Research On The Structure Regulation Of Electrospun Fibrous Membranes And Its Application In Formaldehyde Sensors

As an important organic chemical material, formaldehyde has enormous application in building materials, textiles, chemical industry, medical, pesticide and many other fields. But materials with formaldehyde would gradually release free formaldehyde during usage process. At present, the WHO has blacklisted formaldehyde in potential carcinogen and critical pollution as well as established guidelines for indoor concentration of formaldehyde with a maximal value of 80 ppb while the domestic value is 60 ppb. However, the bottleneck problems, such as in situ, rapidly or selectively on-line detecting formaldehyde at low concentrations, which traditional detecting methods faced, have greatly promoted the development of highly sensitive formaldehyde sensors. Functional nanomaterials have super-high specific surface area and are able to improve both the sensitivity and selectivity of sensors. Three-dimensional (3D) electrospun fibrous membranes not only possess advantages such as high specific surface area, highly porous structure and controllable fibrous structures, but also are able to realize functionalization through regulating chemical composition or structures. Thus, electrospun fibers have great potential in sensor applications.In this thesis, we first retrospected the history and development of electrospinning and primarily introduced the electro-netting technique which evolved from electrospinning. Based on the introduction of various electrospun fibrous structures, the thesis pointed out that the variety of fibers and controllability of fiber structures were the critical factors for sensor applications. After deeply analyzing the main issue of the present formaldehyde sensors, the thesis managed to fabricate quartz crystal microbalance (QCM)'sensors based on multi-structured electrospun fibrous membranes to in situ detecting formaldehyde, did researches on sensing mechanism, investigated the combination of various structural fibrous membranes with QCM, analyzed and compared the detecting results of different sensing membrane structures, revealed the influencing factors of sensing property and finally constructed sensitive sensing surfaces to make rapid and highly sensitive formaldehyde detection possibly. Besides, another colorimetric sensor based on nano-nets was developed to realize visible formaldehyde detecting with low expenses. The main contents of the thesis are as follows:(1) A novel formaldehyde sensor was developed by electrospinning deposition of nanofibrous polyethyleneimine (PEI)/poly(vinyl alcohol) (PVA) membranes as sensitive coatings on QCM. The morphology of the porous 3D PEI/PVA membranes was controllable by tuning the compositions of polymers and solvents in PEI/PVA solutions. The results indicated that the content of PE1 and the composition of solvent would influence not only the morphology of fibers, but also the sensing performance. When the fibrous PEI/PVA (1.6/1 wt/wt) membranes coated sensor was formed from the cosolvent of water and ethanol, the morphology and sensing property were optimum and the sensor showed a fast response to the 10 ppm formaldehyde. The response of fibrous membrane coated QCM sensors was higher than that of corresponded flat membrane coated QCM sensor. Furthermore, the sensors exhibited excellent reproducibility, reversibility and selectivity. According to Langmuir adsorption theory and Sauerbrey equation, we studied the responding dynamics relationship of absorption gases on sensing membranes and deduced the constant of absorption rate; we also discovered that 3D porous membranes structure was beneficial to the diffusion and absorption of gases.(2) In order to further improve the performance of the formaldehyde sensors, we developed nanoporous PEI-polystyrene (PS) composite fibrous membrane modified QCM sensor system. Based on the phase separation mechanism induced by solvent evaporation, nanoporous PS fibers with different concentrations were fabricated and electrospun deposition on the QCM, followed by the functionalization of the sensing PE1 on the membranes. The results showed that the Brunauer-Emmett-Teller (BET) surface area of fibrous membrane (from 11.67 m2/g to 47.25 m2/g) and the corresponding sensitivity of the sensor were both increased with the increased concentration of PS solution (from 7 wt% to 13 wt%). The developed formaldehyde-selective sensors exhibited fast response and low detection limit (3 ppm) at room temperature. The cushion effect to resonance transmission because of PS porous membrane's own porous fiber structure and the sensing material into the underlying effect caused by its thickness determine the loading amount of PS fiber membrane should not be too high, researches indicate that it is appropriate for coating load to be around 500 Hz. Increasing the coating load of PEI in the QCM will provide more adsorption sites of formaldehyde, and thus improve the sensor performance.(3) Nanoporous PEI-TiO2 composite fibrous membrane modified QCM sensor system was successfully constructed. Taking exploring the application of inorganic fibers in QCM sensor as the starting point, we prepared the nanoporous TiO2 fibrous membrane with high BET surface area (68.72 m2/g) through electrospinning a sol-gel titanium tetraisopropoxide/PS composite solution, and followed by calcination. We also explored the deposition technology of inorganic fibrous membrane on the QCM and the following surface modification process. Ethylene glycol was chosen as a dispersing agent for TiO2 nanofibers and inorganic fibers as substrate to be uniform deposited on QCM surface, then followed by surface modification of sensing PEI, and finally we obtained nanostructured PEI. Test results show that the sensor can realize formaldehyde detection with low interference as well as fast response (response time of 120 s), and the detection limit is 1 ppm. The coating load of PEI, TiO2 and ambient temperature all affect the sensing performance. The study found that increasing coating load of PEI on QCM could provide more adsorption points for formaldehyde, and thus will improve sensor performance. Inorganic TiO2 nanofibers have rigidity and high packing density characteristics which are conducive to vibration transmission caused by the gases adsorption into the fibrous membrane, together with TiO2 nanofibers'own response to formaldehyde; we got the conclusion that the response of the sensor increases with the coating load of TiO2 nanofibers. The QCM sensor response to formaldehyde was decreased when the temperature increased, which is accordance with the negative Arrhenius equation, and we concluded that the most appropriate temperature for QCM sensor is between 20-25℃.(4) A nanostructured complex, PEI functionalized polyamide-6 (PA-6) (PEI-PA-6) nano-fiber/net (NFN), is developed as a novel sensing coating on QCM for highly sensitive formaldehyde detection. Based on the structure control of a variety of polymer-based (such as poly(acrylic acid), gelatin, polyurethane and PA-6) spider-web-like NFN membranes and the analysis of forces acting on the charged droplets, a possible formation mechanism of nano-nets was initially proposed. The deposition and modification of materials oriented to determine the PA-6 nano-nets as a substrate material deposit on the QCM surface directly via electro-spinning/netting (ESN), and complete the PEI surface modification, and thus for the first time, we prepared a functionalized NFN membrane on QCM sensor. The responses of the sensors in response to formaldehyde were analyzed in terms of PA-6 NFN membranes morphologies, PA-6 substrate and sensing PEI coating loads, and the comparison with nanoporous fibers. Experimental results show that this new PEI-PA-6 NFN nanostructure based QCM sensor exhibits excellent formaldehyde sensing performances in terms of remarkably low detection limit (50 ppb), rapid response, superior selectivity and good reproducibility. The sensitivity of QCM sensors exposed to 100 ppm of formaldehyde was firstly increased and then decreased with increasing the PA-6 NFN coating load from 0 Hz to 2200 Hz, and the response reaches the highest level at PA-6 NFN coating load of 850 Hz. The sensing response was increased with the amount of PEI deposited on the QCM electrode due to more absorbing sites. Particularly, the response reaches a steady value at higher PEI coating load (>3000 Hz). Finally, a preliminary analysis about the diffusion kinetics of formaldehyde molecules in the fibrous membrane was made combining Fick diffusion equation and Sauerbrey equation. (5) A novel strategy for highly sensitive colorimetric sensor system was developed based on ESN PA-6 NFN. In this thesis, the principle for formaldehyde detection is based on the color changes caused by the reaction between formaldehyde and Methyl Yellow-impregnated ESN PA-6 NFN. The sensor presented a significant reflectance intensity decreasing band at 550 nm which induce the visual color changes from yellow to red after exposure to formaldehyde. Taking the advantages of the NFN structure, such as high specific surface area and porosity, the fabricated sensor strips are easy to operate and possess the highly sensitivity, good selectivity, reasonable reproducibility and allow for detection of foemaldehyde with a low detection limit of 50 ppb observing by naked eye. Additionally, the colorimetric responses are visualized quantitative by using a color-differentiation map prepared from converted RGB (red, green and blue) values. At the wavelength of 550 nm, the intensity of reflected light present a non-linear decreasing as reaction time increases (0-30 min). The as-prepared sensor strips provided a new insight into the design and development of a novel colorimetric sensing system based on the NFN platform.This thesis combines the structure regulation of electrospun fibers with sensing devices to fabricate a formaldehyde sensor with good selectivity; it could provide a certain amount of experimental and theoretical basis for improving the performance of gas sensors. Meanwhile, it opened up a new way for the electrospun fibers in the area of device application; it also has important scientific significance and practical value in environmental monitoring and solving air pollutions.