Functionalization of Nanofibers with Metal Nanoparticles Using Novel Electrospinning-Based Methodologies

We present novel electrospinning-based methodologies to fabricate nanofibers (NFs) functionalized with metal nanoparticles (NPs). The composites have the potential as building blocks of functional fabrics with antimicrobial and catalytic properties for biomedical, filtration, sensor and catalytic applications. In one method, the electrospinning polymer acts as the reducing, protecting and templating agent for the NPs at ambient conditions, hence a genuine one-step process via electrospinning. To date, previous studies involve electrospinning (ES) of the polymer and metal salt solution and the resulting NF composite is subjected to either thermal, chemical or radiolytic (UV and microwave irradiation) reduction process. Others incorporated separately prepared metal NPs (usually entail the several steps and use of organic solvents and corrosive reducing agents) into the polymer solution after which the polymer-NP solution is electrospun. In this study, to the best of our knowledge, an organic solvent- and corrosive reducing agent-free model system involving poly(ethylene oxide) (PEO)-AgNO3-water system to generate Ag NP-PEO polymer fiber nanocomposites via electrospinning all in one-step at ambient conditions, is presented for the first time. The process is clean, green and energy efficient, since it is water based and performed at ambient conditions.

For PEO at molecular weights (MWs) that are electrospinnable (600-2000 kDa) and dissolved in water at concentrations in the range of 2-4 wt %, we show that Ag+ ions in the PEO aqueous solution (at concentrations 2 to 12 wt % AgNO3 based on PEO) transform to Ag NPs at ambient conditions (presumably through the formation of pseudocrown ethers and their complexation with the Ag+ ions), without the use of any additional reducing agent and stabilizer. Such transformation is not possible at PEO MWs lower than 20 kDa. Ultra Violet (UV)/Visible (Vis) absorption spectrophotometry shows that the chemical reduction of Ag+ ions is almost complete within 4 hours and MW dependent (Figure 1). The resulting Ag NPs are spherical with sizes between 5-10 nm, crystalline and are well-dispersed at or near the surface of NF with fiber diameters between 150-300 nm as observed from transmission electron microscopy (TEM) analyses. The incorporation of Ag NPs to the polymer solution significantly improved fiber quality by decreasing fiber diameter and dramatically minimizing bead formation from bead-forming PEO solution as seen from scanning electron microscopy (SEM) (Figures 2A1 and 2B1). These effects we believe to be due in large part to increased solution electrical conductivity and viscosity.

Figure 2. (A1, B1) SEM and (A2, B2) TEM images of nanofibers from pure PEO and PEO-AgNO3 solutions after 4 h reduction ; (C1) XRD and(C2) XPS  spectra of electrospun nanofiber mats from PEO-AgNO3 aqueous solution after 4 h reduction.

 

We report interesting phenomena such as NP protrusions (Figure 2B2) and NP alignment (to form nanochains) on the NF surface, both of which were found to be primarily associated with the applied electrical field. A possible explanation and mechanism for NP protrusions and alignment during electrospinning are proposed. X-ray photoelectron spectroscopy (XPS) (Figure 2C2) and x-ray diffraction (XRD) analyses (Figure 2C1) corroborate the presence of Ag NPs on the NF matrix (Figure 2). We also report the use of these metal NP-functionalized nanofibers as templates to fabricate hollow nanofibers (nanotubes) loaded with silver (Ag) nanoparticles (NPs) by atomic layer deposition (ALD). The resulting Ag NPs are spherical, crystalline and are well-dispersed within the nanotubes as observed from TEM and XRD analyses.