## Materials and Methods  
PLA 6252D (\(\varrho=1.24\) g/cm\({}^{3}\), the glass transition temperature is at about 58 \({}^{\circ}\)C, NatureWorks, Minnetonka, MN, USA), shellac (\(\varrho=1.175\) g/cm\({}^{3}\), Bosheng, Beijing, China), dichloromethane (DCM), dimethyl formamide (DMF) and anhydrous ethanol (Tianjin Da Mao Chemical Reagent, Tianjin, China) were used as the two fiber materials and three solvents, respectively. All reagents were utilized without further purification. A plastic syringe (Foshan Lepton Precision Measurement and Control Technology, Foshan, Guangdong, China) with a stainless steel needle (0.50 mm) was employed as the electrospinning equipment. Methanol and anhydrous ethanol (Tianjin Da Mao Chemical Reagent, Tianjin, China) were used for the detecting and sensing of alcoholic vapors with a low molecular weight.  
An amount of PLA (10% wt%) was dissolved in a mixture of DCM and DMF (9:1 weight ratio) at room temperature, and stirred for 2 h to form a precursor solution. The precursor was then placed in a plastic syringe of 20 mL in volume and a stainless steel needle of 0.50 mm in diameter was used for electrospinning. The high voltage applied between the tip and the collector was set to 18 kV and the distance between them was 15 cm. The flow rate was fixed as 1.0 mL/h. During the electrospinning process, the temperature was set as \(25\pm 2\) (\({}^{\circ}\)C) and the relative humidity as \(65\pm 5\)(%). After a period of time, the PLA electrospinning membrane on the collector (pure aluminum foil) was removed and dried at room temperature for 8 h. Then, an amount of shellac (45% wt%) was dissolved in anhydrous ethanol at room temperature and stirred for 2 h to form a precursor solution. The solution was then placed in the plastic syringe and the stainless steel needle for electrospinning. Then the shellac membrane was collected on the PLA membrane as prepared. The temperature and humidity were the same as those for the preparation of the PLA membrane. Then, the membrane on the collector was removed and dried at room temperature. After that, a layer of PLA electrospinning membrane was electrospun on the dried membrane, and the parameters of electrospinning were the same as those for the preparation of the PLA membrane. Finally, an electrospinning membrane with a PLA/shellac/PLA sandwich-structure was obtained (see Figure 1).  
A 100 mL alcohol such as methanol or absolute ethanol was added to a specific vessel to fill the vessel with mixed vapors containing alcohol. The specific apparatus is shown in Figure 2. The content of alcohol was constant to ensure that the experiment is carried out in an atmosphere of alcoholic vapor with a low molecular weight and constant concentration. The PLA/shellac/PLA sandwich-structural membrane was then placed in the specific apparatus at room temperature, and the resistance of the  
Figure 1: Schematic illustration of a poly (lactic acid) (PLA)/shellac/PLA sandwich-structural membrane.  
photoresistor was measured after a period of time. The thickness was measured to be \(60.2\pm 0.9\)\(\upmu\)m and \(80.4\pm 1.1\)\(\upmu\)m for PLA and shellac, respectively, using a micrometer.  
In order to accurately measure the resistance value of the photoresistor, the PLA/shellac/PLA sandwich-structural membranes were placed in a single pure vapor at room temperature, and started to test the resistance of the photoresistor.  
Scanning electron microscope (SEM) images were taken (SEM, SU8220, Hitachi, Tokyo, Japan). X-ray diffraction (XRD) was carried out on a D/Max-Ultima IV diffractometer (Ultima IV, Rigaku, Tokyo, Japan) over the 20 range of 10\({}^{\circ}\)-80\({}^{\circ}\). Functional groups of the fibers were measured in absorbance mode by an attenuated total reflection-Fourier transform infrared spectrometer (ATR-FTIR, Nicolet IS50, Thermofisher, Massachusetts, USA). The wavenumber was ranged from 400 to 4000 cm\({}^{-1}\). The resistance value of the photoresistor was measured using a multimeter (PM8236, PEAKMETER, Shenzhen, China).