long response times. These gaps motivate a robust, fluorescent sp² COF with highly accessible amidoxime sites for selective, fast UO₂²⁺ capture and sensing.

- Materials design: Construct a sp² carbon-conjugated COF with open 1D channels and dense amidoxime ligands to ensure high stability, strong fluorescence, and selective UO₂²⁺ binding. - Synthesis of TFPT-BTAN: Polymerize 2,4,6-tris(4-formylphenyl)-1,3,5-triazine (TFPT) with 2,2′,2″-(benzene-1,3,5-triyl)triacetonitrile (BTAN) via a Knoevenagel reaction. Optimized conditions: o-dichlorobenzene (o-DCB):4 M DBU (10:1 v/v), 90 °C; obtain highly crystalline TFPT-BTAN (AA-stacked, open 1D channels ~1.5 nm). - Post-synthetic amidoximation: Treat TFPT-BTAN with excess NH₂OH·HCl at 85 °C for 24 h to convert cyano groups to amidoxime, yielding TFPT-BTAN-AO. - Characterization: FT-IR (disappearance of C=O in monomer; C≡N at ~2241 cm⁻¹ in TFPT-BTAN; loss of C≡N and appearance of amidoxime bands at 1403 and 1707 cm⁻¹ after conversion), solid-state ¹³C CP/MAS NMR (cyano C ~113 ppm, triazine C ~171 ppm; amidoxime C ~156 ppm), PXRD (distinct (100) at 5.8° 2θ; Pawley refinement Rp=2.85%, Rwp=4.27%; crystallinity retained after amidoximation), N₂ sorption (BET 1062 m² g⁻¹, pore 1.44 nm for TFPT-BTAN; BET 803 m² g⁻¹ after AO), SEM (porous network retained), TGA (stable to ~320 °C). - Stability tests: Expose TFPT-BTAN-AO to water (100 °C), 1 M HCl, 1 M NaOH, HNO₃ (0.1–5.0 M), and γ-irradiation (50, 200 kGy); assess by FT-IR, PXRD, and adsorption performance; compare with β-ketoenamine COFs (Tp-Bpy, Tp-BD) under strong nitric acid. - Fluorescence sensing: Disperse TFPT-BTAN-AO in water; excite at 277 nm; emission at 460 nm; optimize pH (best at 6.0); measure response time (equilibrium within 2 s); selectivity by adding various metal ions (UO₂²⁺ 20 μM; others 50 μM) and visual inspection under 365 nm UV; calibration 0.02–6.0 μM with R²; determine LOD (3σ/slope) as 6.7 nM. - Interaction studies: FT-IR after UO₂²⁺ loading (O–U–O vibration at 916 cm⁻¹; N–H bending at 1613 cm⁻¹); XPS N 1s and O 1s core levels before/after UO₂²⁺ (appearance of N–U at 401.1 eV; O–U at 531.3 eV; binding energy shifts), indicating coordination of amino and hydroxyl of amidoxime; time-resolved fluorescence (lifetime decreases from 3.1 to 1.8 ns) consistent with PET quenching. - Adsorption/extraction performance: Synthesize amorphous POP analog (POP-TB) and amidoximate it (POP-TB-AO) as control; conduct UO₂²⁺ adsorption isotherms (pH 4.0), fit Langmuir (R²>0.99), determine capacities; kinetics (pseudo-second-order, R²>0.995), time to ~98% saturation; evaluate pH dependence (pH<5 emphasized due to hydrolysis at higher pH), extraction under highly acidic media (e.g., 3.0 M HNO₃), robustness after extreme treatments; quantify removal from ~9.952 ppm down to ppb; calculate distribution coefficient Kd under competitive ion conditions. - Regeneration: Desorb loaded UO₂²⁺ using 1.0 M Na₂CO₃; assess elution efficiency and retained capacity over six adsorption–desorption cycles; verify structural/functional integrity by PXRD and FT-IR; monitor fluorescence cycling visually under UV.

- Structure and porosity: TFPT-BTAN exhibits high crystallinity with AA stacking, open 1D channels (~1.5 nm), BET 1062 m² g⁻¹; after amidoximation, TFPT-BTAN-AO retains crystallinity and porosity (BET 803 m² g⁻¹). - Stability: TFPT-BTAN-AO maintains structure after harsh treatments (water 100 °C, 1 M HCl, 1 M NaOH, HNO₃ up to 5.0 M, γ-irradiation at 50 and 200 kGy); superior stability compared to β-ketoenamine COFs in strong nitric acid. - Fluorescence sensing: Bright blue emission at 460 nm (Φ = 4.3%); rapid response to UO₂²⁺ with equilibrium in 2 s; strong selectivity—only UO₂²⁺ significantly quenches fluorescence among many ions; linear calibration 0.02–6.0 μM at 460 nm (R²=0.993); 87% quenching at 20 μM UO₂²⁺; LOD 6.7 nM, below WHO drinking water limit (63 nM). - Binding mechanism: FT-IR shows O–U–O at 916 cm⁻¹; XPS reveals N–U (401.1 eV) and O–U (531.3 eV) formation with binding energy shifts; lifetime decreases from 3.1 to 1.8 ns indicate PET-mediated quenching; coordination involves amino and hydroxyl of amidoxime. - Extraction performance: Langmuir isotherm fits (R²>0.99); maximum capacity 427 mg g⁻¹ (TFPT-BTAN-AO) vs 353 mg g⁻¹ (POP-TB-AO); among the highest for COFs and surpassing previous COFs (e.g., COF-TpDb-AO 408 mg g⁻¹, ACOF 169 mg g⁻¹, 0-GS-COF 144.2 mg g⁻¹); kinetics fast—~98% saturation in 45 min (vs 85 min for POP-TB-AO to 95%); equilibrium capacity 417 mg g⁻¹ (vs 336 mg g⁻¹ for POP-TB-AO). - Extreme conditions: High adsorption in strong acid—capacity 128 mg g⁻¹ in 3.0 M HNO₃ (greater than COF-IHEP1 at 2 M HNO₃). Performance unchanged after extreme treatments. - Removal to standards: Uranium reduced from 9.952 ppm to 8.45 ppb (pH 2) and 6.17 ppb (pH 12), below US EPA discharge standard (30 ppb). - Selectivity/affinity: Distribution coefficient Kd = 8.3×10⁵ mL g⁻¹ at V/m = 5000 mL g⁻¹, far exceeding benchmark for good adsorbents (10⁴ mL g⁻¹). - Regeneration: Na₂CO₃ (1 M) elution efficiency >95% and retained adsorption capacity >87% after six cycles; PXRD and FT-IR confirm structural integrity; fluorescence sensing regenerable over cycles.

The designed sp² carbon-conjugated COF with open 1D channels and dense amidoxime ligands addresses the dual challenge of stability and performance for UO₂²⁺ detection and extraction. The robust C=C-linked framework provides chemical, thermal, and radiation resistance not achievable with traditional imine/boron-linked COFs, enabling operation in strongly acidic and irradiative environments. The highly accessible, uniformly distributed amidoxime binding sites within ordered channels drive both high capacity and rapid kinetics through efficient diffusion and mass transfer, explaining the superior performance over an amorphous POP analog and prior COFs. The strong fluorescence combined with selective UO₂²⁺-induced PET quenching affords real-time, on-site monitoring with an ultralow detection limit. The reversible coordination chemistry with carbonate elution enables regeneration without structural degradation, making the material practical for repeated use. Collectively, the results validate the materials design strategy and suggest broader applicability to other contaminants through ligand tailoring.

A sp² carbon-conjugated fluorescent COF (TFPT-BTAN-AO) was developed that combines exceptional chemical/thermal/radiation stability with high-density amidoxime ligands in open 1D channels. It enables selective, real-time UO₂²⁺ detection (2 s response, 6.7 nM LOD) and efficient extraction (Langmuir capacity 427 mg g⁻¹; rapid kinetics; high affinity Kd = 8.3×10⁵ mL g⁻¹), maintains performance under extreme conditions (strong acids, bases, heat, γ-irradiation), and is readily regenerated with carbonate over multiple cycles. This represents a first demonstration of regenerable COF-based simultaneous detection and extraction of UO₂²⁺. The approach can be generalized by incorporating other selective ligands for broader contaminant monitoring and remediation.