| Visual detection is a method that uses the naked eye to directly recognize the color change of the detection result to directly read the relevant information of the target analyte.It has the advantages of simple,fast and low cost,and has been received extensive attention in many fields,such as environmental monitoring,food safety,and medical diagnosis.The most commonly used in visual detection are colorimetry and fluorescence.Among them,the ratiometric fluorescence technique in the fluorescence method has attracted much attention because of its advantages such as high sensitivity and obvious change in fluorescence color.Ratiometric fluorescent probes usually contain two emission bands,which can detect analytes quantitatively by the ratio of the two fluorescence intensities.It has a self-calibration function.The ratiometric fluorescence probe constructed by the traditional method mainly uses water-phase quantum dots,carbon dots,organic dyes and the like as fluorophores,and combines them by mixing,coupling and embedding.Due to the tedious assembly process and the harsh experimental conditions,the fluorophore is easily affected by the environment and fluorescence quenching occurs,the stability of the probe is poor,and the accuracy of the results is affected.The signal output mode of the ratiometric fluorescent probe can be divided into two categories,one is the fixed reference single signal response,and the other is the dual signal reverse change.The ratiometric fluorescent probes reported so far mainly focus on single signal changes and have low sensitivity.In order to construct a stable dual-signal reverse ratio fluorescent probe,the following research work has been performed in this paper:?1?A new affinity ligand-driven assembly strategy was developed,in which hydrophobic green?Cd Zn Se/Cd S/Zn S,G-QDs?and red?Cd Se/Cd S/Zn S,R-QDs?quantum dots were directly affinity assembled with mercapto modified dendritic mesoporous silicon carrier?d Si O2?in organic phase,successfully prepared dual-emission layered silica nanocomposites SQSQS.First,green quantum dots are supported on the inner channels of dendritic mesoporous silica spheres by thiol-metal coordination?d Si O2@G-QDs,SQ?.After the silanization modification,the phase transfer of the hydrophobic assembly is completed,and then a silica layer is coated on the outside by the St?ber method?d Si O2@G-QDs@Si O2,SQS?.The sulfhydryl group was modified on the surface of the composite nanosphere,and red quantum dots were assembled?d Si O2@G-QDs@Si O2@R-QDs,SQSQ?.After the phase inversion and controllable coating of the silica protective layer,a dual-emission quantum dot nanocomposite?d Si O2@G-QDs@Si O2@R-QDs@Si O2,SQSQS?was obtained.We can precisely control the fluorescence intensity ratio and color of nanocomposites by changing the concentration of green and red quantum dots.The constructed nanocomposite probes show good fluorescence characteristics of the original quantum dots and good optical/colloidal stability.?2?The study found that the absorption spectra of gold nanoparticles?Au NPs?in the dispersed and aggregated state partially overlap with the green and red emission spectra of the quantum dot nanocomposite,respectively,and have different degrees of quenching effect on the two-color quantum dots.Based on this,the prepared dual-emission quantum dot nanocomposite?SQSQS?was combined with gold nanoparticles to design a ratio fluorescent probe based on the internal filtration effect.With the help of the image analysis software of mobile phone app,a new method of highly sensitive visual detection of melamine based on color and tone changes was developed.Spectral and visual inspections of melamine in milk and milk powder from actual samples were performed with satisfactory results.In addition,based on the reverse signal change of the two-color quantum dots caused by melamine,a logic gate strategy for melamine detection was constructed. |
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