Ag colloids and Ag clusters over EDAPTMS-coated silica nanoparticles: synthesis, characterization, and antibacterial activity against Escherichia coli

Abstract

To produce better antibacterial water-insoluble nanocomposites of silver (Ag), silver-silicon dioxide (Ag-SiO(2)) hybrid and silver colloid (Ag-c) nanoparticles (NPs) were studied. Ag-c NPs were synthesized using reduction of AgNO(3), and Ag-SiO(2) composites were prepared on a core of silica NPs functionalized with ethylenediamino-propyltrimethoxysilane, where Ag clusters were fabricated on amino groups using seed-mediated growth and characterized by transmission electron microscopy and ultraviolet-visible absorption spectroscopy. Antibacterial effectiveness of the Ag-SiO(2) NPs was tested against general Escherichia coli (E. coli ATCC 25922) and E. coli O157:H7 by measuring the growth based on optical density and digital counting of live-dead cells using a fluorescent microscope, and a field emission scanning electron microscope. Minimum inhibitory concentration values were studied against four representative bacteria along with E. coli O157:H7. Results showed that Ag NPs of 6.6 ± 4.5 nm were attached to the surface of SiO(2) NPs (74 ± 13.5 nm), and the Ag-c NPs (3.5 ± 2 nm) showed excellent antibacterial properties.

From the clinical editor: In this paper, the synthesis of Ag colloids and Ag clusters over EDAPTMS-coated silica nanoparticles is reported. Both NPs were examined for antibacterial effectiveness against representative bacteria including E. coli O157:H7 and found to have excellent antibacterial properties.

Figure 1

TEM images (A–D) of Ag-SiO₂ NPs and (E) Ag colloids NPs. (F–H) Size distribution graphs of (F) SiO₂ NPs (mean size = 73.53 nm), (G) Ag NPs linked with SiO₂ (mean size = 6.6 nm), and (H) Ag-c NPs (mean size = 3.5 nm).

Figure 2

UV-visible spectra of (A) aqueous suspension of Ag-SiO₂ NPs at 1000, 100, 10, 1, and 0.1 μg/mL (top to bottom) and (B) aqueous solution of Ag-c NPs at 85, 42.5, 8.5, 0.85, 8.5 × 10⁻², and 8.5 × 10⁻³ μg/mL (top to bottom). (C, D) Antibacterial effect in culture tube after 24 hours’ incubation of (C) Ag-SiO₂ NPs (lefthand tube contains 100 μg/mL Ag-SiO₂ NPs and E. coli, and righthand tube contains only E. coli control); (D) Ag-c NPs (lefthand tube contains 42.5 μg/mL Ag-c NPs with E. coli, and righthand tube contains only E. coli control). (E, F) Growth test of E. coli on agar plates. (E) Lefthand plate contains Ag-SiO₂ NPs + E. coli, and righthand plate contains only E. coli control, which contains ~1.78 × 106 cfu of E. coli. (F) Lefthand plate contains Ag-c NPs + E. coli, and righthand plate contains only E. coli control, which contains ~1.28 × 10⁶ cfu of E. coli.

Figure 3

Growth curve of E. coli (EC) ATCC 25922 (A) in presence of Ag-c NPs at a concentration of 0.85 (dark blue), 8.5 (pink), and 42.5 μg/mL (red) with their positive controls (bacteria only) light blue, yellow, and violet, respectively, and negative control Ag-c NPs only (green line); (B) in presence of Ag-SiO₂ NPs at a concentration of 100 (pink) and 10 (violet) μg/mL, with bacteria only (green and red) and Ag-SiO₂ NPs only (dark blue and light blue), respectively. (C, D) Growth curve for E. coli O157:H7 (C) in presence of Ag-c NPs at concentration of 42.5 μg/mL (pink), bacteria only (yellow), and NPs only (blue); (D) in presence of Ag-SiO₂ NPs at concentration of 100 μg/mL (green), bacteria only (red), and NPs only (light blue).

Figure 4

Colonies count of E. coli ATCC 25922 expressed as a percentage of the number of colonies grown on silver-free LB agar plates. (A) Ag-SiO₂ NPs at 0.1, 1, 10, and 100 μg/mL; (B) colloidal Ag NPs at 0.085, 0.85, 8.5, and 42.5 μg/mL. (C, D) FE-SEM images of (C) damaged E. coli ATCC 25922 and Ag-SiO₂ NPs complex; (D) damaged cell surface of single E. coli bacterium in presence of Ag-SiO₂ NPs.

Figure 5

Fluorescent microscopy images of live and dead bacterial cells in presence of silver NPs, using green and red UV-visible filters, respectively. (A, B) 100 μg/mL Ag-SiO₂ NPs + E. coli ATCC 25922; (C, D) 8.5 μg/mL Ag colloid NPs + E. coli ATCC 25922; (E, F) control having only E. coli ATCC 25922; (G, H) 100 μg/mL Ag-SiO₂ NPs + E. coli O157:H7; (I, J) 8.5 μg/mL Ag colloid NPs + E. coli O157:H7; (K, L) control containing only E. coli O157:H7.

Figure 6

Bar graph of live and dead (A) E. coli ATCC 25922, (B) E. coli O157:H7 in the presence and absence of Ag-c NPs, where bacterial cells counts are expressed as a function of percentage per milliliter.