Layer by Layer treatment

Figure 1. Functionalization of electrospun fibers by Layer-by-Layer treatment.

The layer-by-layer method is based on the adhesion of positively and negatively charged macromolecules on the surface of the base material. Cellulose acetate with its negatively charged surface is an attractive material for this. Otherwise, to initiate the process, the surface of the material may first be activated through chemical or other means. Subsequent alternate introduction of opposing charged molecules are used to build up the layers. Both charged organic and inorganic molecules has been incorporated into electrospun fibers.

An advantage of this method is that the layers of functional molecules added can be selected either to enhance a single property or to introduce multiple properties. Sensors with layers of different functionalities can be constructed using this concept. The high surface area of electrospun nanofibers has prompted many researchers to explore the possibility of constructing highly sensitive sensors in combination with the layer-by-layer assembly technique [Wang et al 2004, Ding et al 2006]. While most layer-by-layer coating is through the use of alternately charged organic molecules, positively charged inorganic nanoparticles may also be coated on negatively charged organic molecules (eg. Polyacrylic acid). Coated nanoparticles on the surface of the fiber have been shown to increase its surface roughness and a final coating of fluoroalkylsilane has used to create a superhydrophobic membrane [Ogawa et al 2007]. With sufficient build-up of the external layer, a tube structure can be fabricated by removal of the inner electrospun fiber. Ding et al [2005] fabricated polyoxometalate nanotubes by thermal removal of the electrospun nanofiber core template.

Table 1. Selected examples of layer-by-layer treated electrospun fibers.

Polymer	Pre-treatment reagent	Alternate layers sequence	Reference
Polystyrene	Concentrated sulfuric acid for sulfonation of polymer surface	Fluorescein isothiocyanate-labeled poly(allylamine hydrochloride) Poly(styrene sulfonate, sodium salt)	Muller et al 2006
Polystyrene	Concentrated sulfuric acid for sulfonation of polymer surface	Polyethylenimine Poly(thymidine) in SSC buffer Poly-adenosine-block-guanosine (polyA15G15) in SSC buffer Polythymidine-block-cytidine in SSC buffer	Muller et al 2006
Cellulose Acetate	None	Poly(allylamine hydrochloride) Hydrolyzed poly[2-(3-thienyl)ethanol butoxy carbonyl-methyl urethane] (fluorescent conjugated polymer)	Wang et al 2004
Cellulose Acetate	None	Polyethylenimine Polyoxometalate (H3PMo12O40)	Ding et al 2006
Cellulose Acetate	None	Polyethylenimine Polyoxometalate (H3PMo12O40)	Ding et al 2006
Cellulose Acetate	None	TiO2 Poly(acrylic acid)	Ogawa et al 2007
Cellulose Acetate	None	TiO2 Poly(acrylic acid) fluoroalkylsilane (FAS, CF3(CF2)7(CH2)2Si(OCH3)3	Ding et al 2004
Cellulose Acetate	NaOH	Chitosan Alginate	Deng et al 2010
Poly(L-lactic acid)	None	Poly(ethyleneimine) pGL3 (plasmid DNA)	Sakai et al 2009

Sandua et al (2022) used layer-by-layer deposition technique to immobilize metal oxide on the surface of electrospun fibers in the construction of membrane for photocatalytic degradation of organic dye. Electrospinning was used to construct a base layer of poly(acrylic acid) (PAA)/β-Cyclodextrin (β-CD) fibers . PAA functions as a polyanion due to its property of negative charge hence the electrospun PAA/β-CD forms the first layer. Metal oxide particles with their positive charge were first sonicated in water before the electrospun fiber membrane was dipped into it. Photocatalytic TiO₂ particles were able to adhere to the electrospun membrane due to the negative charges on PAA. Photocatalytic enhanced metal oxide particles such as WO₃ and Fe₂O₃ were added to PAA anionic solutions to form the next layer on the TiO₂ coated membrane to produce a functional coating. When irradiated with both UV and visible light, photocatalytic degradation of methylene blue with membrane containing TiO₂ only, TiO₂ and WO₃ only, and all three TiO₂, WO₃ and Fe₂O₃ was 65%, 75% and 85% respectively. Photocatalytic activity of TiO₂ depends on irradiation in the UV spectrum. The addition of WO₃ decreases the band gap and facilities photocatalytic activity under visible light. The presence of Fe₂O₃ reduced the band gap of TiO₂ thus increasing the photocatalytic activity under visible and solar light for dye degradation.

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