Research On The Detection Of Heavy Metal Ions Based On The Highly Sensitive Evanescent Field Change Behavior On The Fiber Surface

At present,water environment has been severely polluted due to human activities.During the contamiantions,the excessive intake of poizous heavy metal ions has caused a serious threat and a considerable harzard to human's health.Consequently,there is an urgent demand to develope accurate,selective and fast analytical method for the determination of heavy metal ions,which is also of important research and social significance.Evanescent wave(EW)is an electromagnetic wave formed on the surface of optical fiber when the beam propagates by total internal reflection(TIR).The intensity of the evanescent wave decreases exponentially along the axis with the penetration depth of about 100 nm.As a result,EW can be used to sensitively monitor the interface behavior.Based on evanescent wave at surface of optical fiber,novel fiber-optic chemical sensors(FOCS)have been fabricated through miniaturized and modular design,which can be used for rapid and sensitive detection of heavy metal ions in water environment.Four analytical detection systems were constructed based on two basic sensing principles,including evanescent wave absorption and evanescent wave excited fluorescence.Combined with selective chromogenic reagents or highly sensitive fluorescent probes,the sensing characteristics of EWFOCS for different heavy metal ions were discussed in detail.The main research work of this paper includes the following aspects:(1)A highly sensitive fiber-optic evanescent wave sensor based on EW absorption was developed to detect Pb2+.Blue LED(?=480 nm)was chosen as light source,and dithizone(DTZ)was used as the chromogenic reagent of Pb2+ in the experiment.The produced DTZ-Pb2+chromophores can sensitively interact and absorb EW on the optical fiber's surface.As a result,the concentration of Pb2+was obtained by detecting the change of EW intensity.The results showed that the Pb2+concentration was linear with EW intensity in the range of 0.05-3.0 mg L1.The detection limit of this method was 17 ?g L-1,the response time was 0.27 min,and the reagent consumption was 280 ?L.(2)A highly sensitive fiber-optic evanescent wave sensor conbined with solid phase extraction(SPE)and EW absorption was developed to detect Cu2+.Fe3O4@PDA@KH550 SPE column was prepared for selective adsorption of Cu2+ at pH=8.Green LED(?=510 nm)was employed as light source,and 1,5-diphenylmethyl carbazide(DPC)was used as both chromogenic and eluent reagent for Cu2+.Cu2+was eluted at pH=10 to form DPC-Cu2+chromophores which would contribute to absorption of EW at optical fiber surface.Consequently,the concentration of Cu2+was obtained by detecting the change of the EW intensity.The results showed that the Cu2+concentration was linear with EW intensity in the range of 0.01-6 mg L-1.The detection limit of Cu2+was 3.24 ?g L-1,the response time was 2.07 min,and the reagent consumption was 280 ?L.(3)A highly sensitive fiber-optic evanescent wave sensor based on carbon quantum dot(CQDs)fluorescence quenching was developed for Cu2+determination.Amino-modified carbon quantum dots(CQDs-PEI 1800)were prepared by pyrolysis of citric acid and introduction of polyethyleneimine(PEI 1800).Purple LED(?=365 nm)was selected as light source,and the fluorescence of CQDs-PEI 1800 was excited by the EW on the surface of optical fiber.The fluorescence emission(?=438 nm)could be quenched efficiently by Cu2+and the concentration of Cu2+could be obtained by detecting the change of fluorescence intensity.The experimental results showed that the Cu2+concentration was linear with fluorescence intensity in the range of 0.01-8 mg L-1.The detection limit was 3.33 ?g L-1 the response time was 0.8 min,and the sample dosage was 20 ?L.(4)A highly sensitive fiber-optic evanescent wave sensor based on carbon quantum dots fluorescence quenching was developed for Hg2+ determination.N-doped carbon quantum dots(N-CQDs)were prepared by simple hydrothermal method using citric acid as carbon source and ammonia as N-doped reagent.Purple LED(?=365 nm)was selected as light source,and the fluorescence of N-CQDs were excited by the EW on the surface of optical fiber.The fluorescence emission(?=440nm)could be quenched efficiently by Hg2+ and the Hg2+concentration could be obtained by detecting the change of fluorescence intensity.The experimental results showed that the Hg2+ concentration was was linear with fluorescence intensity in the range of 0.08-1.2 mg L-1.The detection limit was 26.67?g L-1,the response time was 0.26 min min,and the sample dosage is 20 ?L.