With the rapid growth of economy and increasing demand for comfortable living environment, decoration materials are extensively used in the built environment, which brings complex and diverse pollutants including formaldehyde into the indoor environment [1, 2]. Among various types of air pollutants, formaldehyde has received significant attention because of its wide usage and toxicity. Being a reactive compound, formaldehyde can damage proteins, cause genetic mutations, DNA single-strand internal cross-linking and DNA-protein cross-linking, and inhibit DNA damage repair, etc. . Long-term exposure to low-dose formaldehyde can cause chronic respiratory diseases, nasopharyngeal cancer, brain tumors and other diseases [4, 5]. Therefore, a timely and precise detection of formaldehyde concentration is particularly important.
Many types of formaldehyde detection methods, such as spectrophotometry, chromatography, and fluorescence spectroscopy, have been proposed. [6–8]. However, they are generally expensive, and often require a long cycle of sampling and analysis operated by professionals. The measurement results using these methods are usually the mean values over a period of time, which does not reflect the formaldehyde concentration in real time .
Up to now, the research of formaldehyde gas sensor is mostly focused on the development of detection techniques, which require both high sensitivity and short detection period, in the air in real time . For example, Bouchikhi et al. developed a metal oxide thin film-based sensor by vapor deposition of tungsten trioxide (WO3) nanowires (NWs) and metal nanoparticles modified WO3 NWs gas sensing layer on interdigital platinum electrode, and the sensor showed a high sensitivity for formaldehyde gas under both dark and ultraviolet light irradiation conditions . Yin et al developed a polymer thin film sensor by applying a flower-like compound with a heterostructure based on Sn3O4 and reduced graphene oxide (rGO) to achieve a wide detection range of formaldehyde gas . These types of sensors have the advantages of high sensitivity and low detection limit. However, they often suffer from the severe interferences of temperature and humidity changes. Up to now, there are not many studies to minimize the interferences of temperature and humidity. For example, Wang et al. proposed a formaldehyde gas sensor based on Cu-doped Sn3O4 nanoflowers . Its response to 100 ppm formaldehyde is 53 (the ratio of the resistances of the sensor in dry air and the gaseous environment), with a detection limit of 1 ppm, but the changes of humidity have a significant impact on the detection results. An offset of about 10% in response is reported between 25% and 75% RH environments . Zeng et al. prepared a La2O3-In2O3 and nanotube sensor using an electrospinning method, and the sensor has a response value of 101.9 to 50 ppm formaldehyde gas. They reported that when the RH value was lower than 60%, the sensor's response to formaldehyde gas was relatively stable, but when the RH value was higher than 60%, the sensor's response to formaldehyde gas was decreased sharply with the increase of RH values .
Previously, we developed a surface acoustic wave (SAW) formaldehyde gas sensor based on a bi-layer nanofilm of bacterial cellulose (BC) and polyethyleneimine (PEI) . The BC nano layer significantly improves the sensitivity of the PEI film and reduces the response and recovery time for the low concentrations of formaldehyde. The sensor has a frequency shift of 35.6 kHz to 10 ppm formaldehyde gas at room temperature and 30% relative humidity (RH), with both good selectivity and stability. The sensor uses a ST-cut quartz substrate which has a low temperature coefficient of frequency (TCF). However, it shows a poor performance with the change of humidity, because the amine groups of PEI and the hydroxyl groups of BC have strong adsorptions of H2O molecules [16, 17], thus causing a significant mass loading effect of the SAW sensor. One way to solve this problem is to install a humidity sensor next to this formaldehyde SAW sensor which can quantitatively detect humidity to correct the SAW sensor output through offline data analysis. However, this is an indirect compensation increasing the complexity, size and production cost of the formaldehyde sensor. Therefore, under the premise of maintaining the PEI/BC sensing film's sensing performance for formaldehyde gas, solving the problem of its high sensitivity to RH values becomes our key research topic to improve the performance of the formaldehyde gas sensor.
Changes of hydrophobicity is currently a research hotspot in the field of functional materials . It is also important in the field of gas sensing, and a high hydrophobic layer on the top of the sensor can prevent the sensor from interfering with the changes of environmental humidity . The hydrophobic layer can prevent water molecules from easily entering the sensitive film, thereby reducing the frequency shift of the mass loading caused by the water molecules in the formaldehyde gas detection process. Recently, researchers have applied various methods to improve the hydrophobicity of sensors. For example, Lee et al. used an ultrafiltration method to successfully exchange water-based poly-(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT: PSS) solution to organic solvent-based PEDOT: PSS solution, which was applied as a coating on a pressure sensor to increase the contact angle of water droplets and avoid the influence of humidity changes to sensors . Chen et al. used a hydrophobic cysteine-sensitized Cu2O(Cu(I)-Cys) nanocomposite as a sensing layer to fabricate a quartz microbalance gas sensor, which could determine hexanal and 1-octen-3-ol at room temperature, with a good hydrophobicity, sensitivity and selectivity .
Zeolite-based imidazole salt frameworks (ZIFs) are a type of metal organic frameworks (MOFs), which are porous crystalline materials formed by continuous and periodic connection of transition metal ions and imidazole-based organic linkers [22, 23]. Due to its high porosity, thermal/chemical stability, surface functionality and diverse synthesis methods, ZIFs have been widely used in various fields, including gas storage, catalysis, and preparation of various nanostructures [24–26]. Li et al  and Yogapriya et al.  reported that the ZIFs have good hydrophobic properties (high water contact angle), and its hydrophobic properties can be further improved by compounding with other materials such as polyvinylidene fluoride or porous fluorinated graphene. The ZIF-8 is regarded as having the best hydrophobic properties among all the ZIF materials . In this study, we proposed to use MOF ZIF-8 structure to improve the humidity insensitivity of the PEI/BC nanofilms SAW formaldehyde gas sensor. We found that the contact angle of a water droplet on the sensing layer can reach ~ 135°, showing its high hydrophobicity. We also found that the creation of many metal ion (Zn2+) sites on the porous surface of the sensor improves the sensor's response to formaldehyde gas through effective physical adsorptions .