Vibration Frequency Formaldehyde Sensor Based On Surface Modified Nanofibers

Formaldehyde is a common harmful, volatile organic compound, which could cause great hazard to human body. It probably brings damage to human central nervous system and immune system, and it is even easy to induce cancer and other disorders. There are many methods for formaldehyde detection, such as spectrophotometry, chromatography, electrochemical method and so on. However, all the mentioned methods require sophisticated instruments, complex sample handling, and professional operators, which makes it difficult to detect formaldehyde gas in situ, quickly and at low cost under room temperature. Utilizing gas sensor to detect formaldehyde has become a hot spot in recent years, and in many types of sensors, the vibration frequency sensor caused widespread concern of researchers both at home and abroad for the advantages of detecting fast and continuously in situ. Quartz crystal microbalance (QCM), as one of the detecting platforms of vibration frequency sensor, is able to measure tiny mass changes (even nanogram). Whereas, the QCM electrode is not selective for formaldehyde, it’s needed to modify the electrode with a sensitive layer which can selectively absorb formaldehyde gas. It has been proved that the sensitivity and selectivity of sensor will be enhanced with increase of the specific surface area of sensitive materials. Although there were researches on formaldehyde detection by means of vibration frequency QCM sensor, most of them modified flat films on QCM electrode as sensitive materials, which limited the further enhancement of the sensitivity of sensors. Electrospun nanofibrous membrane is characterized by three-dimensional structure, large specific surface area and high porosity. So it can act as an ideal sensitive material. In recent years, the electrospinning technology and QCM platform were combined to produce formaldehyde sensors with high performance. However, there are still some shortcomings:on one hand, the active part of composite nanofibers is embedded easily; on the other hand, the template material is hydrophobic and fluffy. In our work, the QCM platform independently developed by ourselves was combined with the electrospinning technology to fabricate a formaldehyde gas sensor based on QCM with high sensitivity, good selectivity, quick response and good reproducibility by utilizing hydrophilic template material and surface modification technology for nanomaterials.In our research, we first prepared polyacrylonitrile (PAN) nanofibrous membranes (NFM) with high specific surface area (13.89m2/g) and porosity (0.031cm3/g) by method of electrospinning. We set the electrospinning parameters as follows:the concentration of PAN polymer solution was6wt%, the voltage was25kV, the tip to QCM electrode distance was15cm, the flow rate was1.5mL/h, the temperature and relative humidity of the surroundings were25℃and40%respectively. Then we conducted the test of field emission scanning electron microscope (FE-SEM) and specific surface area analysis (BET) for the result PAN NFM.Subsequently, the surface modification of the PAN NFM was realized by drop casting of sensitive materials:polyethyleneimine (PEI) and polyvinylamine (PVAm). After trial electrospin-ning, we found it’s nearly impossible to collect electrospun PEI and PVAm nanofibers. So we decided to use drop-casting method, which could be a good solution to realize the sensitive materials nanostructured. Afterwards, the modified PAN NFM were characterized through FE-SEM method, the result of the observation indicated the sensitive materials (PEI and PVAm) had been successfully modified on the surface of PAN NFM. Moreover, the morphology and structure of the surface modified PAN NFM did not change significantly.At last, the fabricated formaldehyde gas sensor based on QCM was utilized to detect the formaldehyde gas in real time. A self-developed quartz crystal microbalance system named QCM-1000was put into use, and we comparatively studied the influence of the structural differences between flat film and NFM, the sensitive materials loading, and the NFM loading on the performances of the result formaldehyde gas sensor. As a result, the sensitivity of the NFM modified formaldehyde gas sensor based on QCM shows at least two times higher than that of the flat film modified one, and the detecting limit reaches ppb level. We got the gas diffusion coefficient of two kinds of sensitive membrane by simulating the response curve based on the Fick’s law of diffusion, and it shows the theoretical analysis results are in conformity with the experimental data. Within the low loading range, the increasing of the PVAm loading can bring more formaldehyde molecule adsorption sites, thus the sensor has higher sensitivity. However, the influence of the PAN NFM loading is quite different. The PAN NFM loading has a critical value. This is mainly for the reason when the PAN NFM loading is too large, the structure of the membrane is loose, which means less rigidity. As far as we know, the rigidity of sensitive materials is very important for the vibration transmission. Moreover, the selectivity and reproducibility of the result sensor had also been tested, it displays that the response of the fabricated sensor to other VOCs is far less than the response to formaldehyde gas. After one circle test, the sensor can be recovered by exposure to N2.