Metal-organic Framework Material As A Ratio Fluorescence Sensor For Heavy Metal Ion Detection

With the rapid development of science and technology,people’s living standard has been significantly improved.However,due to the discharge of industrial waste water and the use of pesticides and fertilizers,the problem of water pollution is becoming more and more serious,such as heavy metal pollution.Heavy metal ions are a great threat to human life activities and natural environment,and some trace heavy metal elements,such as iron and copper,are essential in human physiological activities.Therefore,the monitoring of heavy metal ions is very important.At present,there are many detection methods for the detection of heavy metal ions,such as inductively coupled plasma mass spectrometry,high performance liquid chromatography and atomic absorption spectrometry.Due to the limitations of these methods,fluorescence sensors have advantages in comparison.Therefore,in order to ensure the performance of the sensor,the selection and construction of fluorescent sensor materials is very important.And the fluorescence sensor based on metal organic framework materials（MOFs）has become the object of our research.As a kind of hybrid material composed of metal ions and organic ligands,metal organic framework material combines fluorescence and porous properties.Because the fluorescence of MOFs is derived from their metal ions and organic ligands,the fluorescence of MOFs is designed and varied.In addition,the porous property and adjustable structure of MOFs enable many fluorescent objects to be well encapsulated into their channels,thus forming corresponding fluorescent materials,which further enriches the fluorescence performance of MOFs.The porosity of MOFs makes them have strong ability to enrich the identified substances.Therefore,MOFs fluorescence materials have great potential as fluorescence sensors.In this paper,two kinds of fluorescence sensors based on MOFs were designed and constructed,and a series of characterization analyses were carried out according to the characteristics of the material itself,and the detection performance of the fluorescence sensors on the analyte,such as selectivity and sensitivity,was studied.The specific research work is as follows:（1）In this paper,by microwave assisted heating,sulfonyl rhodamine B（SRB）molecules were encapsulated into UiO-66’s channels by one-pot method and post-synthesis modification method respectively,thus two kinds of composite fluorescence sensors were prepared（SRB@UiO-66）.Through a series of characterization analysis,it can be found that the microwave-assisted heating one-pot synthesis of SRB@UiO-66 composite material has great advantages in all aspects.The encapsulation of SRB molecules in UiO-66’s channels limits the aggregation of SRB molecules,thus optimizing the fluorescence properties of SRB.In addition,UiO-66 itself emits purple fluorescence,which can be combined with SRB that emits red fluorescence,which is just suitable for the construction of a ratio fluorescence sensor with two emission centers.The anti-interference performance of SRB@UiO-66 as a ratio fluorescence sensor in a variety of complex environments was studied.It was found that the phosphorus as a ratio fluorescence sensor almost had no response to the pH of aqueous solution and common anions.Moreover,sulfonic acid groups on SRB molecules and UiO-66 crystal defects provide good binding sites for Fe³⁺,while UiO-66 itself has a strong enrichment capacity for Fe³⁺.Therefore,by studying the response of SRB@UiO-66 as a ratio fluorescence sensor to Fe³⁺,it can be found that in various metal ion solutions,it has a good selective response to Fe³⁺without interference from other metal ions.It was found that the response of SRB@UiO-66 to Fe³⁺had a linear relationship,and the fluorescence intensity ratio（IUIO-66/ISRB）increased with the increase of Fe3+concentration,and the minimum detection limit could reach 3.693μm.（2）In this paper,the yellow fluorescent carbon dots（CDs）were successfully prepared by hydrothermal method using o-phenylenediamine as raw material.Then the carbon dots,zinc ions and dimethylimidazole were mixed in deionized water to synthesize the composite fluorescence sensor（CDs@ZIF-8）by one-pot method.After a series of characterization analysis,it can be found that the fluorescent carbon dots are successfully encapsulated into the pores of ZIF-8,thus preventing the aggregation of carbon dots and enhancing the fluorescence properties of carbon dots.ZIF-8 itself emits blue fluorescence,and CDs@ZIF-8 can be used as a ratio fluorescence sensor for dual emission centers.The fluorescence emission of CDs@ZIF-8 was studied,and it was found that it really had double fluorescence emission,and its fluorescence performance was the best at 400 nm excitation wavelength.At the same time,the anti-interference performance of CDs@ZIF-8 as a ratio fluorescence sensor to the environment was studied.It can be found that it does not respond to pH and common anions in aqueous solution.Moreover,the amino group on the surface of a fluorescent carbon dot and defects of ZIF-8 crystal make CDs@ZIF-8 have abundant Cu²⁺binding sites,and ZIF-8 itself can make Cu²⁺aggregation.Therefore,the response of CDs@ZIF-8 as a ratio fluorescence sensor to Cu²⁺was investigated.It was found that CDs@ZIF-8 had a specific response to Cu²⁺and were not disturbed by other metal ions.In addition,the response of CDs@ZIF-8 to Cu²⁺is linear,and the fluorescence intensity ratio（IZIF-8/ICDs）increases with the increase of Cu²⁺concentration,and the detection limit can reach 11.712μm at the lowest.