Optical Fiber Surface Plasmon Resonance Sensor Modified By TiO₂/MWCNT For H₂S Gas Detection

Hydrogen sulfide(H₂S)is a colorless,highly toxic and flammable harmful gas,mainly producing from oil refining,coal mining and metal smelting and so on,which is seriously threaten the ecological environment and human health.Low concentration of H₂S gas can severely damage people's eyes,respiratory system and central nervous system,while high concentration of H₂S gas cause respiratory failure and immediate death.Therefore,in order to prevent environmental pollution and protect human life safety,it is urgent to develop a H₂S gas sensor with high sensitivity and on-line detection.Existing H₂S gas detection technologies are mainly divided into electrical and optical categories.Electrical sensors have problems such as complex preparation process,slow response time and continuous power supply.The sensor based on optical fiber Surface Plasmon Resonance(SPR)has the unique advantages of anti-electromagnetic interference,rapid response and remote on-line monitoring,and has been widely studied and applied in gas detection.Aiming at the low detection sensitivity of the traditional SPR gas sensor,the TiO₂/MWCNT composite modified optical fiber SPR sensor for H₂S gas detection was proposed in this thesis.The feasibility was verified by simulation analysis and experimental study.The main research work of this paper is as follows:(1)The excitation principle and propagation characteristics of optical fiber surface plasiton were systematically studied.The TiO₂-modified optical fiber SPR gas sensor was proposed,and the sensing mechanism of the device was studied in depth.The physical model of the sensor was established based on the theory of N-layer film transmission.The structural parameters of the sensor were optimized by MATLAB simulation analysis.The optimal structural parameters of the fiber core diameter,sensing area length and gold film thickness were 600?m,10 mm and 50 nm,respectively.The gas sensing mechanism of the sensor was analyzed based on the chemical and physical adsorption process between the TiO₂sensitive layer and H₂S gas molecules.(2)A terminal reflective fiber SPR probe was prepared based on the optimized structural parameters of the sensor.The TiO₂sensitive layer was fixed on the surface of the probe by layer by layer self-assembly technology.The optical fiber SPR gas sensing system was built,and the refractive index sensing characteristics of the sensor were studied.In the experiment,the thickness of the TiO₂sensing layer was optimized by controlling the deposition period.The experimental results show that the sensitivity of the sensor reaches the maximum when the deposition period of the sensitive layer is 3,and the sensitivity of the sensor is 72.8%higher than that of the pure gold film fiber SPR sensor.The sensitivity of the sensor is 11.05 pm/ppm,and the sensor has a good linear response in the range of 0 ppm?60 ppm H₂S concentration.The response and recovery time are 60 s and 100 s,respectively.And it has good selectivity and long-term stability.(3)The TiO₂/MWCNT composite film was prepared by introducing MWCNT material as the sensitive layer of the sensor to further optimize the performance of the sensor.A fiber SPR sensor modified by TiO₂/MWCNT composite thin film was fabricated and characterized.The results show that a reticulated porous film is formed on the surface of the sensor,and TiO₂nanoparticles are uniformly dispersed on the surface of MWCNT.The effects of composite volume ratio and sensitive film thickness on the sensor performance were studied.The results show that the maximum sensitivity of the sensor is 21.76 pm/ppm when the volume ratio of TiO₂to MWCNT is 1:5 and the deposition period is 3,and the sensor has a good linear response in the range of H₂S concentration from 0 ppm to 60 ppm.The response time and recovery time are 30 s and45 s,respectively.Compared with the TiO₂-modified sensor,the sensitivity of the sensor is improved by 96.9%.