"On-off-on" fluorescent system for detection of Zn 2+ in biological samples using quantum dots-carbon dots ratiometric nanosensor.

In this work, a novel strategy for Zn2+ detection was established by using an "on-off-on" ratiometric fluorescent nanosensor with the help of ethylene diaminetetraacetic acid (EDTA), a common metal chelating agent. The simple and practical ratiometric fluorescent nanosensor was synthesized by covalently binding CdTe quantum dots (QDs) and carbon dots (CDs), possessing two emission peaks at 525 nm and 450 nm under a single wavelength excitation of 360 nm, respectively. The CdTe QDs served as the response signal label, and the CDs, having no response to the analytes, acted as the reference signal. The fluorescence emission of the nanosensor was turned off to create a low-level "off" state when EDTA was added, and turned on dramatically with the addition of the target Zn2+ . Under the optimal conditions, the change of the fluorescence intensity ratio of the nanosensor at 525 nm and 450 nm (ΔF525 /F450 ) had good linearity against the concentrations of Zn2+ within a dynamic linear range of 0.50-40 μM. The limit of detection was as low as 0.33 μM (3σ/K), which was low enough for the detection of Zn2+ in human body. The proposed method was successfully applied to the detection of Zn2+ in human urine and plasma with RSDs less than 10%. The results show that the as-prepared QDs-CDs ratiometric nanosensor has potential application of clinical detection of Zn2+ in the human body.