Silver nanoparticles nucleated in NaOH-treated halloysite: a potential antimicrobial material

Abstract

Despite all recent advances in medical treatments, infectious diseases remain dangerous. This has led to intensive scientific research on materials with antimicrobial properties. Silver nanoparticles (Ag-NPs) are a well-established solution in this area. The present work studied the nucleation of silver on halloysite substrates modified by chemical treatment with NaOH. The resulting stabilized Ag-NPs were characterized by X-ray diffraction, transmission electron microscopy, and energy-dispersive X-ray spectroscopy. The nucleation was characterized by thermogravimetric analysis and differential scanning calorimetry. The antimicrobial properties of the Ag-NPs were investigated against E. coli and S. aureus. The potential of the Ag-NPs for industrial application was tested by dispersing them into low-density polyethylene. The importance of the chemical affinity between matrix and additive was tested through coating the Ag-NPs with dodecanethiol, a non-polar surfactant. The resulting composites were characterized by scanning electron microscopy and in terms of surface antimicrobial activity. The results demonstrate that the Ag-NPs synthesized in this work are indeed antimicrobial, and that it is possible to imbue a polymeric matrix with the antimicrobial properties of Ag-NPs.

Keywords: DIO coating; antimicrobial activity; halloysite; nanocomposites; silver nanoparticles.

Figure 1

Crystal structure of halloysite particles.

Figure 2

Layout of sample dimensions (in mm × mm) and preparation for the polymer antimicrobial test showing (1) film cover, (2) bacterial suspension, (3) LDPE test specimen, (4) Petri dish, and (5) Petri dish cover.

Figure 3

XRD patterns of HNT-8 and Ag/HNT-8.

Figure 4

TEM images of (a) HNT-0, (b) Ag/HNT-0, (c) HNT-4, (d) Ag/HNT-4, (e) HNT-8, and (f) Ag/HNT-8. The insets show the corresponding particle size histogram.

Figure 5

DSC and TGA measurements of Ag-NP formation on HNT-8 substrate from room temperature up to 600 °C in air. TEM image of the samples prepared with the same parameters as those of the DSC/TGA measurements, but the synthesis was stopped at specific temperatures, that is, 65, 105, 230, and 505 °C, to observe the Ag-NP nucleation on the substrates.

Figure 6

Image of AgNP/HNT-8 with different colours after the synthesis process was interrupted at 40, 100, 120, 200, 350, 500, and 700 °C.

Figure 7

Scanning electron microscopy of (a) Ag/HNT-8-doped LDPE and (b) Ag/HNT-8/DIO-doped LDPE.

Figure 8

Molecular structures of polyethylene and dodecanethiol, and scheme of Ag-NPs coated with dodecanethiol.