Research On Highly Selective Ammonia Sensor Based On The Ionic Conduction Mechanism Of Biomass Hydrogel

Ammonia(NH₃)is an air pollutant and a metabolite in human exhaled breath as a biomarker of kidney disease.For the detection of NH₃,gas sensors have received widespread attention due to their small size,simple structure and convenient operation.Regardless of the monitoring of air pollution,the guarantee of air quality in production and living places,or the fields of health assessment and disease detection,higher requirements are put forward on the highly selectivity,highly sensitivity,good stability,portability and environmental friendliness of NH₃sensors.This paper aims at the application scenarios of complex NH₃detection with the influence of humidity and the interference of multiple gases,combined with the special properties of NH₃that is easy to dissolve and ionize in water,constructed a biomass acid/hydrogel sensitive system to create a high humidity NH₃sensor based on ion conduction mechanism.According to the different functions of the two parts in the composite system,different composite sensitive materials of hydrogels and organic acid monomers were designed,The influence of the water content and the ion transport capacity of the hydrogels,as well as the influence of organic acid monomer molecular structure and carboxylic acid ionization ability on NH₃sensing characteristics were stuied.A self-supporting flexible NH₃sensor was fabricated from the design perspective of the new device.The high-humidity NH₃sensor based on ion conductivity provided a novel solution for the highly selectivity and highly sensitivity detection of NH₃,and opened up a new method for detecting NH₃under high-humidity conditions.The main research content of this paper are summarized as follows:1.In view of the reported problems of NH₃sensors being affected by humidity and poor selectivity and based on the characteristics of NH₃that is easy to dissolve and ionize in water,the poly-L-glutamic acid(PAA)and L-glutamic acid(GA)were selected as sensitive material,and the NH₃sensor were made by a simple method of physical blending and then dropped coating to form a film.The PGA/GA sensor has been tested for different relative humidity,different NH₃concentration,multiple gas interference and other gas-sensitive performance indicators.The results show that the sensor has high selectivity and high sensitivity to NH₃,and it is effective when the relative humidity is 80%RH.The response of 50 ppm NH₃reaches 8.40,and the minimum detection limit is 0.5 ppm.The impedance analysis method and oscillation circuit method were used to study the electrical properties and adsorption properties of the sensitive materials,and the ionic conduction mechanism of PGA/GA high humidity NH₃sensing in the presence of different humidity was analyzed,which proposed a feasible research idea for the following research work.2.The main role of different components of sensitive materials in the detection of high humidity NH₃and the influence of different hydrogel materials on the sensitive characteristics of high humidity NH₃sensors were studied.Poly-L-aspartic acid(PAA)was selected as a hydrophilic material to produce a sensor based on PAA/GA composite hydrogel.By comparing the ability of adsorb H₂O and adsorb NH₃about the single component GA,PAA,PGA,composite materials PAA/GA and PGA/GA,it can be clarified that the hydrogel in the composite system is mainly used as a good ionic transport medium and a stable materials,and the small acidic organic molecules are used as sensitive materials for NH₃adsorption.In a high-humidity environment,the hydrophilic hydrogel material adsorbs water to form an ionic-activated hydrogel,which promotes the adsorption of NH₃by small acidic organic molecules.Due to its chemical structure,PAA is more hydrophilic than PGA,which can make acidic organic small molecules GA adsorb more NH₃,thereby further improving the sensitivity of the NH₃sensor.Finally,analysis methods such as complex impedance spectroscopy and equivalent circuit diagram were used to conduct a more detailed analysis of the special sensitive mechanism of detecting NH₃under high humidity conditions,and proposed a humidity-activated ammonia sensitive mechanism.3.Aiming at the baseline drift of PAA/GA and PGA/GA sensors in the process of high-humidity NH₃detection,we were committed to solving the stability problem of high-humidity NH₃sensors,and compared the ammonia sensitivity characteristics of different small molecule organic acids in the composite hydrogel system.We used the in-situ photopolymerization method to make a stable cross-linked hydrogel sensitive material based on citric acid(CA),malic acid(MA)and tartaric acid(TA)and polyethylene glycol diacrylate(PEGDA).The acid dissociation constants of CA,MA and TA are smaller than GA(CA:p Ka₁=3.31,MA:p Ka₁=3.40,TA:p Ka₁=3.04and GA:p Ka₁=2.10).Baseline drift was found in PAA/GA and PGA/GA sensors,but not in CA/PEGDA,MA/PEGDA and TA/PEGDA sensors.It shows that the small acid molecules can adsorb NH₃without causing irreversible chemical reactions with NH₃by controlling the balanced water content of the hydrogel composite and the dissociation constant of the acidic groups.The CA has more multi-calculating groups than MA and TA,so the sensor based on CA/PEGDA shows the most excellent NH₃sensitivity.The CA/PEGDA sensor has the highest response value among the three sensors at 80%RH.The NH₃concentration and response value show a good linear relationship within the range of 0.05-20 ppm NH₃,while exhibiting excellent selectivity and long-term stability.4.In this experiment,cellulose and biomass acid were used as the substrate and NH₃sensitive materials of the flexible sensor to produce self-supporting flexible NH₃sensor,which reduced the complex steps in the production process and avoided the problem of weak connection between sensitive materials and substrate materials.Ethyl cellulose(EC)and hydroxyethyl cellulose(HEC)mainly play the role of ionic transport and stable film formation,while citric acid(CA)is the main NH₃sensitive material.The response of CA/EC/HEC sensor is 3.29 to 5 ppm NH₃,and shows good repeatability,response-recovery characteristics,highly selectivity and long-term stability.The results of the bending test show that the CA/EC/HEC flexible sensor has good flexibility and good sensitivity to NH₃under different bending angles.In this paper,a biomassic acid/hydrogel composite material system which can be used for the detection of NH₃in high humidity environment was constructed for the complex detection environment with multiple interference,and a novel high-selectivity NH₃sensor was successfully developed.A new NH₃sensitive mechanism named humidity-activated was proposed for the first time,and a new flexible NH₃sensor with self-support was designed.The above research results not only provide a new idea for the research of high-selectivity and high-sensitivity NH₃sensor,but also provide the possibility for the practical application of the sensor in complex detection environment.