## 1 Introduction  
Micro/nanostructure-based thin film fabrication technology are used in a wide range of industries and research fields for integration systems and device applications, and are related to energy applications such as flexible electronics, soft photonics, supercapacitors, and surface modification [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]. The characteristics of the high surface-area-to-volume ratio of the hierarchical structure dramatically enhances performance to achieve their applications. For examples, these characteristics provide a large number of active sites that can be utilized for ion transfer in batteries [3], detecting gas with high sensitivity and fast response in sensors [5], and controlling cell behavior using a high-density functional group on the film surface [7].  
A fiber-based porous film fabrication using electrospinningning technology is one of the thin film fabrication methods for the mass production for filters in industries [8, 11]. This paper endeavors to increase the particulate matter (PM) filtration efficiency. In the nanofiber (NF) fabrication process, the viscosity of the polymer solution and intensity of the applied voltage is controlled for the spider-web-like fibrous structure [12]. A nanofiber net filter structure shows a high filtration efficiency of over 99.9%, with a low pressure drop (<100 Pa) [13]. Furthermore, in the post-process of the filter fabrication, triboelectric charges are applied to the NF filter surface to help capture PM below a diameter of 0.3 \(\upmu\)m [14]. In this way, PM filtration is enhanced by controlling geometry during the NF manufacturing step or by applying continuous external static forces to the filter surface.  
Including basic PM filtration performance, the multifunctional filter is designed to provide additional performance using a hybrid structure [15; 16; 17; 18; 19; 20; 21; 22]. Modulating the nanofiber surface (surface functionalization and metal oxide combination) improves the content of the active sites, which can be effective in absorbing volatile organic compounds (VOCs), sulfur dioxide, and carbon monoxide. [PERSON] et al. presented surface functionalization of gelatin nano-fabrics on a paper towel to capture PM and toxic gaseous molecules. The protein chain of gelatin NFs has a molecular functional group that captures gasses such as sulfur dioxide and carbon monoxide, as well as volatile organic compounds [18]. Additionally, the MnO2 embedded PE/PP fiber filters demonstrate air purification for PM 2.5 filtration with excellent catalytic activity for formaldehyde [19]. The nanofiber membrane filter, which is a modulated nanofiber surface functionalization or a metal oxide combined structure, shows superior filtration efficiency and provides gas capture characteristics.  
In this research, a smart filter is fabricated onto the structure of zinc oxide (ZnO) nanowire (NW) arrays on the polyvinylidene dilluoride (PVDF) NF layer. ZnO, a semiconductor with large excitation binding energy (60 meV) at room temperature, is a promising material for a wide range of applications, such as photodetectors or chemical sensors [23; 24]. In addition, the wurtzite crystalline structure of the ZnO is beneficial to grow nanowire geometry at a low-temperature under 100 \({}^{\circ}\)C. With a low-temperature hydrothermal synthesis method, it is possible to fabricate a hierarchical structure on a polymer membrane without thermal damages [2]. The ZnO NWs-PVDF NF layer performance is analyzed by the photo conductivity (in the current-voltage curve and the current-time curve) and the PM filtration characteristics (in the pressure difference, the PM filtration efficiency, and the quality factor). These characteristics demonstrate a multifunctional smart filter application capable of both PM filtration and toxic polar gas detection.