Nitrogen-doped carbon dots coupled with morin-Al: Cleverly design an integrated sensing platform for ratiometric optical dual-mode and smartphone-assisted visual detection of fluoride ion.

Ratiometric fluorescence sensor has high selectivity and good sensitivity; however, its development is limited by intricate design, tedious synthesis, etc. Herein, a facile and effective ratiometric fluorescence sensing platform for fluoride ion (F) detection was developed by simply combining nitrogen-doped carbon dots (N-CDs) and morin-Al based on inner filter effect (IFE). The competitive binding of F to Al obviously decreased morin-Al fluorescence and increased N-CDs fluorescence, attributing to the inhibition of IFE between N-CDs and morin-Al. The as-constructed ratiometric fluorescence sensing platform can be used for F detection with a wide linear range (0.5-150 μM) and a low detection limit (55.8 nM). Interestingly, with the introduction of F into the N-CDs/morin-Al sensing platform, a distinguishable change in fluorescence color from green to blue enabled the N-CDs/morin-Al system to be used as a smartphone-assisted visual sensing platform for F detection with a detection limit of 2.09 μM. This platform was successfully applied for the onsite monitoring of F in various water samples with satisfying results. These findings provide a novel guidance for the facile construction of a ratiometric optical dual-mode and smartphone-assisted sensing platform based on CDs, revealing the broad application prospect of CDs in environmental monitoring field.