## 2 Materials and Methods  
### Nonwoven Nanofibrous Filter Production by Electrospinning  
Electrospinning is a method of production of nanofibers from a viscous polymer solution by pulling outwards the liquid with charge carriers induced at the surface of the solution meniscus by the electric field. Due to the tangential stress, a thin nanometer jet is generated. The polymer solution is fed under low pressure, with a constant flow rate, to a metal capillary nozzle, which is maintained at high voltage. The solvent evaporates from the jet ejected from the nozzle, under controlled conditions (temperature and humidity), and the remaining dry or semidry polymer fiber, with a diameter of the order of a few hundred nanometers, is collected on the surface of a grounded flat substrate or on a rotating cylinder. The deposited several layers of the fibers form a filtration mat. A scheme of setup used for the electrospinning is shown in Figure 1a, and a photograph of the stand in Figure 1b. The electrospinning device consisted of a single capillary nozzle with an inner diameter of 0.4 mm and an outer diameter of 0.5 mm, connected to a high voltage source. The capillary was energized from a high-voltage power supply SL300 30 kV (SPELLMAN, Hauppauge, NY 11788, USA), providing a DC voltage of 13.5 kV of positive polarity. The polymer solution was fed into a syringe of a volume of 2 mL, and infused with a constant flow rate of 1 mL/h to the capillary nozzle by a Fusion 200 High Precision Dual syringe pump (CHEMYX Inc., Stafford, TX 77477, USA). An aluminum frame with a stainless steel mesh, used as the substrate, was placed beneath the capillary outlet, on a heated rotating table H3.1 (DANLAB, Aulum, Denmark), which was grounded. The distance of the capillary nozzle tip to the substrate was 120 mm. The heated table was placed on a movable platform, allowing the simultaneous progressive and rotational motion of the substrate in order to obtain an even distribution of the nonwoven fabric on the surface of several substrates at a time. Polymer nanofibers were produced from a solution of polyvinylidene fluoride (PVDF, Mw = 455,000 g/mol, Sigma-Aldrich, St. Louis, MO 63103, USA) with a mass concentration of 15% (0.9 g PVDF) in a mixture of 2.48 g Dimethylacetamide (DMAC, Sigma-Aldrich, St. Louis, MO 63103, USA) and 2.84 g acetone (Chempur, Piekary Slaskie, Poland). The PVDF solution was obtained by stirring the polymer at room temperature for 8 h using a magnetic stirrer. The electrospinning process was carried out in ambient air of 45-50% relative humidity and at room temperature. The nonwoven fabric was deposited on a mesh scaffold made of stainless steel wires 0.2 mm in diameter, and with 2 mm mesh, embedded in a 15 mm \(\times\) 15 mm aluminum frame. The diameter of the fibers obtained in this process, deposited on the mesh, and estimated from SEM micrographs was between 100 and 700 nm. The thickness of the nanofibrous filter was controlled by the time of electrospinning. Figure 1: Setup for the production of polymer nanofibrous filter by electrospinning: (**a**) schematic; photograph of the stand (**b**). Figure 2 shows a photograph of the PVDF jet pulled out from a capillary nozzle, taken by a continuous illumination (Figure 2a), and illuminated by a stroboscopic lamp of 3 us flash (Figure 2b). A single polymer fiber can be seen in this photograph. The fibers bend (whip) randomly into many loops, forming a cone plume under continuous illumination. The velocity of free nanofibers in the interelectrode space can vary from 2 m/s to 200 m/s [90], depending on the physical properties of the solution and fabrication conditions (electric field). Figure 3 shows a photograph of a clean PVDF nanofibrous filter specimen deposited onto a metal mesh scaffold by the electrospinning process by a time of 10 min. After this time, the estimated weight of the nonwoven nanofibrous filter was about 2.5 mg (11 g/m\({}^{2}\)). Figure 3: Metal mesh scaffold with PVDF nanofibrous filter. Electrospinning time of 10 min. Capillary nozzle voltage +13.5 kV.