TFBG Ammonia Sensing Technology Based On PH Value Modulating Conductivity Of Conductive Polymer

In recent years,the worsening ecological pollution has become one of the focus of social attention.Environmental monitoring,p H monitoring and gas monitoring have become an important part of environmental protection work.Among liquid pollutants,acid rain has always been a troublesome environmental pollution problem,which has a serious negative impact on human living buildings,animal husbandry and breeding industry.Among gaseous pollutants,NH₃ is a common polluting gas,which is widely used in agriculture,scientific research,automobile industry and many other fields.However,as NH₃ is toxic,highly reductive,colorless,flammable and irritating,real-time monitoring of NH₃ is very important.The Occupational Safety and Health Administration（OSHA）of the United States has established a safe ammonia content of less than 25 ppm.As a result,ammonia sensors with low cost,remote monitoring,high sensitivity,good stability,and low detection limit are required.Tilted Bragg fiber grating is a special kind of fiber grating.The grating plane of TFBG is at an Angle with the axial direction of the fiber,which can excite a series of cladding mode formants.These cladding mode formants can sensitively perceive changes in refractive index and conductivity of the outside world,so TFBG has higher sensitivity than other fiber structures.Based on this characteristic,sensors combining TFBG with metal films,polymers,metal oxides,graphene and other functional materials have been developed.The surface evanescent field of TFBG interacts with the material.In the process of detecting the external environment,the evanescent field senses the changes of refractive index and conductivity of the material.Therefore,TFBG sensor has unique advantages and wide applications in temperature and pressure detection,biochemical gas sensing and other aspects.At present,gas sensing materials mainly include metal oxides and highly conductive polymers.This paper uses polyaniline（PANI）,a highly conductive polymer.Compared with metal oxides,highly conductive polymers do not have the limitation of high temperature and do not oxidize easily when exposed to air for a long time,which can maintain stable sensor performance.PANI has excellent sensing characteristics.The higher the doping degree of PANI is,the stronger the acidity is,and the higher the conductivity and refractive index are.The higher the conductivity and refractive index,the higher the degree of PANI doping and the lower the p H value.As a result,a highly sensitive miniaturized p H sensor is proposed in this paper by combining TFBG,which is sensitive to refractive index and conductivity,with PANI,which is highly resistant to oxidation,and exploiting the property that acidity and alkalinity can modulate the conductivity of PANI,changing the transmission spectrum of TFBG.It is employed for p H and gas sensing in complicated situations by detecting ammonia gas at varying concentrations utilizing its p H sensitive characteristic,and it implements a communication band all-fiber integrated sensing system for multipurpose sensor detection.The following is the main focus of this paper:1.The writing process,sensing mechanism,and spectrum properties of TFBG are introduced and discussed briefly.The sensing application of TFBG is introduced in a targeted manner by using external conductivity and refractive index as the mechanism.Highly conductive polymers are briefly discussed in terms of their conductivity mechanism and sensing advantages.PANI’s conductive mechanism,the link between conductivity magnitude and structure,and the characterization of PANI in various states are all covered in depth.PANI’s spectrum characteristics upon deposition are investigated.2.The TFBG-p H sensor was proposed by coating PANI on the surface of TFBG to take advantage of PANI’s high conductivity and sensitivity to a wide range of p H values.In this investigation,the conductivity and refractive index of PANI thin-film material were dramatically raised with the lower p H of the buffer solution,while the apparent variation of transmission spectrum intensity was minimized.When sensor properties were compared with different film thicknesses,it was discovered that the thicker film（556 nm）had a greater sensitivity of 2.14 d B/p H and a higher linearity variation pattern（R²=0.9986）.Furthermore,because the film thickness is only at the nanoscale level and there is no"layering"of material conductivity changes.The sensor is stable and can be cycled for a long period of time through repeated changes in solution p H.3.We devised a doped state（acidic state）PANI film to detect ammonia gas by utilizing the sensitive property of alkaline substances in the doped state of plasmonic acid,as well as the dendritic three-dimensional structure of PANI itself in this state,loose and porous,with a high specific surface area to collect ammonia molecules.During the detecting process,hydrogen ions（H⁺）in PANI mix with adsorbed ammonia molecules to create ammonium ions（NH₄⁺）,resulting in decreased conductivity and a weaker acidity,which affects the transmission intensity of the transmission spectrum.The detection of ammonia necessitates a thicker PANI film,and by comparison,the film’s performance is best at 156 nm.In the range of 25 ppm-500ppm,the linear sensitivity reached 5.2×10^-3 d B/ppm,the detection limit was 10 ppm,and the reaction time could be as rapid as 21 s.Finally,the sensor’s selectivity was demonstrated.