N-doped carbon quantum dots from osmanthus fragrans as a novel off-on fluorescent nanosensor for highly sensitive detection of quercetin and aluminium ion, and cell imaging.

In this work, fluorescent N-doped carbon quantum dots (N-CQDs) have been synthesized by simple hydrothermal heating of natural osmanthus fragrans, without any toxic ingredients or surface chemical modifications. The N-CQDs possess a high quantum yield of 21.9 %, outstanding blue fluorescence, good water dispersity, and excellent optical stability. Because the favorable inner filter effect (IFE) between N-CQDs and quercetin (QT) occurs, the addition of QT to N-CQDs can cause their fluorescence quenching. When Al3+ was added to the N-CQDs/QT system solution, it was found that the inhibition of IFE leads to the fluorescence intensity of N-CQDs/QT system enhancement by virtue of a specific binding of QT to aluminum ion (Al3+). Therefore, we used the N-CQDs as a novel off-on fluorescent nanosensor to detect QT and Al3+. Under optimal conditions, the fluorescent nanosensor can detect QT within the wide linear response in the range of 0.003-80 μmol/L with as low as 1 nmol/L detection limit. For the detection of Al3+, the N-CQDs/QT system showed linearity response toward Al3+ in a range of 0.1∼100 μmol/L and the limit of detection was found at 26 nmol/L. In addition, N-CQDs have been successfully used to efficient quantification QT in human plasma and monitor Al3+ in serum samples. Noteworthy, the N-CQDs demonstrated low toxicity toward T24 cells, which realized sensing QT and Al3+ in the living cells.