Ratiometric dual-emission carbon dots coupled with smartphone for visual quantification of Co2+ and EDTA and biological sensing

Ratiometric fluorescence analysis provides the feasibility for visual quantitative sensing by presenting continuous color changes. Herein, dual-emission carbon dots (D-CDs) have been developed for ratiometric recognition of Co2+ and EDTA, along with successive fluorescence variation of yellow, purplish-pink, purple, light blue, and dark blue. D-CDs have been synthesized by microwave-assisted heating of tetracycline and manifest dual emissions at 562 nm and 610 nm under 450 nm excitation. Intriguingly, the emissions at 562 nm and 610 nm are quenched while a ratio rising emission emerges at 482 nm upon introducing Co2+, which displays ratiometric fluorescence emission characteristics (I482/I562) in the range of 0.25–400 µM with significant fluorescence varying from yellow, purplish-pink, purple, light blue, to dark blue. Subsequent addition of EDTA could ratiometric recover the fluorescence of D-CDs in the range of 50–130 µM with the fluorescence returning from dark blue, light blue, purple, purplish-pink, to yellow. More crucially, integrating continuous color changes and color recognizer APP in the smartphone, visual quantification of Co2+ and EDTA can be realized based on the ratio of blue value and red value (B/R) with linear ranges of 0–400 µM and 0–300 µM, respectively. Furthermore, proposed D-CDs could be extended as a visual biosensing platform for tracking Co2+ and EDTA in living cells, which have potential application prospect in biosystem.