Lignin-Modified Metal–Organic Frameworks for Fluorescence Detection of Cr⁶⁺ and Adsorption of Congo Red

Herein, a lignin-modified metal–organic framework (MIL-NL) was successfully assembled by grafting aminated lignin onto MIL-101-Fe-NH2. With the introduction of aminated lignin via the Schiff base reaction, MIL-NL possesses not only the crystal structure of MIL-101-Fe-NH2 but also that of Fe2O3. The benefits of the amine and hydroxyl groups of the aminated lignin on MIL-NL allow it to interact with different objects through hydrogen bonding. MIL-NL emits blue fluorescence at 435 nm under an excitation light of 380 nm. The MIL-NL was built to detect Cr6+ by fluorescence "off" and ascorbic acid (AA) by fluorescence "on". The detection limits of MIL-NL for Cr6+ were 0.14 (1.5–20 μM) and 0.18 μM (40–200 μM), respectively, and AA in the range of 5–60 nM and 0.5–1.8 μM with LOD values of 0.425 and 15.90 nM, respectively. MIL-NL can detect Cr6+ quickly and sensitively (<1 min). Furthermore, MIL-NL showed a strong adsorption capacity of up to 1495.23 mg/g for Congo red. Multifunctional lignin-modified MOFs offer great potential in the field of environmental detection.