Electrospun Nanofibers: Materials,Synthesis Parameters,and Their Role in Sensing Applications

Since the last decade, electrospinning is garnering more attention in the scientific research community, industries, applications like sensing (glucose, H₂O₂, dopamine, ascorbic acid, uric acid, neurotransmitter, etc.), biomedical applications (wound dressing, wound healing, skin, nerve, bone tissue engineering, and drug delivery systems), water treatment, energy harvesting, and storage applications. This review paper provides a brief overview of the electrospinning method, history of the electrospinning, factors affecting the electrospun nanofibers, and their morphology with different materials and composites (metals, metal oxides, 2D material, polymers and copolymers, carbon-based materials, etc.) used in the electrospinning technique with optical spinning parameters. Moreover, this paper deliberates the application of electrospun nanofibers and fibrous mats for sensing (electrochemical, optical, fluorescence, colorimetric, mechanical, photoelectric, mass sensitive change, resistive, ultrasensitive, etc.) in most illustrative representations. In the end, the challenges, opportunities of the electrospun nanofibers, and new direction for future progress are also discussed.     

Electrospun Nanofibers Membranes for Effective Air Filtration

In modern society, traffic and transportation and the manufacturing industry and construction industries continuously release large amounts of dust and particles into the atmosphere, which can cause heavy air pollution, leading to health hazards. The haze disaster, a serious problem in developing countries such as China and India, has become one of the main issues of global environmental pollution in recent decades. Many air filtration technologies have been developed. Air filtration using electrospun fibers that intercept fine particles/volatile organic gases/bacterium is a relatively new, but highly promising, technique. Due to their interconnected nanoscale pore structures, highly specific surface areas, fine diameters, and porous structure as well as their ability to incorporate active chemistry on a nanoscale surface, electrospun fibers are becoming a promising versatile platform for air filtration. In this review, following a short introduction concerning the need for air filtration and filtration theory and mechanism, electrospun nanofibers membranes for air filtration have been highlighted, including the preparation (electrospinning process) and the parameters relevant to filtration efficacy. Additionally, various types (function) of the electrospun air filtration membranes have been classified in detail. Furthermore, their potential in the filtration of fine particles and chemical pollutants has been discussed. Finally, the challenges of their practical application and the future prospects have been summarized. Given that some advanced electrospun air filtration nanofibrous membranes exist for treating different contaminants from various types of polluted atmosphere, it is believed that they should make a significant contribution in protection against air pollution.

     

Biodegradable Electrospun Poly(lactic acid) Nanofibers for Effective PM 2.5 Removal

In addition to the rapid urbanization and industrialization around the world, air pollution due to particulate matter is a substantial threat to human health. A considerable research effort has been devoted to the development of electrospun polymer nanofibers for air filter applications. Among these new technologies, electrostatic charge‐assisted air filtration is a promising technology for removing small particulate matter (PM). In this investigation, biodegradable electrospun poly(l ‐lactic acid) (PLLA) polymer nanofibers are employed for air filter applications. Electrostatic charges generated from the PLLA nanofiber can significantly enhance air filter applications. Compared with a 3M commercial respirator filter, electrospun PLLA fibrous filters exhibit a high efficiency of 99.3%. Even after 6 h of filtration time, the PLLA filtration membrane still exhibits a 15% improvement in quality factor for PM 2.5 particles than the 3M respirator. This is mainly attributed to the electrostatic force generated from the electrospun PLLA nanofibers, which significantly benefit submicron particle absorption. Due to their biodegradability, ease of fabrication, and relatively high efficiency, electrospun PLLA nanofibers show great promise in applications such as air cleaning systems and personal air purifier applications.     

静电纺丝聚合物纳米纤维研究进展

将静电纺丝技术应用到高分子材料研究中,可以制备聚合物纳米纤维,在众多领域有广阔的应用前景。从静电纺丝聚合物纳米纤维原理、有序性、应用的角度,简述其研究进展,并指出一些仍待解决的问题,同时对电纺聚合物纳米纤维未来发展进行了展望。     

Influence of Heat Treatment of Electrospun Carbon Nanofibers on Biological Response

The main aim of this study is to investigate the effect of fragmentation of electrospun carbon nanofibers (eCNFs) obtained at different temperatures, i.e., at 750 °C, 1000 °C, 1500 °C, 1750 °C and 2000 °C on the cellular response in vitro. In order to assess the influence of nanofibers on biological response, it was necessary to conduct physicochemical, microstructural and structural studies such as SEM, XPS, Raman spectroscopy, HRTEM and surface wettability of the obtained materials. During the in vitro study, all samples made contact with the human chondrocyte CHON-001 cell lines. The key study was to assess the genotoxicity of eCNFs using the comet test after 1 h or 24 h. Special attention was paid to the degree of crystallinity of the nanofibers, the dimensions of the degradation products and the presence of functional groups on their surface. A detailed analysis showed that the key determinant of the genotoxic effect is the surface chemistry. The presence of nitrogen-containing groups as a product of the decomposition of nitrile groups has an influence on the biological response, leading to mutations in the DNA. This effect was observed only for samples carbonized at lower temperatures, i.e., 750 °C and 1000 °C. These results are important with respect to selecting the temperature of thermal treatment of eCNFs dedicated for medical and environmental functions due to the minimization of the genotoxic effect of these materials.     

电纺法制备纳米纤维及其应用研究

静电纺丝(电纺)技术是一种制备直径为数10 nm到数μm纳米纤维的有效方法,介绍了电纺的工作机理,对电纺条件影响纤维形态和纳米纤维应用进行了综述。最后对纳米纤维应用发展方向进行了展望。     

Fabrication and DOE Optimization of Electrospun Chitosan/Gelatin/PVA Nanofibers for Skin Tissue Engineering

Chitosan/gelatin-based nanofibers display excellent biological performance in tissue engineering because of their biocompatible composition and nanofibrous structure with a high surface-to-volume ratio mimicking the native extracellular matrix. In this study, to save time and cost of experiments, a response surface methodology based on Box–Behnken design (BBD) is developed to predict the mean diameter of (chitosan:gelatin)/poly(vinyl alcohol) (PVA) nanofibers in three volume ratios of chitosan:gelatin by considering PVA percentage, applied voltage, and flow rate as input variables. The morphology and chemical composition of nanofibers are investigated through scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR), respectively. The optimum conditions to yield the minimum diameter of nanofibers with chitosan:gelatin ratios of 25:75, 50:50, and 75:25 are found and result in 165, 121, and 92 nm, respectively, which show good accordance with BBD estimated results. The tensile testing indicates that nanofibers containing higher ratio of chitosan:gelatin result in higher tensile stress and lower toughness and tensile strain. The water contact angle analysis (WCA) shows the appropriate hydrophilicity of crosslinked nanofibers. The MTT assay shows excellent cell viability and cell attachment of nanofibers for mouse fibroblast (L929) cells. The results indicate that optimum nanofibers are potent candidates for wound healing applications.     

Electrospun Hybrid Soy Protein/PVA Fibers

Rheological behavior and spinnability of biodegradable materials based on SPI and PVA were studied for the production of electrospun fibers. pH level, processing temperature, and heating time were adjusted to investigate the effects of denaturing of soy protein on the rheology of SPI/PVA solutions. The results show that zero shear viscosity and degree of shear thinning of the SPI solution can be controlled by adjusting pH level and thermal treatment. The continuous production of uniform SPI/PVA fibers was achieved by electrospinning. The presence and amount of soy protein in the electrospun fibers was determined by EMPA and elemental analysis, confirming that the SPI was well incorporated into the PVA and remained in the electrospun fibers.

     

Tunable Mechanical and Electrical Properties of Coaxial Electrospun Composite Nanofibers of P(VDF-TrFE) and P(VDF-TrFE-CTFE)

The coaxial core/shell composite electrospun nanofibers consisting of relaxor ferroelectric P(VDF-TrFE-CTFE) and ferroelectric P(VDF-TrFE) polymers are successfully tailored towards superior structural, mechanical, and electrical properties over the individual polymers. The core/shell-TrFE/CTFE membrane discloses a more prominent mechanical anisotropy between the revolving direction (RD) and cross direction (CD) associated with a higher tensile modulus of 26.9 MPa and good strength-ductility balance, beneficial from a better degree of nanofiber alignment, the increased density, and C-F bonding. The interfacial coupling between the terpolymer P(VDF-TrFE-CTFE) and copolymer P(VDF-TrFE) is responsible for comparable full-frequency dielectric responses between the core/shell-TrFE/CTFE and pristine terpolymer. Moreover, an impressive piezoelectric coefficient up to 50.5 pm/V is achieved in the core/shell-TrFE/CTFE composite structure. Our findings corroborate the promising approach of coaxial electrospinning in efficiently tuning mechanical and electrical performances of the electrospun core/shell composite nanofiber membranes-based electroactive polymers (EAPs) actuators as artificial muscle implants.     

Dual Electric Field Induced Alignment of Electrospun Nanofibers

A dual‐field method is described to produce aligned fibers by electrospinning. By applying a secondary electric field perpendicular to the primary field, control over the orientation of the fibers on the collector is obtained. The dual‐field approach is used in conjunction with dual electrodes and a rotating collector. Both approaches were used to electrospin poly(lactic acid) fibers under conditions intended to produce fibers with a diameter greater then 400 nm to aid observation by optical microscopy and digital photography. Digital photography was used to visualize a large area of aligned fiber and image analysis software was used to quantify the degree of alignment. Fibers aligned with the aid of a dual field showed better alignment than those from a single electric field under otherwise identical conditions.

     

In-Situ Synchrotron SAXS and WAXS Investigation on the Deformation of Single and Coaxial Electrospun P(VDF-TrFE)-Based Nanofibers

Coaxial core/shell electrospun nanofibers consisting of ferroelectric P(VDF-TrFE) and relaxor ferroelectric P(VDF-TrFE-CTFE) are tailor-made with hierarchical structures to modulate their mechanical properties with respect to their constituents. Compared with two single and the other coaxial membranes prepared in the research, the core/shell-TrFE/CTFE membrane shows a more prominent mechanical anisotropy between revolving direction (RD) and cross direction (CD) associated with improved resistance to tensile stress for the crystallite phase stability and good strength-ductility balance. This is due to the better degree of core/shell-TrFE-CTFE nanofiber alignment and the crystalline/amorphous ratio. The coupling between terpolymer P(VDF-TrFE-CTFE) and copolymer P(VDF-TrFE) is responsible for phase stabilization, comparing the core/shell-TrFE/CTFE with the pristine terpolymer. Moreover, an impressive collective deformation mechanism of a two-length scale in the core/shell composite structure is found. We apply in-situ synchrotron X-ray to resolve the two-length scale simultaneously by using the small-angle X-ray scattering to characterize the nanofibers and the wide-angle X-ray diffraction to identify the phase transformations. Our findings may serve as guidelines for the fabrication of the electrospun nanofibers used as membranes-based electroactive polymers.     

Epoxy Resin Nanofibers Prepared Using Electrospun Core/Sheath Nanofibers as Templates

A strategy is described to prepare epoxy resin nanofibers by combining coaxial electrospinning and traditional hot‐curing processes. Core/sheath nanofibers with diameters of 480 ± 80 nm are prepared at flow rates of 0.1 and 2 mL h^?1 for the core (20% w/v EP and 6% w/v curing agent in ethanol/acetone) and sheath (10% PVP in ethanol) fluids. After the curing of the nanofibers and selective removal of the sheath PVP, EP nanofibers with an average diameter of 210 ± 60 nm are obtained. ATR‐FTIR analysis shows that the EP nanofibers display no obvious difference compared with an EP film cast from the core solution. The method presented allows to develop functional EP nanoproducts and to prepare heat‐cured resin nanofibers.

     

静电纺再生丝素纳米纤维的结构与性能

静电纺丝获得的丝素纳米级纤维可作为细胞培养支架,用于纺丝工艺及后处理能改变丝素微细结构,影响其水溶性和力学性能。本文采用XRD、FTIR、固态13CNMR和DSC研究了不同工艺下丝素纳米纤维及经甲醇处理后的微细结构,比较了不同微细结构下的水溶性和力学性能。结果表明,电纺丝的微细结构受纺丝工艺影响,高电压、纺丝液中丝素质量分数大时纺得的电纺丝结晶度高,经甲醇处理后,β化程度提高;w(丝素)=11%、15%时制备的电纺丝断裂强度分别为8.5、11.9 cN/mm;w(丝素)=11%、19%,水溶性由51.2%下降到43.3%;w(丝素)=19%、电压32 kV制得的电纺丝甲醇处理前后水溶性从43.3%下降到6.6%,说明丝素纳米纤维结晶度提高,强度增加、水溶性下降,满足了细胞支架的要求。     

Enhanced Piezoelectric Performance of Electrospun Polyvinylidene Fluoride Doped with Inorganic Salts

A novel approach to preparing electrospun polyvinylidene fluoride (PVDF) nanofibers is proposed, with high piezoelectric performance. PVDF nanofibers are doped with inorganic salts without the use of any postpolarization treatment. Twenty‐six salts are doped into the nanofibers and their piezoelectric properties are studied. The salts are classified into three groups based on their differing piezoelectric enhancement effects. A piezoelectric nanogenerator fabricated with an optimized electrospun PVDF nanofiber mat shows a piezovoltage seven times greater than that of a device based on undoped nanofibers. The simple and low‐cost approach to fabricate these piezoelectric nanofiber mats may broaden the range of industrial applications of these materials in energy‐harvesting devices and portable sensors.     

Emission Based Sensing of Subnanomolar Dissolved Carbon Dioxide Exploiting Electrospun Nanofibers

In this work, the authors report a novel emission-based optical sensor for dissolved carbon dioxide. Ethyl cellulose has been used together with an imidazolium based ionic liquid as matrix material. Sensors were fabricated either in form of continuous thin films or nanofibers. The offered composition exhibited enhanced stability and excellent detection limits for sensing of bicarbonate. The preliminary analysis of calibration plots show that the sensitivity of electrospun nanofibrous membranes to detect bicarbonate is at least 5-fold better than that of the thin films. The stability of the indicator in the employed matrix was superior to the previously reported ones.     

Core‐Sheath Structure in Electrospun Nanofibers from Polymer Blends

Summary: Electrospinning of polymer blends offers the potential to prepare functional nanofibers for use in a variety of applications. This work focused on control of the internal morphology of nanofibers prepared by electrospinning polymer blends to obtain core‐sheath structures. Polybutadiene/polystyrene, poly(methylmethacrylate)/polystyrene, polybutadiene/poly(methylmethacrylate), polybutadiene/polycarbonate, polyaniline/polycarbonate, and poly(methylmethacrylate)/polycarbonate blends were electrospun from polymer solutions. It was found that the formation of core‐sheath structures depends on both thermodynamic and kinetic factors. Incompatibility and large solubility parameter difference of the two polymers is helpful for good phase separation, but not sufficient for the formation of core‐sheath structures. Kinetic factors, however, play a much more important role in the development of the nanofiber morphology. During the electrospinning process, the rapid solvent evaporation requires systems with high molecular mobility for the formation of core‐sheath structures. It was found that polymer blends with lower molecular weight tend to form core‐sheath structures rather than co‐continuous structures, as a result of their higher molecular mobility. Rheological factors also affect the internal phase morphology of nanofibers. It was observed the composition with higher viscosity was always located at the center and the composition with lower viscosity located outside.

TEM image of electrospun polybutadiene/polycarbonate nanofibers at 25/75 wt.‐% ratio after staining by osmium tetroxide. The dark regions are polybutadiene and the light region is polycarbonate.     

Hierarchical Structured Electrospun Nanofibers for Improved Fog Harvesting Applications

Collection of clean water from humid air has attracted immense attention in recent years due to the lack of access to pure drinking water among large section of population in several parts of the world. Hence, there is a persistent demand for the fabrication of robust, scalable membranes for efficient harvesting of pure water, especially in fog‐laden areas. Herein, three different membranes based on neat nanofibers, nanofibers with microparticles, and nanofibers with hierarchical structures (nanopillars) are successfully fabricated using poly(vinylidene fluoride‐co‐hexafluoropropylene) and fluorinated polyhedral oligomeric silsesquioxane composite mixture. Neat nanofibers and nanofibers with microparticles are fabricated by employing direct electrospinning and electrospinning combined with electrospraying process, respectively. Hierarchical structured fibers are fabricated by growing nanopillars on the surface of the fibers using electrospinning combined with template‐wetting method. The wettability properties including water contact angle and hysteresis of these membranes are investigated. Due to the increased surface roughness and low surface energy, the hierarchical fibers exhibit higher contact angle (153°) and lower hysteresis (3°) compared to the neat nanofibers and nanofibers with microparticles. Furthermore, the results demonstrate that the presence of nanopillars on the surface of the nanofibers improves the membrane's water collection efficiency when exposed to humid air.

     

Fabrication and Antibacterial Activity of Electrospun Nylon 6 Nanofibers Grafted With 2-Substituted Vinylimidazoles

The authors present the fabrication of electrospun nanofibers with antimicrobial properties by the UV-initiated grafting (photo-grafting) of 2-substituted vinylimidazoles onto nylon 6 nanofibers. The characterization was performed using IR spectroscopy (ATR-FTIR) and scanning electron microscopy (SEM-EDX). The antimicrobial properties of the grafted electrospun nylon 6 nanofibers were evaluated against Escherichia coli and Staphylococcus aureus as model challenge microorganisms, using the dynamic shake flask method. All the grafted electrospun nylon 6 nanofibers exhibited excellent growth reduction of E. coli (99.94–99.99%) and S. aureus (99.55–99.99%). The electrospun nylon 6 nanofiber composites could be used twice before a decrease in antibacterial activity was observed. The study showed that electrospun nylon 6 nanofiber composites possess a potential for use to control pathogens in water.     

Quick Polymerization from Electrospun Macroinitiators for Making Thermoresponsive Nanofibers

Thermoresponsive nanofibers by very fast grafting of N,N‐isopropylacrylamide (NIPAAm) from electrospun atom transfer radical polymerization (ATRP) macroinitiator are presented in this work. The heterogenous grafting of NIPAAm onto macroinitiator fibers could be done in few minutes, i.e., in less than 5 min. The procedure involved electrospinning of an ATRP macroinitiator and subsequent PNIPAAm grafting using “grafting from” technique. The ATRP Macroinitiator was based on a copolymer of methyl methacrylate (MMA) and 2‐hydroxyethyl methacrylate (HEMA). The growth of the PNIPAAm layer on electrospun fibers was followed by IR‐spectroscopy and SEM analysis. The temperature‐dependent‐phase transition was proven by contact angle measurements and could be shown on the same surface for many cycles.