Dual-functional flexible cationic porphyrin-based covalent organic frameworks for selective adsorption and sensitive detection of Cr (VI)

Due to the strong carcinogenicity, non-degradability and bioaccumulation of Cr (VI), exploring ingenious strategies and renewable materials to simultaneously detect and remove Cr (VI) from industrial wastewater, to reduce its damage to ecosystems, is of great importance. Covalent organic frameworks are considered as excellent adsorbents, but most of them are neutral and rigid materials. Herein, a cationic and flexible COF (Imi-TPP-COF-Br-AR) was fabricated for the first time by integrating porphyrin and charged imidazole into a framework, followed by reduction. Such elaborate design endowed Imi-TPP-COF-Br-AR with excellent light-driven oxidase-like activity, dense ion sites, and stable photoelectric properties. Imi-TPP-COF-Br-AR could purify Cr (VI) polluted water to drinking levels with outstanding adsorption performance (Q_max = 373.14 mg/g), also exhibited broader detection range (0.5–220 μM) and lower limit of detection (LOD, 41 nM) for Cr (VI). Mechanism studies and density functional theory (DFT) calculations disclosed that the electrostatic interaction between cationic material and the negatively charged Cr (VI) anions, and the extended π-conjugated structure cooperatively promoted the remarkable performance of Imi-TPP-COF-Br-AR for Cr (VI). This work provides a novel strategy for the structural and functional design of cationic and flexible COFs materials, especially those can realize dual functions only within one material.