Smartphone-assisted detection of methylene blue using leaf-derived, dual-emissive carbon dots as ratiometric fluorescence probes

Specific assay for methylene blue (MB) in aquaculture is critically required to assess the safety of ecosystems and aquatic products. In this work, a facile method is demonstrated for ratiometric fluorescence determination of MB based on blue/red dual emissive carbon dots (b/r-CDs). b/r-CDs, derived from the leaves of Impatiens balsamina, show weak and strong emission peaks at 432 and 672 nm, respectively, and possess a high photoluminescence quantum yield of 35.7%. The red fluorescence of b/r-CDs at 672 nm, serving as the response signal, can be significantly quenched by MB due to the mechanism of inner filter effect, while the blue fluorescence at 432 nm is insensitive to MB, thus acting as the background reference. With addition of an increased amount of MB, the fluorescence color of b/r-CDs gradually changes from red to blue, which realizes the visual detection. The linear range and limit of detection are measured to be 0.05–10 μM and 15 nM, respectively. Notably, a portable smartphone sensing platform is built for quantitative detection of MB by analyzing the RGB values. Moreover, the practical applications of b/r-CDs for monitoring MB in real water and aquatic products using both fluorescence spectrophotometer and smartphone achieve satisfactory results. Graphical