Design And Performance Research Of Heavy Metal Ion Sensor Based On Optical Fiber SPR

Once heavy metals enter the environment,they cannot be biodegraded,and they will cause soil,air,water and other pollution to varying degrees,among which water pollution is one of the factors that cause great harm to human health.Therefore,it is urgent to prepare sensors for detecting heavy metal ions in water.In this paper,three kinds of optical fiber sensors based on surface plasma resonance(SPR)are designed and fabricated,which adopt two types of structures:reflective and transmissive.Different sensing structures are built by using a variety of special optical fibers,and then different sensitive composite films are combined with optical fiber sensing structures,so as to realize the detection of cadmium ions and trivalent chromium ions and explore all aspects of the performance of the sensor.The main research work of this paper is as follows:(1)A reflective PVA/Ti O₂ composite optical fiber sensor is proposed.Silver(Ag)was deposited on the side and end face of plastic clad multimode fiber(PCMF)by physical vapor deposition(PVD),and then PVA,Ti O₂ and PVA/Ti O₂ sensitive films were coated on the outer surface of PCMF covered by Ag film,and the cadmium ion spectral response was tested and compared to select the best sensitive film.Afterwards,the side and end faces of the PCMF coated with PVA/Ti O₂ sensitive film were observed by scanning electron microscope(SEM)and the sensitive material was analyzed by fourier transform infrared spectroscopy(FTIR).The experimental results show that the sensor coated with PVA/Ti O₂ sensitive film has the highest sensitivity.The sensor can detect Cd²⁺solution with a concentration range of 0-1?M,the maximum sensitivity is 315.2nm/?M,and the minimum detection concentration is 0.006?M(about 0.67 ppb),which is lower than the World Health Organization(WHO)Cd content in drinking water regulated value of 3 ppb.At the same time,the cadmium ion sensor has the advantages of high sensitivity,good stability,fast response speed and excellent selectivity.(2)A thin core fiber(TCF)is fused between two sections of multi-mode fiber(MMF)to form a MMF-TCF-MMF sensing structure,and then a certain thickness of Ag film is evaporated on the TCF to excite the SPR phenomenon,Then the surface of Ag membrane was coated with Cd²⁺ion imprinted polymer(Cd²⁺-IIP)and non-ionic imprinted polymer(NIP).The responses of different composite membrane sensors to cadmium ions were measured and SEM characterization and FTIR analysis were performed.The results show that the coated Cd²⁺-IIP composite film has the highest sensitivity,the sensitivity is 3.39nm/n M and the limit of detection is about 1.08 n M(about 0.12 ppb),which is far lower than the international standard.The transmissive cadmium ion sensor has the advantages of reusability,excellent selectivity,low detection limit and good stability.(3)A small core fiber(SCF)is fused between two multimode optical fibers to form a MMF-SCF-MMF sensing structure,and then 67 nm Ag film is evaporated on the side of SCF by PVD,Then,Cr³⁺ion imprinted polymer(Cr³⁺-IIP)and non-ionic imprinted polymer were coated on the surface of Ag film by pull-up coating machine,and the responses of different sensitive film sensors to chromium ions were measured respectively.At the same time,the sensors coated with Ag-Cr³⁺-IIP sensitive film were tested for selectivity and stability.The results show that the linearity of the sensor is0.994,the sensitivity is 73 nm/?M,and the limit of detection is 0.093?M(about 2.548ppb),which is lower than the guidance value of 50 ppb of Cr content in drinking water of the WHO.Through SEM and EDS analysis,the sensor can be desorbed by hydrochloric acid and can be reused.Its response time is about 70 s,and it also has the advantages of good linearity,good stability and excellent selectivity.