Nanoporous Polystyrene Fibers Functionalized By Polyethyleneimine For Enhanced Formaldehyde Sensing

Formaldehyde is a toxic volatile organic compound, which could cause great harm to human body. Traditional monitoring methods are difficult to test micro amount of formaldehyde in a real-time, accurate, and low-cost way. The QCM sensor is a mass testing device in the level of nanogram. Modifying the surface of QCM electrode with sensitive material to test formaldehyde has drawn the attention of researchers both at home and abroad. However, this technology currently faces a problem:the flat sensing polymer membrane coated on the QCM electrode limits the number of effective adsorption sites for formaldehyde. This research aims at design a new type of efficient formaldehyde sensor based on QCM technique. Through the combination of electrospinning technology and surface modification of nano materials, the research successfully overcame this difficulty, and made a new type of efficeient formaldehyde sensor. Also, we made a tentative investigation on the theoretic relationship between the 3D structure of polystyrene (PS) nano fibers with the performance of the final formaldehyde sensor.The research first fabricated nanoporous PS fibrous mats via electrospinning directly. The solvent selected is THF/DMF with a weight ratio of 1:4. Pretests were carried out to determain the best experimental parameters, including concentration of PS solution (7,10, and 13wt%); voltage (20kV); tip-to-collector distance(15cm); speed of the syringe (4ml/h); temperature and relative humidity in the lab (25℃and 40%RH). Subsequently, FE-SEM test was carried out and an Acrobat Photoshop image analysis software and statistical method were used to analyze fiber diameters. It could be seen that:all of the three kinds of PS fibers had porous structure and bead-on-string structure; under the same fabrication parameters, both the thickness of the fibrous membrane and the average fiber diameters increased with the incensement of the PS concentration. In addition, BET test and Nitrogen Adsorption-Desorption analysis was carried out for the three-dimensional PS fibrous membrane. And the ASAP2020 surface area analyzing software was also used. The results showed that surface area of three kinds of PS fibers increased to 11.67,37.23 and 42.25 m2/g respectively, with the increasement of concentration of PS solutions, which is almost as much as ten times of corresponding flat membranes; it is worth noticing that the average fiber diameters increased at the same time, which should decrease fibers'surface area, indicating that porous structure of PS fibers is the dominant factor influencing the surface area. The width of pores in the PS fibers belonged to macro- and meso-pores. As the increasing of concentration of PS solutions, the pore volume in accordance with a certain pore width increased. What is more, the formation of the porous structure can be explained by phase separation theory.Then, modification of the PS nano fibrous membrane was realized by drop-casting of polyethyleneimine (PEI). A reversible chemical reaction can take place between the hydroxyl groups of PEI molecules and formaldehyde molecules, making PEI the ideal material for formaldehyde testing. Since it is difficult to obtain PEI fibers through electrospinning, a drop-casting method was introduced. The FE-SEM images of PEI-PS membranes indicated that white PEI particles were attached to the PS fibers and there were more particles in the PS fibrous mat with a higher concentration of electrospinning solution; and PEI did not influent the morphology of PS fibers.Finally, the prepared QCM electrode with PEI-PS was tested on the Stanford QCM200 System. Influence of PS solution concentration and amount of PEI load on the performance of the final formaldehyde sensors was tested. The results show that, the sensor performed better with a higher PS solution concentration, with the same load of PEI and PS onto the QCM electrode. It was also observed that more PEI load resulted in better sensing performance, yet PEI of more than 6000 Hz tended to cause overload of the QCM electrode. Moreover, selectivity test was carried out, which demonstrated that the formaldehyde sensor developed in this research had nice selectivity toward VOCs other than formaldehyde. Finally, the research analized all the experimental results comprehensively, and established a tentative theoretical relationship between experimental parameters and the performance of the QCM-based PEI-PS formaldehyde sensor, which would benefit similar research in the future.