Study On The Gas Sensing Performance Of Ion-in-conjugated Squaric Polymers

In recent years,with the issuing of the "Action Plan of Air Pollution Prevention and Control",our country has put forward more and more requirements for the prevention and control of atmospheric pollutes.Effective treatments of air pollutes are inseparable from accurate detection of the toxic and hazardous gases.Hence,gas sensing is more and more desirable.At present,the chemresistive gas sensing materials are mainly from metal oxides.However,the gas sensors from metal oxides have the disadvantages of high operation temperature,high energy consumption,poor selectivity and low sensitivity.In contrast,organic thin film gas sensors have the advantages of room temperature operation,high selectivity,simple preparation and material diversity,which have attracted wide attention of researchers in the sensory filed.Developing the new organic gas sensing materials is of great significance for improving gas sensing performance.Despite that research on organic gas sensing materials has been developed for many years,there are also many problems to be solved,such as slow response,low sensitivity,poor long-tern stability,and poor batch reproducibility,hinder organic materials from practical application.The above problems are caused by the improper interaction between the sensing materials and the gas molecules,which are either too strong or too weak.In this paper,by introducing ion-in-conjugation structure to sensing materials to form the moderate interaction between the materials and the gas molecules to facilitate gas absorption and desorption,the bad sensitivity and slow response of the organic sensor could be alleviate by certain degree.This paper focuses on squaric polymers,including squaramide and squaraine,which are both of unique ion-in-conjugation structures.Squaramides are important compounds to provide hydrogen bond donor/acceptor and widely used in ion recognition.As common organic dyes,squaraines has high molar extinction coefficient,low band gap,and wide spectrum absorption range.There are already a large number of applications based on squaraine in the fields of solar cells,optical information storage,bioanalysis.However,they are rarely reported in gas sensing.Therefore,inspired by the unique properties and wide application of this kind of organic materials,this paper introduces the squaric polymers into the field of gas sensing.A serious of polymers based on squaric acid were designed and synthesized,and a thin film gas sensor was prepared by brush-coating the squaric polymers onto the interdigital electrodes.Furthermore,test the sensing performances of these sensor deviee towards ammonia,nitrogen dioxide and nitric oxide,and discuss the sensing mechanism from the perspective of molecular interaction.This paper is composed by the following chapters:(1)An organic conjugated polymer(PADS)was designed,synthesized and fabricated to an organic thin film gas sensor.An ultra-low limit of detection of 10 ppt towards ammonia is achieved,which is the lowest detection limit of all ammonin sensor up to date.In addition,this sensor can also detect the 20 ppb of NO.It has a long sensing life of at least 240 days,faster response/recovery,higher selectivity and better batch reproducibility compared to benchmarked peers.Besides,a new sensing mechanism based on double hydrogen bonding(DHB)is proposed.This work Provides a new direction for the design of organic materials for gas sensing.(2)An polysquaraine PPDS was designed and synthesized,which possesses the special ion-in-conjugation structure.An ion-dipole interaction occurring between the molecules and PPDS is beneficial to the adsorption and desorption of gas molecules to achieve a good sensing performance.The results show that the PPDS can detect nitrogen dioxide as low as 5 ppb.The response is as high as 921 at 10 ppm and has a good selectivity and stability.(3)Based on the previous work,a squaraine conjugated polymer(PDBS)was designed and synthesized as a sensing material to detect NOZ by introducing an amine group into the polysquaraine chain.The introduced amino and the ion-in-conjugation structure of squaraine can provide the hydrogen bonding and ion-dipole interaction with NO2 molecules.Compared to PPDS,the synergistic effect of hydrogen bonding and ion-dipole interaction reduced the limit of detection to 1 ppb in sensing NO2.In addition,the sensor based on PBDS has a faster response,good selectivity and at least 120 days of sensing life.