Research Of Fiber PH Sensor Based On SPR Principle

pH is an important parameter to measure the solution and great significance in biomedicine,marine monitoring and wastewater treatment.Nowadays,detecting methods for pH mostly use test paper comparison and electrochemical measurement,which have the disadvantages of low detection precision and large volume of measurement electrodes.In addition,traditional fiber optic pH sensors are mostly based on fluorescent labeling,which have the longer detection time,biological contamination and the expensive subsequent demodulation equipment.Therefore,it is necessary to research a fiber sensor with high pH detection accuracy and biologically harmless.The pH-sensitive hydrogel is a polymer material with a network cross-linked structure.When the pH value changes,the hydrogel shrinks or swells,which change its refractive index and enable its sensing function for pH.The combination of hydrogel and fiber structure supports the bio-harmful and highly sensitive pH measurements.Surface plasmon resonance(SPR)sensors are widely used for refractive index measurement due to their high sensitivity.Therefore,in view of the demand of pH detection,combined with the advantages of SPR technology,a fiber optic pH sensor based on SPR principle and hydrogel is proposed.The sensor has advantages of small volume,high sensitivity and short response time.It can achieve high precision and fast pH detection.The main research contents of this paper are as follows:(1)The background of fiber-optic sensors based on pH-sensitive hydrogel is summarized,and the development and research background of fiber-optic SPR sensors are introduced.On this basis,the research contents and research significances of this topic are put forward.(2)The basic principles and sensing characteristics of fiber SPR sensor are studied,and the structural parameters of fiber SPR are optimized by simulation model.Futhermore,the preparation method of polyacrylamide hydrogel was explored,and the optimal ratio was determined to test the pH sensitivity of the gel.(3)The pH sensing characteristics of the polyacrylamide-based gel fiber interference structure were investigated experimentally,and the temperature compensation was realized by the Bragg fiber grating after the interference structure.The performance of the sensor was tested as follows.The sensor can measure pH ranging from 2 to 12,the corresponding sensitivities for pH and temperature are 0.271 nm/pH and 0.009 nm/℃,respectively,with the response time of 24 s.Since the temperature sensitivity differs greatly from the pH sensitivity,temperature crosstalk can be ignored.In order to increase the measuring sensitivity of the sensor,the hydrogel was applied onto the fiber SPR structure for investigation.The results show that the maximum sensitivity of the polyacrylamide-based gel fiber SPR structure is 13 nm/pH.(4)The coating process was optimized,and the layer-sensitive self-assembly technology was applied to the pH-sensitive film coating to achieve controllable film thickness and uniformity.Two kinds of poly electrolytes,sodium alginate and polyethyleneimine,were selected,and the pH sensitive film was successfully coated on the surface of the sensing area by layer-by-layer self-assembly method.The pH sensing characteristics of the self-assembled multilayer optical fiber interference structure were studied experimentally.The results show that the sensitivity of the sensor is 1.7 nm/pH,which is compared with the sensitivity of polyacrylamide hydrogel(0.27 nm/pH).Since the coating process is controllable,the thickness and uniformity of the sensitive film has a better dynamic response due to greatly shortened response time(12 s).The pH sensitive membrane was bonded to the fiber SPR structure.The experimental results show that the sensor has a wider range,a shorter response time,and a higher sensitivity(maximum sensitivity is 38 nm/pH).