Antibacterial properties of silver nanoparticles in three different sizes and their nanocomposites with a new waterborne polyurethane

Abstract

Silver nanoparticles (AgNPs) are strong bactericidal agents but they are also cytotoxic. Embedding them in a polymer matrix may reduce their cytotoxic effect. In the present study, AgNPs in three average sizes were tested for their antibacterial activities and cytotoxicity. Nanocomposites from a new waterborne polyetherurethane (PEU) ionomer and AgNPs were prepared without the use of any crosslinker. It was observed that the antibacterial activity of AgNPs against Escherichia coli started at the effective concentration of 0.1-1 ppm, while that against Staphylococcus aureus started at higher concentrations of 1-10 ppm. Cytotoxicity of AgNPs was observed at the concentration of 10 ppm. AgNPs with smaller average size showed greater antibacterial activity as well as cytotoxicity. The PEU synthesized in this study showed high tensile strength, and the addition of AgNPs at all sizes further increased its thermal stability. The delicate surface features of nanophases, however, were only observed in nanocomposites with either small-or medium-sized AgNPs. PEU-Ag nanocomposites had a strong bacteriostatic effect on the growth of E. coli and S. aureus. The proliferation of endothelial cells on PEU-Ag nanocomposites was enhanced, whereas the platelet adhesion was reduced. The expression of endothelial nitric oxide synthase gene was upregulated on PEU-Ag containing small-sized AgNPs (30 ppm) or medium-sized AgNPs (60 ppm). This effect was not as remarkable in nanocomposites from large-sized AgNPs. Overall, nanocomposites from the PEU and 60 ppm of the medium-sized (5 nm) AgNPs showed the best biocompatibility and antibacterial activity. Addition of smaller or larger AgNPs did not produce as substantial an effect in PEU, especially for the larger AgNPs.

Keywords: antibacterial activity; biocompatibility; polyurethane; silver nanoparticles.

Figure 1

Schematics for the synthesis of waterborne polyetherurethane ionomer.

Figure 2

Antibacterial activity of silver nanoparticles against A) Escherichia coli and B) Staphylococcus aureus, shown as bacterial colony-forming units after exposure to silver nanoparticles of different sizes and concentrations for 24 hours. Double-distilled water served as the control. Silver ion (Ag⁺) of the same concentration was included for comparison. Note: *P < 0.05 relative to double-distilled water.

Figure 3

Cytotoxicity of silver nanoparticles for L929 fibroblasts. The cells were incubated in culture medium containing silver nanoparticles (of different sizes) at the concentrations of 1 ppm (1 p) and 10 ppm (10 p) for 24 hours. Note: *P < 0.05 relative to negative control.

Figure 4

Proliferation of endothelial cells on the waterborne polyurethane (polyetherurethane and polyesterurethane) at 24 and 72 hours. Note: *P < 0.05.

Figure 5

Atomic force microscopy phase images of polyetherurethane-silver nanocomposites containing different sizes and concentrations.

Figure 6

Microbiostatic effects against A) Escherichia coli and B) Staphylococcus aureus by polyetherurethane-silver nanocomposites containing silver nanoparticles of different sizes and concentrations after 24 hours. Notes: *P < 0.05, greater than polyetherurethane; **greater than PEUL15. Abbreviation: PEUL15 polyetherurethane containing 15 ppm large-sized silver nanoparticles.

Figure 7

A) Number and B) mitochondrial activity of endothelial cells grown on polyetherurethane-silver. Notes: *P < 0.05 relative to polyetherurethane; **P < 0.05, ***P < 0.05.

Figure 8

A) The expression of endothelial nitric oxide synthase gene and B) semiquantification of endothelial nitric oxide synthase/GAPDH ratio for endothelial cells cultured on the materials for three days. Note: * P < 0.05 relative to polyetherurethane.