Construction Of Cellulose Nanocrystal-based Chiral Nematic Materials And Its Response Mechanism To Formaldehyde

Volatile organic compounds(VOCs),volatile organic substances with a melting point lower than room temperature and a boiling point of 50-260°C.Among them,aldehyde-based VOCs,such as formaldehyde(HCHO),are an important type of VOCs.Formaldehyde in indoor and outdoor air mainly comes from building decoration materials,various chemicals,synthetic textiles,food preservation.When its content reaches a certain concentration,it will cause damage to human nerves and organs and induce systemic diseases.However,it is still a great challenge and demand to develop a universal,real-time and high-sensitive monitoring system for formaldehyde detection.On the other hand,cellulose nanocrystal(CNC)has gained more and more attention in the field of gas sensing due to its chiral nematic structure,abundant hydroxyl structure and high specific surface area.This thesis aims at the current problem of contradiction between selectivity and sensitivity in formaldehyde gas sensors,combined with the structural advantages of CNC,takes CNC chiral sensitive films as the research object and formaldehyde gas as the detection target,focuses on the construction strategy and mechanism of CNC-based sensitive films with formaldehyde gas response ability.On this basis,systematically explores the response behavior and mechanism of these sensitive films to formaldehyde gas,and finally develops a dual-response channel sensitive films with both response specificity,mechanical practicality and sensitivity.It provides theoretical basis and technical support for the development of formaldehyde gas sensors for efficient and convenient detection,and at the same time,it also provides new ideas for the application of CNC in gas sensor.The main research contents of this thesis are as follows:(1)The CNC-sensitive film was successfully prepared by sulfuric acid hydrolysis combined with evaporation-induced self-assembly(EISA)method.CNC sensitive films are placed in different concentrations of formaldehyde gas atmosphere,formaldehyde molecules are embedded in the chiral nematic structure of CNC through hydrogen bonds,thus the maximum reflected wavelength of CNC sensitive films is increased,and the structural color of sensitive film is redshifted.When the formaldehyde gas concentration increased from 0.14 to 14 mg/m~3,the response range of the CNC sensitive film was 92 nm,and the structural color changed from purple to blue.However,the CNC sensitive film does not respond significantly at low concentrations of formaldehyde gas,the structural color is almost unchanged,and there is no response specificity.(2)Focusing on the improvement of the specificity of the response of CNC sensitive films to formaldehyde gas,glycine(Gly)was used as the intermediate of functional groups to successfully prepare the chiral nematic CNC-Gly sensitive films by two-step esterification method combined with EISA method.The content of Gly added was increased from 20 wt%to 80 wt%,and the strongest reflective peak of the sensitive films shifted from 468 nm red to 682 nm due to space-contributing of Gly.The CNC-Gly sensitive film was placed in an atmosphere of different concentrations of formaldehyde gas,and when the formaldehyde gas concentration was 0.08mg/m~3,the CNC-Gly sensitive film changed from bluish blue to lake green.As the formaldehyde gas concentration increased from 0.08 to 5.0 mg/m~3,the response range of the CNC-Gly sensitive film was 107 nm,and the sensitive film changed from lake green to orange-yellow.By comparing the microscopic morphology and chemical structure of CNC-Gly films under different concentrations of formaldehyde gas,it is pointed out that the synergistic effect of formaldehyde molecules and the Schiff base reaction of amino groups and the binding of hydroxyl hydrogen bonds significantly increases the pitch of CNC-Gly sensitive film,thus sensitive films respond to lower concentrations of formaldehyde gas.Moreover,with the gas concentration increased from0.08 to 5.0 mg/m~3,the response range of the CNC-Gly sensitive film to the maximum reflected wavelength of acetaldehyde,methanol and ethanol gases was 61,36 and 45 nm,respectively,which was less than that of formaldehyde gas.This is due to the aldehyde group in the formaldehyde molecule is a strong electron-rich end,with a strong Schiff base reactivity,which is easily adsorbed on the CNC-Gly sensitive film,thus the sensitive film structure color shows obvious redshift and achieves the effect of specific response,which is more beneficial to the visual detection of formaldehyde gas.(3)To improve the drawbacks of CNC chiral sensitive films with large brittleness,low toughness and poor practicability,the flexible cationic polymer polyethyleneimine(PEI)is constructed by electrostatic interaction method combined with EISA method.By analyzing the microscopic morphology,interface structure,chirality and mechanical properties of CNC-Gly/PEI films,it was concluded that when the PEI addition was 8 wt%,the comprehensive performance of CNC-Gly/PEI sensitive film was optimal,among which the strongest reflection peaks of the sensitive films was 535 nm,and the tensile modulus,tensile strength and elongation at break were709.47 MPa,15.01 MPa and 7.35%,respectively.Moreover,PEI introduces a large number of amino groups,which further enhances the response range and specificity of sensitive films to formaldehyde gas.When the formaldehyde gas concentration was 0.08 mg/m~3,the CNC-Gly/PEI sensitive film changed from bluish blue to light lake green.As the formaldehyde gas concentration increased from 0.08 to 5.0 mg/m~3,the response range of the CNC-Gly/PEI sensitive film was 112nm,and the sensitive film changed from light lake green to light orange-red.With the gas concentration increased from 0.08 to 5.0 mg/m~3,the response range of the CNC-Gly/PEI sensitive film to the maximum reflected wavelength of acetaldehyde,methanol and ethanol gases was 64,33 and 42 nm,respectively,which was even smaller than that of formaldehyde gas.(4)Focusing on improving the sensitivity of CNC sensitive films to formaldehyde gas,Cd Se/CNC-Gly/PEI luminescence sensitive films were constructed by physical blending method and EISA method by using Cd Se quantum dots as fluorescent probes.By adjusting the content of Cd Se quantum dots from 5 wt%to 15 wt%,by analyzing the micromorphology,interface structure,chirality and photoluminescence performance of Cd Se/CNC-Gly/PEI luminescent sensitive films,it is pointed out that when the Cd Se addition is 10 wt%,the comprehensive performance of Cd Se/CNC-Gly/PEI sensitive film is optimal,of which the strongest reflection peak of the sensitive film is 408 nm,the emission wavelength,quantum yield and fluorescence intensity are556 nm,36.7%and 603 a.u.,respectively.With the introduction of Cd Se into the CNC chiral nematic structure,the CNC chiral nematic sensitive film was upgraded from a single-channel response to a dual-channel response.With the increase of formaldehyde gas concentration from0.08 to 5.0 mg/m~3,the response range of the sensitive film reached to 120 nm,the sensitive film changed from dark purple to yellow-green,and the fluorescence intensity increased from 603 to1163 a.u.Through the photoluminescence response channel,it takes only 22 min to complete the detection of formaldehyde gas concentration of 5.0 mg/m~3,and the sensitivity is higher than CNC,CNC-Gly,and CNC-Gly/PEI sensitive films.Furthermore,through the analysis the change of fluorescence intensity with response time during the response,it is revealed that the main stage of the sensitive film response is the linear correlation stage between the growth rate of fluorescence intensity and the response time.Thus,the higher the concentration of formaldehyde gas,the greater the absolute value of the linear slope,and the higher the sensitivity of sensitive films.