| Zinc,one of the essential trace elements in human body,is second only to ferrum in content.As key part of the structural components of a large number of metalloenzyme,zinc plays a very important part in terms of biological functions,including cell proliferation and differentiation,gene expression,signal transduction and neurotransmission.When lacked of zinc,human body would be influenced heavily on growth,neuronal development,and immunity.Besides,some studies indicate that disruption of zinc homeostasis is responsible to several diseases,such as Alzheimer's,epilepsy,diabetes and cancer.So,detection of zinc in human body is an important way to diagnosis these diseases on clinic.Traditional methods for Zn2+ detection,including atomic absorption spectrometry,dithizone colorimetric method,anodic stripping voltammetry,and oscillographic polarography,either are sensitive and selective but laboratory-based and tedious in sample preparation,or are easy to operate but weak in sensitivity and selectivity.Therefore,there is an urgent need to develop novel approaches that are convenient and fast as well as sensitive and selective.Recently,fluorescent nanosensors,like quantum dots(QDs)and carbon dots(CDs)have drawn more and more attentions due to their unique optic characteristics.Compared with traditional organic fluorescent dyes,QDs and CDs possess remarkable advantages,such as size-tunable optical properties,high fluorescent quantum yields and excellent photochemical stabilities.As individual fluorescent nanosensors,both QDs and CDs have had a good performance in detection of different analytes.In recent years,some researchers have established ratiometric fluorescent methods based on QDs with CDs to obtain more sensitive and selective fluorescent nanosensors,attributed to the decrease of error caused by interference of instrumental efficiency and environmental conditions.Despite of the advantages,these ratiometric nanosensors were mostly used based on "on-off" systems,the sensitivity was reduced,resulting from the high-level "on" background signals.Thus,an "off-on" system would be a good strategy to improve sensitivity because of the low-level "off" state.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 QDs and CDs,possessing two emission peaks at 525 and 450 on under a single wavelength excitation of 360 nm,respectively.CdTe QDs were served as the response signal label,and CDs,having no response to the analytes,acted as the reference signal.The nanosensor was turned off to create a low-level "off" state when EDTA was added,and turned on with the addition of the target Zn2+ dramatically.Under the optimal conditions,the fluorescence intensity ratio changes(?F525/F450)had good linearity against the concentrations of Zn2+ within a dynamic linear range of 0.50-40 ?M.The limit of detection was as low as 0.33 ?M(3?/K),which was low enough for the detection of Zn2+ in human body.The intra-day and inter-day relative standard deviations(RSDs)of three different concentrations of Zn2+were all less than 10%.The proposed method was successfully applied to the detection of Zn2+ in human urine and plasma.11.67 ?M Zn2+ in urine and 7.92 ?M in plasma was found using this detection system,the recoveries were in the range of 84.89-108.1%with RSDs less than 10%.The results showed that the as-prepared QDs-CDs ratiometric nanosensor has potential application of clinical detection of Zn2+ in human body. |
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