Design And Experimental Study On Reflective Fiber Mercury Ion Sensor

Heavy metal pollution is one of the most important water environmental protection issues.To be the most toxic element in heavy metal pollution,Hg poses a serious threat to human health.In order to ensure that the content of Hg2+ in water environment is limited within the safe range,it is very important to explore Hg2+ sensors with accurate detection,high sensitivity,simple and efficient use.At present,most of the Hg2+ sensors based on spectroscopy and electrochemical methods have the disadvantages of large volume,high price and cumbersome detection process.The rapid development of optical fiber sensing technology has advantages compring with other sensing technologies,such as electromagnetic interference resistance,small size,and high sensitivity.Based on these,the purpose of this thesis is to study the mercury ion sensor based on optical fiber sensing technology.It focuses on the design and implementation of reflective sensor probe,the preparation and coating of mercury ion sensitive film,and the solution of temperature cross-sensitivity.The main research contents include:(1)Theoretical design and simulation analysis of reflective fiber Hg2+ sensor structure.According to the actual measurement requirements of Hg2+,the reflective single mode fiberno-core(S-N)fiber and reflective surface plasmon resonance(SPR)fiber sensing structures based on the principle of SPR were proposed respectively.The theoretical models of the two kinds of sensors were established to verify the feasibility of the sensing scheme.The optimal structural parameters and sensing characteristics of the sensors were determined through simulation analysis,which laid a theoretical foundation for Hg2+ sensing experiments.(2)Research on selection and coating method of Hg2+ sensitive film.Non-toxic and harmless Chitosan(CS)and polyacrylic acid(PAA)were selected as Hg2+ sensitive membrane materials.In the two sensing methods,the sensitive film was coated on the surface of optical fiber and metal film,respectively.In order to effectively and stably combine the sensitive film onto the optical fiber,the method of treating hydroxylation(-OH)on the surface of the optical fiber and carboxylation(-COOH)on the surface of the metal film were proposed,respectively.Then sensitive film is coated by layer by layer self-assembly technology.(3)Construction of the experimental system and analysis of the sensing characteristics.Experimental studies were carried out by measuring different concentrations of Hg2+ solution using the prepared sensors.The results showed that the reflective S-N fiber sensor had a sensitivity of up to 0.082nm/(?mol/L)in the range of 0?100?mol/L Hg2+ concentration.The reflective SPR fiber sensor had a sensitivity range of 0.5586nm/(?mol/L)in the range of 0?30?mol/L Hg2+concentration.Compared with the reflective S-N optical fiber Hg2+ sensor,the sensitivity of the reflective SPR fiber Hg2+sensor in the low concentration range of Hg2+had been significantly improved.In addition,the two sensors showed good specificity and reproducibility for Hg2+.(4)Analysis and solution of the cross-sensitive problem between Hg2+ concentration and temperature.The temperature sensitivity of the reflective S-N fiber Hg2+ sensor was 0.0187nm/?.By cascading an FBG,the solution temperature and the concentration of Hg2+were measured simultaneously.The experimental results showed that the temperature sensitivity of FBG reached 0.0147nm/?.Through the two-parameter measurement matrix equation,not only the ion concentration and temperature could be measured simultaneously,but also the influence of solution temperature on the accuracy of Hg2+concentration detection could be eliminated.Similarly,the temperature sensitivity of the reflective SPR fiber Hg2+sensor was 0.2158nm/?.Using the method of partially coating the sensitive film in the SPR sensing region,two resonant wavelengths appeared in the output spectrum,which could be used to simultaneously measure the temperature of the solution and the concentration of Hg2+.The above experiments verified that the temperature sensitivity of the sensor was 0.2901nm/? and the sensitivity of Hg2+ concentration was 1.1176nm/(?mol/L).