Lignin carbon dots with engineered graphitic N and sp2 domains: Visible-light-driven oxidase-like activity and green fluorescence for smartphone-assisted dual-mode nitrite detection

Lignin-based CDs with fluorescence and oxidase-like activity were designed firstly • Synergistic engineering of rich graphitic N and large sp 2 domains narrows bandgap • Relationship of structure and visible light-driven oxidase-like activity was revealed • A ratiometric colorimetric/fluorescent system for nitrite detection was constructed Renewable lignin-derived carbon dots with oxidase activity are potential sustainable alternatives for natural enzymes, but their weak visible-light absorption fundamentally inhibits triggering their oxidase-like activity. Herein, we successfully engineered N-doped lignin carbon dots (LCD MA ) with superior visible-light harvesting capacity via enlarging their conjugated sp 2 domain size and graphitic N content. Owing to the synergistic effect of these structures, the bandgap of LCD MA was narrowed to 3.241 eV, which efficiently extended its visible absorption range to 500 nm, and red-shifted its fluorescent light to green region. The superior visible absorption endowed LCD MA with outstanding oxidase-like activity via boosting the generation of photogenerated electrons, which drove the reduction of O 2 to • O 2 − . The LCD MA exhibited a rapid catalytic rate (V max  = 21.64 × 10 −5  mM s −1 ) and strong affinity (K m  = 0.70 mM) for 3,3′,5,5′-tetramethylbenzidine. Owing to the catalytic activity and green fluorescence of LCD MA , we constructed a ratio-metric colorimetric and fluorescence dual-mode solution system for nitrite detection in the linear range of 2–250 μM, whose detection limits were as low as 0.85 μM and 1.60 μM, respectively. Furthermore, we engineered an all-in-one portable sensing platform by encapsulating LCD MA and TMB into hydrogel for visual and quantitative detection of nitrite through smartphone with a detection limit of 8.95 μM. This study not only provided an effective strategy for rationally engineering sustainable carbon dots with oxidase-like activity from renewable biomass, but also delivered a practical and effective analytical tool for food safety monitoring. Download: Download high-res image (370KB) Download: Download full-size image