Green synthesis and antibacterial effects of aqueous colloidal solutions of silver nanoparticles using camomile terpenoids as a combined reducing and capping agent

Abstract

Green synthesis method using camomile extract was applied to synthesize silver nanoparticles to tune their antibacterial properties merging the synergistic effect of camomile and Ag. Scanning transmission electron microscopy revealed that camomile extract (CE) consisted of porous globular nanometer sized structures, which were a perfect support for Ag nanoparticles. The Ag nanoparticles synthesized with the camomile extract (AgNPs/CE) of 7 nm average sizes, were uniformly distributed on the CE support, contrary to the pure Ag nanoparticles synthesized with glucose (AgNPs/G), which were over 50 nm in diameter and strongly agglomerated. The energy dispersive X-ray spectroscopy chemical analysis showed that camomile terpenoids act as a capping and reducing agent being adsorbed on the surface of AgNPs/CE enabling their reduction from Ag(+) and preventing them from agglomeration. Fourier transform infrared and ultraviolet-visible spectroscopy measurements confirmed these findings, as the spectra of AgNPs/CE, compared to pure CE, did not contain the 1109 cm(-1) band, corresponding to -C-O groups of terpenoids and the peaks at 280 and 320 nm, respectively. Antibacterial tests using four bacteria strains showed that the AgNPs/CE performed five times better compared to CE AgNPs/G samples, reducing totally all the bacteria in 2 h.

Keywords: Antibacterial properties; Camomile; Green synthesis; Silver nanoparticles.

Fig. 1

DLS spectrum of size distribution by number for Ag nanoparticles synthesized in the camomile extract

Fig. 2

STEM HAADF image with the corresponding EDX maps of I pure camomile extract used for the synthesis; II Ag particles synthesized in the camomile extract; III Ag particles synthesized with glucose; M chemical formula of terpenoids showing the presence of phosphorus

Fig. 3

Typical FTIR absorption spectra of: a pure camomile extract CE and b camomile extract synthesized silver nanoparticles AgNPs/CE, c AgNPs/G synthesized by the Ag⁺ reduction with glucose

Fig. 4

UV-vis spectra for: CE (black) showing the two peaks corresponding to terpenoids, for AgNPs/CE (red) and AgNPs/G (green)—only one peak from silver is visible (color figure online)

Fig. 5

a Antibacterial test of camomile extract (CE), plant extract with silver nanoparticles (AgNPs/CE) and pure nanoparticles synthesized with glucose (AgNPs/G) against S. aureus. b The percentage of microorganism (S. aureus) reduction exposed to the tested nanosuspensions: AgNPs/G (above) and AgNPs/CE (below). c The flowchart for the evaluation of colony forming unit (CFU)

Fig. 6

The mean zone of inhibition against various bacteria strains using agar diffusion method. The wells contain: camomile extract synthesized silver nanoparticles (AgNPs/CE)—left side or pure nanoparticles (AgNPs/G)—right side, respectively

Fig. 7

MICs of nanomaterials against bacterial strains. Different concentrations of nanosolutions, were estimated from the concentration of silver nitrate, which was used as precursor, and were exposed to S. aureus, P. aeruginosa, B. subtilis and E. coli. White bars on the photograph denote the MICs for this experiment