Preparation Of Colorimetric Nanocellulose Film And Benzene Gas-Sensitive Performance Study

New colorimetric sensors are attracting a lot of attention due to their excellent real-time visualization characteristics.In recent years,cellulose nanocrystals（CNC）with chiral liquid crystal effect have made a splash in the field of colorimetric sensing of gases and humidity due to their easy modification,low toxicity and never fading characteristics.There have been many studies on the colorimetric sensing of gases such as ethanol and formaldehyde by CNC.However,little has been done in the field of colorimetric sensing of benzene gas,which is widely used and has higher toxicity.In this thesis,based on the chiral helical liquid crystal structure of CNC,CNC/Fe composite membrane with colorimetric sensing performance for benzene gas was prepared by introducing the sensitive factor Fe³⁺.The effects of Fe³⁺on the formation of CNC helical liquid crystals as well as the composite membrane morphology and structural color characteristics during the self-assembly process were systematically investigated,and the benzene gas-sensitive performance and mechanism of CNC/Fe were further investigated;In order to improve the detection limit of benzene gas,graphene quantum dots（GQDs）were introduced into the CNC/Fe system,to obtain CNC/Fe/GQDs composite membrane with dual structural color/fluorescence response to low concentration of benzene gas,and the benzene gas-sensitive performance of the composite films was further investigated.With the help of computerized color management system and digital image processing technology,a colorimetric sensing analysis system based on CNC composite films was designed and constructed with the prepared composite membranes as benzene gas sensitive materials,and the relationship between the structural color intensity/fluorescence color intensity of the composite films and benzene gas was further discussed.The details are as follows:（1）The introduction of Fe³⁺not only provides a modulating effect on the color and morphology of the CNC/Fe composite membranes,but also can confer the colorimetric sensing ability of the composite film for benzene gas.Among them,the CF14 composite membrane exhibited a maximum reflection wavelength blue shift from 671 nm to 442 nm compared to the pure CNC film,showing a maximum shift of 229 nm,which was mainly attributed to the electrostatic effect of Fe³⁺.In the system,Fe³⁺filled between the rods of CNC and formed electrostatic bonding,which reduced the electrostatic repulsive force of the system and made the CNC fully contacted,forming more intermolecular hydrogen bonds and eventually leading to the reduction of the pitch of the composite membranes.Since the off-domainπ-electrons on the aromatic ring and the unfilled 3d metal orbitals in Fe³⁺can formσ-bonds by sharing electron density,the spatial site resistance provided by benzene molecules in the composite film after adsorption of benzene gas leads to a larger pitch in the composite membranes,which makes the structural color of the composite film red-shifted.Among them,the CF6 composite membrane（5 m L 2.0 wt%CNC and 100μL 6mmol/L Fe Cl₃）exhibited the best colorimetric response behavior to benzene gas,showing an overall total redshift of 84 nm in the range of benzene gas concentration of 1-24 g/m³.（2）The introduction of GQDs equipped the composite membranes with fluorescence emission performance,and the fluorescence emission intensity of the composite membrane was positively correlated with the amount of GQDs added.On the one hand,there is an abundantπ-πconjugate system in GQDs,so the introduction of GQDs further improves the benzene gas-sensitive performance of the composite membranes and reduces the detection range of benzene gas.Among them,CF6/GQDs-200 exhibited a total shift of 51 nm in the benzene gas concentration of 0-250mg/m³.On the other hand,the phototransfer mechanism between GQDs and benzene gas,when the CF6/GQDs composite membranes after adsorbed benzene molecules is irradiated by excitation light,the electrons excited on GQDs are absorbed by the electron-deficient benzene molecule rings through conjugatedπ-bonds without being excited into the air to form fluorescence.Therefore,the CF6/GQDs composite membranes exhibits fluorescence burst in benzene gas environment.In summary,the CF6/GQDs composite membrane has the dual structural color/fluorescence intensity detection response characteristics for benzene gas,which further improves the detection accuracy of the composite membrane.It is worth noting that the fluorescence property of GQDs can be stabilized in water molecules.Therefore,even in a high humidity environment,the CF6/GQDs composite membranes can still maintain fluorescence response to benzene gas and show a certain degree of resistance to water molecules.（3）The relationship between the color intensity/fluorescent color intensity of CF6/GQDs-200composite membranes and the benzene gas concentration was obtained by image colorimetric analysis and pattern recognition algorithm.A good linear correlation exists between the gas concentration and the structural color intensity of CF6/GQDs-200 composite membranes in the range of benzene gas concentration of 0-125 mg/m³,and the regression coefficient R² reaches0.93575;And in the range of benzene gas concentration of 0-75 mg/m³,there was a good linear correlation between the gas concentration and the fluorescence color intensity of CF6/GQDs-200with the regression coefficient R² reaching 0.86444.The construction mode of this colorimetric sensing system is further improved on the basis of the traditional visual colorimetric method,which is more favorable to the discrimination of benzene gas concentration in the environment.