Synthesis and filtration properties of polyimide nanofiber membrane/carbon woven fabric sandwiched hot gas filters for removal of PM 2.5 particles

Electrospun PA6 multi-stage structured nanofiber membrane with high filtration performance for oily particles

Oily particles pollution poses a tremendous threat to people's health, so it is urgent to develop air filtration materials with the ability of removing fine oily particles effectively. In this study, a nylon 6 multi-stage structured nanofiber membrane (PA6 MSNM) for effective air filtration of fine oily particles was designed and fabricated by adding a certain amount of tetrabutylammonium hexafluorophosphate (TBAHP) via one-step electrospinning. The PA6 MSNMs were composed of coarse trunk fibres and fine branching fibres. Benefiting from the properties of small pore size and high porosity, the resulting PA6 MSNMs exhibited high average filtration efficiency of 99.80% for oily aerosol particles of 0.20-4.59 μm, a low pressure drop of 251 Pa, and the high quality factor of 0.0248 Pa-1. More importantly, its filtration efficiencies for oily aerosol particles of 0.25 and 0.30 μm were up to 99.99% and 100.00%, respectively. It is expected that the multi-stage electrospun nanofiber membranes would have wide application prospects in air filtration, particularly for filtering oily particles.

Scalable, solvent-free transparent film-based air filter with high particulate matter 2.5 filtration efficiency

Over the course of the COVID-19 pandemic, people have realized the importance of wearing a mask. However, conventional nanofiber-based face masks impede communication between people because of their opacity. Moreover, it remains challenging to achieve both high filtration performance and transparency through fibrous mask filters without using harmful solvents. Herein, scalable transparent film-based filters with high transparency and collection efficiency are fabricated in a facile manner by means of corona discharging and punch stamping. Both methods improve the surface potential of the film while the punch stamping procedure generates micropores in the film, which enhances the electrostatic force between the film and particulate matter (PM), thereby improving the collection efficiency of the film. Moreover, the suggested fabrication method involves no nanofibers and harmful solvents, which mitigates the generation of microplastics and potential risks for the human body. The film-based filter provides a high PM2.5 collection efficiency of 99.9 % while maintaining a transparency of 52 % at the wavelength of 550 nm. This enables people to distinguish the facial expressions of a person wearing a mask composed of the proposed film-based filter. Moreover, the results of durability experiments indicate that the developed film-based filter is anti-fouling, liquid-resistant, microplastic-free and foldability.

Recent progress on electrospun nanofibrous polymer membranes for water and air purification: A review

Developing new polymer membranes with excellent thermal, mechanical, and chemical stability has shown great potential for various environmental remediation applications such as wastewater treatment and air filtration. Polymer membranes have been widely investigated over the past years and utilized to overcome severe ecological issues. Membrane-based technologies play a critical role in water purification and air filtration with the ability to act efficiently and sustainably. Electrospun nanofiber membranes have displayed excellent performance in removing various contaminants from water, such as bacteria, dyes, heavy metals, and oil. These nanofibrous membranes have shown good potential to filter the air from tiny particles, volatile organic compounds, and toxic gases. The performance of polymer membranes can be enhanced by fine-tuning polymer structure, varying surface properties, and strengthening overall membrane porosity. In this review, we discuss the involvement of electrospun nanofibrous membranes in different environmental remediation applications. It further reviews the recent progress of polymer membrane development by utilizing nanoparticles and naturally occurring polymers.

Natural Polymers and Their Nanocomposites Used for Environmental Applications

The aim of this review is to bring together the main natural polymer applications for environmental remediation, as a class of nexus materials with advanced properties that offer the opportunity of integration in single or simultaneous decontamination processes. By identifying the main natural polymers derived from agro-industrial sources or monomers converted by biotechnology into sustainable polymers, the paper offers the main performances identified in the literature for: (i) the treatment of water contaminated with heavy metals and emerging pollutants such as dyes and organics, (ii) the decontamination and remediation of soils, and (iii) the reduction in the number of suspended solids of a particulate matter (PM) type in the atmosphere. Because nanotechnology offers new horizons in materials science, nanocomposite tunable polymers are also studied and presented as promising materials in the context of developing sustainable and integrated products in society to ensure quality of life. As a class of future smart materials, the natural polymers and their nanocomposites are obtained from renewable resources, which are inexpensive materials with high surface area, porosity, and high adsorption properties due to their various functional groups. The information gathered in this review paper is based on the publications in the field from the last two decades. The future perspectives of these fascinating materials should take into account the scale-up, the toxicity of nanoparticles, and the competition with food production, as well as the environmental regulations.

ZIF-8/PI Nanofibrous Membranes With High-Temperature Resistance for Highly Efficient PM0.3 Air Filtration and Oil-Water Separation

Air and water pollution poses a serious threat to public health and the ecological environment worldwide. Particulate matter (PM) is the major air pollutant, and its primary sources are processes that require high temperatures, such as fossil fuel combustion and vehicle exhaust. PM_0.3 can penetrate and seriously harm the bronchi of the lungs, but it is difficult to remove PM_0.3 due to its small size. Therefore, PM_0.3 air filters that are highly efficient and resistant to high temperatures must be developed. Polyimide (PI) is an excellent polymer with a high temperature resistance and a good mechanical property. Air filters made from PI nanofibers have a high PM removal efficiency and a low air flow resistance. Herein, zeolitic imidazolate framework-8 (ZIF-8) was used to modify PI nanofibers to fabricate air filters with a high specific surface area and filtration efficiency. Compared with traditional PI membranes, the ZIF-8/PI multifunction nanofiber membranes achieved super-high filtration efficiency for ultrafine particles (PM_0.3, 100%), and the pressure drop was only 63 Pa. The filtration mechanism of performance improvement caused by the introduction of ZIF-8/PI nanofiber membrane is explored. Moreover, the ZIF-8/PI nanofiber membranes exhibited excellent thermal stability (300 C) and efficient water–oil separation ability (99.85%).

High-Performance Washable PM2.5 Filter Fabricated with Laser-Induced Graphene

This study demonstrates a novel application of laser-induced graphene (LIG) as a reusable conductive particulate matter (PM) filter. Four types of LIG-based filters were fabricated based on the laser-induced pyrolysis of thin polyimide (PI) sheets, each pyrolyzed on either a single side or both sides, with or without densification. The LIG filters exhibited a high removal efficiency while maintaining minimal pressure drop compared to a commercial fiberglass filter. The densified LIG (dLIG) filters displayed a higher PM_2.5 removal efficiency (>99.86%) than regular LIG filters. The dLIG filters also exhibited excellent durability when tested for washability by ultrasonication in tap water. After being cleaned and left to dry, the structures of the dLIG filters were well-maintained; their filtration efficiencies were also well-maintained (less than a 7% change in PM_2.5 removal efficiency), and their resistances only marginally increased (less than a 7% increase after five uses). These results demonstrate the robustness and reusability of the dLIG filters and the accessibility of their cleaning (not requiring aggressive cleaning agents). These promising features will enable the application of LIG in economical, scalable, and high-performance air cleaning.

Antibacterial Activity and Protection Efficiency of Polyvinyl Butyral Nanofibrous Membrane Containing Thymol Prepared through Vertical Electrospinning

Human safety, health management, and disease transmission prevention have become crucial tasks in the present COVID-19 pandemic situation. Masks are widely available and create a safer and disease transmission-free environment. This study presents a facile method of fabricating masks through electrospinning nontoxic polyvinyl butyral (PVB) polymeric matrix with the antibacterial component Thymol, a natural phenol monoterpene. Based on the results of Japanese Industrial Standards and American Association of Textile Chemists and Colorists methods, the maximum antibacterial value of the mask against Gram-positive and Gram-negative bacteria was 5.6 and 6.4, respectively. Moreover, vertical electrospinning was performed to prepare Thymol/PVB nanofiber masks, and the effects of parameters on the submicron particulate filtration efficiency (PFE), differential pressure, and bacterial filtration efficiency (BFE) were determined. Thorough optimization of the small-diameter nanofiber-based antibacterial mask led to denser accumulation and improved PFE and pressure difference; the mask was thus noted to meet the present pandemic requirements. The as-developed nanofibrous masks have the antibacterial activity suggested by the National Standard of the Republic of China (CNS 14774) for general medical masks. Their BFE reaches 99.4%, with a pressure difference of <5 mmH2O/cm2. The mask can safeguard human health and promote a healthy environment.

Laminated polyacrylonitrile nanofiber membrane codoped with boehmite nanoparticles for efficient electrostatic capture of particulate matters

Particulate matters (PMs) air pollution is identified as the major threat to public health and climate. High-performance air filter technology based on various electrospun nanofibers is considered as an effective strategy to eliminate the effects of PMs air pollution. However, to date, nearly all the existing micro-/nanofibers are hard to meet both requirements of high PMs removal efficiency and long service life. In this work, we reported the production of laminated polyacrylonitrile(PAN)-boehmite nanoparticles (BNPs) nanofiber structured membrane by the electrospinning process. The dimension of PAN-BNPs nanofiber can be tunable from (0.09 ± 0.03)μm to (0.81 ± 0.11)μm by controlling the PAN and BNPs concentrations in precursors. The optimized PAN-BNPs nanofiber air filter with a basis weight of 1 g m^-2demonstrates the attractive attributes of high PM_2.5removal efficiency up to 99.962% and low pressure drop of 58 Pa. Most importantly, after introducing the BNPs as electret, the removal efficiency is very stable under the air flow rate of 6 l min^-1. This PAN-BNPs nanofiber with a long electrostatic duration time offers an approach for fabricating future high-performance air filters.

Flexible Nanoporous Template for the Design and Development of Reusable Anti-COVID-19 Hydrophobic Face Masks

Since the outbreak of the severe respiratory disease caused by the novel coronavirus (COVID-19), the use of face masks has become ubiquitous worldwide to control the rapid spread of this pandemic. As a result, the world is currently facing a face mask shortage, and some countries have placed limits on the number of masks that can be bought by each person. Although the surgical grade N95 mask provides the highest level of protection currently available, its filtration efficiency for sub-300 nm particles is around 85% due to its wider pore size (∼300 nm). Because the COVID-19 virus shows a diameter of around 65-125 nm, there is a need for developing more efficient masks. To overcome these issues, we demonstrate the development of a flexible, nanoporous membrane to achieve a reusable N95 mask with a replaceable membrane and enhanced filtration efficiency. We first developed a flexible nanoporous Si-based template on a silicon-on-insulator wafer using KOH etching and then used the template as a hard mask during a reactive ion etching process to transfer the patterns onto a flexible and lightweight (<0.12 g) polymeric membrane. Pores with sizes down to 5 nm were achieved with a narrow distribution. Theoretical calculations show that airflow rates above 85 L/min are possible through the mask, which confirms its breathability over a wide range of pore sizes, densities, membrane thicknesses, and pressure drops. Finally, the membrane is intrinsically hydrophobic, which contributes to antifouling and self-cleaning as a result of droplets rolling and sliding on the inclined mask area.

Reduced graphene-oxide filter system for removing filterable and condensable particulate matter from source

Air pollution is one of the most serious problems facing mankind because of its impact on ecosystems and human beings. Although particulate matter (PM) consists of both filterable PM (FPM) and condensable PM (CPM), most research has focused on eliminating only FPM. In this work, we introduce a filter system that removes both FPM and CPM from pollution source with high efficiency. The system consists of two reduced graphene oxide (rGO) filters and a condenser between them that can remove the usual FPM and at the same time CPM-induced FPM that typically leaves the pollution source unabated. The filters, quite effective in removing the PM with their three-dimensional structure, retain the removal capability even at high temperature and in acidic condition that prevail at the pollution source. The proposed rGO system could provide a complete solution for removal of both FPM and CPM from the pollution source.

Recent Advances in Electrospun Sustainable Composites for Biomedical, Environmental, Energy, and Packaging Applications

Electrospinning has gained constant enthusiasm and wide interest as a novel sustainable material processing technique due to its ease of operation and wide adaptability for fabricating eco-friendly fibers on a nanoscale. In addition, the device working parameters, spinning solution properties, and the environmental factors can have a significant effect on the fibers' morphology during electrospinning. This review summarizes the newly developed principles and influence factors for electrospinning technology in the past five years, including these factors' interactions with the electrospinning mechanism as well as its most recent applications of electrospun natural or sustainable composite materials in biology, environmental protection, energy, and food packaging materials.

Hydro-Assisted Self-Regenerating Brominated N-Alkylated Thiophene Diketopyrrolopyrrole Dye Nanofibers-A Sustainable Synthesis Route for Renewable Air Filter Materials

With rising global concerns over the alarming levels of particulate pollution, a sustainable air quality management is the need of the hour. Air filtration research has gained momentum in recent years. However, the research perspective is still blinkered toward formulating new fiber systems for the energy-intensive electrospinning process to fabricate high quality factor air filters. A holistic approach on sustainable air filtration models is still lacking. The air filter model presented in this work uses a simple process involving water-induced self-organization and self-regeneration of nanofibers, and an easy recycling route after the filter life that not only facilitates reuse of the microfibrous scaffold holding the nanofibers but also allows renewal of nanofibers. Three generations of air filters are fabricated and tested, all having high particulate matter (PM)-adsorbing tendency, high filtration efficiency (>95%), and high Young's modulus (≈5 GPa). The renewable air filters offer a sustainable alternative to the present cost-intensive electrospun air filters.

Highly efficient transparent air filter prepared by collecting-electrode-free bipolar electrospinning apparatus

Electrospinning technology has been used for a long time. A jet from a needle was formed by applying high voltage, and then the nanofibers are deposited onto a collecting electrode (usually metal) and the excess charge is conducted away to complete the electrospinning. Alternatively, it is also possible to prevent charge accumulation from hindering the progress of electrospinning by means of charge neutralization. A bipolar electrospinning technique (B-EEM) was developed to induce jets with different charges through a set of high-voltage power supplies of opposite polarity, and the two jets neutralize each other on the insulating mesh, thus completing the electrospinning process. There is no need for a collecting electrode to complete the electrospinning process. We have found that the filters produced by the new technology have better filtration efficiency while maintaining the same transparency in relative to the original technology, and this optimization is due to the distribution modification of the nanofibers on the mesh.

Electrospun Polymer Composite Membrane with Superior Thermal Stability and Excellent Chemical Resistance for High-Efficiency PM2.5 Capture

To address the challenge of high-temperature air filtration, a novel electreted polysulfonamide/polyacrylonitrile-boehmite (PSA/PAN-B) composite nanofiber based filter was developed via electrospinning for effective high-temperature dust removal. In this study, the spinnability of PSA was greatly improved by adding a small amount of PAN as an auxiliary polymer, and the introduction of a boehmite electret further significantly reinforced the properties of PSA fibers. As a result, the PSA/PAN-B membrane exhibited a high filtration efficiency (up to 99.52 ± 0.32%), low pressure drop (45.16 ± 1.39 Pa), excellent flexibility, good mechanical properties, high thermal stability (up to approximately 300 °C), and superior chemical resistance. Through data analysis and 3D simulation, the important benefits of the boehmite electret in the optimization of the PSA fibrous membrane performance were determined: it increases the charge storage capacity, constructs a rough surface morphology, improves the specific surface area, and enhances the mechanical properties. More importantly, the PSA/PAN-B film possessed a robust PM2.5 purification capacity, and the particulate matter removal efficiency was kept unchanged after high-temperature, acid, or alkali treatment-a performance derived from the intrinsic molecular structure of PSA. The PSA/PAN-B composite fibrous membrane, with excellent comprehensive properties, is a promising candidate for air filters, especially in harsh environments, further broadening the applications of PSA and providing new insight into the design of high-performance filters with high-temperature and corrosion resistance.

Membrane technology in air pollution control: prospect and challenge

Rapid growth in environmental awareness raised the strict regulations on air pollution control and led to great developments in air filtration or cleaning technologies. The membrane is one of the promising technologies for air filtration due to its high efficiency, low cost, and easy to scale-up. Two types of air filtration membranes have been used to provide high efficiencies in contaminant removal, namely electro-spun fibrous and microporous polymeric membranes. The fibrous membranes composed of randomly oriented fibers, either micro-or nano-fiber, and have been widely used to remove fine particles, such as aerosols. Meanwhile, the microporous membranes have a pore size that can remove both particulate and molecular, such as toxic gases. This paper reviews the role of membrane technologies in air pollution control. The performances of the membranes are presented comprehensively. Besides, the developments of the membranes to improve the filtration efficiency and reduce air-flow resistance are also discussed.

Highly Efficient, Flexible, and Recyclable Air Filters Using Polyimide Films with Patterned Thru-Holes Fabricated by Ion Milling

We present the first demonstration of the environmental application of a polyimide film (Kapton) with patterned thru-holes as a novel, efficient, flexible, recyclable, and active particulate matter (PM) air filter. The Kapton air filter captures PM using micro-sized thru-holes and the strong electrostatic force because of its high work function. It is highly efficient, transparent, flexible, and heat-resistant. Furthermore, it can be recycled simply by washing it with tap water. The proposed PM filter is a promising candidate for use as a highly efficient and economical recyclable air filter for home appliances, such as air-purifiers, air-conditioners, and humidifiers, as well as industrial filtration systems.

Electrospun polyimide nanofiber-coated polyimide nonwoven fabric for hot gas filtration

In the present work, heat-endurable polyimide nanofibrous membrane-coated commercial polyimide nonwoven fabrics were prepared by electrospinning synthesized polyamic acid solution on polyimide mats followed by a thermal imidization process. Polyimide synthesized was confirmed by Fourier-transform infrared spectroscopy and thermogravimetric analysis tests. A higher viscosity of precursor solution resulted in a nanofibrous network coated on the nonwoven fabrics. The coated filter showed lower dimension of pores and greater portion of small pores. The proportion of pores less than 10 µm reached as high as 71.2%. PM2.5 removal efficiency was increased from 81.4 to 97.2%. However, the pressure drop of air flow over the filters did not show significant changes. It suggested a facile method to improve the filtration performance of commercial polyimide nonwoven fabric filters.

Bio-based and photocrosslinked electrospun antibacterial nanofibrous membranes for air filtration

Air pollution is becoming more and more severe especially in developing countries. It is urgent to seek an effective protection for the unacceptable levels of the air pollution. Up to date, the air filtration is a widely used method to protect us from the air pollution. However, most of the air filtration membranes in fabrication process is required to use harmful organic solvents. Here, we report a human friendly, multifunctional and bio-based chitosan/poly (vinyl alcohol) air filtration membrane via green electrospinning and UV-cured. This green approach avoid use hazardous organic solvents which caused the residual solvents could bring the human subprime damage. Specifically, the superhydrophobic silica nanoparticles is introduced on the nanofibers to formation of the rough surface to increase filtration efficiency. In addition, the Ag nanoparticles (NPs) are fabricated on the surface through UV reduction of AgNO₃ to achieve the aim of antibacterial treatment. The CS/PVA@SiO₂/Ag NPs membranes not only possess excellent filtration performance but efficiently antibacterial activities. As this green, multifunctional and bio-based CS/PVA@SiO₂/Ag NPs air filtration membranes have several superior features like high air filtration performance, biological compatibility and antibacterial, it has great potential application in eco-friendly air filtration materials, especially in personal air filtration materials.

Numerical Comparison of Prediction Models for Aerosol Filtration Efficiency Applied on a Hollow-Fiber Membrane Pore Structure

Hollow-fiber membranes (HFMs) have been widely applied to many liquid treatment applications such as wastewater treatment, membrane contactors/bioreactors and membrane distillation. Despite the fact that HFMs are widely used for gas separation from gas mixtures, their use for mechanical filtration of aerosols is very scarce. In this work, we compared mathematical models developed for the prediction of air filtration efficiency by applying them on the structural parameters of polypropylene HFMs. These membranes are characteristic of pore diameters of about 90 nm and have high solidity, thus providing high potential for nanoparticle removal from air. A single fiber/collector and capillary pore approach was chosen to compare between models developed for fibrous filters and capillary-pore membranes (Nuclepore filters) based on three main mechanisms occurring in aerosol filtration (inertial impaction, interception and diffusion). The collection efficiency due to individual mechanisms differs significantly. The differences are caused by the parameters for which the individual models were developed, i.e., given values of governing dimensionless numbers (Reynolds, Stokes and Peclet number) and also given values of filter porosity and filter fiber diameter. Some models can be used to predict the efficiency of HFMs based on assumptions depending on the conditions and exact membrane parameters.

Preparation and Evaluation of a Polyimide-Coated Ultrafine Gilt Molybdenum Wire and Its Knitted Mesh Used for Electromagnetic Reflectors

In this work, polyimide (PI) was coated onto an ultrafine gilt molybdenum wire in order to protect the gilt surface and prepare an electrically stable wire mesh material which can be widely used in space. The surface of the PI-coated gilt molybdenum wires was characterized using FTIR, SEM, and EDS. Factors such as temperature stability of the PI coating, mechanical properties of the PI-coated gilt molybdenum wires, contact resistance stability, and electromagnetic microwave reflectivity of the their knitted meshes were also investigated. The results indicate that the PI coating conformed uniformly to the surface of the gilt molybdenum wires. The prepared PI coating exhibited excellent temperature stability in the -196 to 300 °C range and could efficiently protect the gilt surface and improve the stability of contact resistance, while the reflection of its wire meshes showed only a slight decrease of 1.4% with the PI coating thickness of 3 μm for electromagnetic microwaves in the S band.

Green electrospun and crosslinked poly(vinyl alcohol)/poly(acrylic acid) composite membranes for antibacterial effective air filtration

Air pollution has become a major environmental concern given the ever increasing levels of particulate matter (PM) and the increased in treatment-resistant bacterial and viral strains. Major efforts are therefore required into the development of air filtration and purification technology as well as novel, alternative antiviral and antibacterial treatment modalities. Here, we report an environmentally friendly method for the generation of multifunctional poly(vinyl alcohol)/poly(acrylic acid) (PVA-PAA) composite membranes via green electrospinning and thermal crosslinking. Superhydrophobic silica nanoparticles were then incorporated into the fibers resulting in a rough surface, after which AgNO₃ was introduced, resulting in the formation of Ag nanoparticles through UV reduction. The PVA-PAA-SiO₂-Ag NPs membranes were found to possess high air filtration performance (with >98% filtration efficiency for PM_2.5) as well as potent antibacterial and antiviral activities. The green synthesis approach avoids the use of hazardous organic solvents, thereby bypassing any potential toxicity concerns caused by organic solvent residues. These newly designed PVA-PAA-SiO₂ NPs-Ag NPs nanofibrous membranes with many superior features (e.g. high filtration efficiency, high tensile strength, biological compatibility, and antibacterial properties) can be applied in eco-friendly air filtration materials, in particular for personal air filtration devices.

Fabrication of Continuous Microfibers Containing Magnetic Nanoparticles by a Facile Magneto-Mechanical Drawing

A facile method termed magneto-mechanical drawing is used to produce polymer composite microfibers. Compared with electrospinning and other fiber spinning methods, magneto-mechanical drawing uses magnetic force generated by a permanent magnet to draw droplets of polymer/magnetic nanoparticle suspensions, leading to fabrication of composite microfibers. In addition, because of the rotating collector, it is easy to control the fiber assembly such as fibrous array in parallel or crossed fibrous structure. The general applicability of this method has also been proved by spinning different polymers and magnetic nanoparticles. The resultant fibers exhibit good superparamagnetic behavior at room temperature and ultrahigh stretchability (~443.8 %). The results indicate that magneto-mechanical drawing is a promising technique to fabricate magnetic and stretchable microfibers and devices.

Mechanically enhanced PES electrospun nanofiber membranes (ENMs) for microfiltration: The effects of ENM properties on membrane performance

The application of electrospun nanofiber membranes (ENMs) as microfilters for the process of water purification requires the substrate to possess suitable strength, permeability, and a smooth surface. Therefore, the fiber homogeneity, inter-fiber adhesion, and surface roughness of the ENMs must be carefully controlled. Concurrently, an understanding of the ENMs' rejection mechanism for contaminants is necessary for the effective application of ENMs. In this study, we demonstrate the fabrication of polyethersulfone (PES) ENMs, which are useful for water purification as water treatment membranes. An optimum fabrication condition that can significantly improve the mechanical property and surface roughness of the PES membrane is also illustrated. This technique induces the solvent remaining on the fiber's surface after the electrospinning process, and the mechanical properties and surface roughness of the membrane are improved by the solvent-induced fusion of the fiber. The fabricated PES ENMs also show higher clean water productivity. Additionally, we show that a particulate contaminant in water is mainly rejected on the ENM surface by using a water filtration test. Based on our conclusions, we suggest the appropriate ENM regeneration method and confirm that the fabricated ENMs show excellent regeneration ability.

Fabrication of a polyvinylidene fluoride tree-like nanofiber web for ultra high performance air filtration

A novel polyvinylidene fluoride tree-like nanofiber web for high performance air filtration was successfully fabricated via one-step electrospinning.