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

Uranium is a key element in the nuclear industry, but its unintended leakage has caused health and environmental concerns. Here we report a sp2 carbon-conjugated fluorescent covalent organic framework (COF) named TFPT-BTAN-AO with excellent chemical, thermal and radiation stability is synthesized by integrating triazine-based building blocks with amidoxime-substituted linkers. TFPT-BTAN-AO shows an exceptional UO22+ adsorption capacity of 427 mg g−1 attributable to the abundant selective uranium-binding groups on the highly accessible pore walls of open 1D channels. In addition, it has an ultra-fast response time (2 s) and an ultra-low detection limit of 6.7 nM UO22+ suitable for on-site and real-time monitoring of UO22+, allowing not only extraction but also monitoring the quality of the extracted water. This study demonstrates great potential of fluorescent COFs for radionuclide detection and extraction. By rational designing target ligands, this strategy can be extended to the detection and extraction of other contaminants.

W ith a low-carbon footprint, nuclear energy has a critical role in the global energy system [1][2][3] . Owing to the widespread use of nuclear power, large-scale uranium mining, nuclear accidents, and improper disposal of nuclear wastes, a large quantity of radioactive uranium has penetrated into the environment mainly in the form of UO 2 2+ 4-6 . Thus, regenerable materials for concurrent UO 2 2+ detection and extraction are demanded for environmental monitoring and protection.
Some porous materials such as porous organic polymers (POPs) 7 , metal-organic frameworks (MOFs) 8 , and hydrogels 4 have been developed for this purpose. However, the performance of amorphous POPs is affected by its irregular pores, burying a large fraction of porosity 9 , and hindering fast mass transfer needed for real-time response 10 . Although MOFs have regular pores and good crystallinity 8 , stability under extreme conditions (acid, base, temperature, and radiation) remains a challenge [11][12][13] . High stability is particularly important for the extraction of UO 2 2+ , as the sample matrix is likely to be strongly radioactive and acidic. Therefore, it remains a synthetic challenge for realtime detection and regenerable extraction of UO 2 2+ .
Recently, considerable attention has been paid to the construction of olefin-based COFs synthesized by the Knoevenagel condensation reaction. Although the sp 2 -carbon bond are very stable, the reversibility of sp 2 -carbon bond formation is poor, making the synthesis of sp 2 -carbon-linked COFs extremely challenging 33 . Since 2016, several examples of sp 2 -carbon COFs have been reported, such as sp 2 c-COF 34 , TP-COF 35 , Por-sp 2 c-COF 33 , and g-C 34 N 6 -COF 36 . However, their application for the detection or extraction of UO 2 2+ has not been explored. More importantly, the exploration of COFs for fluorescence detection of UO 2 2+ is still in its infancy, and most UO 2 2+ -sensing platforms are often hampered by poor selectivity and a long response time [37][38][39][40] .
We herein report a sp 2 carbon-conjugated COF for simultaneous detection and extraction of UO 2 2+ by integrating triazine-based building blocks with amidoxime-substituted linkers. This sp 2 carbon-conjugated COF not only has good luminescence yield, but also excellent chemical and thermal stability. Its selective binding of UO 2 2+ is obtained by introducing amidoxime functional groups as ligands in the open 1D channels. Its real-time fluorescence response to UO 2 2+ can be visually observed and recycling was also confirmed using reversible uranium binding.
Results sp 2 carbon-conjugated COF for reversible uranium binding. To achieve an acid, base, and radiation stable, strongly fluorescent and selective UO 2 2+ -binding COF, we had the following materials design considerations. Most current COFs relied on the boron-oxygen and imine bonds 35 . However, such reversible bonds lead to relatively poor stability. In addition, their weak π-electron delocalization over the framework hinders fluorescence yield 36 . Carbon-carbon double bonds (-C═C-) are more stable and can keep conjugated π-electrons, which can overcome the above challenges 36,41 . In addition, their intrinsic open 1D channels and regular porous structures facilitate exposure of binding sites, boosting rapid diffusion, and mass transfer. Thus, by introducing specific metal-binding sites on the pore walls, the sp 2 carbon-conjugated COF may serve for high performance UO 2 2+ extraction under harsh conditions.
The chemical structure and composition of TFPT-BTAN were determined by Fourier transform infrared (FT-IR) and solid-state 13 C CP/MAS NMR spectroscopy. In the FT-IR spectra of TFPT-BTAN ( Supplementary Fig. 2), the characteristic vibration peak of -CN (at~2241 cm −1 ) was observed for both BTAN monomer and TFPT-BTAN. A stretching vibration peak of C═O (at~1698 cm −1 ) was found in the TFPT monomer and it completely disappeared in TFPT-BTAN, indicating a high degree of condensation. The solidstate 13 C CP/MAS NMR of TFPT-BTAN further confirmed highly efficient condensation, supported by the peaks at~113 and~171 ppm assigned to the carbon atoms in cyano and triazine moieties, respectively ( Supplementary Fig. 3).
To determine the structure of TFPT-BTAN, powder X-ray diffraction (PXRD) experiments were performed (Fig. 2a). A strong peak at 5.8°(2θ) is assigned to the diffraction from the (100) plane, indicating a highly crystalline of TFPT-BTAN 37 . The other peaks at 9.8°, 11.2°, and 26.3°(2θ) are assigned to the diffractions of (110), (200), and (001) planes, respectively. Our PXRD pattern matches well with the AA stacking model of the simulated TFPT-BTAN structure, and the Pawley refined PXRD pattern fits well with experimental data (R p = 2.85% and R wp = 4.27%), as demonstrated by the negligible difference (Supplementary Fig. 4 and Supplementary Table 2). The above results indicate that TFPT-BTAN has open 1D channels (1.5 nm in diameter) and the interlayer distance of the framework is 3.5 Å. To evaluate the details of the pore features of TFPT-BTAN, N 2 adsorption-desorption experiments were performed. The Brunauer-Emmett-Teller (BET) surface area of TFPT-BTAN was determined to be 1062 m 2 g −1 , and the pore-size distribution centered at 1.44 nm based on non-local density functional theory, which matches well with the model (Fig. 2b) 30 .
We prepared TFPT-BTAN-AO by reacting crystalline and porous TFPT-BTAN with NH 2 OH·HCl. The PXRD pattern of TFPT-BTAN-AO shows a diffraction pattern comparable to the one of TFPT-BTAN with a strong diffraction peak at 5.8° (Fig. 2a), indicating that crystallinity was well retained after the amidoximation process. N 2 adsorption-desorption isotherm were performed to verify pore accessibility after the post-modification process, affording isotherms comparable to those of TFPT-BTAN. The BET surface area was evaluated to be 803 m 2 g −1 , indicating that porosity was well retained after the amidoximation process (Fig. 2c).
As shown in Supplementary Fig. 5, the vibration peak of -CN (at 2241 cm −1 ) disappeared in TFPT-BTAN-AO, and the vibration peaks of the amidoxime groups can be observed at 1403 and 1707 cm −1 , confirming the successful amidoximation 1 . Furthermore, solid-state 13 C CP/MAS NMR spectra confirmed this successful conversion, as indicated by the disappearance of the peak at~113 ppm that is assigned to carbon atoms in the cyano groups together with the concomitant appearance of a peak at~156 ppm assigned to the carbon atoms in amidoxime groups ( Supplementary Fig. 3) 1,35 . The obtained TFPT-BTAN and TFPT-BTAN-AO were pale-yellow powders. The scanning electron micrograph of TFPT-BTAN shows a porous network structure (Fig. 2d), which did not change after amidoxime functionalization (Fig. 2e), indicating that TFPT-BTAN-AO can be rapidly and thoroughly penetrated with UO 2 2+ .
Thermogravimetric analysis profiles revealed that both the TFPT-BTAN and TFPT-BTAN-AO were stable up to 320°C, indicating that good thermal stability ( Supplementary Fig. 6). In addition, we treated TFPT-BTAN-AO under different harsh conditions for 12 h to study chemical stability. The FT-IR spectra ( Fig. 2f and Supplementary Fig. 7a) and PXRD ( Fig. 2g and Supplementary Fig. 7b) both show that TFPT-BTAN-AO retained the same characteristic peaks after the treatments, confirming its remarkable chemical stability. In order to reflect the superior stability of TFPT-BTAN-AO compared with other COFs, two β-ketoenamine COFs (Tp-Bpy and Tp-BD) were synthesized by the reported method 30 Table 3). All of these indicated successful synthesis of stable TFPT-BTAN-AO.
Selective sensing of UO 2 2+ . After demonstrating the synthesis, we then studied the sensing performance of our COF. The normalized fluorescence spectra of TFPT-BTAN and TFPT-BTAN-AO dispersed in water are shown in Supplementary Fig. 8. Compared with TFPT-BTAN, the aminoximation process introduced a large amount of -OH and -NH 2 , thereby reducing the conjugation effect in the TFPT-BTAN-AO. As a result, the emission spectrum of TFPT-BTAN-AO blue shifted compared to With a high-quantum yield and specific affinity uranium-binding groups, we then studied its UO 2 2+ -sensing performance. A stock solution of TFPT-BTAN-AO was added to the solution containing UO 2 2+ , and then diluted with ultrapure water for fluorescence measurement ( Supplementary Fig. 9). The fluorescence of TFPT-BTAN-AO was significantly quenched by UO 2 2+ , and the optimal pH for detection was at 6.0 ( Supplementary Fig. 10). Importantly, TFPT-BTAN-AO responded very quickly to UO 2 2+ , and the system can reach equilibrium within 2 s ( Supplementary Fig. 11), much faster than other detection systems (Supplementary Table 4). These results suggest that the 1D channels could promote the rapid diffusion and mass transfer of UO 2 2+ and achieve real-time detection.
To test selectivity, UO 2 2+ (20 μM) was added directly to the TFPT-BTAN-AO aqueous dispersion, whereas other metal ions were added at 50 μM ( Fig. 3 and Supplementary Fig. 12). Only UO 2 2+ caused significant fluorescence quenching, other metal ions have little effect on the UO 2 2+ detection, and it can be visually observed under a portable 365 nm UV lamp, indicating that TFPT-BTAN-AO has good selectivity for UO 2 2+ attributable to the specific affinity between UO 2 2+ and amidoxime groups 1 , and the fluorescence quenching property of UO 2 2+ .
Highly sensitive sensing of UO 2 2+ . To explore the sensitivity, the fluorescence spectra of TFPT-BTAN-AO were measured at different UO 2 2+ concentrations (Fig. 4a). The fluorescence intensity of TFPT-BTAN-AO decreased with the increasing concentration of UO 2 2+ and 87% of the fluorescence was quenched with 20 μM UO 2 2+ . In addition, the fluorescence response to UO 2 2+ was clearly observed under a 365 nm UV lamp. Figure 4b shows a good calibration curve for the fluorescence intensity of TFPT-BTAN-AO at 460 nm versus UO 2 2+ concentration (0.02-6.0 μM) with a high correlation coefficient of 0.993. The limit of detection of the TFPF-BTAN-AO was determined as 6.7 nM UO 2 2+ (Supplementary Fig. 13), well below the World Health Organization contamination limit for UO 2 2+ in drinking water (63 nM) 7 . Therefore TFPF-BTAN-AO can be used for high sensitivity detection of UO 2 2+ .
Interaction between TFPT-BTAN-AO and UO 2 2+ . FT-IR and X-ray photoelectron spectroscopy (XPS) were applied to investigate the effective uranium-binding and fluorescence quenching. The FT-IR spectrum of TFPT-BTAN-AO after treatment with UO 2 2+ shows a new peak at 916 cm −1 , which can be attributed to the vibration of O═U═O ( Supplementary Fig. 14) 42 . In addition, the appearance of N-H bending vibration at 1613 cm −1 after treatment with UO 2 2+ indicates the presence of a chemisorption process 8 . After treating the TFPT-BTAN-AO with UO 2 2+ , distinctive U 4 f-binding energy peaks emerged, revealing that UO 2 2+ was successfully loaded onto the TFPT-BTAN-AO (Supplementary Fig. 15) 43,44 .
In the high-resolution XPS spectrum of N 1 s of the TFPT-BTAN-AO (Fig. 5a), the two binding energy peaks at 399.7 and 398.8 eV are assigned to C-N and C═N, respectively, which correspond to the nitrogen atoms in the TFPT-BTAN-AO framework 42,45 . After treatment with UO 2 2+ , the N 1 s XPS spectrum of the TFPT-BTAN-AO was observed under the same measurement conditions (Fig. 5b). Comparing the N 1 s binding energy peaks in the two samples, a new N-U peak (401.1 eV) formed, and the peak located at 399.7 eV of the TFPT-BTAN-AO moved 0.22 eV to a higher binding energy after treatment with UO 2 2+ . However, the N 1 s peaks at 398.8 eV in Fig. 5a, b show no shift after treatment with UO 2 2+ , revealing that the nitrogen atoms of the C═N did not bind to UO 2 2+ . Comparing the O 1 s peaks in the two samples (Fig. 5c, d), it is clearly observed that a new O-U peak (531.3 eV) formed, and the O 1 s core peak of the TFPT-BTAN-AO moved 0.2 eV to a higher binding energy after treatment with UO 2 2+ . Based on the above results, it is speculated that the adsorption of UO 2 2+ onto the TFPT-BTAN-AO is a chemical process, and both the amino and hydroxyl groups in TFPT-BTAN-AO are coordinated with UO 2 2+ (Fig. 5e), similar to the previously reported binding mode 46 .
The quenching effect of UO 2 2+ on TFPT-BTAN-AO was further studied by time-resolved fluorescence spectroscopy. The fluorescence decay profile shows that TFPT-BTAN-AO has a lifetime of 3.1 ns (Supplementary Fig. 16, red curve). Upon addition of UO 2 2+ , the lifetime decreased to 1.8 ns (blue curve), which is consistent with a decrease in fluorescence intensity. UO 2 2+ induced quenching of TFPT-BTAN-AO likely to proceed by a photoinduced electron transfer (PET) process from the framework to UO 2 2+ .
Efficient extraction of UO 2 2+ . Given the very stable (-C═C-) bonds and abundant selective UO 2 2+ -binding groups on the 1D channels, this material may serve for high performance UO 2 2+ extraction under harsh conditions. To test the importance of the regular porous structure for adsorbing UO 2 2+ , an amorphous analog named POP-TB was synthesized and subjected to the same amidoximation to give POP-TB-AO ( Supplementary  Figs. 17-19). Both the COF-based materials and their amorphous POP analogs were demonstrated very similar FT-IR spectra and nitrogen element contents before and after the amidoximation process ( Supplementary Figs. 20 and 21), indicating similar amidoxime functionalization processes.
The adsorption isotherm of TFPT-BTAN-AO has a much steeper adsorption curve for UO 2 2+ (Fig. 6a), indicating that its affinity for UO 2 2+ is higher than that of corresponding POP-TB-AO 24 . All the adsorption experimental data are in good agreement with the Langmuir isotherm model, and the correlation coefficients are higher than 0.99 ( Supplementary Fig. 22). Surprisingly, the maximum adsorption capacity for UO 2 2+ on TFPT-BTAN-AO (427 mg g −1 ) is much higher than that of POP-TB-AO (353 mg g −1 ), and is located among the top of different types of adsorbents (Supplementary Table 5). Importantly, the superior performance of TFPT-BTAN-AO exceeded all previously reported COFs for UO 2 2+ extraction, like COF-TpDb-AO (408 mg g −1 ) 1 , ACOF (169 mg g −1 ) 43 , and o-GS-COF (144.2 mg g −1 ) 3 . The uranium content loaded on the framework was calculated based on the ICP-MS results, this capacity means that 66.8% accessibility of the amidoxime groups in TFPT-BTAN-AO were used to extract UO 2 2+ . The TFPT-BTAN-AO showed exceptional performance in extraction of B l a n k C o 2 + C a 2 + C d 2 + In addition to the high adsorption capacity, TFPT-BTAN-AO also extracted UO 2 2+ from aqueous solution more rapidly compared with POP-TB-AO (Fig. 6b). All the adsorption kinetics data are in good agreement with the pseudo-second-order model, and the correlation coefficients are higher than 0.995 (Supplementary Fig. 23). It is worth noting that TFPT-BTAN-AO can achieve a saturation capacity of about 98% within 45 min. This is in sharp contrast to the long contact times required for other uranium sorbent materials, which typically range from hours to days 7,[47][48][49] . For comparison, POP-TB-AO took 85 min to reach 95% of its equilibrium adsorption capacity.

C r 3 + S r 2 + M n 2 + F e 3 + C u 2 + N i 2 + A l 3 + Z n 2 + M g 2 + P b 2 + B a 2 + C e 3 + L a 3 + N d 3 + S m 3 + G d 3 + U O
Apart from rapid equilibration, TFPT-BTAN-AO also has higher extraction capacity with an equilibrium capacity of 417 mg g −1 , whereas POP-TB-AO only reaches 336 mg g −1 (Supplementary Fig. 24). Considering that TFPT-BTAN-AO and POP-TB-AO have similar chemical compositions, the high absorption capacity and rapid adsorption kinetics should be attributed to their different pore structures. The rapid kinetics of TFPT-BTAN-AO can be attributed to the hierarchical pores structure and the evenly and densely distributed chelating sites on the pore walls to facilitate rapid diffusion of UO 2 2+ throughout the framework (Supplementary Fig. 25). In contrast, the pore structure in POP-TB-AO is irregular, making it more susceptible to clogging, thus greatly impairing their adsorption performance.
For practical applications, extraction of UO 2 2+ under various harsh conditions is highly desirable. Considering that UO 2 2+ is mainly present in the acidic environment and hydrolysis occurs at a higher pH ( Supplementary Fig. 26), adsorption experiments were performed at pH < 5.0. Supplementary Fig. 27 shows the adsorption capacity of UO 2 2+ on TFPT-BTAN-AO at different pH values. Obviously, the adsorption capacity increases as the system pH value rises from 1.0 to 4.0.
As nuclear fuel reprocessing and wastewater are usually treated under highly acidic conditions, it is necessary to evaluate the effect of solution acidity on UO 2 2+ extraction by TFPT-BTAN-AO ( Supplementary Fig. 27). Surprisingly, TFPT-BTAN-AO showed high adsorption capacity in highly acidic media, the saturated capacity in 3.0 M nitric acid was calculated to be 128 mg g −1 , this capacity is much higher than COF-IHEP1 (70 mg g −1 , 2 M nitric acid) 50 . To further confirm the chemical stability and excellent  Fig. 28). The results verified that the uranium extraction performance of TFPT-BTAN-AO was almost unchanged after treatment under various extreme conditions, indicating that TFPT-BTAN-AO has excellent stability and practical application potential. This feature is a significant advantage over imine-based COFs sorbents that typically suffer from the decomposition of structures under extreme conditions 1,3 . In addition, the uranium concentration reduced from 9.952 ppm to 8.45 ppb and 6.17 ppb at pH values of 2 and 12, respectively (Fig. 6c). It is well below the US Environmental Protection Agency (EPA) uranium-containing wastewater discharge standard (30 ppb) 1 . To evaluate the affinity of TFPT-BTAN-AO toward UO 2 2+ , additional selective extraction experiments with 9.952 ppm UO 2 2+ (V/m = 5000 mL g −1 ) were carried out. The calculated distribution coefficient K d (8.3 × 10 6 mL g −1 ) is much larger than the distinguishing standard (1.0 × 10 4 mL g −1 ), which is generally considered to be a good adsorbent (Fig. 6d), indicating the excellent affinity of TFPT-BTAN-AO for UO 2 2+47 . We attribute the enormous distribution coefficient to the specific affinity for UO 2 2+ provided by the 1D open channel amidoxime groups in TFPT-BTAN-AO. The above results indicate the superiority of TFPT-BTAN-AO as a promising candidate for uranium extraction.
Reversible binding for regeneration. One of the unique advantages of TFPT-BTAN-AO is that the very stable framework provides the most critical foundation for the reversible binding of UO 2 2+ . We found that the fluorescence of TFPT-BTAN-AO can be easily recovered by Na 2 CO 3 (1 M) aqueous solution (Fig. 7a). To confirm the practical reusability of TFPT-BTAN-AO as an adsorbent, we conducted multiple adsorption-desorption experiments and found that sodium carbonate has a high elution efficiency (>95%) even after six cycles (Fig. 7b). In addition, elution of UO 2 2+ with sodium carbonate did not affect the adsorption performance of TFPT-BTAN-AO, and maintained a high adsorption capacity (>87%) even after six cycles. More importantly, owing to the excellent stability of the framework, TFPT-BTAN-AO crystal structure and functional groups were well preserved after recycling, as evidenced by PXRD and FT-IR results (Supplementary Fig. 29 and Fig. 7c). It is worth noting that TFPT-BTAN-AO can be cycled at least six times without noticeable influence of response to UO 2 2+ or sensitivity, and this exceptional regeneration can be observed by the naked eye under a portable UV lamp (Supplementary Fig. 30). So far, this is the first demonstration of COF-based regenerable detection and extraction of UO 2 2+ . The above results indicate that TFPT-BTAN-AO has great potential for simultaneous detection and extraction of UO 2 2+ . Importantly, such regeneration is almost impossible for previously reported imine-based COFs adsorbents.

Discussion
In summary, we have successfully developed a strongly fluorescent COF for real-time detection and efficient extraction of UO 2 2+ . Different from previous COF sorbents relying on imine bond (-C═N-), our COF used very stable carbon-carbon double bonds (-C═C-). Our COF combines strong fluorescence, excellent stability, dense, and evenly distributed amidoxime groups, and highly accessible binding sites through the open 1D channel. With these advantages, TFPT-BTAN-AO achieved real-time sensitive detection, efficient extraction, and efficient regeneration by simply  Synthesis of TFPT-BTAN-AO. The TFPT-BTAN (0.4 g) was swollen in absolute ethanol (40 mL) for 20 min, followed by the addition of NH 2 OH·HCl (1.0 g) and N (CH 2 CH 3 ) 3 (1.5 g). After stirring at 85°C for 24 h, the mixture was filtered, washed with excess water and finally dried at 60°C under vacuum. The pale-yellow solid obtained was expressed as TFPT-BTAN-AO. Elemental analysis: calculated C (66.96%), H (6.09%), N (19.52%), and observed C (65.31%), H (6.22%), N (19.75%).
Synthesis of POP-TB-AO. The POP-TB (0.4 g) was swollen in absolute ethanol (40 mL) for 20 min, followed by the addition of NH 2 OH·HCl (1.0 g) and N (CH 2 CH 3 ) 3 (1.5 g). After stirring at 85°C for 24 h, the mixture was filtered, washed with excess water and finally dried at 60°C under vacuum. The pale-yellow solid obtained was expressed as POP-TB-AO. Elemental analysis: observed C (65.15%), H (6.13%), N (19.58%).

Data availability
All data are either provided in the Article and its Supplementary Information or are available from the corresponding author upon request.