A pH-dependent N, P co-doped carbon dots as fluorescent probe for malachite green assay and its visual application based on fluorescent hydrogel kit

A pH-dependent red fluorescent N, P co-doped carbon dots (N, P-CDs) was firstly developed for rapid, sensitive and visual detection of the toxic dye malachite green (MG). The synthesis of N, P-CDs was carried out by hydrothermal treatment of o-phenylenediamine and phosphoric acid, and the low cytotoxicity of N, P-CDs was verified by Cell Counting Kit-8 (CCK-8) method. The linear decrease of N, P-CDs fluorescence in the pH range of 3.8–5.6 was attributed to the reduction of protonation of nitrogen-containing groups on the surface. And the sensing mechanism for MG was constructed based on the inner filter effect (IFE). The obtained linear ranges were 0.08–1 and 1–50 µmol·L−1 with limit of detections (LODs) of 0.07 and 0.74 µmol·L−1. The proposed method has been successfully applied to the determination of MG in water and fish samples. In addition, a kind of N, P-CDs fluorescent hydrogel kit was prepared for the visual analysis of MG. This method is not only simple, rapid and accurate, but also the prepared fluorescent hydrogel has potential application prospects for on-site qualitative identification and semi-quantitative analysis of MG, which also provides a new idea for monitoring MG in water environment and fish products.