Nanoparticles and nanofiber combination

Nanoparticles have the potential to exhibit novel properties due to quantum size effect. However, nanoparticles often require a carrier for most practical application. While having a smaller surface area compared to nanoparticles, nanofibers are able to function as a scaffold to hold them and prevents their aggregation. A combination of nanofibers and nanoparticles has the potential to maximise the effective functional output from nanoparticles. There are several ways of combining nanoparticles and nanofibers with the nanoparticles either embedded in the nanofiber matrix or adhere to its surface.

Blending

Blending is one of the most straight forward methods of combining nanoparticles with nanofibers. In this method, nanoparticles are mixed into the solution for electrospinning. Sonication is usually carried out for better dispersion of the particles into the solution. Surfactant may also be added for the same purpose. In most studies, nanoparticles are reported to be well dispersed in electrospun fibers when it has been properly dispersed in the solution prior to electrospinning. However, in some cases, the nanoparticles may require additional assistance to ensure uniform dispersion within the fiber matrix. Zhu et al (2022) used ultrasonic-assisted electrospinning for uniform dispersion of starch-capped Ag nanoparticles in polyvinylpyrrolidone (PVP) fibers. In this setup, the mixture of AgNPs and PVP solution was passed through a tube that coiled round an ultrasonic generator before ending with a metal nozzle. As the solution dispersion flow pass the ultrasonic generator, the ultrasonic vibration breaks up any AgNPs aggregates. The solution dispersion then travels a short distance to the nozzle where a high voltage is applied for electrospinning. Comparing the dispersion of the nanoparticles between electrospinning with and without ultrasonic-assistance, it is apparent in 0.6 wt% concentration of AgNPs formed long strips of aggregated nanoparticles within the fibers without ultrasonic-assistance. With ultrasonic assistance, the AgNPs are evenly dispersed in the electrospun fiber. Hence this setup presents a relatively simple method of dispersing nanoparticles in the solution just before electrospinning.

Apparatus scheme of fabricating AgNPs-PVP nanofiber by ultrasonic-assisted electrospinning [Zhu et al 2022].

TEM images of 0.6 wt.% AgNPs loading on 10 wt.% PVP electrospun fibers without Ultrasonic-Assisted (left) and with Ultrasonic-Assisted (right) [Zhu et al 2022].

Electrospinning is not known to have any detrimental effect to the activity of the nanoparticles. However, since the nanoparticles are encapsulated within the fiber matrix, its activity may be reduced. Zohoori et al (2014) loaded electrospun polyamide 66 nanofibers with nanoparticles (TiO₂, SrTiO₃ and ZnO) through solution blending for the purpose of constructing a self-cleaning material. The electrospun fibers were heat-setted on surface of nylon fabric such that any stain on the electrospun composite fibers may undergo photocatalytic degradation. The layered fabric demonstrated self-cleaning property when stained with Direct Green 6 under UV irradiation. The fabric was able to maintain photo-activity after repeated laundering. Other nanoparticles such as CuO [Haider et al 2015] and MgO [Dhineshbabu et al 2014] has also been added into solution for electrospinning to achieve antibacterial effect.

Top. TEM image of CuO nanoparticles in PLGA/CuO electrospun nanofibers; Bottom: Cu2+ ions release in water from the PLGA/CuO hybrid nanofiber scaffolds with respect to incubation time [Haider et al 2015. Journal of Nanomaterials 2015; 2015: 832762, this work is licensed under a Creative Commons Attribution 3.0 Unported License.]

Core-shell fibers

A core-shell fiber morphology allows a targeted distribution of the nanoparticles, either closer to the surface or at the core. Such a fiber structure can be constructed using a coaxial spinneret where the distribution of the material is determined by the feed solution extrusion either at the outer outlet or the core outlet. Bedford et al (2010) constructed a self-cleaning textile fiber by using coaxial electrospinning. In their setup, the core of the fiber was made from cellulose acetate and the sheath was made from TiO₂ nanoparticles. The core-shell fibers with TiO₂ nanoparticles on the surface were able to fully degrade blue dye stains in 7 to 8 hours

Reduction of precursors

Instead of adding nanoparticles into the solution for electrospinning, inorganic salt may be added to the solution and electrospun to form fibers. As inorganic salt may be dissolved in the solution, its dispersion will be more uniform. A subsequent reduction is performed to convert the salt into nanoparticles. Silver nitrate is commonly used for dissolving in the electrospinning solution followed by photo-reduction using UV irradiation [Son et al 2006, Rujitanaroj et al 2010], thermal reduction at 80 °C [Xu et al 2006] or aging [Rujitanaroj et al 2008] to form silver nanoparticles. UV irradiation of silver nitrate incorporated nanofibers showed that the silver nanoparticles were distributed close to the surface of the nanofibers [Son et al 2006, Rujitanaroj et al 2010]. However, thermal reduction of silver nitrate showed uniform distribution of silver nanoparticles throughout the cross-section of the nanofibers [Xu et al 2006].

Surface adhesion

Electrospinning and electrospraying may be used in combination to modify the property of the membrane. Depending on the desired characteristics of the membrane, electrospinning and electrospraying may be carried out consecutively or concurrently. The former would result in a distinct layer of fibers and nanoparticles while the latter will have a uniform distribution of nanoparticles throughout the fibrous membrane. Zhang et al (2022) constructed a superhydrophobic membrane by electrospraying polydimethylsiloxane (DP8)/SiO₂ microspheres onto an electrospun polyvinylidenefluoride (PVDF) membrane. The presence of polydimethylsiloxane (DP8)/SiO₂ microspheres on the surface of the PVDF membrane increases its hydrophobicity such that an ultra-high hydrophobic angle (162.1°) was reached. The PVDF/DP8/SiO₂ composite membrane had a distinct hierarchical structure with the microspheres forming an obvious honeycomb structure on the base fiber membrane layer.

SEM of pure PVDF nanofiber membrane DP8 composite membrane and electrosprayed DP8-X with different SiO2 content. (a-a2) PVDF, (b-b2) DP8, (c-c2) DP8-0.5% SiO2, (d-d2) DP8-1.0% SiO2 [Zhang et al 2022].

To ensure maximum activity of nanoparticles, it is good to have them on the surface of electrospun fibers. Zheng et al (2014) electrospun cellulose directly into an aqueous suspension of magnesium hydroxide nanoparticles. As the cellulose fibers are formed in the aqueous suspension, the magnesium hydroxide nanoparticles adhered on the surface of the fibers. An advantage of this method is that the distribution of the nanoparticles across the bulk nanofibrous structure is likely to be more uniform than having the coating as a separate process.

When combining electrospun fibers and electrosprayed nanoparticles, it is common to have two sets of apparatus for each of them. However, a coaxial nozzle has been used as a simple means to construct fiber-nanoparticles hybrid films where the fiber is produced by electrospinning and the nanoparticles by electrospraying using the same nozzle. Du et al (2023) demonstrated this by using a coaxial nozzle with the inner orifice made of metal and the outer sheath made of polyethylene. The core and sheath solutions were fed separately and charging of both solutions were via the inner metal orifice. The electrospinnable solution was to be fed through the core while the non-electrospinnable solution was to be fed through the sheath. For electrospinning of the inner core solution, only the inner core solution was fed and for electrospraying, only the outer sheath solution was fed. With this simple alternating of solution feeding either through the inner core or outer sheath solution, electrospinning and electrospraying respectively can be carried out. This setup also allows routine core-shell electrospinning by feeding both solutions simultaneously.

Alternatively, an ultrasonic atomizer [Dong et al 2013], airbrush or other spraying device may be used in place of electrospraying. Spraying may be directed at the electrospinning jet such that the particles are attached to the fiber surface prior to deposition on the mat [Xuyen et al 2009] or at the opposite side of a rotating collector [Jaworek et al 2009]. Almost any kind of nanoparticles can be incorporated on the surface the nanofibers using this method. TiO₂, MgO and Al₂O₃ nanoparticles have been coated uniformly across the thickness of the membrane using simultaneous electrospinning and electrospraying [Jaworek et al 2009].

Chemically active nanoparticles on the surface of the nanofibers will increase the activity of the composite. However there is a risk that the nanoparticles may get dislodged from the nanofiber and causes health and environmental risks. Zhang et al [2013] tested the stability of the TiO₂ particles on nylon-6 membrane by sonication in ultrasonic bath for a few minutes. The internal microstructure of the membrane remains unchanged and this demonstrated the adhesion stability of the particles to the membrane. Post spinning process such as heat treatment may be required to ensure better bonding or fusion between the particles and the fibers. Trejo and Frey (2015) conducted a study of carboxylic acid coated iron oxide nanoparticles (CA-Fe₃O₄ NPs) applied to Nylon 6 nanomembranes using simultaneous electrospinning and electrospraying. Their study showed that over 97% of the nanoparticles remain adhered to the fiber surface after 60 minutes of washing. The retention ability of the nanoparticles on the nanofibers is dependent on the pH of the washing solution.

To enhance bonding of nanoparticles to the surface of nanofibers, the nanofibers and particles may be functionalized for surface conjugation. Li et al (2017) electrospun polyacrylonitrile (PAN) containing 3-aminopropyltriethoxysilane (APS) as precursor material. Amine groups were then introduced to the surface of the electrospun PAN/APS nanofibers for conjugation with prepared negatively charged platinum (Pt) nanoparticles. Due to the uniform distribution of amine groups on the surface of the fibers, the negatively charged Pt nanoparticles were electrostatically attracted to the positively charged amine groups with even distribution with a high Pt nanoparticles loading density of up to 5.61% of the whole amount of hybrid nanofibers.

Multi-layered

Simultaneous electrospinning and electrospraying may be used to incorporate functional nanoparticles on the surface of the nanofibers within the membrane. Al-Dhubhani et al (2023) constructed bipolar membranes (BPMs) with its anion exchange layer made from electrospun Fumasep (FAA-3)/polymeric catalysts poly(4-vinylpyrrolidone) (P4VP) and the cation exchange polymer layer made of electrospun sulfonated poly(ether ether ketone) (SPEEK). A water dissociating catalyst for BPMs, MCM-41 with mesoporous structure of a zeolitic-like framework of amorphous silica, was electrosprayed in the form of nanoparticles onto the electrospun membrane. By using multiple spinnerets, Al-Dhubhani et al (2023) was able to control the dispersion of nanoparticles with the nanofibers. Alternating electrospinning and electrospraying with two or more nozzles produces layers of pure nanofibers and nanofibers dispersed with nanoparticles. Having simultaneous electrospinning and electrospraying have the advantage of having the nanofibers as carriers for nanoparticles. This will maintain an interconnected pore structure.

Using a rotating collector and separate nozzles on either side of the collector allows electrospinning and electrospraying to be carried out simultaneously. Depending on the speed of rotation, the thickness of the membrane with nanoparticles in between can be controlled. Using this setup, Tajik et al (2024) electrosprayed BaTiO₃ nanoparticles on electrospun PVDF nanofibers to increase piezoelectric voltage output. With aluminium foil (Al) as the base layer, the output voltage was the highest with simultaneous electrospinning of PVDF fibers and electrospraying of BaTiO₃ nanoparticles with an output voltage of 3.77 V. This was significantly higher than single layered sets of Al-PVDF (2.09 V), Al-BaTiO₃-PVDF (2.03 V) and Al-PVDF-BaTiO₃ (2.69 V). Having multiple thin and alternating stacked layers of PVDF and BaTiO₃ was able to generate better voltage output. This may be due to better contact and flow of charges between the two piezoelectric materials.

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