Fluorescence-colorimetry Dual Mode Sensors For The Detection Of Mercury Ion In Farmland Irrigation Water

Mercury ion(Hg²⁺)is a common and highly toxic heavy metal pollutant,which is difficult to degrade in the natural environment.Hg²⁺has an inhibitory effect on the growth of crops,leading to the reduction of their quality and quantity.In addition,Hg²⁺can accumulate in human body through the food chain,and cause serious damage to several organs.Therefore,it is of great importance to develop Hg²⁺detection methods with high performance for the ecological environment,agricultural production,and human health.Until now,researchers have developed a variety of methods for the determination of Hg²⁺.Among them,inductively coupled plasma mass spectrometry,atomic emission spectrometry,atomic absorption spectrometry and atomic fluorescence spectrometry exhibit highly accurate and reliable,but equipment used for these techniques are expensive and require specialized technicians to operate.The electrochemical method,surface-enhanced Raman scattering spectrometry,fluorescence and colorimetric methods are simple to operate and fast to analyze.However,single detection methods have their own limitations and suffer from false negatives or positives.To address these problems,this thesis proposes to develop a high-performance fluorescence-colorimetry dual-mode sensor to achieve accurate,simple,rapid and visual detection of Hg²⁺in irrigated agricultural water.The specific work is as follows:（1）Firstly,a carboxyfluorescein-labelled aptamer（FAM-apt）was used as fluorescence probe and recognition element.Then,the fluorescence-colorimetry dual-mode sensing strategy for the detection of Hg²⁺by integrating the fluorescence quenching effect and peroxidase-like activity of porous cerium oxide nanorod（P-Ce O₂NR）.The adsorption and desorption of FAM-apt on P-Ce O₂NR could affect the fluorescence signal of FAM-apt and the peroxidase-like catalytic activity of P-Ce O₂NR.In the absence of Hg²⁺,FAM-apt was adsorbed on the surface of P-Ce O₂NR,and the fluorescence signal of FAM-apt was quenched by P-Ce O₂NR.Meanwhile,the peroxidase-like activity of P-Ce O₂NR was enhanced,which drove the oxidation of colorless 3,3’,5,5’-tetramethylbenzidine（TMB）to the blue oxidation state of TMB（ox-TMB）.In the presence of Hg²⁺,Hg²⁺could bind specifically to FAM-apt,which desorbed from the surface of P-Ce O₂NR and the fluorescence of FAM-apt was recovered.At the same time,the enzyme-like catalytic activity of P-Ce O₂NR decreased and the color changed from blue to light blue.On this basis,a fluorescence-colorimetry dual-mode aptasensor was developed applied to detect Hg²⁺in irrigated agricultural water.（2）In order to,meet the needs of the fluorescence signal,peroxidase-like activity,and selective detection in fluorescence-colorimetry dual-mode sensor,three materials（FAM,P-Ce O₂NR and apt）were respectively in Chapter 2.To simplify the construction and detection process,we prepared a novel bifunctional nanocomposite of cerium-aggregated gold nanoclusters（Ce-Au NCs）which exhibited fluorescence property and peroxidase-like activity.Moreover,the selective detection of Hg²⁺could be satisfied by the metalophilic interaction between Hg²⁺and Au NCs.When Hg²⁺was present,Hg²⁺effectively quench the fluorescence signal of the Ce-Au NCs and enhance the peroxidase-like activity of the Ce-Au NCs,and the color of the solution changed from light blue to dark blue.On this basis,we constructed a fluorescence-colorimetry dual-mode sensor for Hg²⁺detection.The method was also used for the detection of Hg²⁺in farmland irrigation water in fields,and satisfactory analytical results were obtained.（3）To achieve on-site detection,a Ce-Au NCs-based fluorescence-colorimetry dual-mode paper-based sensor was constructed combining the Chapter 3 and paper-based sensing technology with the advantages of the simple and rapid analysis.The fluorescence intensity of the paper-based sensor decreased upon the addition of Hg²⁺.Based on this,the image color of the test paper were collected by the developed portable fluorescence detection device and smart phone using a double-hole plate,which can realize rapid and visual qualitative analysis of Hg²⁺by comparing with blank sample.Then,using a single-hole base plate,the Lab mean value of the paper-based fluorescence color was analyzed using computer software Photoshop after collecting the image color of the test paper.A good linear relationship between Hg²⁺concentration and Lab value was obtained for the quantitative detection of Hg²⁺in fluorescence mode.In addition,Hg²⁺could enhance the peroxidase-like activity of Ce-Au NCs.As the concentration of Hg²⁺increased,the color of the paper-based sensor gradually changes from blue to green,and then to yellow.In order to avoid the variability of color recognition by human eyes,a portable colorimeter was introduced to improve the accuracy of colorimetry detection.The color information（Lab value）of the paper was collected by the colorimeter,and the linear relationship between the concentration of Hg²⁺and Lab value was constructed,which realized quantitative colorimetric detection of Hg²⁺.The developed portable fluorescence-colorimetry dual-mode paper-based sensing device could achieve simple and rapid analysis of Hg²⁺,and solved the problem of the analysis of Hg²⁺in farmland irrigation water.