19 Applications of Electrospun Nanofibers

Dr. Anchal Srivastava

Contents of this Unit

1.  Introduction

2.  Nanofiber Reinforcement

3.  Filtration

4.  Catalysis

(i)   Chemical Catalyst

(ii)   Photocatalysts

5.  Chemical Sensors

5.1. Humidity Sensors

5.2. Gas Sensors

6.  Lithium ion batteries

7.  Fuel Cells

8.  Biosensors

9.  Water Treatment

9.1. Heavy metal ion adsorption

9.2. Adsorption of organic compounds

10. Summary

Learning Outcomes

After studying this module, you shall be able to learn the applications of electrospun fibers for varieties of applications.

1. Introduction

With the development of electrospinning, diverse materials have been successfully used in electrospinning for nanofibers including organic materials, organic/inorganic materials, and inorganic materials with the diameter range from microscale down to nanoscale. Additionally, variety of architectures based on electrospun fibers have been realized such as solidified fibers, porous (surface pores or interior pores) fibers, hollow fibers, core–shell fibers, hierarchical structured fibers. Those unique advantages of electrospinning afford the multi-functional properties for diverse applications, including nanofiber reinforcement, filtration, catalysis, electronic devices, lithium-ion battery, fuel cells, biomedical field, etc.

In this section we will present the applications of electrospun nanostructures in nanofiber reinforcement, filtration, catalysis, electronic devices, lithium-ion battery, fuel cells, and biomedical field.

2. Nanofiber Reinforcement:

One of the important applications based on traditional fibers at microscale such as carbon fiber, glass fiber is to act as fiber reinforcement in composite systems. With the aid of these fiber reinforcements, the composite materials exhibit superior structural properties including high modulus and strength to weight ratios. Promoted by these practical applications based on traditional fibers, electrospun nanofibers have been investigated in fiber reinforcement as well, driven by their better mechanical properties than bulk fiber.

Several research papers have been published on the usage of electrospun nanofibers-reinforced composites in dental restorative matrix. It has been also found that reinforcement of nanofibers is suitable to improve the interfacial adhesion between the nanofiber nonwoven and dental methacrylate of BIS-GMA/TEGDMA.

Natural electrospun nanofibers have also been investigated in nanofiber-reinforced composite for biodegradable applications. Researchers have explored the potentialities of ultra-fine cellulose electrospun nanofibers as reinforcing agent of polybutylene succinate (PBS) for biodgradable composite and electrospun cellulose nanofibers to reinforce poly vinyl alcochol (PVA) films for biodegradable composite.

3. Filtration:

Filtration is one of the important fields of engineering. It was estimated that future filtration market would be up to US $700b by the year 2020. For high filtration efficiencies, it is generally necessary that the size of the channels and pores in the filter materials should match the scale for the particles or droplets that is to be captured in the filter. Taking the advantage of very high specific surface area, good interconnectivity of porous, low basis weight, high permeability, and controllable pore sizes of the elctrospunn nanofibers, electrospun nanofiber non-wovens have been widely used in air and liquid filtration applications. Usually, the particle filtration occurs via multiple collection mechanism such as sieving, direct interception, inertial impaction, diffusion, and electrostatic collection.

Till now, many highly efficient nanofilters made up of nanofibers have been realized. For example, few authors have demonstrated the high-efficiency particulate air (HEPA) filters based on Nylon 6 electrospun nanofibers (diameter: 80–200 nm; basis weight 10.75 g/m2) which exhibits minimum removal efficiency of 99.97 % of particles greater than or equal to 300 nm in diameter. Another report suggest the separation of micro–sub-micron polystyrene particles (0.5, 1, 6, and 10 lm) from water based on electrospun Nylon 6 nanofibers (diameter 30–100 nm). As the diameter of the polystyrene particles is greater than or equal to 1 lm, the removal efficiency can be 100 %. When the diameter of polystyrene is 0.5 lm, the removal efficiency is higher than 90 %.

It is well known that heating, ventilating, and air conditioning (HVAC) air filters usually operated in dark, damp, and ambient temperature conditions, which is susceptible for bacterial, mold, and fungal attacks, resulting in unpredictable deterioration and bad odor. To solve this problem, functionalization of the surface of filtering media with antimicrobial agents for long-lasting durable antimicrobial functionality is of current interest. In 2007, Jeong and Youk explored the electrospun polyurethane cationomer (PUCs) nanofiber mats with different amounts of quaternary ammonium groups in antimicrobial air filter. They found that PUCs exhibited very strong antimicrobial activities against Staphylococcus aureus and Escherichia coli (Fig. 1).

Fig. 1. Antimicrobial activities of PUCs after incubation for 24 h: (a-1) blank and (a-2) PUCs against Staphylococcus aureus, and (b-1) blank and (b-2) PUCs against Escherichia coli.

4. Catalysis:

It is well known that nanostructured materials have opened new possibilities for creating and mastering nanoobjects for novel advanced catalytic materials. In general, catalysis is a molecular phenomenon and the reaction occurs on an active site. A crucial step in catalysis is how to remove and recycle the catalyst after the reaction. The immobilization of catalysts in materials with large surface area advances an interesting solution to this problem. Taking the large surface area and high porosities, electrospun nanofibers, as a novel catalysts or supports for catalysts, have been widely investigated in catalytic field.

4.1. Chemical Catalysts:

In 2004,    Hou    and    Reneker    firstly    demonstrated    simple     in    situ    electrospinning route       for directly depositing Fe nanoparticles with different sizes on carbon nanofibers as shown in Fig. 2. In their study, PAN and Fe (acetylacetonate)3 were used as the precursors; carbonization and hydrogen reduction were used to convert the precursors into the Fe/carbon composite fibers. With those Fe nanoparticles as chemical catalysts, carbon nanotubes can grow on the surface of carbon nanofibers with controllable length by supplying the hexane vapor with different time as shown in Fig. 3.

Fig. 2. TEM images of the carbon nanofibers containing Fe nanoparticles (black dots), made from precursor PAN fibers and Fe (acetylacetonate)3 by calcination. The molar ratio of Fe (acetylacetonate)3 to PAN is a 1:2 and b 1:1, respectively. The size of Fe nanoparticles is 10 nm a and 20 nm b, respectively.

 

Fig. 3. Carbon nanotubes with controllable length on carbon nanofibers by controlling the supplying time of hexane vapor. The time of (a) and (b) was 3 and 5 min, respectively. The argon flow rate was 600 mL/min.

4.2. Photocatalysts:

Photocatalytic performances based on diverse electrospun ceramic (TiO2, ZnO, SnO2, and so on) nanostructures (nanofibers and nanotubes) have been carried on. With the development of the photocatalysts, scientists found the fast recombination rate of the photogenerated electron/hole pairs hinders this technique in practice. To solve this problem, three general routes have been developed:

Route 1:

Coupling semiconductor of metal oxides with different band-gap widths is an effective route to enhance the photocatalytic activity by increasing the separation of photogenerated electron/hole pairs and extending the energy range of the photoexcitation by virtue of the different Fermi levels of the ingredients in the composite materials. Few work suggest that the photocatalytic performances based on ZnO/TiO2 composite (15.76 wt.%) nanofiber toward the decomposition of Rhodamine B (RhB) exhibited the most efficient photocatalytic activity in contrast to other ZnO/TiO2 composite fibers, TiO2 particles, and pristine TiO2 nanofibers. The enhanced photocatalytic performances can be illustrated as follows: Upon light-activation, the electron transfers from the conduction band of ZnO to that of TiO2. Simultaneously, the hole transfer from the valence band of TiO2 to that of ZnO. Thus, efficient charge separation increased the lifetime of the charge carriers and reduces the recombination of the hole-electron pairs in the composite nanofibers, resulting in increasing the quantum efficiency as shown in Fig. 4.

Fig. 4. Schematic diagram illustrating the principle of charge separation and photocatalytic activity of the ZnO/TiO2 composite nanofiber systems.

Route 2:

Loading noble metals (Au, Pt, Ag) on the surfaces of semiconductor can also improve the photocatalytic activities, which can reduce the recombination of the photogenerated electron/hole pairs and lengthen the photogenerated electron/hole pairs lifetime through the conduction band electron trapping. It has been observed that the optimal photocatalytic activity of Ag–ZnO nanofibers could exceed that of pure ZnO nanofiber by a factor of more than 25 (Fig. 5a) and the photocatalytic activity of the composite can be tuned by adjusting the Ag contents (Fig. 5b). The mechanism can be explained as follows: As the Ag–ZnO systems illuminated by UV light with photon energy higher than the band gap of ZnO, electrons in the valence band can be excited to the conduction band with the same amount of holes left in the valence band. For the work function of ZnO (5.2 eV) is higher than that of Ag (4.26 eV), the electrons will migrate from ZnO to Ag for the Fermi-level equilibration, increasing the lifetime of the photogenerated electron/hole pairs (Fig. 5c). Additionally, the Ag nanoparticles can also act as the bridge to transfer the photogenerated electrons to the dye in solution.

Route 3:

Construction of mesoporous structures within the semiconductor to provide large surface area and mesochannels in which the large surface area can offer more active adsorption sites and photocatalytic reaction centers and the mesochannels can act as light-transfer path for introducing incident photo flux onto the inner surface of samples, resulting in enhanced photocatalytic activity.

Fig. 5. (a) Kinetics of the photodegradation of an aqueous solution of RhB by Ag–ZnO composite fibers with different Ag contents. (b) Degradation rate constants for composites with different Ag contents. (c) Schematic illustrations of the band structure–related photocatalytic mechanism for Ag/ZnO composite.

5. Chemical Sensors:

1D nanofibers fabricated by electrospinning have specific surface approximately one to two orders of the magnitude larger than flat films, making them good candidates for sensing applications.

5.1. Humidity Sensors

Humidity sensors are very important driven by their practical applications in environment monitoring industrial process control, our daily life, and so on. Till known, many humidity sensors based on nanostructure materials have been successfully obtained for the large surface areas of nanoscale structures. Excellent sensing performances could be attributed to the unique structures of electrospun nanofibers: (1) The large surface of the nanofiber makes the absorption of water molecules on the surface of sample easily; (2) 1D continuous structures of the fibers can facilitate fast mass transfer of the water molecules to and from the interaction region as well as improve the rate for charge carriers to transverse the barriers induced by molecular recognition along the fibers; (3) In contrast to traditional 2D nanoscaled films, the interfacial areas between the active sensing region of the nanofibers and the underly substrate is greatly reduced. These advantages result in significant gain in the sensing signal and good stability. Due to such fabulous properties of electrospun fibers, diverse ultra-sensitive humidity nanosensors have been developed based on electrospun nanofibers including pristine metal oxide fibers, basic metal salt-doped metal oxide fibers, coupled metal oxide fibers, and conducting polymer fibers.

5.2. Gas Sensors

Gas sensor, as a special device to detect the presence of different gases, gained special attention for monitoring environmental pollution. Usually, gas-sensing performance features such as sensitivity, selectivity, response and recovery, stability, durability, reproducibility, and reversibility are influenced by the intrinsic properties of the sensing materials used. Till now, many attempts have been carried out to prepare gas sensors based on electrospun nanofibrous membranes including acoustic wave, resistive, photoelectric, and optical gas sensor.

6. Lithium-Ion Batteries

Among diverse energy storage devices, rechargeable lithium-ion batteries with high energy density, long cycle lives, flexible design, low self-discharge, high operating voltage, and no ‘‘memory effect,’’ are regarded as the effective solution to the ever increasing demand for high–energy density electrochemical power sources.

Fig. 6. Schematic illustration of lithium-ion battery, consisting of anode and cathodeseparated by one electrolyte containing dissociated lithium salts.

Typical lithium-ion battery consists of an anode and a cathode separated by an electrolyte containing dissociated lithium salts, enabling the transfer of lithium ions between the two electrodes as shown in Fig. 6. Usually, the electrolyte membrane is a porous separator film to prevent the physical contact between the anode and cathode. As the battery is being charged, external electrical power source injects electrons into the anode. Simultaneously, the cathode gives up some of its lithium ions, which move through the electrolyte to the anode and remain there; thus, electricity is stored in the battery in the form of chemical energy. As the battery is discharging, the lithium ions move back across the electrolyte to the cathode, releasing the electrons to the outer circuit for the external electrical work. In principle, normal powder materials have a long diffusion path for lithium ions and show electrode reaction kinetics, which reduce their practical applications. Electrospun nanofibers gain special attentions in fabricating electrodes within lithium-ion battery driven by the shorter diffusion path in contrast to the commonly employed powder materials, fast intercalation kinetics for the large surface-to volume ratio, and decreased charge-transfer resistance at the interface between the electrolyte and active electrode materials for the large number of lithium insertion sites can be provide based on it. Furthermore, electrospun-based separator within lithium-ion battery also attracts immensely attentions for their small pore size and large porosity, which can decrease the cell resistance.

7. Fuel Cells

Similar to the lithium-ion batteries, fuel cells also contain three parts: cathode (positive side), anode (negative), and electrolyte. In general, fuel cells are devices in which chemical energy from a fuel is converted into electrical in the presence of catalyst (Fig. 7). Hydrogen is the most common fuel, but hydrocarbons such as natural gas and alcohols like methanol are sometimes used. Fuel cells are different from batteries in that they require a constant source of fuel and oxygen to run, but they can produce electricity continually for as long as these inputs are supplied. Fuel cells are mainly classified by the type of electrolyte they used (e.g., proton. exchange membrane (PEM) fuel cells, alkaline fuel cells, phosphoric acid fuel cells, molten carbonate fuel cells, and solid oxide fuel cells). Recently, electrospinning technique has been induced into the fuel cells for fabricating high efficient catalysts and proton exchange membranes.

Fig. 7. Schematic illustration of fuel cells, consisting of anode and cathode separated by electrolyte.

In principle, electrospun nanofibers-based catalysts and proton exchange membranes offer the good opportunity for high efficient fuel cells driven by their large surface-to-volume rate and controlled diversity in compositions.

8. Biosensors

Electrospun products gain special attention for sensitive detection of clinical, environmental, and food safety analytes driven by their extremely large surface area for providing a number of binding sites available for biological recognition element immobilization and diverse functionalization. Currently, electrospun nanofibers, nanowires, and nanotubes are frequently investigated for use in biosensors. For example, it has been observed that poly(vinyl alcohol) (PVA) nanofibers containing acetylcholinesterase (AChE) by electrospinning a melt of PVA, AChE, and bovine serum albumen (BSA) as an enzyme stabilizer. The AChE-modified nanofibers exhibited a 40 % activity recovery after electrospinning. Additionally, the enzymes within the nanofibers had a higher stability (the immobilized AChE retained > 34 % of its initial activity when stored at 30 °C for 100 days and retained 70 % of its initial activity after ten consecutive reactor batch cycles) in acidic solutions when compared to free enzymes. Recently, the incorporation of molecularly imprinted polymers (MIPs) within nanofiber networks to construct high-sensitivity analytical systems has also been developed. For example, Haupt and Gheber fabricated PVA nanofibers to support MIP (imprinted with the fluorescent amino acid derivative dansyl-Lphenylalanine) nanoparticles. Kim and Chang fabricated polyimide nanofibers imprinted using a diamine monomer template for binding and detecting estrone. Wang and Yu reported a facile route to fabricate gold nanoparticlespoly(vinyl alcohol) (Au NPs-PVA) hybrid water-stable nanofibrous mats with tunable densities of Au NPs and further demonstrate the potential application of as-prepared Au NPs-PVA nanofibrous mats as efficient biosensor for the detection of H2O2.

9. Water Treatment

9.1 Heavy Metal Ion Adsorption

Heavy metals are a serious biological problem in aquatic systems. Adsorption and filtration are the commonly used methods for removal of these contaminants. Recently, electrospun silk nanofiber mats have been applied in removing heavy metal ions. Higher adsorption capacity (1.65–2.88 mg/g) was obtained based on electrospun nanofiber mats in contrast to that (0.71 mg/g) of the conventional materials like wool silver for the larger surface area of the fiber mats. In addition to those polymeric nanofiber mats, metal oxide nanofiber mats have also applied in heavy metal adsorption.

9.2 Adsorption of Organic Compounds

Organic materials in drinking water can pose health hazards and also need to be removed. Kaur et al. explored the removal of phenolphthalein as a model organic molecule from water using a poly(methylmethacrylate) (PMMA) nanofiber membrane functionalized with phenylcarbomylated and azidophenylcarbomylated b-cyclodextrins. The results obtained showed that the functionalized nanofibrous membranes were able to effectively capture the PHP molecules. Metal oxide fiber mats also exhibited good ability in organic compounds removal from water for its filtration and photocatalytic ability. Zhang and Sun used titanium oxide nanofiber mats to remove humic acid in water. In their study, TiO2 nanowire membrane achieved near 100 and 93.6 % removal rate of humic acid and total organic carbon (TOC), respectively, via a concurrent filtration and photocatalytic oxidation. The TiO2 nanowire membrane also showed excellent antifouling ability owing to the photodegradation of foulants by the TiO2 nanowire membrane.

10.  Summary:

• In this module, you study the applications of electrospun fibers in various fields due to their excellent properties.

Suggested Reading

(i) Text book of “Electrospun Nanofibers and Their Applications” by Ji-Huan He, Yong Liu, Lu-Feng Mo, Yu-

Qin Wan and Lan Xu.

(ii) Text book  of  “One-Dimensional  Nanostructures  Electrospinning  Technique  and  Unique Nanofibers”

by Zhenyu Li. Ce Wang.

(iii) Text book of “An Introduction to Electrospinning and Nanofibers” by Seeram Ramakrishna, Kazutoshi

Fujihara, Wee-Eong Teo, Teik-Cheng Lim, Zuwei Ma.