The Preparation Of Fluorescent Nanomaterials For Detection Of Water Pollutants

The fluorescence detection method has the advantages of rapid response,high sensitivity and good selectivity.It has been widely used in the analysis and detection of water pollutants.The fluorescent dyes used early have some defects such as narrow excitation spectrum,wide fluorescence emission spectrum and asymmetric distribution,and easy photobleaching.With the rapid development of nanomaterials,new fluorescent nanomaterials with higher fluorescence quantum yields,superior light stability have gradually replaced traditional materials.Although researchers have used fluorescent materials such as telluride quantum dots and metal nanoclusters to achieve some research results,these fluorescent materials have the disadvantages of poor chemical stability,high toxicity,and high cost.Therefore,the development of a new type of fluorescent probe material is important.We designed a series of fluorescent probes with good light stability,strong chemical inertness and low toxicity using biocompatible chitosan,carbon quantum dots and metal-organic framework（MOFs）materials,and successfully applied them to detection of water pollutants.Based on this,the main research contents of this paper are as follows:1.A nonconjugated polymer fluorophore（FCPs）was synthesized by cross-linking chitosan with glutaraldehyde via Schiff base reactions.The results revealed that the synthesized FCPs exhibited excellent photostability and water solubility.More importantly,FCPs possessed dramatically enhanced fluorescence intensity originated from the n-π*transitions of the Schiff base sub-fluorophore groups（e.g.,C=N）that can be selectively and sensitively quenched by Cr₂O₇^2-through oxidative damages to C=N group.An effective EDTA masking agent approach was employed to minimize ionic interferences.In the presence of high concentration of interference ions including Cr（Ⅲ）,the quenching FCPs fluorescence is capable of selectively determining Cr（Ⅵ）within a concentration range up to 50 μM and a detection limit of 0.226 μM.Finally,the analytical performance of GCPF was also confirmed by analyzing real surface water and industrial samples,which proved its potential application value in the actual water testing.2.The N-doped CQDs were synthesized using citrate acid as the carbon source and ethylenediamine as the N source by a facile hydrothermal route.It is further identified that under the induction effect of glutaraldehyde（GD）,the aldehydes of the GD could react with the amino groups on the surface of the carbon quantum dots,and the N-doped CQDs could generate self-assembled CQDs nanospheres with fluorescence.Subsequently,we found that the crosslinked carbon nanospheres have obvious pH-dependent fluorescence properties,the results showed that there is a good linear relationship between the shift of emission wavelength and the pH when the pHis between 2.29 to 7.16.However,common cations,anions,and small molecules in organisms can not cause the shift in the emission wavelength of crosslinked carbon nanospheres.The results have shown that H⁺ can damages the C=N group of crosslinked carbon nanospheres,resulting in an increase in the surface oxidized carbon content and more surface defects,which causes a fluorescence shift effect associated with the surface state.This work uses fluorescence emission wavelength shift to quantitative sensor of pH,providing a new strategy for fluorescence detection mechanism.3.Using 2,5-dihydroxyterephthalic acid（H4DOBDC）as an organic linking unit,isopropyl titanate（Ti（i-OPr）₄）as a metal structural unit,a Ti-MOF nanoflowers have been successfully synthesized by a two-step hydrothermal method for the selective detection and facile adsorption of Al（Ⅲ）in solution.Benefiting from its high specific surface area and excellent fluorescence properties,Ti-MOF nanoflowers were used as fluorescent probes for the quantitative detection of Al（Ⅲ）,its fluorescence intensity gradually increases with increasing Al（Ⅲ）concentration.The linear range for Al（Ⅲ）detection is 0.4-15μM and the detection limit is as low as 0.075 μM.The interference of Fe（III）toward the detection of Al（Ⅲ）can be shielded by adding ascorbic acid.More significantly,the Ti-MOF nanoflowers exhibited strong adsorption capacity for Al（Ⅲ）with a maximum adsorption of 25.85 mg g^-1,a fast adsorption rate and a pseudo-second-order kinetics model.The dual functional Ti-MOF nanoflowers can not only precisely detect trace levels of Al（Ⅲ）,but also simultaneously realize its efficient decontamination.4.By using the strong interaction between ligand H4DOBDC and Al（Ⅲ）,a flower Al-MOF material was synthesized via a facile one-step solvothermal route using glacial acetic acid as a solvent.This structure is favorable for the rapid diffusion of measured ions and the enhancement of contact area between the target ion and the fluorescent probe,consequently improving the sensitivity of fluorescent detection.The results showed that the emission wavelength of Al-MOF is 517 nm at the excitation wavelength of 372 nm.Phosphate ions can quench the fluorescence of Al-MOF by complexation with the material,while other common 11 cations(Cd²⁺,Pb²⁺,Hg²⁺,etc.)and seven anions(NO₃^-,SO₄^2-,CO₃^2-,etc.)have small effect on the fluorescence intensity of Al-MOF.Based on this,we used Al-MOF as fluorescent probe for the quantitative detection of phosphate ions based on fluorescence quenching.In the concentration range of 1-40 μM,there was a good linear relationship between the fluorescence intensity of Al-MOF and phosphate ions concentration.The detection limit of phosphate was as low as 0.66 μM.The analytical method has the advantages of simple preparation,high selectivity,good stability and reproducibility,and it is expected to be applied to rapid detection of phosphate ions in actual water environment.