Nuclear energy plays a crucial role in the global energy system, but its use necessitates careful management of uranium, a key element in the nuclear industry. Unintended uranium leakage poses significant health and environmental risks, primarily in the form of UO₂²⁺. Existing materials for UO₂²⁺ detection and extraction, such as porous organic polymers (POPs) and metal-organic frameworks (MOFs), have limitations. Amorphous POPs suffer from irregular pores hindering mass transfer, while MOFs, despite their regular pores, often lack stability under extreme conditions (acidic, basic, high temperature, radiation). These limitations necessitate the development of highly stable and responsive materials for real-time UO₂²⁺ detection and efficient, regenerable extraction. Covalent organic frameworks (COFs) offer a promising alternative. COFs are porous crystalline polymers possessing excellent chemical and thermal stability, tunable functionality, and flexible topological connectivity. Their tunable porosity and large surface area are ideal for radionuclide extraction. Post-modification allows for the strategic placement of functional units to optimize performance. While some COFs have shown promise in UO₂²⁺ extraction, limitations exist due to the instability of their covalent bonds (e.g., boron-oxygen and imine bonds) under harsh conditions. These bonds are susceptible to degradation by irradiation, acids, and bases, hindering regeneration and practical applications. Recently, sp² carbon-conjugated COFs, synthesized through Knoevenagel condensation, have gained attention due to their inherent stability, however their application in UO₂²⁺ detection and extraction remains largely unexplored. This study introduces a novel sp² carbon-conjugated COF, designed for simultaneous detection and extraction of UO₂²⁺. The COF integrates triazine-based building blocks with amidoxime-substituted linkers, resulting in a highly stable, fluorescent material with selective UO₂²⁺ binding. This combination aims to overcome the limitations of previous COFs, enabling real-time detection and efficient, regenerable extraction of UO₂²⁺ under various challenging conditions.

The literature extensively documents the need for effective methods to detect and remove uranium from contaminated environments. Several porous materials have been explored, including POPs, MOFs, and hydrogels. However, each material class presents inherent challenges. POPs often lack the ordered porosity for efficient mass transport, leading to slow response times. MOFs, while possessing well-defined pore structures, frequently exhibit instability in harsh environments, a crucial factor when dealing with radioactive and often acidic samples. The existing COFs developed for uranium extraction mostly rely on Schiff base linkages, which prove vulnerable to decomposition under extreme conditions, limiting their reusability. Recent advances in sp² carbon-linked COFs offer increased stability due to the robust carbon-carbon bonds, but their application in uranium remediation is relatively understudied. Therefore, there is a clear gap in the literature for a material that offers both exceptional stability and high performance in UO₂²⁺ extraction and detection.

The synthesis of TFPT-BTAN-AO involved a two-step process. First, 2,4,6-tris(4-formylphenyl)-1,3,5-triazine (TFPT) and 2,2′,2″-(benzene-1,3,5-triyl)triacetonitrile (BTAN) were polymerized via Knoevenagel condensation to yield a cyano-based COF (TFPT-BTAN). The reaction conditions (solvent, catalyst, temperature) were optimized to achieve high crystallinity. Subsequently, amidoximation of TFPT-BTAN using NH₂OH·HCl introduced amidoxime groups, resulting in TFPT-BTAN-AO. The successful synthesis and structural integrity were confirmed through various characterization techniques including FT-IR, solid-state ¹³C CP/MAS NMR, PXRD, and N₂ adsorption-desorption analyses. PXRD patterns indicated high crystallinity and the presence of open 1D channels. N₂ adsorption confirmed a high BET surface area (1062 m² g⁻¹ for TFPT-BTAN and 803 m² g⁻¹ for TFPT-BTAN-AO). SEM images showcased the porous network structure. The chemical and thermal stability of TFPT-BTAN-AO was evaluated by subjecting it to various harsh conditions (water at 100 °C, 1 M HCl, 1 M NaOH, γ-ray irradiation). The material retained its structural integrity and functional groups after these treatments. The sensing performance was assessed by measuring fluorescence quenching upon the addition of UO₂²⁺. The selectivity was determined using various metal ions. The sensitivity was evaluated using different concentrations of UO₂²⁺, resulting in a calibration curve and a detection limit calculation. The interaction between TFPT-BTAN-AO and UO₂²⁺ was investigated using FT-IR and XPS. Time-resolved fluorescence spectroscopy was employed to study quenching mechanisms. The efficiency of UO₂²⁺ extraction was compared with an amorphous analogue (POP-TB-AO). Adsorption isotherms and kinetics were studied, and the results were fitted to Langmuir and pseudo-second-order models, respectively. The adsorption capacity and kinetics were examined under various pH conditions and in the presence of high concentrations of nitric acid. The reusability and regeneration of TFPT-BTAN-AO were evaluated through multiple adsorption-desorption cycles using a 1 M Na₂CO₃ solution. PXRD and FT-IR analysis confirmed the structural integrity of the COF after multiple cycles. Synthesis methods for model compounds, TFPT-BTAN COF, TFPT-BTAN-AO, POP-TB, and POP-TB-AO are described in detail in the supplementary information.

The synthesized sp² carbon-conjugated COF, TFPT-BTAN-AO, displayed exceptional properties for uranium detection and extraction. Key findings include: * **High UO₂²⁺ adsorption capacity:** TFPT-BTAN-AO achieved a maximum UO₂²⁺ adsorption capacity of 427 mg g⁻¹, surpassing many previously reported COFs. This high capacity is attributed to the abundant amidoxime groups strategically located within the accessible 1D channels. * **Ultra-fast response time:** The COF exhibited an ultra-fast response time of 2 s, enabling real-time UO₂²⁺ detection. * **Ultra-low detection limit:** A very low detection limit of 6.7 nM UO₂²⁺ was achieved, well below the WHO contamination limit for drinking water. * **Excellent selectivity:** TFPT-BTAN-AO showed high selectivity for UO₂²⁺ over other competing metal ions, owing to the specific interaction between UO₂²⁺ and the amidoxime groups. * **Exceptional stability:** The sp² carbon-conjugated structure imparted exceptional stability to TFPT-BTAN-AO against harsh conditions, including high temperatures, strong acids (even 5.0 M nitric acid), strong bases, and high doses of γ-ray irradiation. This superior stability significantly outperforms previous imine-based COFs. * **Efficient and regenerable extraction:** The material demonstrated efficient UO₂²⁺ extraction, with approximately 66.8% accessibility of the amidoxime groups involved in UO₂²⁺ binding. Importantly, the TFPT-BTAN-AO could be regenerated effectively using a 1 M Na₂CO₃ solution, maintaining high adsorption capacity (>87%) even after six cycles. The regeneration process did not compromise the structure or functional groups of the COF, as evidenced by PXRD and FT-IR analysis. * **Superior Performance Compared to Amorphous Analogue:** A direct comparison with an amorphous analogue (POP-TB-AO) highlighted the significant impact of the ordered porous structure on adsorption capacity and kinetics. TFPT-BTAN-AO consistently outperformed POP-TB-AO in both aspects.

The results demonstrate that the rational design of the sp² carbon-conjugated COF, incorporating abundant and accessible amidoxime groups within a highly stable framework, has led to a significant improvement in UO₂²⁺ detection and extraction. The combination of strong fluorescence, high stability, and fast response time makes TFPT-BTAN-AO a highly promising material for environmental remediation. The superior performance compared to the amorphous analogue underscores the critical role of the ordered porous structure in facilitating rapid mass transfer and preventing pore clogging. The exceptional regeneration capability allows for multiple reuse, making it economically viable for large-scale applications. The success of this strategy highlights the potential of COFs as customizable platforms for the detection and removal of various environmental contaminants by modifying the functional groups to target specific pollutants.

This study successfully synthesized a highly stable and efficient fluorescent COF, TFPT-BTAN-AO, for the simultaneous detection and extraction of UO₂²⁺. The material's exceptional properties, including high adsorption capacity, rapid response, excellent selectivity, remarkable stability, and efficient regeneration, make it a promising candidate for real-world uranium remediation applications. The modular nature of COF synthesis opens avenues for developing similar materials tailored to other environmental contaminants by employing specific ligands.

While TFPT-BTAN-AO demonstrates significant advantages, some limitations should be considered. The synthesis process might require optimization for scale-up, particularly in the amidoximation step. Further research is needed to evaluate the long-term stability of the material under continuous exposure to extremely harsh conditions and the potential leaching of the functional groups. The study focused primarily on UO₂²⁺; the efficacy of TFPT-BTAN-AO for extracting uranium in complex environmental matrices warrants further investigation.