| As a member of the family of carbon materials,carbon dots(CDs)are considered as the candidates to replace traditional quantum dots due to their outstanding characteristics such as diverse synthetic precursors,good hydrophilicity,low toxicity and tunable photoluminescence properties,etc.They have set off a revolution in the fields of sensing,optical devices,biomedicine,cell imaging and so on.Particularly,they have visible multi-colors fluorescence under the ultraviolet lamp,so they have become a popular research object in the field of visual sensing.However,many efforts are focused on single emission CDs,which makes the sensors based on these CDs only show the intensities enhancing or weakening of fluorescence,and the visual effect shows the fluorescence colors from on to off or off to on,which has the inevitable shortcomings such as poor visual effect,low sensitivity and so on.That seriously hinders its wide application in visual sensing.A few reports of dual-emission CDs(D-CDs)can not be widely used because of its emissions too close or large half peak width,therefore,it is difficult to produce ideal visualization effect and high sensitivity.This also directly leads to the fact that most of the current CDs-based sensors only have a single fluorescence signal changes,which makes the prepared sensors vulnerable to various factors.The basic reason of these problems lacks the effective synthesis strategies of multi-emission CDs(M-CDs).Effective synthesis strategies can realize one-step synthesis of D-/M-CDs.D-/M-CDs not only has the excellent properties of multi-colors fluorescence under a single excitation wavelength,but also avoids the tedious steps of dual/multi-signals in the optical probe design process.What’s more,the excellent visualization effect of dual/multi-signals displays unlimited potential in the field of real-time detection,especially the colorimetric and fluorescent dual-mode visualization sensors.Thus,for realizing the construction of dual-mode sensor based on as-prepared CDs,it is necessary to give full play to the potential of the CDs,and use the responses of analytes to the UV-vis absorption spectrum signal to produce good visualization effect under the condition of instrument-free.Thus,in this work,the D-/M-CDs were synthesized by using two strategies of recessive signal doping and molecular fluorescence memory effects.Based on the as-prepared CDs,the colorimetric and fluorescence dual-mode sensors have been designed to realize the visual determination of the analyte.The main contents are summarized as follows:(1)Based on the recessive signal doping strategy,the europium doped dual-emission CDs(Eu-CDs)with a recessive signal was synthesized.The colorimetric and fluorescence dual-mode sensor was designed by bonding with Chromium Black T(EBT),for high sensitivity and visualization detection of dipicolinic acid(DPA),a anthrax biomarker.Firstly,Eu-CDs was synthesized by a one-step solvothermal method using glutathione as precursor,Eu in europium nitrate as the recessive signal and formamide as the solvent.The prepared Eu-CDs has low toxicity and good biocompatibility,and has been successfully used as fluorescent labeling of A549 cells.And the magenta appears when Eu-CDs coordinates with EBT.Based on the ligand displacement strategy,DPA reduces magenta and makes blue appear,which develops a ratiometric colorimetric method of DPA.Under 270 nm excitation,DPA makes the red recessive signal of Eu in EBT-CDs@Eu appear,based on the absorbance-energytransfer-emission(AETE)effect,and the ratiometric fluorescent assay of DPA has been developed.Based above,dual-mode and dual-ratiometric optical probe was designed successfully.The limit of detection(LOD)are 10.6 n M and 50 n M for ratio colorimetric and fluorescence assay,respectively.More prominently,naked-eye determination of DPA concentrations without instrument is as low as 1.0 μM.Furthermore,its practicability was verified by quantifying the DPA in Bacillus subtilis spores and human urine [relative standard deviation(RSD)≤2.35%].The sensor shows great superiority in on-site analysis of DPA,especially in limited instruments conditions.(2)Based on the molecular fluorescence memory effects strategy,the CDs with blue,yellow and red emissions in the visible region under a single excitation wavelength were synthesized.Due to its special solvation effect and excellent photoluminescence properties,the parpared CDs can be directly used as the colorimetric and ratiometric fluorescence dual-mode sensor to realize the visual detection of water in organic solvents.Using oxalic acid as carbon source,Nile Blue A as nitrogen source and the parent of molecular fluorescence functionalization,ethanol as the solvent,multi-emission CDs(T-CDs)with special solvation effect and temperature-dependent carbonization were synthesized via one step thermal pyrolysis method.Its red emission has a high quantum yield(QY)(the absolute QY in ethanol is as high as 73.6%).The molecular fluorescence genetic strategy and luminescence mechanism of T-CDs were studied under comparable and controllable conditions.It was further proved by column separation chromatography that different fluorescence emissions of T-CDs originate from different emission energy states.Experimental results showed that the red fluorescence of T-CDs could be attributed to the inheritance of precursor.Moreover,the prepared T-CDs have good photochemical stability,salt tolerance and low toxicity,and have been successfully used in the imaging of Hela cells.In addition,due to the special solvation effect of T-CDs,we have successfully established a dual-mode analysis assay for the visual determination of water in organic solvents.The LOD in ethanol is as low as 0.17%(v/v).The assay has also been verified in other solvents such as DMSO,acetone,ethylene glycol and DMF,and has been successfully applied to the detection of water in spirit and alcohol samples(RSD≤1.0%).These results broaden the application scope of CDs and provide a simple method for semi-quantitative detection of water in organic solvents. |
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